United States Patent [19]
`Kou
`
`[11]
`[45]
`
`lllllllllllllllllIlllllllllllllllllllllllllllllllllllllllllllllllllllllllll
`
`US005172375A
`Patent Number:
`Date of Patent:
`
`7 5,172,375
`Dec. 15, 1992
`
`[54] MULTIPLE ACCESS SATELLITE
`COMMUNICATION SYSTEM FOR
`MINI-EARTH STATION NETWORKS
`[75] Inventor: Yukari Kou, Tokyo, Japan
`[73] Assignee: NEC Corporation, Japan
`[21] App]. No.: 543,677 '
`[22] Filed:
`Jun. 25, 1990
`
`Related US. Application Data
`Continuation-impart of Ser. No. 542,205, Jun. 22, 1990,
`
`[63]
`
`'
`
`abandoned.
`
`[30]
`
`Foreign Application Priority Data
`
`Jun. 22, 1989 [JP]
`
`Japan ........................ .2 ..... .. 1458186
`
`[51] 1m. 01.5 .............................................. .. H04J 3/16
`[52] US. Cl. ........................ .. 370/95.3; 370/ 104.1
`[58] Field of Search ................. .. 370/95.3, 94.1, 104.1,
`"370/95.l, 94.2
`
`[56]
`
`References Cited
`U.S. PATENT DOCUMENTS
`
`3.806,879 4/1974 Schmidt et a1. ................ .. 370/104.l
`
`4,381,562 4/1983 Acampora . . . . . . .
`
`. . . . .. 370/97
`
`370/104.1
`4,731,783 3/1988 Fontanes
`.. 370/95.3
`4,742,512 5/1988 Akashi et a1.
`4,775,974 10/1988 Koboyoshi ....................... .. 370/94.l
`
`OTHER PUBLICATIONS
`A. Fujii et al., IEEE Global Telecommunications Confer
`ence, “AA/TDMA-Adaptive Satellite Access Method
`
`for Mini-Earth Station Networks”, vol. 3 of 3, pp.
`
`
`
`1494-1499 (1986). Primary Examiner——Douglas W, Olms
`
`Assistant Examiner-T. Samuel
`Attorney, Agent, or Firm-Ostrolenk, Faber, Gerb &
`Soffen
`
`ABSTRACT
`[57]
`A ‘multiple access satellite communication system in
`which a plurality of mini-earth stations each transmit
`data packets to hub earth station via a satellite over an
`inbound channel, while the hub station transmits to the
`mini-earth stations in a broadcast mode over outbound
`channels. Each mini-earth station selectively uses a
`?xed assignement access method, random access
`method, and demand assignment access method, de
`pending on the frequency of the transmission from a
`terminal connected thereto, the amount of transmitted
`data, and whether or not the transmission from the
`terminal is periodic. The hub station outputs a reception
`response signal to the outbound channels by determin
`ing whether or not a conflict of data packets or a trans
`mission error‘ has occurred on the inbound channels.
`Packets for re-transmission are sent out to the inbound
`channels by the demand assignment access method at all
`times, with no regard to the access method used at the
`time of transmission.
`7
`
`3 Claims, 18 Drawing Sheets
`
`SATELLITE(S)
`
`//
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`
`T1,T2ZMIN| EARTH STATION
`A1,A2,P1,P2,I USER TERMINAL
`HI HOST TERMINAL
`
`

`

`U.S. Patent
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`29915:1ceD
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`US. Patent
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`1
`
`MULTIPLE ACCESS SATELLITE
`COMMUNICATION SYSTEM FOR MINI-EARTH
`STATION NETWORKS
`
`This application is a continuation-in-part of applica
`tion Ser. No. 07/542,205, ?led Jun. 22, 1990 and now
`abandoned.
`
`30
`
`5,172,375
`2
`data occurs, the exclusive timeslot of the mini-earth
`station is continuously occupied by the retransmission
`data until the latter is out. Then, the subsequent data are
`accumulated in the mini-earth station with the result
`that the interval between the delivery of data from the
`terminal to the mini-earth station and the arrival of that
`data at the hub station is undesirably increased.
`A random access method or slotted ALOHA method
`is a second method and allows a mini-earth station to
`transmit data by using any time slot every time the data
`is generated. The problem with this method is that
`packets sent from a plurality of mini-earth stations are
`apt to collide in the same time slot. In the event of
`collision, the hub station returns the NAK signal to all
`the mini-earth stations indicating that the packets in the
`time slot of interest were not received correctly. In
`response to the NAK signal, the mini-earth stations
`which transmitted the data in collision re-transmit them
`after the lapse‘ of a period of time which is determined
`by using a random number, for example. So long as the
`frequency at which the individual mini-earth stations
`transmit packets is relatively low and packets are sent at
`random, this scheme substantially minimizes the need
`for re-transmission and thereby insures high through
`put. Nevertheless, as the frequency of transmission from
`the individual mini-earth stations increases, the fre
`quency of collision also increases to lower the through
`put. When the transmission frequency from the mini
`earth stations further increases, even the re-transmitted
`packets collide degrading throughput to a critical ex
`tent.
`A demand assignment access method is a third
`method known in the art. A mini-earth station imple
`mented with this method sends a request for the reser
`vation of the number of slots to be used to a hub station
`every time a terminal associated with the mini-earth
`station produces a transmission request. On receiving
`the reservation request, the hub station assigns time
`slots which the mini-earth station that sent the request
`can use, i.e. reserved time slots. Reserved slot assign
`ment information is returned to all the mini-earth sta
`tions. This approach is desirable when indidual mini
`earth stations send-a great amount of data needing a
`plurality of slots, to a hub station. Even when the trans
`mission frequency from the mini-earth stations in
`creases, this method eliminates'the collision of packets
`particular to the slotted ALOHA method. However,
`each mini-earth station cannot transmit at all until it
`receives reserved slot assignment information from the
`hub station. The demand assignment access method,
`therefore, needs a longer interval between the transmis
`sion of data from a transmitting terminal to its associ
`ated mini-earth station and the arrival thereof at a re
`ceiving terminal than the ?xed assignment access or
`slotted ALOHA method.
`As discussed above, the ?xed assignment access
`method, random access ‘method and demand assignment
`access method each have advantages and disadvan
`tages. Efforts have heretofore been made to combine
`these different methods in order to make the most of
`their advantages. For example, a combined random and
`demand access method is taught by Fujii et al in a paper
`entitled "AA/TDMA-ADAPTIVE SATELLITE
`ACCESS METHOD FOR MINI-EARTH STATION
`NETWORKS”, IEEE Global Telecommunications
`Conference Record, pp. 42.4.1-42.4.6, December, 1986.
`A mini-station implemented by the above-mentioned
`combined random and demand access method deter
`
`BACKGROUND OF THE INVENTION
`The present invention relates to a multiple access
`satellite communication system in which a hub earth
`station and a plurality of mini-earth stations communi
`cate over common shared channels via a satellite and,
`more particularly, to a mini-station-to-hub earth station
`access system.
`In a POS (Point-of-Sales) system, banking/?nancial
`credit card veri?cation or similar computer communi
`cation system, user terminals are connected to mini
`earth stations to communicate with a host computer
`which is connected to a hub earth station. While each
`mini-earth station transmits information to only the hub
`station via a satellite over an inbound channel, the hub
`station sends a response to the information to all the
`mini-earth stations over an outbound channel. Each
`mini-earth stations selects only the information meant
`therefor out of the received signal and transfers it to the
`associated terminal. Speci?cally, information is inter
`changed only between the mini-earth stations and the
`hub station.
`The individual mini-earth stations transmit informa
`tion to the hub station using time slots which are the
`divisions of one frame time. The access of each mini
`earth station to a time slot may be implemented by any
`one of three different methods known in the art, as
`follows.
`A ?rst method is a so-called ?xed assignment access
`method which assigns exclusive time slots ?xedly to the
`individual mini-earth stations. Every time a transmis
`sion request to the hub earth station occurs, each mini
`earth station transfers a packet to the hub station by
`using the exclusive time slot assigned thereto. This kind
`of scheme is advantageously applicable when the termi
`nals connected to the individual mini-earth stations
`output a transmission request constantly. However,
`when the transmission request occurs irregularly, the
`method is not desirable from the standpoint of efficient
`use of time slots.
`Generally, satellite channels are lower in quality than
`wired communication channels and, therefore, bring
`about transmission errors. It has been customary, there
`fore, to cause a transmitting station to transmit a CRC
`(Cyclic Redundancy Check) code or similar error de
`tection code together with data and cause a receiving
`station to constantly monitor the error detection code
`to see if packets have been transmitted without errors.
`On detecting an error in the data of a received packet,
`the hub station returns a Not Acknowledgement
`(NAK) signal indicative of a particular time slot associ~
`ated with the packet with an error to all the mini-earth
`stations, urging the transmitted mini-earth station to
`re-transrnit. In response to the NAK signal, the mini
`earth station of interest re-transmits the data for which
`the NAK signal is meant by using the exclusive time slot
`thereof. Although the mini-earth station may have al
`ready received data following the data in question, the
`re-transmission data is sent out prior to the subsequent
`data. Stated another way, once the re-transmission of
`
`40
`
`

`

`10
`
`3
`mines the lengths of data to be transmitted to a hub
`station and thereby classifies them as either short data
`and long data. Short data has a length smaller than a
`certain threshold value and can be transmitted in, for
`example, one time slot, while long data has a length
`greater than the threshold value and cannot be so trans-
`mitted. When long data is fed from the terminal to the
`mini-earth station, the mini-earth station sends a reser-
`vation request to the hub station for reserving the num-
`ber of time slots which it needs to send the long data.
`Concerning short data, the mini-earth station sends it to
`the hub station by using a time slot which is not assigned
`to itself or any other mini-earth station as a reserved
`time slot, i.e. a random access slot.
`The combined random and demand access method
`executes a unique re-transmission procedure when short
`data sent from a certain mini-earth station has collided
`with short data sent from another mini-earth station.
`Specifically, when a mini-earth station receives a NAK
`signal from a hub station after the transmission of data 20
`by the random access method, it determines that the
`satellite traffic is heavy and transmits short data left
`non-transmitted at the time of arrival of the NAK signal
`by the demand assignment access method together with
`long data. The mini-earth station sends re-transmission
`data by the demand assignment access method also.
`After an Acknowledgement (ACK) signal has returned
`from the hub station in reply to the re-transmission data
`transmitted by the demand assignment access method
`and all the short data sent before the arrival of the NAK
`signal, the mini-earth station again begins transmitting
`subsequent short data to the hub station by the random
`access method.
`
`As stated above, when the supply of short data to the
`individual mini-stations increases, the combined random
`and demand access method inhibits random packet
`transmission so as to reduce the probability of collision.
`This, coupled with the fact that the delay ascribable to
`the second and successive transmissions of the same
`data is reduced, allows the combined method to be 40
`advantageously applied to a case wherein the amount of
`data to be sent from each mini-station fluctuates notice-
`ably.
`On the other hand, the terminals connectable to the
`mini-earth stations include terminals of the type gener-
`ating data constantly. Data fed from this type of termi-
`nal to the associated mini-earth station should prefera-
`bly be transmitted to the hub station by the fixed assign-
`ment access method, as stated earlier. The combined
`random and demand access method cannot meet this
`need. Especially, the combined method of Fujii et al. is
`not adequate when it
`is desired to send data from a
`terminal of the type generating data constantly to the
`hub station prior to data from the other terminals.
`
`SUMMARY OF THE INVENTION
`
`It is therefore an object of the present invention to
`provide a multiple access satellite communication sys-
`tem which can accomodate terminals different in the
`frequency of data generation and in the amount of data 60
`generation per transmission by combining the combined
`random and demand access method and the fixed as-
`signment access method.
`_
`A multiple access satellite communication system of
`the present invention comprises a single hub earth sta-
`tion, and a plurality of mini-earth stations connected to
`the hub earth station by satellite channels. The mini-
`earth stations each transmit a data packet only to the
`
`65
`
`5,172,375
`
`4
`hub earth station over an inbound channel by time divi-
`sion multiple access. The hub earth station transmits
`identical data to all the mini-earth stations over out-
`bound channels. The mini-earth stations each accom-
`modate a tenninal which generates data to be transmit-
`ted in a slot of the inbound channel by a fixed assign-
`ment access method, and a terminal generates data to be
`transmitted in a slot of the inbound channel by a de-
`mand assignment access method or a random access
`method. The hub station comprises a receiving circuit
`for producing packet data by demodulating a signal on
`the inbound channel, generating an error detection
`signal packet data by packet data by detecting errors in
`the packet data, and extracting from the packet data
`free from errors received data and_ reservation slot re-
`quest information which any of the mini-earth stations
`may transmit, a circuit for generating a frame timing
`signal for the outbound channel, a control circuit for
`producing reservation assignment
`information in re-
`sponse to the reservation slot request information and
`producing a reception response in response to the error
`detection signal, a circuit for multiplexing the reserva-
`tion assignment information and the reception response
`with data fed from a host terminal which is connected
`to the hub earth station, and a circuit for transmitting
`the multiplexed data over the outbound channels after
`modulating the multiplexed data. The mini-earth sta-
`tions each comprise a receiving circuit for demodulat-
`ing a signal on the outbound channel to separate the
`frame timing signal for the outbound channel, the reser-
`vation assignment infonnation, the reception response .
`signal, and the data from the host terminal, a circuit for
`generating an in-station frame timing signal exclusive
`for the mini-earth station in response to the frame timing
`signal, a slot supervising circuit for outputting slot as-
`signment information on the basis of the in-station frame
`timing signal and reservation assignment information, a
`first buffer for temporarily storing data to be transmit-
`ted by the demand assignment access method, a second
`buffer for temporarily storing data to be transmitted by
`the random access method, a third buffer for temporar-
`ily storing data to be transmitted by the fixed assign-
`ment access method, an access method detecting circuit
`for transferring, the data to the third buffer, when data
`inputted from any of the terminals connected to the
`mini-earth station is data to be transmitted by the fixed
`assignment access method, and for transferring other
`data to a message length detecting section. The message
`length detecting section is for detecting, when a re-
`transmission mode signal is OFF, a data length of the
`input data, transferring to the second buffer short data
`which can be transmitted in one time slot, dividing long
`data which carmot be transmitted in one slot into a
`plurality of data in portions, transferring a first one of
`the data portions to the second buffer, transferring the
`other data portions to the first buffer, and outputting a
`first request signal representative of a request for reser-
`vation of a necessary number of slots, and transferring,
`when the re-transmission mode signal is ON, all of the
`input data to the first buffer and outputting the first
`request signal with no regard to the data length. A data
`selecting circuit is for reading, when the re-transmission
`mode signal is OFF, data out of the first buffer when the
`slot assignment information is indicative of a demand
`assignment access slot, data out of the second buffer
`when the slot assignment information is indicative of a
`random access slot, and data out of the third buffer
`when the slot assignment information is indicative of a
`
`

`

`5,172,375
`
`5
`fixed assignment access slot, and, when the re-transmis-
`sion mode signal is ON, reading data out of the first and
`second buffers when the slot assignment information is
`indicative of a demand assignment access slot and data
`out of the third buffer when the slot assignment infor-
`mation is indicative of a fixed assignment access slot. A
`re-transmission control circuit is for storing the data
`read by the data selecting circuit, discarding, when the
`reception response is positive (ACK), the data associ-
`ated with the ACK response and transferring, when the
`reception response is negative (NAK), the data associ-
`ated with the NAK response to the first buffer while
`outputting a second request signal representative of a
`request for reservation of a necessary number of slots,
`turning the re-transmission mode signal ON when a
`reception response in reply to a packet transmitted in
`the random access slot is negative, and turning the re-
`transmission signal OFF when a reception response to
`re-transmitted data is positive. A reservation request
`adding circuit is for producing the reservation slot re-
`quest information in response to the first and second
`reservation requests and adding the reservation slot
`request information to an output of the data selecting
`means. A circuit sends an output of the reservation
`request adding circuit to the inbound channel.
`BRIEF DESCRIPTION OF THE DRAWINGS
`
`The above and other objects, features and advantages
`of the present invention will become more apparent
`from the following detailed description taken with the
`accompanying drawings in which:
`FIG. 1 shows a satellite communication network to
`which the present invention is applicable;
`FIGS. 2 to 4 show formats of signals which are trans-
`mitted in the network of FIG. 1;
`FIG. 5 is a block diagram schematically showing a
`specific construction of a hub station included in the
`network implemented by the present invention;
`FIG. 6 is a block diagram schematically showing a
`specific construction of a mini-earth station of the net-
`work implemented by the present invention;
`FIG. 7 is a block diagram schematically showing a
`specific construction of a receiver installed in a hub
`station;
`FIG. 8 is a block diagram schematically showing a
`control circuit also included in the hub station;
`FIGS. 9A to 9C indicate the function of a slot assign-
`ment table supervisor built in the hub station;
`FIG. 10 is a block diagram schematically showing a
`specific construction of a message length detector in-
`stalled in a mini-earth station;
`FIG. 11 is a block diagram schematically showing a
`specific construction of a data buffer built in the mini-
`earth station;
`FIG. 12 is a block diagram schematically showing a
`specific construction of a data selection unit and a reser-
`vation request adding section included in the mini-earth
`station;
`FIGS. 13A and 13B are flowcharts demonstrating
`specific operations of a control circuit in the data selec-
`tion unit;
`FIG. 14 is a block diagram schematically showing a
`receiver installed in the mini-earth station;
`FIG. 15A is a block diagram schematically showing a
`specific construction of a slot supervisor installed in the
`mini-earth station;
`FIG. 15B shows the format of a slot assignment table
`loaded in the slot supervisor;
`
`6
`FIG. 16 is a block diagram schematically showing a
`specific construction of a re-transmission controller
`included in ihe mini-earth station;
`FIGS. 17A to 17C are flowcharts representative of
`specific operations of a reception response checking
`section forming a part of the re-transmission controller;
`and
`FIG. 18 shows a specific operation of the multiple
`access satellite communication system in accordance
`with the present invention.
`DESCRIPTION OF THE PREFERRED
`EMBODIMENT
`
`Referring to FIG. 1 of the drawings, a satellite com-
`munication network to which the present invention is
`applicable is shown and has a single hub station C, a
`plurality of mini-earth stations T1 and T2, and a satellite
`S. User terminals A1 and P1 are connected to the mini-
`earth station T1 while user terminals A2 and P2 are
`connected to the mini-earth station T2. The User termi-
`nals P1 and P2 access an inbound channel by the fixed
`assignment method and the user terminals Al and A2
`g access the inbound channel by the combined random
`and demand access method. A host terminal H is con-
`nected to the hub station C and may comprise a com-
`puter or a packet switch, for example. The user termi-
`nals interchange data via the mini-earth stations, satel-
`lite, hub station, and host terminal.
`The formats of packets which are transmitted over
`inbound and outbound channels will be described with
`reference to FIGS. 2, 3 and 4.
`FIG. 2 shows the format of a packet which is sent
`from the mini-earth station to the hub station over an
`inbound channel. In the figure, one frame time is di-
`vided into fifteen time slots. Some of the time slots are
`fixedly assigned to the individual mini-earth stations, so
`that each mini-earth station may use the assigned time
`slot exclusively. The other time slots are selectively
`used for demand assignment access and random access
`as designated by the hub station frame by frame. The
`packet which each mini-earth station transmits in the
`associated time slot is made up of a preamble (PRE) for
`recovering a carrier and a clock, a unique word (UW)
`indicative of the beginning of a message, a field (PL)
`representative of an effective packet
`length, a field
`(ADR) showing the address of the transmitting mini-
`earth station, a field (RSV) indicative of the number of
`reservation slots requested by the transmitting mini-
`earth station which is represented by the field (ADR), a
`control field (CRL) describing a transmission/reception
`sequence number, a user data section (I) representative
`of data sent from the user tenninal via the mini-earth
`station, a frame check sequence (FCS) for detecting
`transmission errors, and a guard time (GT) having a
`predetermined length for guaranteeing an interval be-
`tween successive packets. The address of the mini-earth
`station, the address of the user terminal for which the
`packet is meant, and the address of the transmitting user
`terminal are included in the user data section (1). Each
`packet, inclusive of the guard time (GT), has to have
`the same length as the time slot. Hence, when the user
`data section (I) is short, dummy bits (DMY) are inserted
`in the packet.
`FIG. 3 shows the format of a signal which the hub
`station C sends to all the mini-earth stations over the
`outbound channel. As shown, the hub station C trans-
`mits a frame timing signal (FR) and a frame control
`section (FC) at predetermined intervals. The frame
`
`

`

`5,172,375
`
`7
`timing signal FR is constituted by a unique word which
`is distinguishable from the other signals. On receiving
`the frame timing signal, each mini-earth station estab-
`lishes slot synchronization for transmitting packets to
`the hub station. As shown in FIG. 3, the packets to be
`sent from the hub station to the mini-earth stations are
`formatted according to HDLC (High-Level Data Link
`Control) procedures. Specifically, each packet has a
`flag sequence (F) indicative of the beginning of the
`packet and the end of the immediately preceding
`packet, an address field (ADR) representative of the
`destination mini-earth station address, a control field
`(CRL), a user data section (I), and a frame check se-
`quence (FCS).
`FIG. 4 shows the format of the frame control section
`(FC). The format includes a reception response field
`(RRP) in which the hub station sends the previously
`mentioned ACK/NAK signal to show the individual
`mini-earth stations whether or not it has received data
`in the reception slots without errors. In the figure, ax to
`an are representative of ACK or NAK associated with
`the received signals in the slots 1 to 15, respectively. A
`random access slot assignment field (RA) designates a
`random access information slot. While each mini-earth
`station receives the frame control section (FC), it sends
`a packet in the next frame by using a slot which is desig-
`nated by that field and on the random access basis. The
`frame control field (FC) has a reserved slot assignment
`field RSA. In FIG. 4, slots designated by ASN1 and
`ASN2 are assigned to the mini-earth stations having
`addresses (ADR1) and (ADR2),
`respectively. The
`frame control field (FC) further has a frame check se-
`quence (FCS).
`Referring to FIG. 5, a specific construction of the
`hub station for practicing the present
`invention is
`shown. In the following description, let the signal lines
`and the names of signals be identified with each other.
`As shown, an electromagnetic wave from the satellite is
`received by a high-frequency section or RF section 501
`and transformed into an intermediate frequency (IF)
`band thereby. The IF signal is fed from the RF section
`501 to a receiver 502 which then demodulates it to
`produce a baseband signal. Specifically, the receiver
`502 extracts the effective packet length (PL) and suc-
`cessive data up to the frame check sequence (FCS),
`FIG. 2, from the received baseband signal by using the
`unique word as a reference. By using the frame check
`sequence (FCS), the receiver 502 determines whether
`or not data has been received without errors. If the
`received data is free from errors, the receiver 502 trans-
`fers (CRL) and (1), FIG. 2, to a data buffer 503. In
`response, the data buffer 503 converts the data having
`been inputted thereto at the timing particular to the hub
`station into data having a timing particular to the host
`terminal, and then transfers the data to the host termi-
`nal. If the received data has any error, the receiver 502
`discards it. When the reservation slot request field
`(RSV), FIG. 2, includes a time slot reservation request,
`the receiver 502 feeds it to a control circuit 504 together
`with the address (ADR) of the requesting mini-earth 60
`station.
`
`The control circuit 504 generates an ACK/NAK
`signal on the basis of the error detection information fed
`from the receiver 502 and then delivers the reception
`response field (RRP), FIG. 4, to a multiplexer 507. 65
`Further, by referencing the reservation request infor-
`mation fed from the receiver 502, the control circuit 504
`produces information for assigning the requested num-
`
`8
`her of slots to the mini-earth station of interest, i.e. the
`reserved slot assignment field (RSA), FIG. 4. In this
`instance,
`the slots fixedly assigned to the individual
`mini-earth stations are e