4,763,325
`[11] Patent Number:
`United States Patent 15
`
`
`Wolfe et al. Aug. 9, 1988eeeEeeeel[45] Date of Patent:
`
`[75]
`
`4,322,845
`3/1982 Fennel, Jr. et al... 370/104
`
`4,504,946
`3/1985 Raychaudhuri....
`we 370/104
`4,513,416 4/1985 Fujiwara oo...
`sssesesesteeeee 370/104
`7/1986 Kunzinger Aenavneaveneesecncsusceeee 370/104
`4,599,720
`Primary Examiner—Douglas W.Olms
`Attorney, Agent, or Firm—Sughrue, Mion, Zinn,
`Macpeak & Seas
`
`[54] DEMAND ASSIGNED REFORMATTING
`WITH AN OVERFLOW AREA FOR TIME
`DIVISION MULTIPLE ACCESS
`COMMUNICATION
`Inventors: William H. Wolfe, Burke; William P,
`Osborne, Leesburg, both of Va.
`[73] Assignee: Comsat Telesystems, Inc., Fairfax,
`[57]
`ABSTRACT
`Va.
`A time division multiple access communication system
`[21] Appl. No.: 772,536
`in which the repetitive frame is divided into fixed por-
`tions preallocated to separate stations and an overflow
`[22] Filed:
`Sep. 4, 1985
`[51] Unt, Chat oecessescssscssssssesescenessensseoes H04J 3/16—_portion. Each station retains control of its own preal-
`
`[52] U.S. Cle oo.eceeescsessssscesrsseeseonee 370/104; 370/95
`located portion. Whenever one of the stations needs
`[58] Field of Search................... 370/99, 104, 79, 95;
`space that is not availablein its preallocated portion,it
`455/12
`requests a reference station for part of the overflow
`portion, whichis relinquished as soon as the require-
`ment stops.
`
`[56]
`
`References Cited
`U.S. PATENT DOCUMENTS
`
`4,204,093
`
`5/1980 Yelr uu.eessessscessscessscarencenses 370/104
`
`5 Claims, 3 Drawing Sheets
`
`
`
`
`Pereper—lef
`[eee]one)
`
`PePRR[===]sr]
`
`
`
`
`
`
`
`1
`
`APPLE 1005
`
`APPLE 1005
`
`1
`
`

`

`US.Patent
`
`Aug. 9, 1988
`
`Sheet 1 of 3
`
`4,763,325
`
`FIG .|
`
`2,
`
`gs
`
`
`TELEPHONE
`
`LINES
`
`1.0
`
`NO. OF CALLS
`
`NO. OF CHANNELS
`
`FIG 2
`
`0
`
`0
`
`0.1
`0.01
`GRADE OF SERVICE
`
`0.2
`
`2
`
`

`

`US. Patent
`
`Aug. 9, 1988
`
`Sheet 2 of 3
`
`4,763,325
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`

`Aug.9, 1988
`
`Sheet 3 of 3
`
`4,763,325
`
`US. Patent
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`

`1
`
`4,763,325
`
`DEMAND ASSIGNED REFORMATTING WITH AN
`OVERFLOW AREA FOR TIME DIVISION
`MULTIPLE ACCESS COMMUNICATION
`
`BACKGROUND
`
`The invention relates generally to communication
`systems. In particular, the invention relates to the dy-
`namic reformatting of a time division multiple access
`frame dependent upon the demandoftheattachedsta-
`tions.
`large-scale communication systems
`Many modern,
`rely upon geosynchronoussatellites acting as transpon-
`ders between the transmitting and receiving stations.
`Although originally used for point-to-point communi-
`cation between two groundstations, more recentsatel-
`lite communication systems link together a substantial
`numberof groundstations, offering selective communi-
`cation between any pair of the groundstations. Such a
`system is schematically illustrated in FIG. 1 for N
`groundstations 10 linked together by a communication
`satellite 12 in geosynchronous orbit. The illustrated
`system is designed for telephone communications with
`each station 10 being associated with a telephone re-
`gional office. Whenever a telephone connection is de-
`sired between two telephonelines connected to differ-
`ent regional offices, the call is routed through the asso-
`ciated groundstation 10 and is transmitted from there,
`through the satellite 12, to the appropriate receiving
`groundstation 10.
`Older satellite communication systems relied upon
`frequency allocation between the transmitting ground
`stations 10. However, more recent multi-point systems,
`particularly those designed to support telephone/data
`communications, have adopted a TDMA(time division
`multiple access) approach. Sucha system is disclosed by
`Maillet in U.S. Pat. No. 3,649,764. In a TDMA system,
`data is not transmitted continuously but is time multi-
`plexed. Thetransmission is divided into time frames 14
`and 16 with each frame being further subdivided, ac-
`cording to a predetermined format, into traffic bursts
`TB. Both data and voicesignals are transmitted in digi-
`tal form. The frames repeat often enough that a tele-
`phone conversation can be made to appear continuous
`and instantaneous.
`In the illustrated example, each
`ground station 10 is assigned one traffic burst. The
`transmission of the traffic bursts from the individual
`groundstations 10 are synchronized so that they arrive
`at the satellite 12 in the proper time sequence to form
`the up-link frame 14. The communication satellite 12
`receives the up-link frame 14 and retransmits the frame
`as the down-link framep 16. Although the satellite 12
`amplifies and frequency shifts the up-link frame 14 into
`the downlink frame 16 and perhaps uses part of the
`frame for housekeeping purposes, the satellite 12 can be
`viewed as a passive transponderwith the up-link frame
`14 being identical to the down-link frame 16. It is of
`course to be appreciatedthat the frames 14 and 16illus-
`trated in FIG. 1 are only onepair of a nearly continuous
`series of up-link frames and down-link frames,
`the
`frames in each series being separated by the minimum
`necessary time.
`The entire down-link frame 16 is received by each of
`the N stations 10 so tht each station 10is receiving the
`transmissions of every other station 10. The individual
`traffic burst TB must contain additional information
`
`20
`
`25
`
`60
`
`65
`
`2
`indicating for which of the groundstations 10 the trans-
`mission is intended.
`In a TDMAsystem,a reference station 18 is usually
`present to provide some degree of coordination be-
`tween the ground stations 10. At a minimum,the refer-
`ence station 18 must synchronize the groundstations 10
`so that the frames 14 and 16 are synchronized between
`the stations 10 and furthermore it synchronizes the
`traffic bursts TB within the frame.
`Oneofthe difficulties of a telephone-based communi-
`cation system is the fluctuation in the loadsof the vari-
`ous groundstations 10. These fluctions may be either
`statistical or predictable. A statistical fluctuation arises
`because the ground stations 10 has no control on the
`number of requests for a telephone connection andthis
`numberstatistically varies with time. A predictable
`fluctuation would arise from different times of day for
`ground stations 10 located in different
`time zones.
`Nonetheless, for a consumer-based telephone/data sys-
`tem, there must be a high probability that, when a con-
`nection is demanded, channel capacity is available. If
`the frame formatis fixed, this requirementfor availabil-
`ity means that there must be a large amount ofexcess
`capacity within each ofthe traffic bursts TB. This in
`turn implies a relatively high bandwidth system.
`Bandwidth is both scarce and,in the case ofthe satel-
`lite 12, expensive to support because of the correspond-
`ingly increased power level. Alternatively, for a fixed
`bandwidth,
`the excess capacity required for a high
`availablity with a fixed format implies a decreased num-
`ber of reliably available channels.
`In view ofthe problemsofa fixed allocation between
`the multiple groundstations 10, demand assigned multi-
`ple access (DAMA)has been developed. By DAMAis
`meant that the allocation of time or bandwidth between
`the groundstations 10 is dynamically allocated accord-
`ing to a real-time demand for channel capacity de-
`manded bythe individual ground stations 10. Demand
`assigned multiple access has been traditionally used in
`single channelpercarrier satellite communication sys-
`tems, that is, frequency division rather than the time
`division illustrated in FIG. 1. Examples of these systems
`include the SPADE system, which has been imple-
`mented in the INTELSAT network. In the SPADE
`system, each earth station 10 communicates with all
`other stations 10 via a wide band commonsignalling
`channel. All call requests are communicated via this
`channel amongall the stations 10 in the network. The
`different carrier channels, corresponding to different
`frequencies, are allocated to the different groundsta-
`tions 10, according to these requests. Each station 10
`maintains a data base that represents the frequency
`assignments for all carrier frequencies in the transpon-
`der of a satellite 12. The SPADEsystem represents a
`decentralized approach to channel allocation.
`Other satellite systems have been designed for cen-
`tralized control of single channel per carrier satellite
`communication networks. For example, a master con-
`trol computer located in a reference station 18 polls
`each of the earth stations 10 in the network for call
`requests and thereafter assigns satellite frequencies as
`required to set up the desired calls. Both of the de-
`scribed DAMAsystems have been used with frequency
`division rather
`than time division communication.
`However, demand assignment for a TDMA system is
`described by Edstrom in U.S. Pat. No. 3,848,093. It is
`not felt that either the centralized or the decentralized
`approachesare totally appropriate for a TDMAsystem.
`
`5
`
`

`

`4,763,325
`
`3
`A totally decentralized system does not makeefficient
`use of the channel capacity, assuming that there must be
`a high probability for completing a call request. A to-
`tally centralized system such as that of Yeh in U.S.Pat.
`No. 4,204,093, or of Rothauser et al. in U.S. Pat. No.
`4,096,355, although efficient in cali channel capacity,
`introduces excessive complexity and delays caused by
`the rapidly changing system configuration. Torng in
`U.S. Pat. No. 4,383,315 and Fennel, Jr. et al in U.S. Pat.
`No. 4,322,845 disclose a mixture of centralized and
`decentralized control. These problems with totally cen-
`tralized or decentralized control will now be explained.
`In a demandassigned TDMAnetwork,the process of
`establishing a communication link between earth sta-
`tions 10 requires the originating earth station to process
`the incoming call request from the telephone lines to
`determine the destination forthis call. This call process-
`ing will result in a request for a portion of the TDMA
`frame in which to carry thetraffic associated with the
`call, whether it be for voice or data communication. If
`a full duplex connectionis required, as is the case for a
`typical voice call, then two requests will be generated.
`For a typical satellite transponder, between two and
`four call requests per second can be expected. Larger
`systems are designed with multiple transpondersso that
`multiple frames are being received simultaneously. The
`allocation, or management of the TDMAframe, can be
`either centralized at the reference station 18 or decen-
`tralized among the groundstations 10.
`A full evaluation of the benefits and disadvantages of
`the two approaches requires the introduction of some
`com unication terminology. Grade of service (GOS)is
`the probability that a call request cannot be honored by
`a station 10 because no space can beallocated to it.
`Obviously in a consumer market,
`the overall GOS
`should be minimized to prevent the undue occurrence
`of busy signals. As the numberof calls approaches the
`numberofavailable channels, the grade of service dete-
`riorates, that is, GOS increases. A typical relation be-
`tween the percentage usage of the channels and the
`gradeof service is shownin FIG.2, presentedsolely for
`illustrative purposes. Such curves vary depending upon
`system design. For an economically efficient system,
`the number of calls should approach the number of
`channels. However, this increased efficiency inevitably
`degrades the grade of service. On the other hand, a low
`value for the grade ofservice is desirable for high qual-
`ity service, but it is economically expensive. An erlang
`is another measure of channel usage, particularly appro-
`ptiate for TDMAsystems. An erlang is the number of
`call-seconds per second for the system as whole. Obvi-
`ously, a higher numberof erlangs implies an efficiently
`used system. Because there are multiple channels han-
`dling multiple calls ina TDMA system, a TDMAsys-
`tem typically has an erlang value greater than one.
`If the frame management functions for a TDMA
`system are centralized at the reference station 18, then
`the resultant system efficiently uses the available capac-
`ity. For instance, for a TDMA network having a raw
`capcity of 465 full duplex circuits, a fully centralized
`network can support 425 erlangsoftraffic with a grade
`of service GOS=0.01. Although these parameters are
`impressive, such a system nonetheless has several draw-
`backs. The 2-4 call requests per second will require a
`very large computerat the reference station 18. The call
`requestsall pass through the communicationsatellite 12
`located approximately 36,000 miles above the ground
`stations 10 and the reference station 18. As a result, the
`
`— 0
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`15
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`4
`delays associated with the propagation of the request to
`the reference station 18 and of the reply to the request-
`ing station 10 can becomeappreciable, approaching |
`second. Each ground station 10 must reconfigure its
`timing controls to conform to a reconfigured TDMA
`frame. If this reconfiguration is occuring at the rate of
`2-4 times a second, the frame managementprocessing at
`each of the groundstations 10 becomes appreciable and
`additional channel capacity must be provided for the
`frequent call
`requests and resultant reconfiguration
`data. It is to be rememberedthatin a frequency division
`system, the frequencies are individually allocated so
`that the reallocation of one frequency does not require
`a complete reallocation ofall the frequencies.
`If, on the other hand, frame managementweretotally
`decentralized, each earth station 10 would have one
`segmentortraffic burst of the TDMA frame for which
`it had the management responsibility. With this ap-
`proach, the total network traffic capacity would be a
`function of the numberofstations 10 in the network
`since each station must maintain a separate reserve ca-
`pacity to satisfy the required grade of service. If the
`previously described TDMA network of 465 circuits
`was required to maintain the same grade of service
`among 30stations 10, the fully decentralized approach
`would support 320 erlangs of full duplex traffic, a re-
`duction from the 425 erlangs of the totally centralized
`approach. However,
`if the number ofstations is in-
`creased to 100, the maximum full duplex traffic that
`could be supported falls further to 166 erlangs. Thus,
`system delays and complexity are reducedin the fully
`decentralized TDMAsystem butonly at the expense of
`a significantly reduced traffic capacity.
`SUMMARYOF THE INVENTION
`
`Accordingly,it is an object of this invention to pro-
`vide a time division multiple access communication
`sytem of low complexity.
`It is a further object of this invention to provide a
`TDMAcommunication system that efficiently uses the
`available channel capacity.
`It is yet another object ofthis invention to provide,in
`a single demand assigned multiple access communica-
`tion system, the best features of centralized and of de-
`centralized frame management.
`The invention can be summarized as a method of
`frame managementin a time division multiple access
`communication system in which a fixed time frameis
`divided into segments that are assigned to separate sta-
`tions. Eachstation is responsible for the management of
`its own segment. The frameis further provided with an
`overflow area. Whenevera station overflows the capac-
`ity of its own assigned segment, a request is made to a
`central station to assign a small slot in the overflow area
`to that station. The control of the slot reverts to the
`central station when its use by the station terminates.
`BRIEF DESCRIPTION OF THE DRAWINGS
`
`FIG. 1 is an illustration of a time division multiple
`access communication system.
`FIG.2 is a graph illustrating the relationship between
`the utilization of channels in a communication network
`and the resultant grade of service.
`FIG.3 is a timing diagram fora TDMAsystem ofthe
`present invention.
`FIG. 4 is a block diagram for the electronics in a
`ground station.
`
`6
`
`6
`
`

`

`5
`FIG.5 is a block diagram for the electronics in the
`referencestation.
`
`4,763,325
`
`6
`The overflow area of the TDMA frameis also di-
`vided into a numberofslots. Each ofthe overflow slots
`is assigned to oneofthetraffic bursts TB’}-TB’y. Un-
`used slots represent spare overflow capacity. Each of
`the overflow traffic bursts TB’; consists of a preamble
`PREandan overflow sub-burst SB. The overflow sub-
`burst SB is the same size as would be required for a
`preallocated sub-burst SB; and thus corresponds to a
`single telephonecall.
`In operation, whenever the i-th ground station 10
`receives a request for a telephone/data connection from
`one ofits lines, it first attempts to find preallocated
`spare capacity in its owntraffic burst TB; so thatit can
`add another sub-burst SB; thereto. If the spare capacity
`is available, the call can be completed throughthesatel-
`lite 12 without the assistance ofthe referencestation 18.
`If, however, existing traffic sub-bursts SB|-SBx com-
`pletely fill the traffic burst TB; so that no preallocated
`spare capacity remains, then the groundstation 10sig-
`nals the reference station 18 that a channelis required.
`The reference station 18 then attempts to find spare
`capacity in the overflow segment for the insertion of
`another overflow traffic bursts TB’; If the overflow
`spare capacity is available,
`the reference station 18
`makestheallocation and then transmits this fact to the
`requesting ground station 10. The ground station 10
`then associates the telephonecall with the sub-burst SB
`of the newlyallocated overflow traffic burst TB’. If the
`overflow spare capacity was notinitially available, the
`reference station 18 so notifies the requesting ground
`station 10 and the telephonecall is blocked.
`Once the call has been completed, the ground station
`10 originating thatcall receives a call clear message and
`then sendsan overflow slot release messageto the refer-
`ence station 18. The reference station 18 reassigns the
`now vacant overflow traffic burst to its spare capacity.
`All information concerning the destination of the
`sub-bursts SB and SB;is transmitted from the ground
`stations 10 in the broadcast channel.
`Thusit is seen that the formatting control is both
`centralized and decentralized. The control is decentral-
`ized in the respect that each of the groundstations 10
`exercises control of its own preallocated traffic burst
`TB; Controlis centralized in the respect that the refer-
`ence station 18 maintains control overall of the over-
`flow segment. However,the centralized control by the
`referencestation 18 is required only when the respec-
`tive preallocated traffic bursts TB; does not have spare
`capacity. Thus a large fraction of the control is decen-
`tralized.
`By theuseofthe invention, although the GOSin the
`preallocated traffic bursts TB|~TBy are in the range of
`0.05-0.30, the overall GOSis about 0.01.
`The dynamic reformatting capability of this invention
`is being built into a communication system which ini-
`tially includes 30 ground stations. A simplified elec-
`tronic structure for each groundstation is shown in the
`block diagram of FIG. 4. Terrestrial interface equip-
`ment is composed of any number of plesiochronous
`CEPT-32interface units (CIUs) 30 and asynchronous/-
`synchronouspulse-stuffing CEPT-32 interface units 32,
`operating at 2048 kilobits per second. The plesiochro-
`nous CIU 30 buffers data and signaling information
`between the groundstation and theterrestrial telephone
`network. One plesiochronous CIU 30 provides for a
`duplex interface for one CEPT-32 terrestrial digital
`trunk. Inherent in the design of the plesiochronous CIU
`30 is a split-channel capability. This capability allows
`
`15
`
`DETAILED DESCRIPTION OF THE
`PREFERRED EMBODIMENT
`In the time division multiple access (TDMA) system
`of the present invention, the TDMA frameis divided
`into a preallocated segment and an overflow segment.
`Every station is given a portion of the preallocated
`segment over which it exercises control. Whenever a
`station requires additional channel capacity that cannot
`be satisfied by its preallocated portion, it requests the
`reference station to allocate it part of the overflow
`segment. The reference station thus controls the over-
`flow segment.
`One embodimentof the format for a TDMA frame of
`the present invention is shown in FIG. 3. The frame
`begins with two reference bursts RB, and RB, which
`are separately assigned to each of two reference sta-
`tions. Two independent referencestations are provided
`to provide redundancy and thus additional reliablity.
`Only onereferencestation at any one timeis exercising
`control over the system. It is only when thefirst refer-
`ence station fails that the second reference station as-
`serts control over the system. Accordingly, the remain-
`ing discussion will assume only a single controlling
`reference station 10. The reference bursts RB, and RBg
`contain the transmissions of the reference stations 18.
`These transmissions include information about its own
`health as well as control informationfor the satellite 12
`and for the individual ground stations 10. The control
`informaionallowsall groundstations 10 to be synchro-
`nized with the reference station 18.
`A preallocated segment
`is subdivided into traffic
`bursts TB|-TBy. Eachofthetraffic bursts are assigned
`to one of the N ground stations 10. The size of the
`individual traffic bursts TB|-TBy varies according to
`the needsof the individual ground stations 10. Thesize
`at the traffic bursts TB|-TBy are chosen so that the
`GOS of each traffic burst
`is held in the range of
`0.05-0.30. These sizes are selected by the reference
`station 18 but only ona fairly infrequentbasis, for exam-
`ple, half-hourly. In between the adjustment times, the
`i-th ground station 10 to whichthetraffic bursts TB;is
`assigned has controlofthattraffic burst TB;.
`The formatof a preallocatedtraffic burst TB;is also
`shownin FIG.3. It begins with a preamble PRE, used
`for control information. This control
`information is
`used by the groundstation 10 to acquire a burst and to
`identify the beginning of the data portion of the burst.
`Thetraffic burst TB; contains a broadcast channel. This
`broadcast channelis offixed size and is alwaysallocated
`to the respective ground station 10. Typically,
`the
`broadcast channelis used for control and status infor-
`mation transmission and not for telephonecalls. It is
`thus assumed that
`the broadcast channel
`is always
`needed by the respective groundstation 10.
`Following the broadcast channel are a plurality of
`slots that are assigned to variable length sub-bursts
`SB;-SBx. Unusedslots are kept in a spare area. Each of
`the sub-bursts SB;-SBx is associated with one tele-
`phone/data call. The ground station maintains control
`over the allocation of the sub-bursts to the slots. As
`illustrated in FIG. 3, it is assumed that the sub-bursts
`SB\-SBx have been compacted to the left so that all
`spare slots appear on the right. Compaction is not re-
`quired for the practice of the invention.
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`4,763,325
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`7
`for individual channels to be removed from multiple
`CEPT-32 trunks to form a TDMAburst on the transmit
`side and for individual channels to be collected from
`multiple TDMA bursts and hence from multiple
`sources, to form a single CEPT-32 trunk on the receive 5
`side. The plesiochronous CIU 30 is designed to operate
`in either synchronous or plesiochronous mode, thatis,
`in either a master or slave mode.
`The asynchronous/synchronous pulse-stuffing CIU
`32 provides a duplex interface to the groundstation for 10
`an asynchronous 2.048 megabyte per second datastring
`andis fully compatible with CCITT Recommendations
`G.703 and G.912. The pulse-stuffing CIU 32 performs
`bit stuffing on the incoming data stream to synchronize
`it to the groundstation. In addition, it compresses this 15
`continuous data stream into a burst suitable for trans-
`mission overthesatellite. On the receive side, the high
`speed data bursts received from the satellite are ex-
`panded into a continuous low speed data stream. Bit
`destuffing is then performedto restore the rate and the 20
`content of the data stream, which is then sent to the
`terrestrial telephone network.
`Onthesatellite side of the ground station, transmis-
`sion data is modulated by a MODEM34atan IF fre-
`quency of 70 MHzandsentto a transmitter, not shown,
`which further modulates the IF data to the RF transmis-
`sion frequency. An RF receiver, not shown, receives
`the down link transmissions from thesatellite. There
`are, in fact, four separate transponders and the receiver
`provides separate IF receive lines FQ1-FQ4 for the
`four channels. An IF switch 36 selects one of these four
`IF lines FQ1-FQ4for connectionto the receive input of
`the MODEM 34.
`The MODEM 34 provides the functions of interfac-
`ing the data, clock and controlsignals. It is a QPSK 35
`modulator/demodulator
`and provides
`the carrier
`source for the 70 MHz IF carrier. The MODEM 34
`further provides control and IF loop back switches.
`When the MODEMis receiving data from thesatellite,
`it recovers the clock from the received signal.
`A differential driver/receiver interface 33 converts
`single-ended bus related signals on the telephone net-
`work side into a differential form for transmission
`within the chassis.
`Frame managementprocessor equipmentis the heart
`of a groundstation. It is a microprocessor-based subsys-
`tem which controls the operation of the groundstation.
`The frame management processor equipment performs
`several principal functions. The equipment acquires and
`keeps track of the satellite frame with a nominal frame
`length of 18 ms. It keeps the local TDMA clock fre-
`quency locked to the received signal. The frame man-
`agement processor equipment
`transmits the ground
`stationstraffic bursts at their assigned timepositionin its
`allocated time slot. It multiplexes voice, data and, if 55
`desired, video sub-bursts derived from digital trunks in
`the terrestrial telephone network. The frame manage-
`ment processor equipment accepts call requests from
`the telephone network and establishes a sub-burst for
`those requests. The assignment of the sub-bursts must be
`made in conjunction with the destination groundstation
`and additionally, if the overflow pool is to be used, in
`conjunction with the referencestation.
`The components of the frame management processor
`equipment 38 will be described as follows. A signal and 65
`coding module 42 performs two primary functions.
`First of all, it performs the error correction encoding
`and decodingofthe traffic data in order to improve the
`
`60
`
`8
`bit errorrate of the traffic channel overthesatellite link.
`The second function of the signal and coding module
`42, the function of importancefor this invention, is that
`of message handling for all control information ex-
`changed between processors of different ground sta-
`tions and the referencestation. This control information
`is transmitted in the broadcast channelthatis the first
`sub-burst of every burst, except for the overflow traffic
`burst TB’; of the overflow pool.
`A high speed module 44 performs those functions
`that deal directly with the serialized satellite data and
`the high speed, 29.952 MHz clock. These functions
`include theserializing/deserializing of data between the
`serial data link and the parallel buses of the frame man-
`agement processor equipment 38. The high speed mod-
`ule 44 transmits the framestart timing and retimes the
`data received from the demodulatorto the local clock.
`It further generates and adjusts the local clock.
`A control RAM 46 in conjunction with a frame man-
`agement CPU 48 forms a microprocessor based con-
`trolsequence generator. The events occuring during
`each TDMAframeare controlled by this sequence of
`control and address signals. The sequence lasts the
`length of the frame. The sequence can be changed by
`the frame management CPU 48 on a frame-by-frame
`basis. Two independent control sequences are gener-
`ated, one each for the transmit side and the receive side
`pipelines of the frame management processor equip-
`ment 38. The sequences consist of a linked series of
`control words. Each control word corresponds to a
`frame-related event. The events occur during eachsat-
`ellite frame and the duration of those events are stored
`as wordsin the control RAM 46. Twelveofthe 32-bits
`within the control words are addresses for the buses to
`the CIUs 30 and 32. Other bits are used as control bits
`for various activities including the loading of a counter
`which controls the length of time an address stays on
`the bus. Otherbits are used for system control functions
`such as gating the carrier and turning the error correc-
`tion on and off.
`The frame management CPU 48 is a general purpose
`microcomputer system, designed specifically for use as
`an intelligent controller/computer in various roles
`throughoutthe entire communication system. The same
`basic hardware moduleis usedin four different applica-
`tions as a CPU.
`The frame management CPU 48 provides three pri-
`mary functions. It controls the structure ofthe frame by
`loading the control RAM 46 with microinstructions
`which form the sequence of outputs produced by that
`control RAM 46. It acquires and maintains synchroni-
`zation of the frame by monitoring the reference burst
`position and by adjusting the system clock frequency
`throughits interface with the high speed module 44.It
`also monitors the status of the modules attached toits
`bus 50 and generates status messages for the reference
`station. These messagesare sent throughthesatellite via
`the signal and coding module 42. The status messages
`include requests to the reference groundstation for an
`additional sub-burst TB’; in the overflow pool. When a
`call in the overflow pool has been completed, the dis-
`connection requests is likewise sent from the frame
`management CPU 48 to the referencestation in a status
`message. The reference station also makes use of the
`information contained in the status messages to perform
`its periodic reformatting of the preallocated traffic
`bursts TB;.
`
`8
`
`8
`
`

`

`9
`A signal management CPU 52 is a microprocessor-
`based subsystem physically identical to the frame man-
`agement CPU 48. The signal management CPU 52
`transfers and formats signalling data between the CIUs
`30 and 32 andthesatellite channel. Thus, a request for
`a call connection enters the frame management proces-
`. sor 38 through the signal management CPU 52.
`The referencestation is used to maintain network
`synchronization and control of the TDMA burst time
`plan. The electronics of the referencestation, shown in
`FIG. 5, provides the following functions. It provides
`the control mechanism so that all ground stations can
`access thesatellite and not interfere with one another.
`Furthermore, it provides the TDMA frame manage-
`mentfunctions for operating the communication system
`in a demand assign mode. The equipmentof the refer-
`ence station closely resembles the equipment shownin
`FIG.4 for the ground stations. The MODEM 34forthe
`reference station is the same as for a groundstation.
`A frame managementprocessor equipment 60is iden-
`tical to the frame management processor equipment 38
`of the ground station with the major exception that the
`referencestation does not contain a signal management
`CPU 52. The DDRinterface 40 within the frame man-
`agement processor equipment 60, instead of being con-
`nected to CIUs 30 and 32 as in the groundstations,is
`instead connected to a frame supervisory processor
`signal and coding module 62. Otherparts of the frame
`supervisory processor are a frame supervisory CPU 64,
`a clock distribution unit 66 and an ETHERNETinter-
`face module 68. The frame supervisory processor signal
`and coding module 62 is the same as the signal and
`coding modules 42 within the ground stations. There
`are some differences in functionality when the signal
`and coding module 62 is used in the referencestation.
`Somecircuitry in the signal and coding module 42 of
`the ground station is not used in the reference station
`and vice versa. The only forward error correction cir-
`cuitry used in the frame supervisory processoris for
`correction of the broadcast channel. The forward error
`correction circuitry for traffic data is not needed since
`the reference station does notcarry traffic. The signal
`and coding module 62 in the frame supervisory proces-
`sor contains an additional circuit which is not used in a
`groundstation. This circuitry providesthe aperture for
`monitoring the position error of traffic bursts and is
`used to generate a correction signal