Trunked radio repeater system

• US 4,939,746 A
• Filed: 04/14/1988
• Issued: 07/03/1990
• Est. Priority Date: 06/03/1987
• Status: Expired due to Term

• Current Assignee: General Electric Company, Com-Net Ericsson Canada Corp. (TE Connectivity Limited), Com-Net Ericsson Critical Radio Systems, Inc. (TE Connectivity Limited)
• Original Assignee: General Electric Company
• Inventors: Childress, Jeffrey S.
• Primary Examiner(s): Safourek, Benedict V.
• Application Number:
  US07/181,441
• Time in Patent Office: 810 Days
• Field of Search: 370/110.1, 370/110.3, 370/92, 370/93, 455/33, 455/34, 455/38, 455/53, 455/54, 455/56, 379/58, 379/59, 379/60, 379/63
• US Class Current: 455/509
• CPC Class Codes: H04L 1/0083 Formatting with frames or packets; Protocol or part of protocol for error control
• CPC Class Codes: H04L 1/1614 using bitmaps
• CPC Class Codes: H04W 84/08 Trunked mobile radio systems

Abstract

A digital trunked radio repeater system provides substantial improvements in timeliness of channel acquisition and channel drop, and in reliability of critical control signalling. The system uses a much higher digital signalling rate than is typically found in prior art systems, and uses a control channel to convey digital channel request and assignment messages between the central site and mobile transceivers. The mobile radio transceivers transmit channel requests on the control channel (if no response is received, the mobile retries during a retry time window which increases in duration in dependence on the number of retries). The mobile transceiver switches to a working channel in response to an assignment message received on the control channel. Subaudible digital signals transmitted on the control channel and on active working channels allow late entry, shifting to higher priority calls, and other advanced functions. Message and transmission trunking capabilities are both present so as to maximize working channel usage without compromising channel access for high priority communications. During transmission trunking, called and calling transceivers return to the control channel after each transmission (and called transceivers may be inhibited from transmitting) but grant higher priority to calls from the other transceivers being communicated with to ensure continuity over an entire conversation. Additional functions and fault tolerant features further increase the versatility and reliability of the system.

Claims

1. A method for achieving reliable and prompt communications with a trunked radio repeater system having a digital control channel and plural working channels, which working channels are assigned for temporary use of individual radio units specified by digital control signals on the control channel, said method comprising the steps of:

(1) operating a first radio unit on a first working channel in response to digital assignment signals passed over said control channel;

(2) operating a second radio unit on a second working channel in response to further digital assignment signals passed over said control channel;

(3) passing a stream of digital signals over said first working channel to said first radio unit indicating said second working channel assignment; and

(4) operating said first radio unit on said second working channel in response to said stream of signals transmitted by said transmitting step (3).

2. A method as in claim 1 wherein said passing step (3) comprises:

(a) continuously passing subaudible digital signalling over said first working channel; and

(b) interrupting said subaudible digital signals with at least one burst of high-speed digital signalling embedded within said subaudible signals, said high-speed signalling indicating said second working channel assignment.

3. A method as in claim 2 wherein said passing step (3) futher includes:

(c) subsequent to said passing step (b), indicating said second working channel assignment with said subaudible digital signals.

4. A method as in claim 3 wherein said indicating step (c) is performed only once.

5. A method as in claim 1 wherein said passing step (3) is repeated over a preset time period.

6. A method as in claim 1 further comprising the step of:

transmitting digital signals indicating a working channel assignment exactly twice on one of said first and second working channels each time a working channel different from said one channel is assigned, once at a high data rate, and the second time at a lower data rate.

7. A trunked radio repeater system for achieving reliable and prompt communications, said system having a digital control channel and plural working channels which working channels are assigned for temporary use of individual radio units specified by digital control signals on the control channel, said system comprising:

means for operating a first radio unit on a first working channel in response to digital assignment signals passed over said control channel;

means for operating a second radio unit on a second working channel in response to further digital assignment signals passed over said control channel; and

means for passing a stream of digital signals over said first working channel to said first radio unit indicating said second working channel assignment,

means wherein said first-mentioned operating means is also for operating said first radio unit on said second working channel in response to said stream of digital signals passed over said first working channel by said passing means.

8. A system as in claim 7 wherein said passing means comprises:

means for continuously passing subaudible digital signalling over said first working channel; and

means for interrupting said subaudible digital signals with at least one burst of high-speed digital signalling embedded within said subaudible signals, said high-speed signalling indicating said second working channel assignment.

9. A system as in claim 8 wherein said subaudible signalling passing means encodes subaudible digital signals with an indication of said second working channel assignment.

10. A system as in claim 9 wherein said subaudible signalling passing means encodes said subaudible signals with said second working channel assignment indication only once.

11. A system as in claim 7 wherein said passing means passes digital signals indicating said second working channel assignment for a preset time period less than the time period said second working channel remains assigned.

12. A method of priority scanning in a trunked radio repeater system having a digital control channel and plural working channels, which working channels are temporarily assigned for use of individual radio units specified by digital control signals on the control channel, said method including the steps of:

(1) assigning a first working channel to a first radio unit by passing digital assignment signals over said control channels;

(2) passing a burst of digital signals substantially at 9600 bits per second over all assigned ones of said plural working channels indicating the assignment of said first working channel;

(3) subsequently to said passing step (2), passing digital signals substantially at 150 bits per second over all assigned ones of said plural working channels indicating the assignment of said first working channel; and

(4) retuning a further radio unit from a working channel assigned thereto to said first working channel in response to said signals passed by said passing step (3) and/or by said passing step (4).

13. A method as in claim 12 wherein said passing step (3) includes passing a subaudible digital signalling frame in which every two bits are inverted and retransmitted.

14. A method as in claim 12 wherein:

said passing step (3) includes reserving time slots within said 150 bits per second digital signalling for high-speed data bursts; and

said passing step (2) includes filling at least one of said reserved time slots with high-speed digital signalling.

15. A trunked radio repeater system having a digital control channel and plural working channels, which working channels are temporarily assigned for use of individual radio units specified by digital control signals on the control channel, said system including:

means for assigning a first working channel to a radio unit by passing digital assignment signals over said control channel;

means for passing a burst of digital signals substantially at 9600 bits per second over all assigned ones of said plural working channels indicating the assignment of said first working channel;

means cooperating with said first-mentioned passing means for passing subaudible digital signals substantially at 150 bits per second over all assigned ones of said plural working channels indicating the assignment of said first working channel; and

means for retuning a further radio unit from a working channel assigned thereto to said first working channel in response to said digital signal burst and/or subaudible digital signals passed over said working channel assigned thereto.

16. A system as in claim 15 wherein said subaudible signal passing means includes means for transmitting a subaudible digital signalling frame in which every two-bits are inverted.

17. A system as in claim 15 wherein:

said subaudible signal passing means includes means for reserving at least one time slot within said 150 bits per second subaudible digital signalling for high-speed data bursts; and

said digital signal burst passing means includes means for filling at least one of said reserved time slots with high-speed digital signalling.

18. A method of communicating control signals over active working channels of a trunked radio repeater system having a digital control channel and plural working channels, which working channels are temporarily assigned for use of individual radio units specified by digital control signals on the control channel, said method including the steps of:

(1) receiving subaudible digital signalling over a working channel and substantially simultaneously receiving other signals sent over said working channel at a rate which is higher than the rate of said subaudible signalling, said subaudible signalling defining a frame structure;

(2) receiving high-speed data bursts embedded within said subaudible digital signalling at predetermined positions within said signalling frame structure; and

(3) muting audio receiver circuitry during the times said high-speed data bursts are received.

19. A method as in claim 18 wherein said subaudible signalling defines a frame structure wherein every two bits are inverted and repeated, and said high-speed data bursts occupy 4-bit time slots within said frame structure.

20. A method as in claim 19 wherein said subaudible signalling frame structure has the following format:

(a) 2-bit times reserved for said high-speed data bursts;

(b) two additional bits;

(c) an idle frame bit which is set whenever no radio units are on the working channel;

(d) an 11-bit group identification field;

(e) 2-bit times reserved for said high-speed data bursts;

(f) two additional bits;

(g) a further idle frame bit which is set whenever no radio units are on the working channel;

(h) a further 11-bit group identification field;

(i) 2-bit times reserved for said high-speed data bursts;

(j) 2-bit times reserved for high-speed confirmation signals from radio units on the working channel; and

(k) a 12-bit error-checking field.

21. In a trunked radio repeater system of the type having a digital control channel and plural working channels, which working channels are temporarily assigned for use of individual radio units specified by digital control signals on the control channel, an improvement comprising:

means for receiving subaudible digital signalling over a working channel and substantially simultaneously receiving other signals sent over said active working channel at a rate which is higher than the rate of said subaudible signalling;

means for receiving high-speed data bursts embedded within said subaudible digital signals at predetermined positions within said signalling; and

means cooperating with said high-speed data bursts receiving means for muting audio receiver circuitry during times said high-speed data bursts are being received.

22. A system as in claim 21 wherein said subaudible signalling defines a frame structure wherein every two bits are inverted and repeated, and said high-speed data bursts occupy 4-bit time slots within said frame structure.

23. A system as in claim 22 wherein said subaudible signalling frame structure has the following format:

(a) 2-bit times reserved for said high-speed data bursts;

(b) two additional bits;

(c) an idle frame bit which is set whenever no radio units are on the working channels;

(d) an 11-bit group identification field;

(e) 2-bit times reserved for said high-speed data bursts;

(f) two additional bits;

(g) a further idle frame bit which is set whenever no radio units are on the working channel;

(h) a further 11-bit group identification field;

(i) 2-bit times reserved for said high-speed data bursts;

(j) 2-bit times reserved for high-speed confirmation signals from radio units on the working channel; and

(k) a 12-bit error-checking field.

24. A method for achieving reliable and prompt communication within a trunked radio repeater system having a digital control channel and plural working channels, which working channels are assigned for temporary use of individual radio units specified by digital control signals on the control channel, said method comprising the steps of:

monitoring digital signals passed over an assigned working channel after operation of a radio unit has been established thereon, said digital signals being passed over said working channel first substantially at 9600 bits per second and subsequently at a subaudible data rate, and

dropping operation of a radio unit from said working channel in response to a detected change in said digital signals.

25. Apparatus for achieving reliable and prompt communication within a trunked radio repeater system having a digital control channel and plural working channels, which working channels are assigned for temporary use of individual radio units specified by digital control signals on the control channel, said apparatus comprising:

means for monitoring digital signals transmitted over an assigned working channel after operation of a radio unit has been established thereon, said digital signals including a subaudible signal stream having 9600 bits per second high-speed digital signal bursts embedded therein; and

means for dropping operation of a radio unit from said working channel in response to said subaudible signal stream and/or said high-speed signal bursts.

26. A method for achieving reliable and prompt communication within a trunked radio repeater system having a digital control channel and plural working channels, which working channels are assigned for temporary use of individual radio units specified by digital control signals on the control channel, said method comprising the steps of:

requesting assignment of a working channel by passing digital request signals from a first radio unit to a control side over said control channel;

assigning a specific working channel to said first radio unit and to at least a second radio unit by passing digital assignment signals over said control channel;

passing a digital handshake signal over said assigned working channel from said first radio unit to said control site;

substantially simultaneously with said passing step, passing a confirmation message from said control site to said second radio unit; and

releasing said first and second units for communication over said assigned working channel by sending digital release signals on said assigned working channel in response to successful exchange of said handshake signal.

27. A method as in claim 26 wherein said digital assignment signals include a channel assignment message pair, one message in said pair identifying said first radio unit, the other message in said pair identifying said second radio unit.

28. A method as in claim 26 wherein all said digital signals are transmitted at substantially 9600 bits per second.

29. A method as in claim 26 wherein said control signals, request signals and handshake signals include triple data redundancy and said assignment signals include six-fold redundancy of data representing the called party and the assigned channel.

30. A method as in claim 26 further comprising the steps of:

passing digital, new channel assignment updated messages over the assigned working channel first at a high data rate and subsequently at a lower subaudible data rate; and

monitoring said new channel assignment messages in each radio unit and responsively dropping off the assigned working channel if a higher priority incoming message is directed thereto.

31. A method for achieving reliable and prompt communication within a trunked radio repeater system having a digital control channel and plural working channels, which working channels are assigned for temporary use of individual radio units specified by digital control signals on the control channel, said method comprising the steps of:

requesting assignment of a working channel by passing digital request signals from a first radio unit to a control site over said control channel;

assigning a specific working channel to said first radio unit and to at least one second radio unit by passing digital assignment signals from said control site to said radio units over said control channel;

confirming said assignment by sending digital confirmation signals on said assigned working channel from said control site;

passing a digital handshake signal over said assigned working channel between said first radio unit and said control site substantially simultaneously with said confirming step; and

releasing at least one of said units for communication by sending digital release signals over said assigned working channel in response to said handshake signal.

32. A method as in claim 31 wherein said second unit is enabled to communicate by receipt of said release signals.

33. A method as in claim 31 wherein said assigning step includes the step of passing a channel assignment message pair over said control channel, one message in said pair identifying said first radio unit, the other message in said pair identifying said second radio unit.

34. A method as in claim 31 wherein said request signals, assignment signals, acknowledgement signals, digital handshake signals and release signals are all digitally encoded and transmitted at substantially 9600 bits per second over said control channel and assigned working channel, respectively.

35. A method as in claim 31 wherein'"'"'

said control signals, said request signals and said confirmation signals each include triple data redundancy, and

said assignment signals include six-fold redundancy of data representing the called party and the assigned channel.

36. A method as in claim 31 further comprising the steps of:

transmitting digital new channel assignment update messages on the assigned working channel first at substantially 9600 bits per second and subsequently at a much lower, subaudible data rate, and

monitoring said update messages in each unit and responsively dropping off the assigned working channel if a higher priority incoming message is directed thereto.

37. A method for achieving reliable and prompt communication within a trunked radio repeater system having a digital control channel and plural working channels, which working channels are assigned for temporary use of individual radio units specified by digital control signals on the control channel, said method comprising the steps of:

A. sending, from a first radio unit to a control site over the control channel, request signals including

a. string of dotting bits to establish bit sync,

b. a predetermined word sync code repeated three times, and

c. a request message of 40 bits repeated three times, each message including

i. 3 bits identifying the message type,

ii. 11 bits identifying the called unit,

iii. 14 bits identifying the calling unit, and

iv. 12 bits of BCH error detection and correction data;

B. sending, from said control site to said first radio unit over said control channel, assignment signals including

a. predetermined word sync code,

b. a pair of assignment messages, each being repeated three times and having 40 bits including

i. 3 bits identifying the message type,

ii. 8 bits providing one-half the identity of the calling unit;

iii. 5 bits identifying the assigned working channel,

iv. one bit identifying whether transmission trunking is in effect,

v. 11 bits identifying the called group or individual unit, and

vi. 12 bits of BCH error detection and correction data;

C. sending, from said control site over the assigned working channel, confirmation signals including

a. a predetermined

b. a confirmation message of 32 bits repeated three times including

i. 4 bits identifying the type of message,

ii. 1 bit identifying whether transmission trunked operation is in effect,

iii. 1 bit identifying whether a group or individual call is being made,

iv. 11 bits identifying the called group or individual unit, and

v. 14 bits identifying the calling unit, and

vi. 12 bits of BCH error detection and correction data;

D. sending, from said first unit over the assigned working channel, digital acknowledgement signals including

a. a relatively longer string of dotting bits, followed by

b. communications over the working channel to the called group or individual unit, and

E. in response to step D(a) and prior to step D(b), sending, from said control site over the assigned working channel, digital release signals including

a. a predetermined word sync code, and

b. two unit-keyed/unmute messages, each repeated three times and being 32 bit messages including

i. 4 bits identifying the type of message,

ii. 1 bit identifying whether trunked operation is in effect,

iii. 14 bits identifying the calling unit, and

iv. 12 bits identifying the calling unit, and

38. A method as in claim 37 further comprising the step of:

subsequent to step E, sending digital signals over said assigned working channel including new channel assignment messages first substantially at 9600 bits per second and subsequently at a subaudible data rate, said digital signals being monitored by each unit residing on that working channel for higher priority calls directed to that unit or a predefined group including such unit.

39. A method as in claim 38 further comprising the step of:

subsequent to step E, sending a channel assignment update message over said control channel from the control site to permit late entry of additional called units to the assigned working channel.

40. A method as in claim 39 further comprising the step of:

at the conclusion of communication by said first unit, the transmission from the first unit over said assigned working channel of an unkeyed message including

a. a strin gof dotting bits,

b. a block of 128 bits repeated four times, each data block being of the form

i. a string of dotting bits,

ii. a predetermined word sync code,

iii. four 8 bit bytes, each transmitted with triple redundancy and defining a 32 bit message having 4 bits identifying the type of message, a 2-bit block count, 14 bits identifying the calling unit and 12 bits of BCH error detection and correction data.

41. A method as in claim 40 further comprising the step of:

responsive to said unkeyed message, sending, from said control site over said assigned working channel a substantially elongated string of dotting bits, and

all units then assigned to the working channel dropping therefrom and reverting to the control channel in response to said elongated string of dotting bits on the working channel.

42. A system of apparatus for achieving reliable and prompt communication within a trunked radio repeater system having a digital control channel and plural working channels, which working channels are assigned for temporary use of individual radio units specified by digital control signals on the control channel, said system comprising:

means for requesting assignment of a working channel by passing digital request signals from a first radio unit to a control site over said control channel;

means for assigning a specific working channel to said first radio unit and to at least a further radio unit by passing digital assignment signals over said control channel;

means for confirming the operation of said first radio unit on said assigned working channel by passing a string of synchronization signals from said first radio unit to said control site over said working channel;

means for confirming said working channel assignment to said further radio unit by passing digital confirmation message over said assigned working channel from said control site to said further radio unit at substantially the same time said string of synchronization signals are being passed from said first radio unit to said control site; and

means for releasing at least one of said first and further unit for communication over said assigned working channel by sending digital release signals on said assigned working channel in response to successful receipt of said synchronization signals.

43. A system as in claim 42 further comprising:

means for passing new channel assignment message over the assigned working channel, and

means for monitoring said update messages in each unit and responsively dropping off the assigned working channel if a higher priority incoming message is directed thereto.

45. A system as in claim 42 wherein both messages in said pair partially identify said first radio unit.

47. A system as in claim 42 further comprising:

means for sending, from the calling unit on the assigned working channel, a digital unkey message, and

means responsive to said unkey message, for sending, from said control site on said assigned working channel, a digital drop signal which causes all units to drop off the then-assigned working channel thus freeing it for immediate further assignment.

44. A system as in claim 44 wherein said assigning means includes means for passing a channel assignment two-message pair over said control channel, the first message in said pair identifying said first radio unit, the second message in said pair identifying said second radio unit.

46. A system as in claim 44 wherein said passing means includes means for passing at least one high-speed data burst in addition to subaudible digital signals over said assigned working channel both indicating assignment of another working channel.

48. A system for achieving reliable and prompt communication within a trunked radio repeater system having a digital control channel and plural working channels, which working channels are assigned for temporary use of individual radio units specified by digital control signals on the control channel, said system comprising:

means for requesting assignment of a working channel by passing digital request signals from a first radio unit to a control site over said control channel;

means for assigning a specific working channel to said first radio unit and to at least one further radio unit by passing digital assignment signals from said control site to said radio units over said control channel;

means for confirming said assignment by sending digital confirmation signals on said assigned working channel from said control site;

means for acknowledging successful receipt of said assignment signals by at least one of said units sending digital acknowledgement signals to said control site over said assigned working channel in response to said confirmation signals; and

means for releasing at least one of said units for communication by sending digital release signals over said assigned working channel in response to said acknowledgement signals.

49. A system as in claim 48 wherein said request signals, assignment signals, acknowledgement signals and release signals are all digitally encoded and transmitted at substantially 9600 bps over said control channel and assigned working channel, respectively.

50. A system as in claim 48 wherein:

said control signals, said request signals and said confirmation signals each include triple data redundancy, and

said assignment signals include six-fold redundancy of data representing the called party and the assigned channel.

51. A system for achieving reliable and prompt communication within a trunked radio repeater system having a digital control channel and plural working channels, which working channels are assigned for temporary use of individual radio units specified by digital control signals on the control channel, said system comprising:

A. means for sending, from a first radio unit to a control site over the control channel, request signals including

a. a string of dotting bits to establish bit sync,

b. a predetermined word sync code repeated three times, and

c. a request message of 40 bits repeated three times, each message including

i. 4 bits identifying the message type,

ii. 11 bits identifying the called unit,

iii. 14 bits identifying the calling unit, and

iv. 12 bits of BCH error detection and correction data;

B. means for sending, from said control site to said first radio unit over said control channel, assignment signals including

a. a predetermined word sync code,

b. a pair of assignment messages, each being repeated three times and having 40 bits including

i. 3 bits identifying the message type,

ii. 8 bits providing one-half the identity of the calling unit;

iii. 5 bits identifying the assigned working channel,

iv. 1 bit identifying whether transmission trunking is in effect,

v. 11 bits identifying the called group or individual unit, and

vi. 12 bits of BCH error detection and correction data;

C. means for sending, from said control site over the assigned working channel, confirmation signals including

a. a predetermined word sync code, and

b. a confirmation message of 32 bits repeated three times including

i. 4 bits identifying the type of message,

ii. 1 bit identifying whether transmission trunked operation is in effect,

iii. 1 bit identifying whether a group or individual call is being made,

iv. 11 bits identifying the called group or individual unit,

v. 14 bits identifying the calling unit, and

vi. 12 bits of BCH error detection and correction data;

D. means for sending, from said first unit over the assigned working channel, digital acknowledgement signals including

a. a relatively longer string of dotting bits, followed by

b. communications over the working channel to the called group or individual unit, and

E. means operative in response to said longer string of dotting bits for sending, from said control site over the assigned working channel, digital release signals including

a. a predetermined word sync code, and

b. two unit-keyed/unmute messages, each repeated three times and being 32 bit messages including

i. 4 bits identifying the type of message,

ii. 1 bit identifying whether trunked operation is in effect,

iii. 14 bits identifying the calling unit, and

iv. 12 bits of BCH error detection and correction data.

52. A method for achieving reliable and prompt communication within a trunked radio repeater system having a digital control channel and plural working channels, which working channels are assigned for temporary use of individual radio units specified by digital control signals on the control channel, said method comprising the steps of:

requesting assignment of a working channel;

monitoring digital working channel assignment signals passed over said control channel issued in response to said requested assignment;

confirming said channel assignment by passing confirmation digital signals over said assigned working channel between said control site and a first of said radio units and by passing different confirmation signals between said control site and a second of said radio units; and

releasing said assigned working channel for communication by sending digital release signals over said assigned working channel in response to successful exchange of said confirmation signals.

53. A method as in claim 52 wherein said release signals and said different confirmation signals are sent from said control site, and said first-released confirmation signals are sent from said first radio unit.

54. A method as in claim 52 wherein all said digital signals are transmitted at substantially 9600 bits per second.

55. A method as in claim 52 wherein said confirmation and release signals include triple data redundancy and said assignment signals include six-fold redundancy of data representing the called party and the assigned channel.

56. A method as in claim 52 further comprising the steps of:

passing new channel assignment messages on the assigned working channel, and

monitoring said assignment messages in each radio unit and responsively dropping off the assigned working channel if a higher priority incoming message is directed thereto.

57. A method as in claim 56 wherein said passing step includes:

transmitting each new channel assignment message over said working channel at substantially 9600 bits per second; and

subsequently transmitting each new channel assignment message at a subaudible rate.

58. A system of apparatus for achieving reliable and prompt communication within a trunked radio repeater system having a digital control channel and plural working channels, which working channels are assigned for temporary use by individual radio units specified through digital control signals on the control channel, said system comprising:

means for requesting assignment of a working channel;

means for assigning a specific working channel by passing digital assignment signals over said control channel;

means for confirming said channel assignment by passing confirmation digital signalling over said assigned working channel between said control site and a first of said radio units and passing different confirmation signals between said control site and a second of said radio units; and

means for releasing said assigned working channel for communication thereover by sending digital release signals on said assigned working channel in response to successful exchange of said confirmation signals.

59. A system as in claim 58 further comprising:

means for transmitting subaudible new channel assignment messages on the assigned working channel, and

means for monitoring said subaudible assignment messages in each unit and responsively dropping off the assigned working channel if a higher priority incoming message is directed thereto.

60. A system as in claim 59 wherein said confirming step includes:

passing each new channel assignment message over said working channel at substantially 9600 bits per second; and

subsequently passing each new channel assignment message only once at a subaudible rate.

61. A system as in claim 58 further comprising:

means for subsequently transmitting digital channel assignment late entry messages on the control channel to permit the late entry of a called unit to an ongoing working channel communication.

62. A system as in claim 58 further comprising:

means for sending, from the calling unit on the assigned working channel, a digital unkey message, and

means responsive to said unkey message, for sending, from said control site on said assigned working channel, a digital drop signal which causes all units to drop off the then-assigned working channel thus freeing it for immediate further assignment.

63. A radio unit for achieving reliable and prompt communication within a trunked radio repeater system having a digital control channel and plural working channels, which working channels are assigned for temporary use of individual radio units specified by digital control signals on the control channel, said radio unit comprising:

means for requesting assignment of a working channel by passing digital request signals to a control site over said control channel;

means for detecting digital working channel assignment signals over said control channel;

means for passing digital synchronization signals over said assigned working channel,

means for detecting digital release signals on said assigned working channel after successfully passing said synchronization signals and for unmuting communication over the assigned working channel in response to said detected release signals;

means for detecting new channel assignment digital signals on the assigned working channel, said assignment signals including one of (1) a high-speed burst embedded in a subaudible digital signalling frame on said working channel, and (2) subaudible signals on said working channel, subsequent to said high-speed burst and part of said subaudible signalling frame, and

means for responsively dropping off the assigned working channel if a higher priority incoming message is detected.

64. A system as in claim 63 further comprising:

means for sending, from the calling unit on the assigned working channel, a digital unkey message in response to release of a PTT switch, and

means responsive to a digital drop signal received on the assigned working channel for dropping off the then-assigned working channel thus freeing it for immediate further assignment.

65. A radio unit for achieving reliable and prompt communication within a trunked radio repeater system having a digital control channel and plural working channels, which working channels are assigned for temporary use of individual radio units specified by digital control signals on the control channel, said radio unit comprising:

A. means for sending, to a control site over the control channel, request signals including

a. a string of dotting bits to establish bit sync,

b. a predetermined word sync code repeated three times, and

c. a request message of 40 bits repeated three times, each message including

i. 3 bits identifying the message type and type of requested communication,

ii. 1 bits identifying the called unit,

iii. 14 bits identifying the calling unit, and

iv. 12 bits of BCH error detection and correction data;

B. means for detecting on said control channel, assignment signals including

a. a predetermined word sync code,

b. a pair of assignment messages, each being repeated three times and having 40 bits including

i. 3 bits identifying the message type,

ii. 8 bits providing one-half the identity of the calling unit;

working channel,

iv. one bit identifying whether transmission trunking operation is in effect,

v. 11 bits identifying the called group or individual unit, and

vi. 12 bits of BCH error detection and correction data;

C. means for detecting on the assigned working channel, confirmation signals including

a. a predetermined word sync code, and

b. a confirmation message of 32 bits repeated three times including

i. 4 bits identifying the type of message,

ii. 1 bit identifying whether transmission trunked operation is in effect,

iii. 1 bit identifying whether a group or individual call is being made,

iv. 11 bits identifying the called group or individual unit,

v. 14 bits identifying the calling unit, and

vi. 12 bits of BCH error detection and correction data;

D. means for sending, over the assigned working channel, digital acknowledgement signals including

a. a relatively longer string of dotting bits, followed by

b. communications over the working channel to the called group or individual unit, and

E. means for detecting on the assigned working channel, digital release signals including

a. a predetermined word sync code, and

b. two unit-keyed/unmute messages, each repeated three times and being 32 bit message of the form

i. 4 bit identifying the type of message,

ii. 2 bits providing a block count,

iii. 14 bits identifying the calling unit, and

iv. 12 bits of BCH error detection and correction data.

66. Apparatus for achieving reliable and prompt communication within a trunked radio repeater system having a digital control channel and plural working channels, which working channels are assigned for temporary use of individual radio units specified by digital control signals on the control channel, said apparatus comprising:

transceiver means at a central site for transmitting and receiving radio frequency signals over said control channel and over said working channels; and

means at said central site operatively coupled to said transceiver means for continually transmitting subaudible signals over an assigned working channel while the working channel remains asigned, said subaudible signals including a predetermined digital data field which is changed to a new value each time a new assignment of that working channel is made, said new value indicating that the working channel has been reassigned.