Demand-based connection management integrated services access terminal (ISAT) for satellite communication system

• US 5,841,765 A
• Filed: 10/10/1996
• Issued: 11/24/1998
• Est. Priority Date: 10/10/1996
• Status: Expired due to Term

• Current Assignee: Gilat Satellite Networks Limited
• Original Assignee: Skydata, Inc.
• Inventors: Gross, Todd W., Fielding, Dennis E., Arnold, John P.
• Primary Examiner(s): NGUYEN, CHAU T
• Application Number:
  US08/728,636
• Time in Patent Office: 775 Days
• Field of Search: 370/321, 370/329, 370/436, 370/330, 370/344, 370/343, 370/341, 370/316, 370/319, 370/480, 370/400, 370/351, 370/356, 370/254, 370/255, 370/496, 370/353, 370/352, 458/12.1, 458/450, 458/509, 458/510, 375/222
• US Class Current: 370/319
• CPC Class Codes: H04B 7/18582 Arrangements for data linking, i.e. for data framing, for error recovery, for multiple access
• CPC Class Codes: H04B 7/18584 Arrangements for data networking, i.e. for data packet routing, for congestion control data switching networks H04L12/00

Abstract

A demand-based connection management satellite communication network includes a plurality of geographically distributed earth stations, each earth station having a frame relay protocol-based switch for interfacing message signals between terminal devices and modem unit. One of the earth stations is a hub station whose modem unit includes plural demodulators tuned to carrier frequencies transmitted from other, remote stations. Each remote station has at least one hub-programmable demodulator and/or a programmable modulator. When an originating station receives a request from a terminal device for connectivity with a terminal device at a destination station, it transmits a connectivity request message to the hub station. In response, the hub station checks a demodulator and/or modulator frequency assignment database for demodulator availability. If each originating and destination station has an available demodulator and modulator pair, the hub transmits tuning messages to the two stations, which tune their programmable modem components to each other'"'"'s carrier frequency, and thereby provide bidirectional connectivity between the two remote stations.

Claims

1. A method of providing communication connectivity between any two of a plurality of geographically distributed terminal devices by way a satellite communication network having a communication satellite comprising the steps of:

(a) providing a plurality of geographically distributed earth stations, each earth station having a frame relay protocol-based switch for interfacing message signals between terminal devices served by said each earth station and a modulator and demodulators employed by said each earth station, said modulator having a transmission carrier frequency associated with said each earth station;

(b) coupling said modulator to an uplink port of the frame relay protocol-based switch of said each earth station;

(c) installing at one of said earth stations, termed a hub station, a plurality of demodulators, each of which is operative to demodulate messages modulated on carrier frequencies transmitted from other earth stations, termed remote stations, and coupling respective ones of said plurality of demodulators to respectively different downlink ports of the frame relay protocol-based switch of said hub station;

(d) at each remote station, installing a tunable demodulator which is operative to demodulate messages modulated on a carrier frequency transmitted from another earth station, and coupling said tunable demodulator to a downlink port of the frame relay protocol-based switch of said each remote station;

(e) in response to a request from a terminal device coupled to an originating remote station of said remote stations for communication connectivity with a terminal device coupled to a destination remote station of said remote stations, causing the modulator of said originating remote station to transmit a communication connectivity request message to said hub station; and

(f) at said hub station, in response to said communication connectivity request message, transmitting to said originating remote station an originating remote station demodulator tuning message for causing the tunable demodulator at said originating remote station to be tuned to the transmission carrier frequency associated with said destination remote station, and transmitting to said destination remote station a destination remote station demodulator tuning message for causing the tunable demodulator at said destination remote station to be tuned to the transmission carrier frequency associated with said originating remote station.

2. A method according to claim 1, where step (d) further comprises installing a further demodulator at said each remote station, said further demodulator being tuned to the transmission carrier frequency associated with hub station, and being operative to demodulate a demodulator tuning message transmitted thereto from said hub station.

3. A method according to claim 1, wherein step (a) comprises coupling a plurality of terminal device ports of said frame relay protocol-based switch to terminal devices served by said each earth station.

4. A method according to claim 1, wherein step (f) further comprises transmitting, as part of said originating remote station demodulator tuning message, information representative of the bit rate of messages modulated on the transmission carrier frequency associated with said destination remote station, and transmitting as part of said destination remote station demodulator tuning message, information representative of the bit rate of messages modulated on the transmission carrier frequency associated with said originating remote station.

5. A method according to claim 1, further including the step of:

(g) at said destination remote station, causing the frame relay protocol-based switch of said destination remote station to controllably route a downlink-directed message coupled thereto from a demodulator thereof to a virtual port for coupling to a terminal device, said virtual port being selected in accordance with the contents of said downlink-directed message signals.

6. A method according to claim 5, wherein a terminal device port is coupled to a terminal device multiplexer, which controllably interfaces message signals between a plurality of virtual ports and a terminal device port.

7. A method according to claim 5, wherein a terminal device port is coupled to an audio signalling circuit multiplexer which controllably interfaces audio message signals between a plurality of virtual ports and terminal device port.

8. A method according to claim 1, wherein said terminal devices include audio, data and video signaling devices.

9. A method according to claim 5, wherein said terminal devices include audio, data and video signalling devices and wherein step (g) comprises causing said frame relay protocol-based switch to route message signals therethrough such that audio frames are given highest priority, video teleconferencing frames are given the next highest priority, and data frames are given lowest priority.

10. A method according to claim 1, wherein step (d) comprises installing a first demodulator at said each remote station, said first demodulator being tuned to the transmission carrier frequency associated with said hub station, and being operative to demodulate a demodulator tuning-message transmitted thereto from said hub station, and a second demodulator which is operative to demodulate messages modulated on a carrier frequency transmitted from another earth station, and coupling said second demodulator to a downlink port of the frame relay protocol-based switch of said each remote station,

and wherein step (f) comprises, in response to said communication connectivity request message, transmitting to said originating remote station an originating remote station demodulator tuning message for causing the second demodulator at said originating remote station to be tuned to the transmission carrier frequency associated with said destination remote station, and transmitting to said destination remote station a destination remote station demodulator tuning message for causing the second demodulator at said destination remote station to be tuned to the transmission carrier frequency associated with said originating remote station.

11. A method according to claim 10, wherein said each earth station includes a communication control processor associated with said each remote station'"'"'s frame relay switch which stores table entries indicating whether there currently exists an active communication link to said destination station, and wherein step (e) comprises, in response to a call being placed from a terminal device, causing said communication control processor to examine said table entries and, in response to said table entries indicating no existing active communication link, generating a link request message for transmission to said hub station, said link request message designating which remote station is the destination station.

12. A method according to claim 11, wherein step (e) comprises repeatedly transmitting said link request message at a predetermined repetition rate for a prescribed number of repeat tries.

13. A method according to claim 11, wherein step (e) further comprises, in response to said table entries indicating the existence of an active communication link between said originating and destination remote stations, causing said originating remote station'"'"'s control processor to forward a call routing message to an idle voice circuit in said destination remote station, so that said idle voice circuit may seize a line to a called terminal device and thereby complete a communication path between a calling terminal device at the originating remote station and said called terminal device at the destination remote station.

14. A method according to claim 11, wherein step (e) further comprises transmitting a call request message over said active communication link between said originating and destination remote stations, said call request message being repeatedly transmitted a predetermined number of times, or until the hub station establishes a communication link through said network between the originating and destination remote stations, and the call is received and processed by said destination remote station.

15. A method according to claim 11, wherein said hub station includes a network management facility computer and an associated network configuration database, and wherein, in step (f), said network management facility computer examines a network configuration database, to determine availability of a tunable demodulator in each of the originating and destination remote stations.

16. A method according to claim 15, wherein step (f) further comprises, in response to either or both of said originating and destination remote stations containing no available tunable demodulator, causing a voice circuit in said originating remote station to time out, returning a busy signal to a calling party'"'"'s telephone.

17. A method according to claim 14, wherein step (f) further comprises coupling a call request message received by said destination remote station to be coupled through its frame relay switch to the destination remote station'"'"'s control processor, said destination remote station'"'"'s control processor forwarding a call routing packet to an idle voice circuit, so that said idle voice circuit may seize a line to a called terminal device and thereby complete a communication path between a calling terminal device at the originating remote station and said called terminal device at the destination remote station.

18. A method according to claim 17, wherein step (f) further comprises causing said destination remote station to return a call accepted message containing information representative of the established communication link to said originating remote station.

19. A method according to claim 18, wherein step (f) further comprises causing the tunable demodulators in each of the originating and destination remote stations to be maintained tuned to their assigned frequencies as long as any voice calls are present.

20. A method according to claim 19, wherein step (f) further comprises in response to termination of call traffic between said originating and destination remote stations, causing said originating remote station to forward a message to said hub station, indicating that all traffic between the linked stations has been completed, and causing said hub station'"'"'s network management facility computer to update its network configuration database, so as to allow said tunable demodulators in said originating and destination remote stations to be tuned to frequencies associated with other remote stations.

21. A satellite communication system for providing communication connectivity between any two of a plurality of geographically distributed terminal devices by way a satellite communication network having a communication satellite comprising:

a plurality of geographically distributed earth stations, each earth station having a frame relay protocol-based switch for interfacing message signals between terminal devices served by said each earth station and a modulator and demodulators installed in said each earth station;

said modulator having a transmission carrier frequency associated with said each earth station, and being coupled to an uplink port of the frame relay protocol-based switch of said each earth station;

one of said earth stations, termed a hub station, having a plurality of demodulators, each of which is operative to demodulate messages modulated on carrier frequencies transmitted from other earth stations, termed remote stations, and being coupled to respectively different downlink ports of the frame relay protocol-based switch of said hub station;

each remote station having a tunable demodulator which is operative to demodulate messages modulated on a carrier frequency transmitted from another earth station, and being coupled to a downlink port of the frame relay protocol-based switch of said remote station; and

wherein

in response to a request from a terminal device coupled to an originating remote station of said remote stations for communication connectivity with a terminal device coupled to a destination remote station of said remote stations, the modulator of said originating remote station being operative to transmit a communication connectivity request message to said hub station; and

wherein

said hub station, in response to said communication connectivity request message, being operative to transmit, to said originating remote station, an originating remote station demodulator tuning message for causing the demodulator at said originating remote station to be tuned to the transmission carrier frequency associated with said destination remote station, and to transmit, to said destination remote station, a destination remote station demodulator tuning message for causing the demodulator at said destination remote station to be tuned to the transmission carrier frequency associated with said originating remote station.

22. A satellite communication system according to claim 21, wherein said each remote station includes a further demodulator which is operative to demodulate a demodulator tuning message transmitted thereto on the transmission carrier frequency associated with said hub station.

23. A satellite communication system according to claim 21, wherein a plurality of terminal device ports of said frame relay protocol-based switch are coupled to terminal devices served by said each earth station.

24. A satellite communication system according to claim 21, wherein said hub station is operative to transmit, as part of said originating remote station demodulator tuning message, information representative of the bit rate of messages modulated on the transmission carrier frequency associated with said destination remote station, and to transmit as part of said destination remote station demodulator tuning message, information representative of the bit rate of messages modulated on the transmission carrier frequency associated with said originating remote station.

25. A satellite communication system according to claim 21, wherein the frame relay protocol-based switch of said destination remote station is operative to cause the frame relay protocol-based switch of said destination remote station to controllably route a downlink-directed message coupled thereto from a demodulator thereof to a virtual port for coupling to a terminal device, said virtual port being selected in accordance with the contents of said downlink-directed message signals.

26. A satellite communication system according to claim 25, wherein a terminal device port is coupled to a terminal device multiplexer which controllably interfaces message signals between a plurality of virtual ports and a terminal device port.

27. A satellite communication system according to claim 25, wherein a terminal device port is coupled to an audio signaling circuit multiplexer which controllably interfaces audio message signals between a plurality of virtual ports and terminal device port.

28. A satellite communication system according to claim 21, wherein said terminal devices include audio, data and video signaling devices.

29. A satellite communication system according to claim 25, wherein said terminal devices include audio, data and video signaling devices and wherein said frame relay protocol-based switch is operative to route message signals therethrough such that audio frames are given highest priority, video teleconferencing frames are given the next highest priority, and data frames are given lowest priority.

30. A satellite communication system according to claim 21, wherein said hub station is operative to store information representative of whether the tunable demodulator of said originating remote station and the tunable demodulator of said destination remote station are available for establishing communication connectivity between said originating and destination earth stations.

31. A satellite communication system according to claim 21, wherein a respective remote earth station is operative to store information representative of whether communication connectivity exists between said respective remote station and another remote earth station.

32. A satellite communication system according to claim 21, wherein said each remote station includes a first demodulator that is tuned to the transmission carrier frequency associated with hub station, and being operative to demodulate a demodulator tuning-message transmitted thereto from said hub station, and a second demodulator which is operative to demodulate messages modulated on a carrier frequency transmitted from another earth station, said second demodulator being coupled to a downlink port of the frame relay protocol-based switch of said each remote station, and wherein

said hub station is operative, in response to said communication connectivity request message, to transmit, to said originating remote station, an originating remote station demodulator tuning message for causing the second demodulator at said originating remote station to be tuned to the transmission carrier frequency associated with said destination remote station, and to transmit, to said destination remote station, a destination remote station demodulator programming message for causing the second demodulator at said destination remote station to be tuned to the transmission carrier frequency associated with said originating remote station.

33. A satellite communication system according to claim 32, wherein said each earth station includes a communication control processor associated with said each remote station'"'"'s frame relay switch which stores table entries indicating whether there currently exists an active communication link to said destination remote station, and which is operative, in response to a call being placed from a terminal device, for causing said communication control processor to examine said table entries and, in response to said table entries indicating no existing active communication link, for generating a link request message for transmission to said hub station, said link request message designating which remote station is the destination remote station.

34. A satellite communication system to claim 33, wherein said originating remote station is operative to repeatedly transmit said link request message at a predetermined repetition rate for a prescribed number of repeat tries.

35. A satellite communication system method according to claim 33, wherein said originating remote station is operative, in response to said table entries indicating the existence of an active communication link between said originating and destination remote stations, to forward a call routing message to an idle voice circuit in said destination remote station, so that said idle voice circuit may seize a line to a called terminal device and thereby complete a communication path between a calling terminal device at the originating remote station and said called terminal device at the destination remote station.

36. A satellite communication system according to claim 33, wherein said originating remote station is operative to transmit a call request message over said active communication link to said destination remote station, said call request message being repeatedly transmitted a predetermined number of times, or until the hub station establishes a communication link through said network between the originating and destination stations, and the call is received and processed by said destination remote station.

37. A satellite communication system according to claim 33, wherein said hub station includes a network management facility computer and an associated network configuration database, and wherein said network management facility computer is operative to examine a network configuration database, to determine availability of a tunable demodulator in each of said originating and destination remote stations.

38. A satellite communication system according to claim 37, wherein said originating remote station is operative, in response to either or both of said originating and destination remote stations containing no available tunable demodulator, to causing a voice circuit in said originating remote station to time out, and return a busy signal to a calling party'"'"'s telephone.

39. A satellite communication system to claim 36, wherein said destination remote station is operative to couple a call request message received to be coupled through its frame relay switch to said destination remote station'"'"'s control processor, said destination remote station'"'"'s control processor forwarding a call routing packet to an idle voice circuit, so that said idle voice circuit may seize a line to a called terminal device and thereby complete a communication path between a calling terminal device at the originating remote station and said called terminal device at the destination remote station.

40. A satellite communication system according to claim 39, wherein said destination remote station is operative to return a call accepted message containing information representative of the established communication link to said originating remote station.

41. A satellite communication system according to claim 40, wherein said originating and destination stations are operative to maintain their tunable demodulators tuned to their assigned frequencies as long as any voice calls are present.

42. A satellite communication system according to claim 41, wherein said originating remote station, in response to termination of call traffic with said destination remote station, is operative to forward a message to said hub station, indicating that all traffic between the linked stations has been completed, and wherein said hub station'"'"'s network management facility computer is operative to update its network configuration database, so as to allow said tunable demodulators in said originating and destination remote stations to be tuned to frequencies associated with other remote stations.

43. A method of providing communication connectivity between any two of a plurality of geographically distributed terminal devices by way a satellite communication network having a communication satellite comprising the steps of:

(a) providing a plurality of geographically distributed earth stations, each earth station having a frame relay protocol-based switch for interfacing message signals between terminal devices served by said each earth station and a modulator and demodulators employed by said each earth station;

(b)coupling said modulator to an uplink port of the frame relay protocol-based switch of said each earth station;

(c) installing at one of said earth stations, termed a hub station, a demodulator, which is operative to demodulate messages modulated on a control carrier frequency transmitted from other earth stations, termed remote stations, and coupling said demodulator to a downlink port of the frame relay protocol-based switch of said hub station;

(d) at each remote station, installing a tunable demodulator which is operative to demodulate messages modulated on a carrier frequency transmitted from another earth station, and coupling said tunable demodulator to a downlink port of the frame relay protocol-based switch of said each remote station;

(e) in response to a request from a terminal device coupled to an originating remote station of said remote stations for communication connectivity with a terminal device coupled to a destination remote station of said remote stations, causing the modulator of said originating remote station to transmit a communication connectivity request message to said hub station on said control carrier; and

(f) at said hub station, in response to said communication connectivity request message, transmitting to said originating remote station an originating remote station tuning message for causing the modulator at said originating remote station to be tuned to a transmission carrier frequency associated with said originating remote station, and for causing the demodulator at said originating remote station to be tuned to the transmission carrier frequency associated with said destination remote station, and transmitting to said destination remote station a destination remote station tuning message for causing the tunable demodulator at said destination remote station to be tuned to the transmission carrier frequency associated with said originating remote station, and for causing the tunable modulator at said destination remote station to be tuned to the transmission carrier frequency associated with said destination remote station.

44. A method according to claim 43, where step (d) further comprises installing a further demodulator at said each remote station, said further demodulator being tuned to said control carrier frequency, and being operative to demodulate a tuning message transmitted thereto from said hub station.

45. A method according to claim 44, wherein step (a) comprises coupling a plurality of terminal device ports of said frame relay protocol-based switch to terminal devices served by said each earth station.

46. A method according to claim 43, wherein step (f) further comprises transmitting, as part of said originating remote station tuning message, information representative of the bit rate of messages modulated on the transmission carrier frequency associated with said destination remote station, and transmitting as part of said destination remote station tuning message, information representative of the bit rate of messages modulated on the transmission carrier frequency associated with said originating remote station.

47. A method according to claim 43, further including the step of:

(g) at said destination remote station, causing the frame relay protocol-based switch of said destination remote station to controllably route a downlink-directed message coupled thereto from a demodulator thereof to a virtual port for coupling to a terminal device, said virtual port being selected in accordance with the contents of said downlink-directed message signals.

48. A method according to claim 47, wherein a terminal device port is coupled to a terminal device multiplexer, which controllably interfaces message signals between a plurality of virtual ports and a terminal device port.

49. A method according to claim 43, wherein said terminal devices include audio, data and video signalling devices.

50. A method according to claim 43, wherein step (d) comprises installing a first demodulator at said each remote station, said first demodulator being tuned to the control carrier frequency associated with hub station, and being operative to demodulate a tuning-message transmitted thereto from said hub station, and a second demodulator which is operative to demodulate messages modulated on a carrier frequency transmitted from another earth station, and coupling said second demodulator to a downlink port of the frame relay protocol-based switch of said respective remote station,

and wherein step (f) comprises, in response to said communication connectivity request message, transmitting to said originating remote station an originating remote station tuning message for causing the second demodulator at said originating remote station to be tuned to the transmission carrier frequency associated with said destination remote station, and transmitting to said destination remote station a destination remote station tuning message for causing the second demodulator at said destination remote station to be tuned to the transmission carrier frequency associated with said originating remote station.

51. A method according to claim 50, wherein each earth station includes a communication control processor associated with said each remote station'"'"'s frame relay switch which stores table entries indicating whether there currently exists an active communication link to said destination remote station, and wherein step (e) comprises, in response to a call being placed from a terminal device, causing said communication control processor to examine said table entries and, in response to said table entries indicating no existing active communication link, generating a link request message for transmission to said hub station, said link request message designating which remote station is the destination remote station.

52. A method according to claim 51, wherein said hub station includes a network management facility computer and an associated network configuration database, and wherein, in step (f), said network management facility computer examines a network configuration database, to determine availability of a tunable modulator and demodulator pair in each of the originating and destination remote stations and idle carrier frequencies.

53. A method according to claim 52, wherein step (f) further comprises causing tunable the modulator and demodulator pair in each of the originating and destination remote stations to be maintained tuned to their assigned frequencies as long as any calls are present.

54. A method according to claim 53, wherein step (f) further comprises in response to termination of call traffic between said originating and destination remote stations, causing said originating remote station to forward a message to said hub station, indicating that all traffic between the linked stations has been completed, and causing said hub station'"'"'s network management facility computer to update its network configuration database command the remote stations modulators off and verify, so as to allow said tunable modulators and demodulators in said originating and destination remote stations to be tuned to frequencies associated with other remote stations and the carrier frequencies to be returned to an idle state for reassignment.

55. A satellite communication system for providing communication connectivity between any two of a plurality of geographically distributed terminal devices by way a satellite communication network having a communication satellite comprising:

a plurality of geographically distributed earth stations, each earth station having a frame relay protocol-based switch for interfacing message signals between terminal devices served by said each earth station and a modulator and demodulators installed in said each earth station;

said modulator being tunable to a transmission carrier frequency associated with said each earth station, and being coupled to an uplink port of the frame relay protocol-based switch of said each earth station;

one of said earth stations, termed a hub station, having a demodulator, which is operative to demodulate messages modulated on a control carrier frequency transmitted from other earth stations, termed remote stations, and being coupled to a downlink port of the frame relay protocol-based switch of said hub station;

each remote station having a tunable demodulator which is operative to demodulate messages modulated on a carrier frequency transmitted from another earth station, and a tunable modulator which is operative to modulate messages on a carrier frequency for transmission to another earth station, said tunable demodulator being coupled to a downlink port of the frame relay protocol-based switch of said remote station; and

wherein

in response to a request from a terminal device coupled to an originating remote station of said remote stations for communication connectivity with a terminal device coupled to a destination remote station, the modulator of said originating remote station is operative to transmit a communication connectivity request message to said hub station on said control carrier frequency; and

wherein

said hub station, in response to said communication connectivity request message, is operative to transmit, to said originating remote station, an originating remote station tuning message for causing the demodulator at said originating remote station to be tuned to the transmission carrier frequency associated with said destination remote station, and causing the modulator at said originating remote station to be tuned to the transmission carrier frequency associated with said originating remote station, and is operative to transmit, to said destination remote station, a destination remote station demodulator tuning message for causing the demodulator at said destination remote station to be tuned to the transmission carrier frequency associated with said originating remote station, and for causing the modulator at said destination remote station to be tuned to the transmission carrier frequency associated with said destination remote station.

56. A satellite communication system according to claim 55, wherein said each remote station includes a further demodulator which is operative to demodulate a tuning message transmitted thereto on said control carrier frequency from said hub station.

57. A satellite communication system according to claim 55, wherein a plurality of terminal device ports of said frame relay protocol-based switch are coupled to terminal devices served by said each earth station.

58. A satellite communication system according to claim 55, wherein said hub station is operative to transmit, as part of said originating remote station tuning message, information representative of the bit rate of messages modulated on the transmission carrier frequency associated with said destination remote station, and to transmit as part of said destination remote station tuning message, information representative of the bit rate of messages modulated on the transmission carrier frequency associated with said originating remote station.

59. A satellite communication system according to claim 55, wherein the frame relay protocol-based switch of said destination remote station is operative to cause the frame relay protocol-based switch of said destination remote station to controllably route a downlink-directed message coupled thereto from a demodulator whereof to a virtual port for coupling to a terminal device, said virtual port being selected in accordance with the contents of said downlink-directed message signals.

60. A satellite communication system according to claim 59, wherein a terminal device port is coupled to a terminal device multiplexer which controllably interfaces message signals between a plurality of virtual ports and a terminal device port.

61. A satellite communication system according to claim 55, wherein said terminal devices include audio, data and video signaling devices.

62. A satellite communication system according to claim 55, wherein said hub station is operative to store information representative of whether a tunable modulator--demodulator pair of said originating remote earth station and a tunable modulator--demodulator pair of said destination remote station are available for establishing communication connectivity between said originating and destination earth stations.

63. A satellite communication system according to claim 55, wherein said each remote earth station is operative to store information representative of whether communication connectivity exists between said each remote station and another remote earth station.

64. A satellite communication system according to claim 55, wherein said each remote station includes a first demodulator that is tuned to said control carrier frequency associated and is operative to demodulate a tuned-message transmitted thereto from said hub station, and a second, demodulator which is operative to demodulate messages modulated on a carrier frequency transmitted from another earth station, said programmable demodulator being coupled to a downlink port of the frame relay protocol-based switch of said respective remote station.

65. A satellite communication system according to claim 64, wherein said each earth station includes a communication control processor associated with said each remote station'"'"'s frame relay switch, which stores table entries indicating whether there currently exists an active communication link to said destination station, and which is operative, in response to a call being placed from a terminal device, for causing said communication control processor to examine said table entries and, in response to said table entries indicating no existing active communication link, for generating a link request message for transmission to said hub station, said link request message designating which remote station is the destination station.

66. A satellite communication system according to claim 55, wherein said hub station includes a network management facility computer and an associated network configuration database, and wherein said network management facility computer is operative to examine a network configuration database, to determine availability of idle carrier frequencies, and of a tunable modulator--demodulator pair in each of said originating and destination stations.

67. A satellite communication system according to claim 66, wherein said originating and destination remote stations are operative to maintain their tunable modulator--demodulator pairs tuned to their assigned frequencies as long as any voice calls are present.

68. A satellite communication system according to claim 67, wherein each of said originating and destination remote stations, in response to termination of call traffic therebetween, is operative to forward a message to said hub station, indicating that all traffic between the linked stations has been completed, and wherein said hub station'"'"'s network management facility computer is operative to update its network configuration database, so as to allow said tunable modulator--demodulator pair in said originating and destination stations to be tuned to carrier frequency pairs associated with other remote stations.