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United States Patent5195091
Farwell , ; et al.March 16, 1993
Title
Adaptive synchronization arrangement
Abstract
A CDMA cellular radio-telephone system (FIG. 2) has switching systems (201) synchronized to public telephone network (100) timing signals (600), and radio telephones (203) and cell base stations (202) synchronized to a different clock (1000). Transmission delays between the cell base stations and the telephone network are variable. Switching systems include digital communications interfaces (264) to the telephone system, whose connections to the telephone system are synchronized to the telephone system, and whose connections to the cells are nominally also synchronized to the telephone system but whose processor (602) operates for each call within predefined windows (1302, 1402) of phase relationships to the operation of the cell that is handling the call, and occasionally adjusts (FIGS. 13-16) its phase relationships to the operation of the telephone system to achieve and maintain its operation within the predefined windows. Packet-switched communications (350) between the cells and the switching systems absorb the phase relationship fluctuations and the timing adjustments in inter-packet intervals. Circuit-switched signal communications between the switching systems and the telephone system absorb the timing adjustments by means of vocoder (604)-implemented slips--bit insertions or deletions--in the communications traffic bit stream.
Inventors:Farwell; Charles Y. (Denver, CO), Hearn; Michel L.  (Broomfield, CO), Heidebrecht; Richard M.  (Boulder, CO), Ho; Kelvin K.  (Somerset, NJ), Spencer; Douglas A.  (Boulder, CO)
Assignee:AT&T Bell Laboratories (Murray Hill, NJ)
Appl. No.:727492
Filed:July 9, 1991
Current U.S. Class:370/336 370/349 370/350 370/518 370/519 375/356 455/438 455/442 455/502 
Field of Search:370/93,94.1,100.1,103,18,95.3,95.1 375/38,40,106,107 455/33.1,33.2,33.4,51.1,51.2,54.1,56.1,69 379/60,63
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Other References
T H. Murray "The Evolution of DDS Networks: Part I", Telecommunications, Feb. 1989, pp. 39-47. .
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Primary Examiner: Safourek; Benedict V.
Attorney, Agent or Firm:Volejnicek; David
Claims

We claim:
1. A communications system comprising:
a first unit for receiving incoming communication traffic at times dictated by first clock signals having a nominal frequency and a first phase;
a second unit for transmitting incoming communication traffic at times dictated by clock signals having the nominal frequency;
a third unit for interfacing communications between the first and the second units by transmitting to the first unit incoming communication traffic received from the second unit at times dictated by clock signals having the nominal frequency and having a second phase that is offset by an adjustably fixed amount from the first phase;
a communications medium connecting the second unit with the third unit and conveying communication traffic transmitted by the second unit to the third unit for reception, the communications medium having a fluctuating transmission delay;
first means for determining whether the third unit receives incoming communication traffic from the second unit within predetermined windows of time prior to the times of transmission by the third unit of the received incoming communication traffic; and
second means responsive to a determination that receptions of the incoming communication traffic at the third unit fall outside of the windows for either increasing or decreasing the amount of the offset of the second phase from the first phase to move the receptions into the windows.

2. The system of claim 1 wherein:
the second means adjust the offset at commencing of a communication in one step by any amount required to move the receptions substantially into centers of the windows, and adjust the offset during the communication in a series of sequential steps.

3. The system of claim 1 further comprising:
third means cooperative with the second means for inserting additional traffic into the incoming communication traffic received by the third unit and transmitting the additional traffic to the first unit while the amount of the offset is being increased, and for deleting a portion of the incoming communication traffic received by the third unit from the incoming communication traffic transmitted to the first unit while the amount of the offset is being decreased.

4. A communications system comprising:
a first unit for transmitting outgoing communication traffic at times dictated by first clock signals having a nominal frequency and a first phase;
a second unit for transmitting outgoing communication traffic at times dictated by clock signals having the nominal frequency;
a third unit for interfacing communications between the first and the second units by transmitting to the second unit outgoing communication traffic received from the first unit at times dictated by clock signals having the nominal frequency and having a second phase that is offset by an adjustably fixed amount from the first phase;
a communications medium connecting the second unit with the third unit and conveying communications traffic transmitted by the third unit to the second unit for reception, the communications medium having a fluctuating transmission delay;
first means for determining whether the second unit receives outgoing communication traffic from the third unit within predetermined windows of time prior to the times of transmission by the second unit of the received outgoing communication traffic; and
second means responsive to a determination that receptions of the outgoing communication traffic at the second unit fall outside of the windows for either increasing or decreasing the amount of the offset of the second phase from the first phase to move the receptions into the windows.

5. The system of claim 4 wherein:
the second means adjust the offset at commencing of a communication in one step by any amount required to move the receptions substantially into centers of the windows, and adjust the offset during the communication in a series of sequential steps.

6. The system of claim 4 further comprising:
third means cooperative with the second means for inserting additional traffic into the outgoing communication traffic transmitted from the first unit and causing the third unit to receive the additional traffic while the amount of the offset is being increased, and for deleting a portion of the outgoing communication traffic transmitted by the first unit from the outgoing communication traffic received by the third unit while the amount of the offset is being decreased.

7. A communications system comprising:
a first unit for transmitting outgoing communication traffic and receiving incoming communication traffic at times dictated by first clock signals having a nominal frequency and a first phase;
a second unit for transmitting received incoming and outgoing communication traffic at times dictated by second clock signals having the nominal frequency;
a third unit for interfacing communications between the first and the second units by transmitting to the second unit outgoing communication traffic received from the first unit at times dictated by third clock signals having the nominal frequency and having a second phase that is offset by an adjustably fixed first amount from the first phase, and by transmitting to the first unit incoming communication traffic received from the second unit at times dictated by fourth clock signals having the nominal frequency and having a third phase that is offset by an adjustably fixed second amount from the first phase;
a communications medium connecting the second unit with the third unit and conveying communication traffic transmitted by, respectively, the second unit and the third unit to, respectively, the third unit and the second unit for reception, the communications medium having a fluctuating transmission delay;
first means for determining whether the second unit receives outgoing communication traffic from the third unit within first predetermined windows of time prior to the times of transmission by the second unit of the received outgoing communication traffic;
second means for determining whether the third unit receives incoming communication traffic from the second unit within second predetermined windows of time prior to the times of transmission by the third unit of the received incoming communication traffic; and
third means responsive to a determination that either receptions of the outgoing communication traffic at the second unit fall outside of the first windows or receptions of the incoming communication traffic at the third unit fall outside of the second windows for either increasing or decreasing the amount of the offset of either the second phase or the third phase from the first phase to move the receptions that fall outside of their corresponding windows into the corresponding windows.

8. The system of claim 7 wherein:
the third means are responsive to a determination by the first means that receptions of the outgoing communication traffic at the second unit lag the first windows, for decreasing the amount of the offset of the second phase from the first phase.

9. The system of claim 8 wherein:
the third means are further responsive to a determination by the first means that receptions of the outgoing communication traffic at the second unit lead the first windows, for increasing the amount of the offset of the second phase from the first phase.

10. The system of claim 7 wherein:
the third means are responsive to a determination by the second means that receptions of the incoming communication traffic at the third unit lag the second windows, for increasing the amount of the offset of the third phase from the first phase.

11. The system of claim 10 wherein:
the third means are further responsive to a determination by the second means that receptions of the incoming communication traffic at the third unit lead the second windows, for decreasing the amount of the offset of the third phase from the first phase.

12. The system of claim 7 wherein:
the third means adjust the first amount and the second amount of offset at commencing of a communication each in one step by any amount required to move the receptions that fall outside of their corresponding windows into the corresponding windows, and adjust the first amount and the second amount of offset during the communication in a series of sequential steps by an integral multiple of a same predetermined amount during each step.

13. The system of claim 7 further comprising:
fourth means cooperative with the third means for inserting additional traffic into the outgoing communication traffic transmitted from the first unit and causing the third unit to receive the additional traffic while the amount of the offset of the second phase is being increased and deleting a portion of the outgoing communication traffic transmitted by the first unit from the outgoing communication traffic received by the third unit while the amount of the offset of the second phase is being decreased, and for inserting additional traffic into the incoming communication traffic received by the third unit and transmitting the additional traffic to the first unit while the amount of the offset of the third phase is being increased and deleting a portion of the incoming communication traffic received by the third unit from the incoming communication traffic transmitted to the first unit while the amount of the offset of the third phase is being decreased.

14. The system of claim 7 wherein:
the first unit is for transmitting a stream of outgoing communication traffic and receiving a stream of incoming communication traffic;
the second unit is for transmitting packets of received incoming communication traffic, and receiving packets of outgoing communication traffic for transmission of the outgoing communication traffic;
the third unit includes
fourth means responsive to receipt of the stream of outgoing communication traffic from the first unit for packetizing the received outgoing communication traffic and transmitting the packets of the received outgoing communication traffic to the second unit at times dictated by the third clock signals, and
fifth means responsive to receipt of the packets of the incoming communication traffic from the second unit for depacketizing the received incoming communication traffic and transmitting the depacketized received incoming communication traffic toward the first unit at times dictated by the fourth clock signals;
the first means are for determining whether the second unit receives the packets of outgoing communication traffic from the third unit within the first predetermined windows; and
the second means are for determining whether the third unit receives the packets of incoming communication traffic from the second unit within the second predetermined windows.

15. A call-traffic processing apparatus for a cellular radio-telephone system that includes the apparatus, a communications medium connected to the apparatus and having a fluctuating transmission delay, at least one cell connected to the communications medium and each for transmitting first packets containing first frames of coded incoming call traffic received from a radio telephone to the apparatus across the medium and for transmitting to the radio telephone outgoing call traffic received across the medium from the apparatus in second packets containing second frames of coded outgoing call traffic, and a mobile-telephone switching system for interconnecting cells with each other and with a telephone network by routing a first digital stream of incoming call traffic received from the apparatus to a destination and by routing a second digital stream of outgoing call traffic received from a source to the apparatus, and wherein the first and the second digital streams are synchronized with first clock signals having a nominal frequency and a first phase and derived from the telephone network, and the transmissions of the incoming and the outgoing call traffic by the cell are synchronized with second clock signals having the nominal frequency, the call-traffic processing apparatus comprising:
outgoing vocoder means for receiving the second digital stream of outgoing call traffic synchronously with the first clock signals, coding the received outgoing call traffic, and transmitting second frames of the coded outgoing call traffic synchronously with third clock signals having the nominal frequency and a second phase that is offset by an adjustably fixed first amount from the first phase,
incoming vocoder means for receiving first frames of the coded incoming call traffic synchronously with fourth clock signals having the nominal frequency and a third phase that is offset by an adjustably fixed second amount from the first phase, decoding the received incoming call traffic, and transmitting the second digital stream of incoming call traffic synchronously with the first clock signals;
outgoing processing means for receiving the second frames of the coded outgoing call traffic from the outgoing vocoder means, packetizing the received second frames into the second packets, and transmitting the second packets to the cell synchronously with fifth clock signals having the nominal frequency and a fourth phase that is offset by an adjustably fixed third amount from the first phase;
incoming processing means for receiving the first packets from the cell, depacketizing the received first packets into the first frames, and transmitting the first frames to the incoming vocoder means synchronously with sixth clock signals having the nominal frequency and a fifth phase that is offset by an adjustably fixed fourth amount from the first phase;
clock signal generating means for deriving the third, the fourth, the fifth, and the sixth clock signals from the first clock signals;
first means for determining whether the incoming processing means receive the first packets within first predetermined windows of time prior to the times of transmission of the first frames by the incoming processing means to the incoming vocoder means;
second means for determining whether the cell receives the second packets within second predetermined windows of time prior to the times of transmission by the cell of the received outgoing call traffic to the mobile telephone; and
third means responsive to a determination that either receptions of the first packets fall outside of the first windows or receptions of the second packets fall outside of the second windows for causing the clock signal generating means to either increase or decrease the amounts of the offsets of either both the second and the fourth phases or both the third and the fifth phases, with respect to the first phase, to move the packet receptions that fall outside of their corresponding windows into the corresponding windows.

16. The apparatus of claim 15 wherein:
the third means are responsive to a determination by the first means that receptions of the first packets at the incoming processing means lag the first windows, for causing the clock signal generating means to increase the amounts of the offsets of both the third and the fifth phases from the first phase by a same amount.

17. The apparatus of claim 16 wherein:
the third means are responsive to a determination by the first means that receptions of the first packets at the incoming processing means lead the first windows, for causing the clock signal generating means to decrease the amount of the offset of both the third and the fifth phases from the first phase by a same amount.

18. The apparatus of claim 15 wherein:
the third means are responsive to a determination by the second means that receptions of the second packets at the cell lag the second windows, for causing the clock signal generating means to decrease the amounts of the offsets of both the second and the fourth phases from the first phase by a same amount.

19. The apparatus of claim 18 wherein:
the third means are responsive to a determination by the second means that receptions of the second packets at the cell lead the second windows, for causing the clock signal generating means to increase the amounts of the offsets of both the second and the fourth phases from the first phase by a same amount.

20. The apparatus of claim 15 wherein:
the third means cause the clock signal generating means to adjust either the first and the third amounts of offset or the second and the fourth amounts of offset, in one step by any amount required to move any packet receptions that fall outside of their corresponding windows into the corresponding windows at a commencing of a communication, and cause the clock signal generating means to adjust either the first and the third amounts of offset or the second and the fourth amounts of offset by an integral multiple of a same predetermined amount in each step of a series of sequential steps to move packet receptions that fall outside of their corresponding windows into the corresponding windows during the communication.

21. The apparatus of claim 15 further including:
means cooperative with the third means for causing the outgoing vocoder means to insert traffic additional to the outgoing call traffic received by the outgoing vocoder means into the second frames transmitted by the outgoing vocoder means while the amounts of the offsets of the second and the fourth phases are being increased, causing the outgoing vocoder means to delete a portion of the outgoing call traffic received by the outgoing vocoder means from the second frames transmitted by the outgoing vocoder means while the amounts of the offsets of the second and the fourth phases are being decreased, causing the incoming vocoder means to insert traffic additional to the incoming call traffic received by the incoming vocoder means into the first digital stream transmitted by the incoming vocoder means while the amounts of the offsets of the third and the fifth phases are being increased, and causing the incoming vocoder means to delete a portion of the incoming call traffic received by the incoming vocoder means from the first digital stream transmitted by the incoming vocoder means while the amounts of the offsets of the third and the fifth phases are being decreased.

22. A method of operating a communications system comprising a first unit for transmitting incoming communications, a second unit for receiving incoming communications, a third unit for interfacing communications between the first and the second units, and a communications medium having a fluctuating transmission delay and connecting the first unit with the third unit, the method comprising the steps of:
transmitting incoming communication traffic from the first unit at times dictated by first clock signals having a nominal frequency;
conveying the communication traffic transmitted by the first unit via the communications medium to the third unit for reception;
receiving the transmitted incoming communication traffic at the third unit;
transmitting from the third unit to the second unit the incoming communication traffic received from the first unit at times dictated by clock signals having the nominal frequency and having a first phase that is offset by an adjustably fixed amount from a second phase;
receiving incoming communication traffic transmitted from the third unit at the second unit at times dictated by clock signals having the nominal frequency and the second phase;
determining whether the third unit receives incoming communication traffic from the first unit within predetermined windows of time prior to the times of transmission by the third unit of the received incoming communication traffic; and
either increasing or decreasing the amount of the offset of the first phase from the second phase, in response to a determination that receptions of the incoming communication traffic at the third unit fall outside of the windows, to move the receptions into the windows.

23. The method of claim 22 wherein
the step of either increasing or decreasing the amount of the offset comprises the steps of:
adjusting the offset at commencing of a communication in one step by any amount required to move the receptions substantially into centers of the windows; and
adjusting the offset during the communication in a series of sequential steps.

24. The method of claim 22 further comprising the steps of:
inserting additional traffic into the incoming communication traffic received by the third unit and transmitting the additional traffic to the second unit while the amount of the offset is being increased; and
deleting a portion of the incoming communication traffic received by the third unit from the incoming communication traffic transmitted to the second unit while the amount of the offset is being decreased.

25. A method of operating a communications system comprising a first unit for transmitting communications, a second unit for receiving outgoing communications, a third unit for interfacing communications between the first and the second units, and a communications medium having a fluctuating transmission delay and connecting the third unit with the second unit, the method comprising the steps of:
transmitting outgoing communication traffic from the first unit at times dictated by clock signals having a nominal frequency and a first phase;
receiving at the third unit the outgoing communication traffic transmitted by the first unit;
transmitting from the third unit to the second unit outgoing communication traffic received from the first unit at times dictated by clock signals having the nominal frequency and having a second phase that is offset by an adjustably fixed amount from the first phase;
conveying the communication traffic transmitted by the third unit via the communications medium to the second unit for reception;
receiving at the second unit the outgoing communication traffic transmitted by the third unit;
transmitting outgoing communication traffic from the second unit at times dictated by clock signals having the nominal frequency;
determining whether the second unit receives outgoing communication traffic from the third unit within predetermined windows of time prior to the times of transmission by the second unit of the received outgoing communication traffic; and
either increasing or decreasing the amount of the offset of the second phase from the first phase, in response to a determination that receptions of the outgoing communication traffic at the second unit fall outside of the windows, to move the receptions into the windows.

26. The method of claim 25 wherein
the step of either increasing or decreasing the amount of the offset comprises the steps of:
adjusting the offset at commencing of a communication in one step by any amount required to move the receptions substantially into centers of the windows; and
adjusting the offset during the communication in a series of sequential steps.

27. The method of claim 25 further comprising the steps of:
inserting additional traffic into the outgoing communication traffic transmitted from the first unit and causing the third unit to receive the additional traffic while the amount of the offset is being increased; and
deleting a portion of the outgoing communication traffic transmitted by the first unit from the outgoing communication traffic received by the third unit while the amount of the offset is being decreased.

28. A method of operating a communications system comprising a first unit for transmitting outgoing communication traffic and receiving incoming communication traffic, a second unit for transmitting received incoming and outgoing communication traffic, a third unit for interfacing communications between the first and the second units, and a communications medium having a fluctuating transmission delay and connecting the second unit with the third unit, the method comprising the steps of:
transmitting outgoing communication traffic from the first unit and receiving incoming communication traffic at the first unit at times dictated by first clock signals having a nominal frequency and a first phase;
transmitting received incoming and outgoing communication traffic from the second unit at times dictated by second clock signals having the nominal frequency;
receiving at the third unit the outgoing communication traffic transmitted by the first unit;
transmitting from the third unit to the second unit outgoing communication traffic received from the first unit at times dictated by third clock signals having the nominal frequency and having a second phase that is offset by an adjustably fixed first amount from the first phase;
conveying the outgoing communication traffic transmitted by the third unit via the communications medium to the second unit for reception;
conveying the incoming communication traffic transmitted by the second unit via the communications medium to the third unit for reception;
receiving at the third unit the incoming communication traffic transmitted by the second unit;
transmitting from the third unit to the first unit incoming communication traffic received from the second unit at times dictated by fourth clock signals having the nominal frequency and having a third phase that is offset by an adjustably fixed second amount from the first phase;
determining whether the second unit receives outgoing communication traffic from the third unit within first predetermined windows of time prior to the times of transmission by the second unit of the received outgoing communication traffic;
determining whether the third unit receives incoming communication traffic from the second unit within second predetermined windows of time prior to the times of transmission by the third unit of the received incoming communication traffic; and
either increasing or decreasing the amount of the offset of either the second phase or the third phase from the first phase, in response to a determination that either receptions of the outgoing communication traffic at the second unit fall outside of the first windows or receptions of the incoming communication traffic at the third unit fall outside of the second windows, to move the receptions that fall outside of their corresponding windows into the corresponding windows.

29. The method of claim 28 wherein
the step of either increasing or decreasing the amount of the offset comprises the step of:
decreasing the amount of the offset of the second phase from the first phase, in response to a determination that receptions of the outgoing communication traffic at the second unit lag the first windows.

30. The method of claim 29 wherein
the step of either increasing or decreasing the amount of the offset further comprises the step of:
increasing the amount of the offset of the second phase from the first phase, in response to a determination that receptions of the outgoing communication traffic at the second unit lead the first windows.

31. The method of claim 28 wherein
the step of either increasing or decreasing the amount of the offset comprises the step of:
increasing the amount of the offset of the third phase from the first phase, in response to a determination that receptions of the incoming communication traffic at the third unit lag the second windows.

32. The method of claim 31 wherein
the step of either increasing or decreasing the amount of the offset further comprises the step of:
decreasing the amount of the offset of the third phase from the first phase, in response to a determination that receptions of the incoming communication traffic at the third unit lead the second windows.

33. The method of claim 28 wherein
the step of either increasing or decreasing the amount of the offset comprises the steps of:
adjusting the first amount and the second amount of offset at commencing of a communication each in one step by any amount required to move the receptions that fall outside of their corresponding windows into the corresponding windows; and
adjusting the first amount and the second amount of offset during the communication in a series of sequential steps by an integral multiple of a same predetermined amount during each step.

34. The method of claim 28 further comprising the steps of:
inserting additional traffic into the outgoing communication traffic transmitted from the first unit and causing the third unit to receive the additional traffic while the amount of the offset of the second phase is being increased;
deleting a portion of the outgoing communication traffic transmitted by the first unit from the outgoing communication traffic received by the third unit while the amount of the offset of the second phase is being decreased;
inserting additional traffic into the incoming communication traffic received by the third unit and transmitting the additional traffic to the first unit while the amount of the offset of the third phase is being increased; and
deleting a portion of the incoming communication traffic received by the third unit from the incoming communication traffic transmitted to the first unit while the amount of the offset of the third phase is being decreased.

35. The method of claim 28 in a communications system wherein the first unit is for transmitting a stream of outgoing communication traffic and receiving a stream of incoming communication traffic, and the second unit is for transmitting packets of received incoming communication traffic, and receiving packets of outgoing communication traffic for transmission of the outgoing communication traffic, wherein:
the step of receiving at the third unit the outgoing communication traffic transmitted by the first means comprises the steps of
receiving the stream of outgoing communication traffic from the first unit, and
packetizing the received outgoing communication traffic;
the step of transmitting from the third unit to the second unit outgoing communication traffic comprises the step of
transmitting the packets of the received outgoing communication traffic to the second unit at times dictated by the third clock signals;
the step of receiving at the third unit the incoming communication traffic transmitted by the second unit comprises the steps of
receiving the packets of the incoming communication traffic from the second unit, and
depacketizing the received incoming communication traffic;
the step of transmitting from the third unit to the first unit incoming communication traffic comprises the step of
transmitting the depacketized received incoming communication traffic toward the first unit at times dictated by the fourth clock signals;
the step of determining whether the second unit receives outgoing communication traffic comprises the step of
determining whether the second unit receives the packets of outgoing communication traffic from the third unit within the first predetermined windows; and
the step of determining whether the third unit receives incoming communication traffic comprises the step of
determining whether the third unit receives the packets of incoming communication traffic from the second unit within the second predetermined windows.

36. A method of processing call traffic in an interface arrangement of a cellular radio-telephone system that includes the arrangement, a communications medium connected to the apparatus and having a fluctuating transmission delay, at least one cell connected to the communications medium and each for transmitting first packets containing first frames of coded incoming call traffic received from a radio telephone to the arrangement across the medium and for transmitting to the radio telephone outgoing call traffic received across the medium from the arrangement in second packets containing second frames of coded outgoing call traffic, and a mobile-telephone switching system for interconnecting cells with each other and with a telephone network by routing a first digital stream of incoming call traffic received from the arrangement to a destination and by routing a second digital stream of outgoing call traffic received from a source to the arrangement and wherein the first and the second digital streams are synchronized with first clock signals having a nominal frequency and a first phase and derived from the telephone network, and the transmisions of the incoming and the outgoing call traffic by the cell are synchronized with second clock signals having the nominal frequency, the method comprising the steps of:
receiving the second digital stream of outgoing call traffic synchronously with the first clock signals;
coding the received outgoing call traffic;
transmitting second frames of the coded outgoing call traffic synchronously with third clock signals having the nominal frequency and a second phase that is offset by an adjustably fixed first amount from the first phase;
receiving the second frames of the coded outgoing call traffic;
packetizing the received second frames into the second packets;
transmitting the second packets to the cell via the communications medium synchronously with fifth clock signals having the nominal frequency and a fourth phase that is offset by an adjustably fixed third amount from the first phase;
receiving the first packets from the cell via the communications medium;
depacketizing the received first packets into the first frames;
transmitting the first frames synchronously with sixth clock signals having the nominal frequency and a fifth phase that is offset by an adjustably fixed fourth amount from the first phase;
receiving the first frames of the coded incoming call traffic synchronously with fourth clock signals having the nominal frequency and a third phase that is offset by an adjustably fixed second amount from the first phase;
decoding the received incoming call traffic;
transmitting the second digital stream of incoming call traffic synchronously with the first clock signals;
determining whether the interface arrangement receives the first packets within first predetermined windows of time prior to the times of transmission of the first frames by the interface arrangement;
determining whether the cell receives the second packets within second predetermined windows of time prior to the times of transmission by the cell of the received outgoing call traffic to the mobile telephone; and
either increasing or decreasing the amounts of the offsets of either both the second and the fourth phases or both the third and the fifth phases, with respect to the first phase, in response to a determination that either receptions of the first packets fall outside of the first windows or receptions of the second packets fall outside of the second windows, to move the packet receptions that fall outside of their corresponding windows into the corresponding windows.

37. The method of claim 36 wherein
the step of either increasing or decreasing the amounts of the offsets comprises the step of:
increasing the amounts of the offsets of both the third and the fifth phases from the first phase by a same amount, in response to a determination that receptions of the first packets at the interface arrangement lag the first windows.

38. The method of claim 37 wherein
the step of either increasing or decreasing the amounts of the offsets further comprises the step of:
decreasing the amounts of the offsets of both the third and the fifth phases from the first phase by a same amount, in response to a determination that receptions of the first packets at the interface arrangement lead the first windows.

39. The method of claim 36 wherein
the step of either increasing or decreasing the amounts of the offsets comprises the step of:
decreasing the amounts of the offsets of both the second and the fourth phases from the first phase by a same amount, in response to a determination that receptions of the second packets at the cell lag the second windows.

40. The method of claim 39 wherein
the step of either increasing or decreasing the amounts of the offsets further comprises the step of:
increasing the amounts of the offsets of both the second and the fourth phases from the first phase by a same amount, in response to a determination that receptions of the second packets at the cell lead the second windows.

41. The method of claim 36 wherein
the step of either increasing or decreasing the amounts of the offsets comprises the steps of:
adjusting either the first and the third amounts of offset or the second and the fourth amounts of offset, in one step by any amount required to move any packet receptions that fall outside of their corresponding windows into the corresponding windows at a commencing of a communication; and
adjusting either the first and the third amounts of offset or the second and the fourth amounts of offset by an integral multiple of a same predetermined amount in each step of a series of sequential steps to move packet receptions that fall outside of their corresponding windows into the corresponding windows during the communication.

42. The method of claim 36 further comprising the steps of:
inserting traffic additional to the outgoing call traffic into the second frames while the amounts of the offsets of the second and the fourth phases are being increased;
deleting a portion of the outgoing call traffic from the second frames while the amounts of the offsets of the second and the fourth phases are being decreased;
inserting traffic additional to the incoming call traffic into the first digital stream while the amounts of the offsets of the third and the fifth phases are being increased; and
deleting a portion of the incoming call traffic from the first digital stream while the amounts of the offsets of the third and the fifth phases are being decreased.
Description
CROSS-REFERENCE TO RELATED APPLICATIONS

B. D. Bolliger, T. P. Bursh, Jr., M. K. Dennison, M. J. English, C. Y. Farwell, M. L. Hearn, R. M. Heidebrecht, K. K. Ho, K. Y. Ho, D. M. Kissel, P. E. Miller, R. D. Miller, A. S. Mulberg, L. N. Roberts, M. A. Smith, K. F. Smolik, D. A. Spencer, K. W. Strom, and J. S. Thompson, and R. A. Windhausen, "Wireless Access Telephone-to-Telephone Network Interface Arrangement", Ser. No. 07/727,498 filed on even date herewith and assigned to the same assignee;

C. Y. Farwell, M. L. Hearn, R. M. Heidebrecht, K. K. Ho, and D. A. Spencer, "Adaptive Synchronization Arrangement", Ser. No. 07/727,491, filed on even date herewith and assigned to the same assignee; and

B. D. Bolliger, T. P. Bursh, Jr., K. K. Ho, A. S. Mulberg, L. N. Roberts, K. F. Smolik, D. A. Spencer, K. W. Strom, and J. S. Thompson, "Mobile Telephone System Call Processing Arrangement", Ser. No. 07/727,520 filed on even date herewith and assigned to the same assignee.

TECHNICAL FIELD

This invention relates generally to telecommunications arrangements wherein transmission delays between communicating units are not predetermined.

BACKGROUND OF THE INVENTION

It sometimes occurs in digital telecommunications systems that customer-premises communications equipment is timed independently of the network communications equipment (e.g., the public switched telephony network) that interconnects the customer-premises equipment. A particularly significant example thereof is the code-division multiplexed-access (CDMA) radio-telephone system, which is an important type of digital cellular mobile-telephone system. In the CDMA system, nodes that contain radios, i.e., the mobile radio-telephones and cell-site base stations (cells for short), are synchronized to clock signals received by the cells from a global-positioning system (GPS) satellite, whereas the radio-telephone switching systems which interconnect the base stations with each other and with the public telephone network by means of digital communications are synchronized to clock signals which may also be received from the GPS satellite but are distributed by the telephone network.

For purposes of this discussion, two series of events, signals, or operations are considered to be synchronized with each other, or synchronous, if (a) they either occur at the same nominal frequency or one occurs at a frequency that is an integral multiple of the frequency of the other, and (b) they occur in a fixed phase relationship with one another. Operations that are not synchronous are considered to be asynchronous for purposes of this discussion.

The independent timing of the operation of different units of a communication system destroys the assumption that the units provide call traffic to each other at a predetermined steady and unvarying frequency at steady and unvarying points in time i.e. a fixed phase. Rather, independent timing results in the units providing call traffic to each other at a rate and at points in time that fluctuate about a fixed frequency and phase. This asynchrony must be compensated for somehow.

Independent timing is but one cause of this asynchrony. Another cause that may be present in communication systems, such as the CDMA radio-telephone system mentioned above, is the lack of a predetermined and fixed transmission delay between the communicating units. Assuming that both the originating and the destination units are timed either by a common clock or by different clocks that are synchronized with each other, if the transmission delay between the units is fixed and pre-determinable, it can be compensated for in the communication system design such as to allow the units to operate synchronously with each other. But if the delay cannot be predetermined but is variable and fluctuates, the net effect is the same as if the units were independently timed. The fluctuation in the delay may be a result of, for example, occasional changes in the transmission paths that are followed by communications moving between communicating units, or variances in the communication traffic load that flows between--and that must be handled by--the communicating units. This asynchrony must likewise be compensated for.

A partial though inadequate solution to the problems caused by independent timing is to conduct communications between the communicating units in analog instead of digital form. Analog communications can be received asynchronously with their transmission. And while the asynchrony may introduce errors or "glitches" into the communications, the problem is often tolerable for voice-only communications. Thus, in the CDMA radio-telephone system, the radio-telephone switching systems may also be synchronized to the GPS satellite clock signals and hence operate synchronously with the radio-telephones and base stations, if the switching systems are interfaced to the telephone network via analog voice-only communications. Of course, such an arrangement suffers all of the disadvantages that are associated with analog communications, such as low quality and capacity and susceptibility to interference, plus the problem of asynchrony-induced glitches that make the arrangement unsuitable for data communications.

Likewise, a partial though inadequate solution to the problems caused by fluctuating transmission delays is to circuit-switch communication traffic, whereby the dependency of transmission delay on communication traffic load is avoided. However, circuit-switching is inefficient or undesirable for other reasons in many applications. Furthermore, circuit switching does not eliminate fluctuation of transmission delay that is caused by changes in the transmission path, such as will typically arise during CDMA call "soft handoff".

SUMMARY OF THE INVENTION

This invention is directed to solving these and other disadvantages of the prior art. Broadly according to the invention, in a telecommunications system that interconnects communicating units via a transmission medium that experiences fluctuating transmission delay (such as a packet-switched communications medium, for example) there is provided an interface between the communicating units which is nominally synchronized with ones of the units but which operates within a predefined window, i.e., a range, of phase relationships to the operation of the others of the units and occasionally adjusts its otherwise-fixed phase relationship with the operation of the ones of the units to achieve and maintain its operation within the predefined window. The operations of the various units thereby effectively become synchronized with the operations of the interface arrangement, and thus are able to proceed substantially as if they were synchronized with each other and interconnected by a transmission medium having a fixed transmission delay.

More specifically, in a communications system that comprises a first unit for either receiving incoming communication traffic or transmitting outgoing communication traffic at times dictated by first clock signals having a nominal frequency and a first phase, a second unit for either transmitting incoming communication traffic to the first unit or transmitting outgoing communication traffic received from the first unit at times dictated by clock signals having the nominal frequency, a third unit for interfacing communications between the first and the second units, and a communications medium that connects the second unit with the third unit to convey communication traffic therebetween and that has a fluctuating transmission delay, the interfacing operations are conducted as follows. The third unit either transmits to the first unit incoming communication traffic received from the second unit or transmits to the second unit outgoing communication traffic received from the first unit at times dictated by clock signals having the nominal frequency and having a second phase that is offset by an adjustably fixed amount from the first phase. A determination is made of either whether the second unit receives outgoing communication traffic from the third unit within predetermined windows of time prior to the times of transmission by the second unit of the received outgoing communication traffic, or whether the third unit receives incoming communication traffic from the second unit within predetermined windows of time prior to the times of transmission by the third unit of the received incoming communication traffic. Then, in response to a determination that receptions of either communication traffic type fall outside of the respective predetermined windows, the amount of the offset of the second phase from the first phase is either increased or decreased, as necessary, in order to move the subject receptions into the respective windows.

Specifically according to an illustrative embodiment of the invention, in a CDMA radio-telephone system having radio-telephone switching systems and base station (cells) interconnected by a packet transmission medium, each radio-telephone switching system includes a digital communications interface arrangement whose connections to the telephone network are synchronized to the operation of the telephone network, and whose connections to the base stations (cells) are nominally also synchronized to the operation of the telephone system but which operates for each individual call within a predefined window of phase relationships to the operation of a base station that is handling the call and occasionally adjusts its phase relationship with the operation of the telephone system to compensate for transmission delay fluctuations experienced by the medium so as to achieve and to maintain its operation within the predefined window. Since the interface arrangement utilizes packet-switched communications between the base stations and the switching system, the phase relationship fluctuations and the timing adjustments are absorbed and hidden by variations in inter-packet intervals. Further according to the illustrative embodiment, however, the interface arrangement utilizes circuit-switched communications between the switching system and the telephone network, wherein the timing adjustments are absorbed by slips--bit insertions or deletions-- in the communications bit stream.

The perceived asynchrony between the operations of the switching systems and telephone network and the base stations (cells) which results from transmission delay fluctuations, that are caused by variations in the communications traffic load carried by the medium or by changes in the transmission paths taken by the communication traffic during soft handoff, are compensated for and accommodated by the interface arrangement, as required for proper digital communications system operation.

While the discussion of an illustrative embodiment that follows makes a distinction between level-3 "packets" and level-2 "frames", for purposes of clarity, the use of the term "packet" herein and in the claims is intended to encompass either or both "packets" and "frames".

These and other advantages and features of the invention will become apparent from the following description of an illustrative embodiment of the invention considered together with the drawing.

BRIEF DESCRIPTION OF THE DRAWING

FIG. 1 is a block diagram of a conventional cellular radio-telephone system;

FIG. 2 is a block diagram of a cellular radio-telephone system that incorporates an illustrative embodiment of the invention;

FIG. 3 is a block diagram of a cell of the system of FIG. 2;

FIG. 4 is a block diagram of a cell interconnect module of the system of FIG. 2;

FIG. 5 is a block diagram of a speech coding module of the system of FIG. 2;

FIG. 6 is a block diagram of a speech processing unit of the module of FIG. 5;

FIG. 7 is a block diagram of a LAPD frame of the system of FIG. 2;

FIG. 8 is a block diagram of a modified LAPD frame of the system of FIG. 2;

FIG. 9 is a block diagram of a level-3 protocol used for carrying voice and/or signalling information in the frames of FIGS. 7 and 8;

FIG. 10 is a block diagram of a level-3 protocol used for carrying signalling information in the frames of FIGS. 7 and 8;

FIGS. 11-14 are a flow diagram of received-packet processing functions of the processor of the unit of FIG. 6;

FIG. 15 is a flow diagram of transmit-packet processing functions of the processor of the unit of FIG. 6;

FIG. 16 is a flow diagram of clock adjustment functions of a cluster controller of the cell of FIG. 3;

FIG. 17 is a flow diagram of clock adjustment functions of the processor of the unit of FIG. 6 performed at step 970 of FIG. 11;

FIG. 18 is a flow diagram of clock adjustment functions of the processor of the unit of FIG. 6 performed at step 912 of FIG. 11;

FIG. 19 is a timing diagram of packet-transmission clock-adjustments performed at call setup for a service circuit of the unit of FIG. 6;

FIG. 20 is a timing diagram of packet-reception clock-adjustments performed at call setup for a service circuit of the unit of FIG. 6;

FIG. 21 is a timing diagram of packet-transmission clock-adjustments performed during an established call for a service circuit of the unit of FIG. 6;

FIG. 22 is a timing diagram of packet-reception clock-adjustments performed during an established call for a service circuit of the unit of FIG. 6;

FIG. 23 is a signalling diagram of setup of a mobile-originated call in the system of FIG. 2;

FIG. 24 is a signalling diagram of setup of a network-originated call in the system of FIG. 2;

FIG. 25 is a signalling diagram of a mobile-originated disconnection of a call in the system of FIG. 2;

FIG. 26 is a signalling diagram of a network-originated disconnection of a call in the system of FIG. 2;

FIG. 27 is a signalling diagram of the beginning of a soft-handoff of a call in the system of FIG. 2;

FIG. 28 is a signalling diagram of the end of a soft-handoff wherein a master cell drops off;

FIG. 29 is a signalling diagram of the end of a soft-handoff wherein a slave cell drops off;

FIG. 30 is a signalling diagram of a mobile-originated disconnection of a call during soft-handoff in the system of FIG. 2;

FIG. 31 is a signalling diagram of a network-originated disconnection of a call during soft-handoff in the system of FIG. 2;

FIG. 32 is a signalling diagram of a semi-soft-handoff of a call in the system of FIG. 2;

FIG. 33 is a signalling diagram of a CDMA-to-CDMA hard-handoff of a call in the system of FIG. 2;

FIG. 34 is a signalling diagram of a CDMA-to-analog hard-handoff of a call between cells served by the same digital cellular switch in the system of FIG. 2; and

FIG. 35 is a signalling diagram of a CDMA-to-analog hard-handoff of a call between cells served by different digital cellular switches in the system of FIG. 2.

DETAILED DESCRIPTION

Before commencing a discussion of an illustrative implementation of the invention, it may be helpful to consider an existing cellular mobile radio-telephone system to serve as a basis for comparison. Such a system is shown in FIG. 1. A description of such a system may be found in K. W. Strom, "On the Road with AUTOPLEX System 1000", AT&T Technology, Vol. 3, No. 3, 1988, pp. 42-51, and W. J. Hardy and R. A. Lemp, "New AUTOPLEX Cell Site Paves The Way For Digital Cellular Communications", AT&T Technology, Vol. 5, No. 4, 1990, pp. 20-25.

The system of FIG. 1 includes a plurality of geographically-dispersed service nodes known as cell sites, or cells 102 for short, each one of which provides radio-telephone services to wireless user terminals, known as mobile radio-telephones 103, in its vicinity. To provide radio-telephone service between mobile radio-telephones 103 served by different cells 102, and between mobile radio-telephones 103 and the public telephone network 100, cells 102 are interfaced to each other and to network
100 through mobile radio-telephone switching nodes referred to herein as digital cellular switches (DCSs) 101. Each switch 101 is illustratively the AT&T Autoplex.RTM. cellular telecommunications system digital cellular switch. Each digital cellular switch 101 is connected to a plurality of different cells 102 by communication trunks 107, and is connected to network 100 by communication trunks 106. Each trunk 106 and 107 is illustratively a DS0 (64 Kbps time-division multiplexed) channel, a plurality of which are implemented by a DS1 facility which may be transported via land line (T1 line), optical transmission, microwave, etc., facilities. Control over the system of FIG. 1 and coordination of the activities of the various cells 102 and DCSs 101 is exercised by an Executive Cellular Processor (ECP) 105, which is connected to each cell 102 and cellular switch 101 through an Interprocess-Message Switch (IMS) 104 by control links 108. ECP 105 and IMS 104 together make up an ECP complex
134. ECP complex 134 and DCS 101 make up a mobile switching center (MSC) 199. ECP 105 and IMS 104 are illustratively the AT&T Autoplex ECP and the AT&T Autoplex IMS (which includes a plurality of cell site node processors, digital switch node processors, and database node processors, interconnected by an IMS ring), and links 108 are illustratively RS-449 data links within MSC 199. Alternatively, control links 108 may be implemented as 64 Kbps DS0 channels on DS1 facilities between cells 102
and mobile switching center 199.

Each mobile radio-telephone 103 typically comprises an analog FM radio-telephone capable of operating at any one of a plurality of radio frequency pairs. Each cell 102 comprises a plurality of analog FM radios 143 each operating at one of the radio frequency pairs of the mobile radio-telephones 103. Radios 143 of adjacent cells 102 operate at different frequency pairs, to avoid interfering with each other. However, each mobile radio-telephone 103 is typically capable of operating at any of the frequency pairs of all of the cells 102.

In an alternative embodiment, digital radios and radio-telephones operating in time-division multiple-access (TDMA) mode are substituted for the analog FM radios and radio-telephones. Vocoding functions can be a part of the radio units in this embodiment, or can be located at switches 101.

While in a cellular system, a mobile radio-telephone's receiver scans a set of predetermined paging channels. After locking onto the strongest paging channel, the mobile radio-telephone 103 gets instructions from the system and receives incoming calls. A mobile radio-telephone 103 also transmits on a channel to originate a call. When a call is established (incoming or outgoing) the receiver is assigned to a particular voice channel and instructed to tune to that transmit and receive frequency pair. At the same time, a connection is established between the cell 102 and the telephone network 100 through a digital cellular switch 101, which completes the voice path for the telephone conversation.

Once this voice connection is established, the radio signal levels are monitored by the cell's radio 143. As the mobile radio-telephone 103 moves from one cell into another, the serving cell 102 detects the reduction in signal strength and requests that measurements be made by surrounding cells 102. If these measurements indicate that another cell 102 can provide better service, then the voice connection is switched to that cell 102 through a process known as "hard handoff". The process of hard handoff is under control of ECP 105 and requires that a DCS 101 first form a 3-way connection which extends the voice circuit from the serving trunk 106 to radio channels in both the serving cell 102 and the target cell 102. When this connection has been confirmed, the radio-telephone 103 is instructed to retune to the frequency of the assigned radio 143 in the target cell 102. Upon confirmation of the radio-telephone's communication with the target cell 102, the DCS 101 is then instructed to remove the voice connection to the original serving cell 102, leaving the connection between the new serving (target) cell 102 and the serving trunk 106. The telephone conversation continues largely uninterrupted through this handoff process. Meanwhile, the original voice channel is made available for use by another subscriber.

Hard handoffs performed in this way use processor capacity in both the ECP complex 134 and the digital cellular switch 101. For the duration of the 3-way connection, the hard handoff also uses additional switch fabric (TDM bus 130) capacity. If the target cell 102 containing the selected radio 143 is connected to a switching module 120 other than the one containing the serving trunk 106, then the connection must be extended through a time-multiplexed switch (TMS) 121, using additional switching fabric in that switch element. As the number of cells 102 in a system grows larger, the number of handoffs increases and uses and increasing proportion of the system processor and switch fabric resources, thus reducing the system's overall capacity.

Each cell 102 is configured around a high-speed time-division multiplexed (TDM) bus 140. TDM bus 140 is illustratively the 2.048 MHz TDM bus of an AT&T Definity.RTM. communications system Universal Module, and physically comprises one or more TDM buses each having 256 time-slots per frame. Illustratively, multiple TDM buses are used simultaneously by units connected thereto and logically operate as a single TDM bus having a multiple of 256 time-slots per frame. Each time slot has a rate of
64 Kbps. Within a cell 102, radios 143 are connected to TDM bus 140. Radios 143 accept communications for radio transmission from, and supply received radio communications to, TDM bus 140 in DS0 channel format at a rate of 64 Kbps. The input to, and output from, each radio is full-rate pulse-code-modulation (PCM)-coded speech. Also connected to TDM bus 140 are one or more interfaces 142, each one of which couples TDM bus 140 to trunks 107. Illustratively, trunks 107 are carried by T1 facilities employing the DS1 communication format and operating at a rate of 1.544 Mbps, and so interfaces 142 are DS1 interfaces. The DS1 and the aforementioned DS0 format are described by T. H. Murray in "The Evolution of DDS Networks: Part 1", Telecommunications, February 1989, pp. 39-47. An interface 142 accepts from TDM bus 140 communications that have been supplied by a plurality of radios 143, multiplexes them into the DS1 format, and transmits them onto trunks 107. In the reverse direction, interface 142 receives from trunks 107 communications formatted in the DS1 format, demultiplexes them, and supplies them to TDM bus 140 for conveyance to radios 143. TDM bus 140 operates under control of a controller 141, which allocates time slots on bus 140 to individual ones of the radios 143 and interfaces 142. Illustratively, controller 141 makes these allocations on the basis of control information supplied thereto by ECP complex 134 over a control link 108; alternatively, controller
141 may have a database that allows it to make the allocations autonomously.

Each digital cellular switch 101 comprises one or more digital switching modules (DSMs) 120. A module 120 structurally resembles a cell 102 in that it comprises a TDM bus 130 which is similar to TDM bus 140, a controller 131 which provides the same TDM bus control functions as controller 141, and a plurality of interfaces 132 connected to bus 130 which provide the same functionality as interfaces 142. On the basis of control communications originating from ECP complex 134, controller 131
causes communications to be switched by TDM bus 130 between interfaces 132. Each trunk 107 extending from a cell 102 is terminated at a switching module 120 by an interface 132. Other interfaces 132 at a module 120 terminate trunks 106, which are duplicates of trunks 107 but extend to public telephone network 100.

If switch 101 includes more than one module 120, it also includes a time-multiplexed switch (TMS) 121. Then a TMS interface 133 is connected to TDM bus 130 in each module 120 and terminates a link 109 which extends to TMS 121. Interface 133 is illustratively the Module Control Complex (MCC) of an AT&T Definity communications system Universal Module. TMS 121 provides direct switched interconnection between modules 120 of one mobile radio-telephone switch 101. Interconnection between modules
120 of different mobile radio-telephone switches 101 is provided by public telephone network 100 or by trunks that interconnect switches 101 directly.

Overall control of a digital cellular switch 101 and coordination of activities between its modules 120 and 121 is exercised by a DCS controller 161. DCS controller 161 is in direct communication with ECP complex 134 over a control link 108. Controller 161 has its own control connection to TMS 121 through link 150, and to controllers 131 of switching modules 120 through link 150 and TMS interfaces 133. Controller 161 is illustratively the 501 CC processor of an AT&T Definity communications system.

Turning now to FIG. 2, it shows an illustrative example of a cellular mobile radio-telephone system constructed according to the invention. Same numerical designations as were used in FIG. 1 are used in FIG. 2 to designate elements that are common to both systems.

FIG. 2 shows a system topology that resembles the one of FIG. 1 in many respects, though it is not identical. The system of FIG. 2 includes a plurality of geographically-dispersed cells 202, each one of which provides radio-telephony services to mobile radio-telephones 203 in its vicinity. As used herein, cell 202 refers either to a geographically separate cell site or to one of a plurality of "faces" on a given cell site, where a "face" is a cell sector as is typically implemented by using directional transmit antennas at a cell site. The operation of all mobile radio-telephones 203 and cells 202 is synchronized to a common master clock, such as to timing signals generated and broadcast by a global positioning system satellite. Interconnection between cells 202, and between cells 202 and public telephone network 100, is accomplished by digital cellular switches 201, in two stages. First, individual cells 202 are connected to one or more cell interconnect modules (CIMs) 209 of a DCS 201 by trunks 207. Cell interconnect modules 209 of individual DCSs 201 are each in turn connected to each speech coding module (SCM) 220 of that DCS 201 by fiber-optic packet-switched trunks 210. Digital cellular switches 201 are each connected to public network 100 by a plurality of trunks 106, analogously to FIG. 1, and directly to each other by trunks 206 that functionally duplicate trunks 106. The operation of switches 201 is synchronized to master timing signals (not shown) of public telephone network 100. Further analogously to FIG. 1, cells 202 and digital cellular switches 201 operate under control of ECP complex 134, to which they are connected by control links 108. Likewise, the various modules 209 and 220 of a DCS 201 are connected by control links 208 to a common DCS controller 261 and operate under its control. Physically, DCS controller 261 is illustratively again the 501 CC processor.

In the system of FIG. 2, some, but not necessarily all, mobile radio-telephones 203 are digital radio-telephones. While illustratively shown as mounted in a vehicle, a mobile radio-telephone 203 may be any portable radio-telephone, and may even be a stationary radio-telephone. The digital radio-telephones use voice-compression techniques to reduce the required digital transmission rate over the radio channel. Each digital radio-telephone includes voice-compression circuitry in its transmitter and voice-decompression circuitry in its receiver. Each radio-telephone is capable of operating at any one of a plurality of wideband radio frequency pairs.

For handling non-packetized traffic analogous to that handled by the system of FIG. 1, side-by-side with packetized traffic, a DCS 201 of the system of FIG. 2 includes the elements shown in dashed lines: a TMS 121 connected by trunks 109 to modules 209 and 220, and trunks 106 connecting CIMs 209 directly to public telephone network 100. Their use is enlightened further below.

Digital radio-telephones 203 may operate in one or more of time-division multiple-access (TDMA) mode or code-division multiple-access (CDMA) mode or some other digital or analog radio mode. TDMA is a technique, known in the art, that provides multiple users access to a radio channel (frequency) by dividing that channel into multiple time slots. A single user can be assigned to one or more of these time slots. A TDMA radio 203 is illustratively the TIA IS54 digital cellular radio. TDMA employs different frequencies in adjacent cells and therefore requires the "hard handoff" procedure described previously.

In the present illustrative example, digital radio-telephones 203 are assumed to operate in CDMA mode, or as a fallback in the FDMA (analog) mode. CDMA is a direct-sequence spread-spectrum technique which allows reuse of the frequencies in the territories served by adjacent cells 202. Consequently, adjacent cells 202 need not, and do not, operate at different radio frequencies, but re-use the same frequencies. When moving from the vicinity of one cell 202 to the vicinity of another cell 202, a mobile radio-telephone 203 may undergo a "hard handoff" procedure, described previously. However, a CDMA mobile radio-telephone 203 in the system of FIG. 2 may alternatively and preferentially undergo a "soft handoff" procedure, during which it communicates with both of the cells 202 on the same frequency pair at the same time. The CDMA technique and its associated procedures and equipment are also known in the art. The basic principle of direct-sequence code-division multiple-access is the use of a plurality of individual and distinct high-speed digital signals which are absolutely or statistically orthogonal to each other, each to modulate one of a plurality of low-speed (i.e., baseband) user signals and to combine the plurality of modulated signals into common digital signals which then are used to control radio frequency modulation functions. Recovery and separation of the original baseband signals is accomplished using the corresponding digital modulation signals to demodulate within a time-synchronous manner. For a description of CDMA see, e.g., U.S. Pat. No. 4,901,307, and published international patent applications WO 91/07020, WO 91/07036, and WO 91/07037.

A cell 202 is shown in FIG. 3. Similarly to a cell 102 of FIG. 1, cell 202 includes TDM bus 140 operating under control of controller 241, and DS1 interfaces 242 couple TDM bus 140 to trunks 207. Controller 241 is illustratively the control complex of an AT&T Autoplex Series II cell site. It functionally duplicates controller 141 of a cell 102, but now performs additional functions, described below, on account of the fact that cell 202 comprises a plurality of digital radios 243. Every digital radio's signal input and output are interfaced to TDM bus 140 by corresponding one or more channel elements 245 and a cluster controller 244. A channel element 245 is an interface to digital radios 243 serving an individual user. Channel elements 245 provide signal processing functions--baseband and spread-spectrum (CDMA) signal processing functions in this example--for individual calls being transmitted and received by their associated radios 243.

Each cluster controller 244 includes a C-bus 390. C-bus 390 is illustratively a conventional computer input and output (I/O) bus, and channel elements 245 are connected to C-bus 390 as computer I/O devices. C-bus 390 and channel elements 245
operate under control of a controller 393. Controller 393 is illustratively a general-purpose microprocessor, and it is served by a bus 391 which is illustratively a conventional microprocessor main bus. Bus 391 is connected to C-bus 390 by a C-bus interface 392 which functions as an I/O interface of conventional design. Controller 393 orchestrates data movement between channel elements 245 and cell 202 TDM bus 140 (illustratively, one transfer in each direction for each channel element 245 every
20 msecs.), performs operation, administration, and maintenance (OA and M) functions on cluster controller 244, handles cell-site signalling and other specialized functions, and performs level-2 and level-3 protocol formatting and deformatting functions on data (call traffic and signalling) passing between channel elements 245 and TDM bus 140. A memory 394 is connected to bus 391 and serves as a scratch-pad traffic-buffer memory and an instruction memory for controller 393. Also connected to bus 391
is an HDLC controller 395. It performs HDLC formatting and deformatting functions on traffic flowing between channel elements 245 and TDM bus 140, including traffic conversion between byte-oriented form used in cluster controller 244 and bit-oriented form used on TDM bus 140, including bit stuffing and LAPD flag insertion functions. HDLC controller 395 receives and transmits HDLC serial bit streams from/to TDM bus 140 through a TDM bus interface 396, of conventional design, which connects controller
395 to bus 140.

Compressed call traffic and signalling are transported between channel elements 245 and cluster controller 244 in the form of segments of byte-oriented information. Each channel element 245 transmits and receives a segment of byte-oriented information at regular intervals, illustratively every 20 msecs. Cluster controller 244 formats each segment of byte-oriented information in LAPD protocol format which includes a level-3 protocol, for transmission to DCSs 201. While any suitable level-3 protocol may be used, illustrative level-3 protocols 350 and 351 are shown in FIGS. 9 and 10.

FIG. 9 shows a protocol 350 that is used to convey either call traffic or signalling or both, while FIG. 10 shows a protocol 351 that is dedicated to conveying a particular type of signalling. Both protocols 350 and 351 are carried by frames of FIGS. 7 and 8. A level-3 protocol data unit carried over a level-2 protocol is commonly referred to as a packet, and a level-2 protocol data unit is commonly referred to as a frame. Protocol 350 of FIG. 9 comprises at least the information fields
320-327. Additional fields for other types of information may be included in packet 350, but these are not germane to the present discussion. Sequence number field 320 carries a sequential number of this packet 350 within the sequence of packets transmitted in a given direction. In the case of packet 350 outgoing to a channel element 245 from a DCS 201, the sequence numbers begin at 0 at the start of every new call. In the case of packets 350 incoming from a channel element 245 to a DCS 201, the sequence numbers are derived from the master timing signals to which all mobile telephones 203 and cells 202 are synchronized. Packet type field 321 identifies the packet type as either a traffic packet, corresponding to packet 350 of FIG. 9, or a signalling packet, corresponding to packet 351 of FIG. 10. Clock adjust field 322 carries information from cluster controllers 244 to DCSs 201 that is used to compensate for real and virtual drift between the master clock to which mobile telephones 203
and cells 202 are synchronized and a master clock to which public telephone network 100 and DCSs 201 are synchronized. field 322 is used only in the reverse direction, and is null in the forward direction. Air CRC field 323 is the result of a conventional check-sum, computed by a mobile telephone 203 over its transmitted traffic, and is sent by mobile telephone 203 along with that traffic. Signal quality field 324 carries reports computed by channel elements 245 on the quality of call-traffic signals that they are receiving from mobile telephone 203. Fields 323 and 324 are also used only in the reverse direction and are null in the forward direction. Power control field 325 carries information from a cell 202 concerning the trend of power control instructions sent by a channel element 245 to its corresponding mobile telephone 203. Normally, this field is also used only in the reverse direction, but is used in both directions during soft handoff, as will be explained further below. Voice/signalling type field 326 identifies the type of information that is carried by packet 350: voice traffic only, voice plus signalling, or signalling only. And voice/signalling data field 327 carries call voice traffic or signalling information, or a mix of both, to and from channel elements 245.

A signalling packet 351, shown in FIG. 10, is simpler than traffic packet 350 of FIG. 9: it has fields 321 and 328-331 that are relevant to this discussion. Packet type field 321, and already discussed in conjunction with FIG. 9, identifies packet 351 as a signalling packet. Message type field 328 identifies the type of signalling carried by packet 351. Channel element ID field 329 identifies the particular channel element 245 participating in this message exchange. Frame selector ID field 330 identifies a particular virtual port on a processor 602 (see FIG. 6) participating in this message exchange. These fields 329 and 330 may be used for security, maintenance, performance tracking, billing, routing, etc. Channel element 245 and frame selector IDs are assigned administratively at system configuration time, and remain fixed thereafter. And signalling data field 331 carries the signalling information that is being conveyed.

A cluster controller 244 couples a plurality of channel elements 245 to TDM bus 140. Each cluster controller 244 communicates on TDM bus 140 through an allocated input and an output "pipe". The allocation is administrable, and is typically done at system initialization. Each "pipe" illustratively constitutes a plurality of (e.g., four) time slots (i.e., four 64 Kbps channels) on TDM bus 140. In the reverse (inbound) direction, cluster controller 244 queues traffic segments received from channel elements 245, formats them into packets, wraps the packets into inverted-HDLC-format LAPD (level-2 protocol) frames, and transmits the LAPD frames one after another into its allocated output "pipe" on TDM bus 140. In the forward (outbound) direction, cluster controller 244 receives LAPD frames from its allocated input "pipe" on TDM bus 140, terminates the LAPD protocol, deformats the packets, and then distributes the contents of these packets to channel elements 245 according to an address field embedded in the received frames. As a consequence of the operations of cluster controllers 244, frames being conveyed to and from them are statistically multiplexed onto TDM bus 140, thereby greatly increasing the traffic-carrying capacity of the bandwidth of TDM bus 140 over alternative transmission techniques.

An illustrative LAPD frame 300 is shown in FIG. 7. For purposes of this discussion, it comprises a plurality of fields 301-305: a flag field 301, used for delimiting frames; a Data Link Connection Identifier (DLCI) field 302; a control field 303
which specifies the type of LAPD frame this is; a user data field 304 which contains the level-3 protocol (packet) 350 or 351 referred to above; and a frame check sequence (FCS) field 305, used for error checking. The DLCI field 302 is the frame end-to-end address field. It contains a virtual link number or index (DLCI) that associates the frame with a particular call. In the forward direction, the DLCI identifies a particular channel element 245; in the reverse direction, the DLCI identifies a particular one of a plurality (illustratively two) of virtual ports of processor 602 which correspond to a particular speech processing unit 264 service circuit 612 (see FIG. 6). Within a cluster controller 244, the DLCI identifies the channel element
245 which is the source or destination of the frame. In this embodiment, DLCIs are assigned to ports and channel elements administratively at system configuration time, and remain fixed thereafter.

The transmission of frames to and from cluster controllers 244 is effected using the frame-relay technique of transmission, whereby protocol termination of the frames occurs only at the transmission endpoints, thereby greatly increasing the efficiency and speed of those frame transfers through the system of FIG. 2. The frame-relay technique is described in U.S. Pat. No. 4,894,822. It is hereby incorporated herein by reference.

Advantageously, in order to provide radio telephone services to conventional analog or digital TDMA mobile telephones 103 within the same system, analog FM or TDMA digital radios 143 may also be connected to TDM bus 140 in cells 202, in the manner described for cells 102, as suggested by the dashed blocks in FIG. 3. Alternatively, conventional cells 102 may be used side-by-side with cells 202 within the system of FIG. 2. TDMA traffic may be carried through the system of FIG. 2 either in circuit-switched form, like the analog radio traffic, or in packet-switched form, like the CDMA traffic.

In the cell 202 of FIG. 3, DS1 interfaces 242 perform their conventional functions of gathering 64 Kbps time slots from TDM bus 140 and multiplexing them into DS1 format for transmission on trunks 207, and vice versa. It is important for purposes of this application that each interface 242 ensure that the delay undergone by signals of every DS0 channel within interface 242 be constant; many commercial DS1 interfaces, such as the AT&T TN 464C, do in fact meet this condition. On account of the functions performed by cluster controllers 244, frames are statistically multiplexed onto trunks 207 and the format of facilities that implement trunks 207 is, from a logical perspective, no longer the purely conventional DS1 format of facilities that implement trunks 107 of FIG. 1: as opposed to comprising 24 independent DS0 channels, as it does on DS1 facilities, each facility now comprises multiple independent "pipes" each consisting of the bandwidth of one or more DS0 channels. Each of the "pipes" carries the LAPD frames created by or destined for a single cluster controller 244. The traffic-carrying capacity of the bandwidth provided by trunks 207 is thereby greatly increased over alternative transmission techniques, such as the conventional circuit-switching technique. Any remaining trunks 207 (i.e., DS0 channels) that are not bundled into "pipes" continue to be used on an independent individual circuit-switched basis, e.g., to carry communications to and from conventional radios 143.

A cell interconnect module (CIM) 209 is shown in FIG. 4. Cell interconnect module 209 is illustratively founded on the Universal Module of the AT&T Definity communications system. It includes a local area network (LAN) bus 250 operating under control of a controller 251. Universal DS1 (UDS1) interfaces 252 connect trunks 207 to LAN bus 250. Each interface 252 includes a DS1 trunk interface 442 which duplicates the DS1 facility-interface circuitry of DS1 interface 242, and a packet processing element (PPE) 401, interconnected by a concentration highway 400. Concentration highway 400 is a time-division multiplexed bus of 64 time slots each having a 64 Kbps rate. The DS1 trunk interface 442 performs the functions of gathering 64
Kbps time slots from concentration highway 400, inverting the inverted HDLC format (discussed in conjunction with cell 202 of FIG. 3) back to normal, and multiplexing the data into DS1 format for transmission on trunks 207, and vice versa.

PPE 401 performs LAPD frame-relay functions between concentration highway 400 and LAN bus 250. PPE 401 includes a translation table 411 that contains a board and a port address for each DLCI 302. Translation table 411 is administered at initialization. PPE 401 is administered to receive LAPD frames 300 on designated time slots of concentration highway 400. For each LAPD frame 300 received on concentration highway 400, PPE 401 uses the contents of the frame's DLCI field 302 to find the corresponding board and port address in table 411. The board and port addresses identify the intended recipient of frame 300 on LAN bus 250. PPE 401 then strips flag field 301 from frame 300 and prepends the found board and port addresses to the frame to form a modified LAPD frame 310 shown in FIG. 8. A comparison with FIG. 7 shows flag field 301 to have been replaced by board address 311 and port address 312. PPE 401 then transmits modified LAPD frame 310 on LAN bus 250. In the other direction, PPE 401 examines modified LAPD frames 310 transmitted on LAN bus 250 for its board address 311. It receives any frame 310 having the looked-for address 311, strips the addresses 311 and 312 from frame 310, replaces them with flag field 301 to form a LAPD frame 300, and then transmits frame 300 on concentration highway 400. The stripped-off port address 312 identifies to PPE 401 the particular time slots on which that particular frame 300 is to be transmitted.

Also connected to LAN bus 250 of cell interconnect module 209 are expansion interfaces (EIs) 253. Each expansion interface 253 couples an optical fiber trunk 210 to LAN bus 250. Expansion interfaces 253 merely act as routing elements. Each expansion interface 253 includes a LAN bus interface 450 which monitors LAN bus for modified LAPD frames 310 having a pre-administered DLCI 302, board address 311, and port address 312. Interface 450 captures any frame 310 having the looked-for DLCI
302, board address 311, and port address 312, strips off the prepended board address 311, and stores the frame 310 in a FIFO buffer 451. FIFO buffer 451 outputs the prepended port address 312 and DLCI 302 of the frame 310 to a translation table 452, and outputs fields 302-305 of frame 310 to a translation inserter 453. Table 452 is a pre-administered table of board and port addresses of speech coder modules 220. Table 452 uses the port address 312 and DLCI 302 that it receives from FIFO buffer 451 as a pointer to find a new board address 311 and port address 312 for the frame 310, and sends the new addresses 311 and 312 to translation inserter 453. Inserter 453 prepends the new board and port addresses 311 and 312 received from table 452 to the frame 310 fields that it received from FIFO buffer 451, and sends the new frame 310 to fiber interface 454. If no corresponding addresses are found in and sent from table 452, inserter 453 merely discards the received frame 310. Fiber interface 454
transmits the frame 310 on optical fiber trunk 210. Any desired protocol and transmission format may be used on trunks 210. In the reverse direction, fiber interface 454 receives frames 310 on trunk 210 and stores them in a FIFO buffer 455. LAN bus interface 450 extracts the stored frames 310 from FIFO buffer 455 and transmits them on LAN bus 250. Consequently, expansion interface 253 merely transmits on LAN bus 250 those frames 310 that it receives on the attached fiber trunk 210. These frames
310 have board addresses 311 that identify the destination interfaces 252 on LAN bus 250, and port addresses 312 that are not looked for by any expansion interfaces 253 on LAN bus 250.

For purposes of handling conventional, circuit-switched, cellular radio telephone communications, cell interconnect module 209 includes elements shown in dashed lines in FIG. 4. Specifically, CIM 209 includes a TDM bus 230 which duplicates TDM bus 130, and each UDS1 interface 252 includes a time-slot interchanger (TSI) 402 which couples concentration highway 400 to TDM bus 230. TSI 402 performs conventional time-slot interchange functions. It receives designated 64 Kbps channels (time slots) on concentration highway 400 and TDM bus 230 and transmits them on designated time slots of TDM bus 230 and concentration highway 400, respectively. TSI 402 is programmed on a per-call basis. For the purpose of switching these conventional communications, TDM bus 230 is coupled by a TMS interface 133 and trunk 109 to a TMS 121 (see FIG. 2), in the manner described for FIG. 1. For the purpose of connecting these conventional communications to public telephone network 100, TDM bus 230 is also coupled by a DS1 interface 132 and a trunk 106 to network 100.

A speech coder module 220 of a digital cellular switch 201 is shown in FIG. 5. Each DCS 201 comprises one or more identical modules 220. Module 220 is illustratively the Universal Module of AT&T Definity communications system. Module 220
includes TDM bus 130 and a LAN bus 260 which is a duplicate of LAN bus 250, both operating under control of a controller 231. As in FIG. 1, TDM bus 130 is connected by DS1 interfaces 132 and trunks 106 to public telephone network 100. Fiber trunks 210
from cell interconnect modules 209 are connected to LAN bus 260 by expansion interfaces 263 which duplicate expansion interfaces 253. Each cell interface module 209 of a DCS 201 is connected to each speech coder module 220 of that DCS 201. Interconnection between DCSs 201 is provided by network 100 through trunks 106.

Buses 260 and 130 are interconnected through a plurality of call-processing nodes referred to herein as speech processing units (SPUs) 264. Based on the board address 311 prepended to each frame 310 by expansion interfaces 253 of cell interconnect modules 209, each speech processing unit 264 receives frames 310 that are addressed to it, depacketizes their contents (i.e., terminates their protocol), performs various processing functions--including speech decompression--on the contents of each received frame, and outputs the processed frame contents on TDM bus 130 in time slots which are assigned to calls on a call-by-call basis. In the reverse direction, a speech processing unit 264 receives communications over TDM bus 130 in time slots which are assigned to calls on a call-by-call basis, performs various processing functions--including speech compression--thereon, packetizes the processed communications, includes in each frame a DLCI 302 identifying a particular channel element
245 of a particular cell 202, prepends to each frame board and port addresses 311 and 312 that identify the frame's destination on LAN bus 260, and transmits the frames 310 on LAN bus 260.

As a consequence of the operations of cell interconnect modules 209 and speech coder modules 220, frames 310 being conveyed between them are statistically multiplexed onto, and frame-relayed over, trunks 210, thereby greatly increasing the traffic-carrying capacity of the bandwidth provided by trunks 210 over alternative transmission techniques such as circuit-switching.

As was mentioned in conjunction with FIG. 3, DCS 201 optionally includes a TMS 121 for servicing conventional radio telephone communications. Speech coder module 220 is connected to TMS 121 by a trunk 109 and a TMS interface 133, in the manner described for switching modules 120 of FIG. 1.

An illustrative speech processing unit 264 is shown in FIG. 6. Each SPU 264 includes a LAN bus interface 601. It monitors frames 310 traversing LAN bus 260 for pre-administered board addresses 311, and captures those having the sought-for addresses 311. LAN bus interface 601 includes a buffer 620. Upon capturing a frame 310, LAN bus interface 601 appends to it a time stamp, stores it in the buffer 620, and issues an interrupt to a processor 602. The time stamp is the present count of a counter 623, discussed further below.

The port address 312 of a frame 310 identifies one of a plurality of service circuits 612 implemented by SPU 264. A service circuit 612 is assigned to a call either for the duration of the call or until a hard handoff occurs. Each service circuit 612 has its own audio-processing circuitry. But all service circuits 612 are served on a time-shared basis by processor 602, which performs frame-selection and protocol-processing functions for all service circuits 612 of an SPU 264. The functions performed by processor 602 on frames 310 received from LAN bus interface 601 are shown in FIGS. 11-14, and 17-18, and functions performed by processor 602 on traffic segments (hereinafter also referred to as traffic frames) received from service circuits 612 are shown in FIG. 15. Processor 602 performs each of these functions for each service circuit 612 every 20 msecs. The performance of the functions is interrupt-driven, by interrupt signals provided by an adaptive synchronization circuit 611 and interface 601.

The exchange of traffic frames of incoming and outgoing call traffic is carried on between processor 602 and service circuits 612 through buffers 603 of processor 602. Each service circuit 612 has its own corresponding buffer 603. A buffer 603
buffers traffic frames passing between processor 602 and a vocoder 604 of a service circuit 612 to compensate for minor differences and fluctuations in the timing of input and output operations of processor 602 and vocoder 604.

Each service circuit 612 has its own vocoder 604. Vocoders 604 provide voice compression and decompression functions. Each is a digital signal processor that receives a traffic frame of compressed speech from processor 602 via buffer 603 at regular intervals (e.g., every 20 msecs.) and decompresses the traffic frame into a predetermined number (e.g., 160 bytes) of pulse-code-modulated (PCM) speech samples. Each byte has a duration of 125 usecs. in this example, referred to as a "tick". In the opposite direction, a vocoder 604 receives 160 bytes of PCM speech samples, performs speech compression functions thereon, and outputs a traffic frame of the compressed speech to processor 602 via buffer 603 at regular intervals (every 20 msecs.). Exchanges of traffic frames between vocoder 604 and processor 602 are timed by clock signals generated by vocoder 604 internal input and output clocks 621 and 622, while receipt and transmission of PCM samples by vocoder 604 are timed by clock signals generated by a clock circuit 600. Clocks 621 and 622 are edge-synchronized with circuit 600 clock signals at system initialization and service circuit 612 reset. Vocoders are well known in the art. Each vocoder 604 is illustratively implemented using the AT&T 16A digital signal processor (DSP) which embodies the Qualcomm, Inc. QCELP low-bit-rate variable-rate speech encoding/decoding algorithm. The QCELP algorithm provides for sending minimal information during periods of low or no speech activity. The frame transport mechanism of this embodiment ideally adapts to time-varying traffic loads.

In the case of a system handling both CDMA and TDMA traffic wherein the TDMA traffic is also frame-relayed, some of the service circuits 612 are dedicated to handling the TDMA traffic, and their vocoders 604 are illustratively the AT&T 16A digital signal processor programmed according to the TIA IS-54 standard for TDMA communications.

PCM samples on their way from vocoders 604 pass through tone-insertion circuits 605. Each service circuit 612 has its own tone-insertion circuit 605. Upon command from processor 602, a tone-insertion circuit 605 momentarily blocks and discards PCM samples output by vocoder 604, and in their place substitutes PCM samples of whatever Touch-Tone signals were specified by the command. Tone-insertion circuit 605 has no effect on PCM samples being input to vocoder 604. Operation of tone-insertion circuit 605 is synchronized with the output of vocoder 604 by clock signals generated by clock circuit 600.

Tone-insertion circuits 605 are followed in the sequence of service circuit 612 circuitry by echo cancellers 606. Each service circuit 612 has its own echo canceller 606. Each cancels echoes of telephone network 100-bound call traffic from telephone network 100-originated call traffic, by keeping an attenuated copy of the vocoder-generated network-bound traffic and subtracting an appropriately-delayed copy from received network-bound traffic. Echo cancellers are well known in the art. Timing of echo canceller 606 operations is controlled by clock signals generated by clock circuit 600.

Echo cancellers 606 receive network-originated traffic from, and transmit network-bound traffic to, a concentration highway 607. Concentration highway 607 is a passive serial TDM bus that carries 64 Kbps time slots. Each echo canceller 606 is statically assigned its own input time slot and its own output time slot on concentration highway 607.

Concentration highway 607 is coupled to TDM bus 130 by a TDM bus interface 608. Interface 608 performs time-slot interchange (TSI) functions between highway 607 and bus 130. Its operation is timed by clock signals generated by circuit 600, and is controlled by a translation and maintenance (XLATION. AND MTCE.) unit 609. Unit 609 performs highway 607-to-bus 130 time-slot assignment functions on a per-call basis, under the direction of controller 231 of that speech coder module 220. Unit 609
communicates with controller 231 via a control channel implemented by bus 130. This control channel is interfaced to unit 609 through interface 608 and bus 613. Unit 609 provides maintenance functions to LAN bus interface 601 via control link 616.

Unit 609 exerts control over interface 608 via a translation and maintenance control bus 613, to which both are connected. Similarly, processor 602 controls circuits 601, 603-606, and 611 via a processor control bus 610. Communications between processor 602 and unit 609 are facilitated by a buffer 614 which couples bus 610 with bus 613.

Clock circuit 600 is connected to TDM bus 130 and derives timing information therefrom, in a conventional manner. Clock circuit 600 distributes this information, in the form of clock signals of various rates, including 2.048 MHz, 8 KHz, and 50
Hz (corresponding to intervals of 500 nsec., 125 usec., and 20 msec. intervals, respectively), all of which are synchronized with each other, via a clock bus 615 to circuits 604-606, 608, and 611, in order to synchronize their operation with TDM bus
130. Clock circuit 600 also distributes this information to LAN bus interface 601 for bit-time synchronization of LAN bus 260. Operation of TDM bus 130 is synchronized to network 100--hence, clock circuit 600 synchronizes operations of the various elements with the master clock of network 100.

Adaptive synchronization circuit 611 uses the clock signals obtained from clock circuit 600 to generate clock signals which are synchronized in frequency with, but are offset in phase--in amounts controlled by processor 602--from, the 20 msec. clock signals generated by clock circuit 600. These offset clock signals are used to time the operations of processor 602. The generation and use of these offset clock signals is explained further below. Physically, circuits 611 and 600 may be implemented as a single device.

Circuit 611 also includes a present-time counter 623. Counter 623 increments its count once every PCM sample tick, e.g., once very 125 usecs. This count is reset by every 50 Hz clock pulse from clock circuit 600, e.g., every 20 msecs. Counter
623 thus indicates present time relative to signals generated by clock circuit 600. A second portion of counter 623 keeps a modulo-8 count that is incremented by the 20 msec. clock pulses that reset the 125 usec. count. Counter 623 provides its counts to LAN bus interface 601 for use as a time stamp of received frames 310.

Discussion now returns to processor 602 and its packet-and frame-processing functions. (Level-2 protocol processing is commonly referred to as frame processing, while level-3 protocol processing is commonly referred to as packet processing.) The functions performed by processor 602 on frames 310 received from LAN bus 260 are shown in FIGS. 11-14. Processor 602 performs these functions for each service circuit every 20 msecs. Performance of different ones of these functions for a particular service circuit 612 is triggered by receipt of corresponding receive interrupt signals from LAN bus interface 601 and adaptive synchronization circuit 611.

As was mentioned above, upon receiving a frame addressed to the corresponding SPU 264, LAN bus interface 601 appends a time stamp to the received frame, stores the received frame in buffer 620, and issues an interrupt to processor 602. Upon being invoked by the receive interrupt signal from LAN bus interface 601, at step 900, processor 602 retrieves the received frame from buffer 620 of LAN bus interface 601, at step 902. Processor 604 then performs conventional level-2, i.e., LAPD protocol, processing on the frame, at step 904. This processing may include acknowledging receipt of the frame. Upon completing level-2 processing, processor 604 checks control field 303 to see if this is a level-2 only frame (e.g., a loop-around test frame), at step 906. If so, processing of the frame is completed, and processor 602 merely returns to the point of its invocation, at step 908. But if this is not a level-2 only frame, i.e., its user data field 304 carries a level-3 protocol, processor
602 uses the frame's DLCI 302 to select from its memory the stored call state of the call to which the frame pertains, at step 910. Next, processor 602 checks, at step 911, packet type field 321 of the received level-3 protocol to determine the packet type: traffic or signalling. If field 321 identifies the packet as a signalling packet, it means that the packet carries cell-to-switch signalling information, i.e., signalling intended for DCS 201. Processor 602 therefore performs the signalled function, at step 970. This may be any one of 3 functions: to update call state by either setting up or tearing down a call or adding or removing a second cell in soft handoff, to insert tones into the telephone network-bound portion of the call, or to perform initial clock synchronization (discussed in conjunction with FIG. 17). Processor 602 then returns to the point of its invocation, at step 946. Voice/signalling packets 350 are sent and received at 20 msec. intervals, while signalling-only packets 351 may be sent at any time as required to send signalling information.

If field 321 identifies the packet as a traffic packet, processor 602 performs clock adjustment and synchronization functions, at step 912, to shift the offset of clock signals generated by circuit 611 from clock signals generated by circuit 600
by an amount determined by processor 602 or dictated by clock adjust field 322 of the received packet. These are described in conjunction with FIG. 18. Processor 602 then checks voice/signalling type field 326 of the received level-3 packet, at step
914, to identify the type of information carried by the packet: voice only, voice plus signalling, or signalling only. If the traffic packet is a voice-only packet, processor 602 checks the retrieved call state to determine if the call is in soft handoff, at step 916. If not, processor 602 checks air CRC field 323 of the frame (containing the result of a check-sum computed over the CDMA transmission between cell 202 and mobile telephone 203), at step 918. If the air CRC does not check out, it means that the packet carries defective information, and so processor 602 discards the packet, at step 923, and then returns, at step 946. Vocoder 604 will mask the loss of that traffic. If the air CRC checks out at step 918, processor 602 checks signal quality field 324 of the packet to determine whether the voice quality meets a predetermined threshold value, at step 919. If the voice quality does meet the threshold value, processor 602 marks the packet as "good" by appending a command thereto, at step 920, stores the packet of voice information in buffer 603 which is allocated to the appropriate service circuit 612, at step 922, and then returns to the point of its invocation, at step 926. If the voice quality does not meet the minimum threshold value, processor 602 marks the packet as "bad", at step 921, stores the packet in buffer 603 of the appropriate service circuit 612, at step 922, and then returns, at step 946.

During the procedures just described, processor 602 uses contents of sequence number field 320 of the received packet to detect and handle lost or out-of-sequence packets, in a conventional manner.

Returning to step 916, if the call is in "soft handoff", processor 602 should be receiving two packets for the call every 20 msecs., each from a different cell 202 but generally carrying identical information. So processor 602 checks whether it has yet received both duplicate packets, at step 932. The duplicate packets are identified by having the same sequence number in field 320. If not, meaning that processor 602 has received either only one of the expected duplicate packets, or has received packets from both cells but bearing different sequence numbers, processor 602 checks the sequence number of the just-received packet, at step 933, to determine whether its sequence number is greater than, equal to, or less than the expected sequence number. If the sequence number of the received packet is greater than the expected sequence number, processor 602 stores the received packet, at step 934, updates the associated call's state to indicate that one of the packets that will be expected in the future has been received, at step 935, and returns, at step 946. Updating of the call state at step 935 includes storing the contents of power control field 325 of the received packet. If the sequence number of the received packet is equal to the expected sequence number, processor 602 proceeds to steps 918 et seq. to process the packet as described previously. And if the sequence number of the received packet is less than the expected sequence number, processor 602 discards the received packet, at step 936, and then returns, at step 946. Again, vocoder 604 will mask the loss of that traffic.

Returning to step 932, if processor 602 finds that it has received both expected packets, processor 602 updates the call state to so indicate, at step 938. This includes storing the contents of power control field 325 of the received packet. It then retrieves the first-received expected packet (now stored in a buffer 603) and compares the air CRC and the signal quality indicia of both packets to determine which packet is better, at step 940. Processor 602 then checks the voice quality field of the better packet to determine whether the voice quality meets a predetermined threshold value, at step 941. If not, processor 602 marks the better packet as "good" by appending a command thereto, at step 943; if so, processor 602 marks the better packet as "bad", at step 942. Processor 602 then discards the worse packet and stores the better packet in buffer 603 of the corresponding call channel, at step 944. Processor 602 then returns, at step 946.

Turning to FIG. 12, following step 946, when processor 602 is invoked at step 950 by a receive interrupt signal RX.sub.-- INT.sub.-- X for a particular (Xth) service circuit 612, processor 602 checks buffer 603 corresponding to that service circuit 612 to determine if buffer 603 is empty, at step 951. If not, processor 602 retrieves the contents of that buffer 603 and passes the retrieved contents to vocoder 604 of that service circuit 612, at step 952. If buffer 603 is empty, processor
602 invokes a function in vocoder 604 of the appropriate service circuit 612 to mask the loss of the voice segment carried by the discarded packet, at step 953. Vocoder 604 masks the loss by generating at its output to circuit 605 PCM samples that it generates as a function of previously-received packets. Processor 602 then returns to the point of its invocation, at step 954.

Returning to step 914, a traffic packet that carries signalling information is encountered by processor 602 only during "soft handoff", as under normal circumstances signalling is sent directly to mobile telephone 203 from cell 202 involved in a given call. If the traffic packet carries only signalling information, processor 602 proceeds to step 955 of FIG. 13. There, processor 602 checks further contents of voice/signalling type field 326, to determine the signalling direction: forward and/or reverse. If the direction is forward, identifying the signalling as being originated by a cell 202 and destined for a mobile telephone 203, processor 602 merely stores the packet, at step 956, and then returns, at step 970. If both signalling directions are indicated, processor 602 stores the forward signalling, at step 957, and then proceeds to step 958. If the direction is reverse, identifying the signalling as being originated by a mobile telephone 203 and destined for cells 202, processor 602 checks, at step 958, whether it has received signalling packets from both sides (i.e., from both of the cells 202 involved in the "soft handoff"). If not, processor 602 stores the packet, at step 960, and then updates the corresponding call's state to indicate that a signalling packet from one side has been received, at step 962. Processor 602 then returns, at step 970. If the check at step 958 reveals that signalling packets from both sides have been received, processor 602 updates the corresponding call's state to so indicate, at step 964, and then compares the air CRC and signal quality fields 323 and 324 of the two packets to determine which packet carries the better quality signals, at step 966. Processor 602 then discards the worse packet and stores the better one, at step 968, and then returns, at step 970.

Returning to step 914, if processor 602 determines that the packet carries both voice and signalling information, processor 602 proceeds to step 985 of FIG. 14, and performs signalling-processing steps 985-998 of FIG. 14 which duplicate steps
955-968 of FIG. 13, and then proceeds to step 932 of FIG. 11 to perform the voice-processing steps.

The functions performed by processor 602 on traffic frames (segments of voice information) received from vocoders 604 are shown in FIG. 15. Processor 602 performs these functions for each service circuit 612 every 20 msecs. The performance of the functions for a particular service circuit 612 is also interrupt-driven, by receipt of a corresponding transmit interrupt signal provided by adaptive synchronization circuit 611.

Upon being invoked by a transmit in