(12) United States Patent
`Reichman et al.
`
`US006240073B1
`US 6,240,073 B1
`May 29, 2001
`
`(10) Patent N0.:
`(45) Date of Patent:
`
`(54) REVERSE LINK FOR A SATELLITE
`COMMUNICATION NETWORK
`
`(75) Inventors: Arie Reichman, Kfar Saba; Shaul
`Laufer, Tel Aviv; Avi Barda, Hod
`Hasharon; Sorin Goldenberg,
`Jerusalem, all of (IL)
`
`(73) Assignee: Shiron Satellite Communications
`(1996) Ltd., Petach Tikva (IL)
`
`( * ) Notice:
`
`Subject to any disclaimer, the term of this
`patent is extended or adjusted under 35
`U.S.C. 154(b) by 0 days.
`
`(21) Appl. No.: 08/970,922
`(22) Filed:
`Nov. 14, 1997
`
`(51) Int. Cl.7 .................................................... .. H04J 13/06
`(52) US. Cl. ......................... .. 370/319; 370/437; 375/202
`(58) Field of Search ................................... .. 370/352, 353,
`370/354, 355, 356, 335, 342, 368, 365,
`465, 319, 431, 433, 437, 320, 321, 343,
`478, 436; 375/202, 205, 201
`
`(56)
`
`References Cited
`
`PUBLICATIONS
`
`J .L. Massey, Some NeW Approaches To Random—Access
`Communications, Reprinted from Performance ’87, pp.
`551—569,1988 ’P.J. Courtois and G.Latouche, Eds. NeW
`York : Elsevier Science, 1998,pp.354—368.
`J .L Massey and P. Mathys, “The Collision Channel Without
`Feedback,” IEEE Trans. Inform. Theory, vol.IT—31,
`pp.192—204, Mar. 1985.
`N. Abramson, “Multiple Access in Wireless Digital Net
`Works,” Proceedings of the IEEE, vol. 82 No. 9 Sep. 1994
`pp. 1360—1370.
`N. Abramson, VSAT Data Networks, in Proc. IEEE. vol. 78,
`No. 7, Jul. 1990, pp.1267—1274.
`M.B.Pursley , “Frequency—Hop Transmission for Satellite
`Packet Switching and Terrestrial Packet Radio NetWorks,”
`IEEE Transactions on Information Theory, vol. IT—32, No.
`5, Sep. 1986, pp 652—667.
`
`K.Yang . and G. L. Stuber, “Throughput Analysis Of The
`Slotted Frequency —Hop Multiple—Access NetWork,” IEEE
`Journal on Selected Areas in Communications. vol. 8. No. 4,
`May 1990.,pp 588—601.
`S. W. Kim and W. Stark, Optimum Rate Reed—Solomon
`Codes For Frequency Hopped Spread Spectrum Multiple
`Access Communication Systems. IEEE Transactions on
`Communications, vol. 37,No2 Feb. 1989, pp 138—144.
`Amir M. Y. Bigloo, T. A Gulliver and V.K. Bhargava, “A
`Slotted Frequency —Hopped Multiple—Access NetWork With
`Packet Combining,” IEEE Journal of Selected Areas in
`Communications, vol. 14, No. 9, Dec. 1996,pp 1859—1865.
`
`(List continued on neXt page.)
`
`Primary Examiner—Huy D. Vu
`(74) Attorney, Agent, or Firm—Darby & Darby
`(57)
`ABSTRACT
`
`The present invention is a return link for a satellite com
`munication system. The return link described herein is
`suitable for use in any type of communications netWork such
`as netWorks used for Internet access purposes. The return
`link in combination With a forWard link forms a complete
`tWo Way communication system via satellite. The return link
`comprises tWo separate communication schemes used in
`combination to implement the return link of the satellite
`system. The ?rst communication scheme uses a random
`access method based on a non synchronous frequency
`hopping code division multiple access technique (NS/FH/
`CDMA). The second communication scheme uses a channel
`assignment method based on a frequency division multiple
`access (FDMA) technique. Data generated by a user is
`transmitted utilizing one of the tWo communication schemes
`in accordance With the content and amount of data gener
`ated. Messages requiring a relatively loW transmission rate,
`such as short bursty messages, utilize the random access
`transmission method. On the other hand, messages requiring
`a higher transmission rate, such as video conferencing,
`utiliZe the channel assignment method.
`
`36 Claims, 13 Drawing Sheets
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`US 6,240,073 B1
`Page 2
`
`OTHER PUBLICATIONS
`ETS 300 421 “Digital Broadcasting Systems for Television,
`Sound and Data Services; Framing Structure, Channel Cod-
`ing and Modulation for 11/12 GHZ Satellite Services” Dec.,
`1994 pp 1003—1011.
`
`S. B. Wicker V. K. Bhargava, “Reed Solomon Codes and
`Then Apphcanons ’Chapter 6 IEEE Press’ 1992'
`S. Laufer and J. Snyders, FeedforWard Multiple Access
`Satellite Communications, IEEE Journal On Selected Areas
`In Communications, vol.
`10,
`No.6, Aug. 1992.
`
`

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`May 29, 2001
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`U.S. Patent
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`May 29, 2001
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`U.S. Patent
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`May 29, 2001
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`U.S. Patent
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`U.S. Patent
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`May 29, 2001
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`Sheet 12 0f 13
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`US 6,240,073 B1
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`

`
`U.S. Patent
`
`May 29, 2001
`
`Sheet 13 0f 13
`
`US 6,240,073 B1
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`
`

`
`US 6,240,073 B1
`
`1
`REVERSE LINK FOR A SATELLITE
`COMMUNICATION NETWORK
`
`FIELD OF THE INVENTION
`
`The present invention relates generally to satellite com
`munications and more particularly relates to a satellite based
`multiple access reverse communication link suitable for an
`Internet access network.
`
`BACKGROUND OF THE INVENTION
`
`Currently, communication systems around the World are
`growing rapidly due to the increasing need for data com
`munication bandWidth. In particular, satellite communica
`tion systems are currently experiencing rapid groWth due to
`groWing customer demand for satellite based data commu
`nications. Most of the current demand and estimated future
`demand Will be for Internet and other netWork based data
`communication applications. A major factor in these com
`munication systems is the bandWidth capacity demanded by
`the user. Applications in Widespread use today, such as video
`conferencing, LAN/WAN and document delivery require
`high speed forWard and return link data capacities.
`Currently, hoWever, these capabilities are not provided by
`present day Internet via satellite systems.
`It is currently estimated that there are approximately 13
`million hosts and 16 million users on the Internet. The
`groWth rate has been approximately 10 million neW users a
`year for the past four years. At the same time, the number of
`Intranets (Internet netWork protocols applied Within an
`enterprise or company for sharing information) are groWing
`at an even faster rate. Currently, accessing the Internet via
`satellite has gained recognition as a fast and reliable solution
`for fast Internet access. Current commercially available
`Internet via satellite solutions such as DirecPC are based on
`an asymmetrical approach in Which the data link to the user
`is via satellite While the return link to the user is via
`telephone lines using commercially available telephony
`modems. The disadvantages of these asymmetrical systems
`is outlined beloW.
`The asymmetric approach via satellite, in Which the user
`receives data from the Internet via satellite, yet sends data to
`the Internet via telephone lines, does not take advantage of
`a major feature of satellite communications: Wide area
`coverage. The asymmetric link is based on a terrestrial
`connection and therefore limits the ability of the fast con
`nection to the Internet to those places in Which telephone
`lines and Internet service providers are available and have
`sufficient grade of service.
`The data rate of asymmetric Internet via satellite com
`munication systems enables basically e-mail and broWsing
`applications only. This structure is mainly targeted to con
`sumer markets Where the user is limited to sending data from
`their home at relatively loW speeds. There are, hoWever,
`many users such as small of?ce/home of?ce (SOHO) that
`desire high speed data communications in both directions
`yet cannot afford having dedicated lease lines for their
`Internet connection. In the United States alone there are
`approximately 3.5 million small businesses of Which only
`10% can justify an expensive leased line. Thus, there are a
`large group of users looking for an on demand economical,
`fast and reliable connection to the Internet With a grade of
`service similar to that of a leased line.
`Typical applications that require high data rates in both
`directions include video conferencing, LAN/WAN systems,
`Internet applications, document delivery, audio applications
`such as Internet Phone, commercial Web sites, net gaming,
`
`15
`
`25
`
`35
`
`45
`
`55
`
`65
`
`2
`point of presence, terminal equipment, Net Meeting and
`collaboration softWare. All the above mentioned applica
`tions are currently not adequately served by the currently
`available asymmetric satellite communication solutions.
`Spread spectrum communication systems have been used
`in a variety of ?elds for some time noW. In spread spectrum
`communication systems, the bandWidth of the transmitted
`signal is much greater than the bandWidth of the information
`to be transmitted. The carrier signal in such systems is
`modulated by a function that serves to Widen or spread the
`bandWidth of the signal for transmission. On the receive
`side, signal is remapped or decoded into the original infor
`mation bandWidth to reproduce the desired output signal.
`Spread spectrum systems can be categoriZed into direct
`sequence systems, frequency hopping systems, time hop
`ping systems and hybrid systems Which are combinations of
`the above three.
`In frequency hopping systems a carrier frequency is
`shifted or hopped in discrete increments in a pattern dictated
`by a predetermined code or sequence, e.g., a pseudo noise
`sequence or code. The resulting consecutive and time
`sequential frequency pattern is called a hopping pattern and
`the duration of each hop frequency is called a chip. The
`transmitted information is embedded in the codes or embed
`ded Within each frequency in the carrier Wave by a modu
`lation scheme such as PSK or FSK.
`In reproducing the information signal of the receiver a
`synchroniZation acquisition process is performed in Which
`the code pattern utiliZed by the receiver is synchroniZed With
`the code pattern generated and used in the transmitter. Using
`this, de-spreading and demodulation are performed on the
`spread spectrum signal to extract the transmitted data. A
`local reference signal is used that has a frequency corre
`sponding to the same code pattern used in the transmitter for
`every chip. The received signal and the local reference are
`mixed in order to perform a correlation or de-spreading
`process for converting the spread spectrum signal into a
`signal having a frequency bandWidth Wide enough to extract
`the data information. More information describing the
`operation of spread spectrum systems can be found in
`“Spread Spectrum Systems,” by R. C. Dixon published by
`John Wiley and Sons, Inc., 1976.
`Multiple user systems use multiple access techniques to
`alloW users to share resources such as time and frequency.
`When the traf?c from each user in the netWork is approxi
`mately steady it is possible to divide a single high capacity
`multiple access channel into a plurality of smaller orthogo
`nal channels corresponding to individual user requirements.
`This can be accomplished either on a frequency basis using
`FDMA, on a time basis using TDMA or using CDMA. In
`addition, various combinations of FDMA and TDMA can
`also be used to minimiZe cost in large netWorks. FDMA and
`TDMA techniques are suitable solutions as long the traf?c
`from each user is relatively stable. CDMA is a multiple
`access technique Which uses spread spectrum communica
`tions. CDMA communications can be synchronous if all
`users are mutually synchroniZed in time.
`TDMA communication systems are also knoWn for pro
`viding multiple access. Theses systems partition the channel
`time in a ?xed predetermined manner. They are ef?cient
`When the user population includes only a relatively small
`number of users having high duty cycles. HoWever, many
`modern communication systems need to provide communi
`cation among interactive data terminals Which operate in
`loW duty cycle burst modes. Thus, TDMA is not particularly
`suited to this kind of communication.
`
`

`
`US 6,240,073 B1
`
`10
`
`15
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`3
`In the typical modern interactive network, however, the
`traf?c from individual terminals in the system varies as a
`function of time due to random traf?c demands by different
`users at each terminal. In addition, the set of terminals active
`in the network can vary from moment to moment. In such
`systems it may be desirable to assign channel capacity to
`users on demand by means of a demand assigned multiple
`access (DAMA) architecture. In a DAMA system a separate
`channel called the request channel is used by individual
`users to request capacity as needed. The capacity can then be
`allocated in response to requests by a central master con
`troller implemented by a common algorithm running in each
`terminal.
`ADAMA system however introduces additional overhead
`into the multiple access channel due to the process of
`requesting and assigning capacity. In addition, the demand
`assignment process introduces a delay which can degrade
`the performance under the channel.
`In some DAMA networks the total number of potential
`data terminals sharing the request channel is much larger
`than the number of terminals active at any given point in
`time. Thus, subdividing a DAMA request channel into
`smaller ?xed allocation sub channels is impractical. It is thus
`necessary to design a request channel architecture based
`upon a random access technique which allows for the
`possibility of a small subset of active transmitters selected
`from a much larger set of potential transmitters. Two random
`access techniques are currently available for such applica
`tions which include ALOHA multiple access and CDMA.
`The ?rst data network to be based upon a random access
`protocol was ALOHANET which went into operation
`throughout the state of Hawaii in 1970. The ALOHA system
`was the ?rst random access multi-point packet data com
`munication system. The system uses a single radio channel
`shared by plurality of stations or data terminals. When a
`station generates a packet, the ALOHA system transmits the
`packet on the common radio channel. Since more than one
`station may attempt to transmit a packet at the same time
`several transmissions may overlap. The overlapping trans
`missions are said to collide if any portion of two transmis
`sions overlap. When a collision occurs each station waits a
`random period of time before attempting to gain access to
`the channel again.
`To increase the utiliZation of the radio channel, the slotted
`ALOHA system was proposed in which the channel is
`partitioned into time slots equal to a packet length wherein
`each station can only transmit a packet at the beginning of
`a slot. In this way overlapping transmissions are forced to
`completely overlap. Using a slotted approach almost
`doubles maximum channel utiliZation compared to the
`unslotted ALOHA system.
`To reduce the effects of collisions in the slotted ALOHA
`system a slot reservation scheme was proposed. The channel
`was partitioned into frames each containing a reservation
`slot for transmitting a reservation packet and data slots for
`transmitting data packets. Each station transmits a reserva
`tion packet on a random access basis requesting slots needed
`for data packet transmission. If the request is granted data
`slots of a subsequent frame are assigned to the requesting
`station which subsequently transmits data packets on its
`assigned slots.
`Satellite communications can provide point to point chan
`nels or broadcast and multiple access channels. A satellite is
`well suited to provide one to many i.e., broadcast, channels
`and many to one, i.e., multiple access, channels from and to
`an earth station. The architecture of the network used in very
`
`4
`small aperture terminal (VSAT) data networks is almost
`always designed around a single large hub earth station
`transmitting data in a broadcast channel to a large number of
`VSATs as shown in FIG. 1. Considering Network A, for
`example, the VSATs 20 in such a network transmit data in
`packets to the hub station 18 using the multiple access
`capability of the satellite channel 17.
`The communications from the hub station of a VSAT
`network to the VSATs themselves is easily con?gured using
`a conventional communication technique such as TDM or
`FDM. Currently, TDM is widely used for multiplexing the
`hub to the VSAT terminals, notwithstanding the fact that
`there are differences in data rate, modulation techniques and
`transmission formats among the various VSAT networks.
`The multiple access link from the VSATs to the hub,
`however, is currently subjected to a greater degree of varia
`tion. The choice of multiple access technique from the
`VSATs to the hub is currently the primary feature distin
`guishing one network from another.
`Recently, however, it has become commonplace to build
`VSAT networks composed of hundreds and thousands of
`more small VSAT terminals. The traf?c in these networks is
`typically in the form of single data packets originating from
`interactive users or bursts of data packets originating from
`some type of ?le transcript protocol. As the number of
`stations in the network increases, the more the traf?c from
`the single station will appear to ?uctuate due to random user
`demand. In such networks, the use of FDMA or TDMA
`becomes impractical while the use of DAMA would impose
`an unreasonable amount of overhead in the network. Thus,
`to provide multiple access to these types of packet data
`networks, the access techniques of direct sequence (DS)
`spread spectrum multiple access, i.e., DS-CDMA and
`ALOHA, are used. Both these multiple access techniques
`however suffer from disadvantages. Direct sequence spread
`spectrum systems require the hub receiving station to have
`a digital matched ?lter operating at the high speed chip rate
`for each of the possible transmitters with each using a
`different spreading sequence. In a network with a large
`number of VSATs this becomes unwieldy and expensive to
`maintain. ALOHA systems suffer from relatively low capac
`ity and high average power requirements.
`SUMMARY OF THE INVENTION
`The present invention is a reverse link for a satellite
`communication system. The reverse link described herein is
`suitable for use in any type of communications network such
`as networks used for Internet access purposes. The reverse
`link in combination with a forward link forms a complete
`two way communication system via satellite. The reverse
`link comprises two separate communication schemes used in
`combination to implement the reverse link of the satellite
`system. The ?rst communication scheme uses a random
`access method based on a non synchronous frequency
`hopping code division multiple access technique (NS/FH/
`CDMA). The second communication scheme uses a channel
`assignment method based on a frequency division multiple
`access (FDMA) technique. Data generated by a user is
`transmitted utiliZing one of the two communication schemes
`in accordance with the content and amount of data gener
`ated. Messages requiring a relatively low transmission rate,
`such as short bursty messages, utiliZe the random access
`transmission method. On the other hand, messages requiring
`a higher transmission rate, such as video conferencing,
`utiliZe the channel assignment method.
`There is therefore provided in accordance with the present
`invention a multiple access communications system for use
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`US 6,240,073 B1
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`in a satellite communication network, comprising a plurality
`of user terminals for generating data to be transmitted over
`the multiple access communication system, at least one hub
`for receiving data over the multiple access communication
`system from the plurality of user terminals, transmitter
`means Within each user terminal for receiving data to be
`transmitted from the user terminal to the hub, the transmitter
`means including ?rst communication means for transmitting
`short bursty data in combination With second communica
`tion means for continuous transmission of data, sWitching
`means coupled to the transmitter means for sWitching trans
`mission betWeen the ?rst communication means and the
`second communication means in accordance With pre
`de?ned criteria and receiver means Within the at least one
`hub adapted to receive data transmitted by the plurality of
`terminals utiliZing either the ?rst communication means or
`the second communication means.
`The ?rst communication means can comprise non syn
`chronous multiple access communication means or non
`synchronous frequency hopping code division multiple
`access communication means. The second communication
`means can comprise frequency division multiple access
`communication means. The receiver means comprises
`means for receiving data transmitted by the plurality of user
`terminals using the ?rst communication means, the ?rst
`communication means utiliZing non synchronous frequency
`hopping code division multiple access communications,
`means for receiving data transmitted by the plurality of user
`terminals using the second communication means, the sec
`ond communication means utiliZing frequency division mul
`tiple access communications and means for receiving pre
`amble and synchroniZation message data transmitted by the
`plurality of user terminals precedent to transmissions utiliZ
`ing the ?rst communication means.
`The sWitching means comprises means for sWitching
`transmission from the ?rst communication means to the
`second communication means either in accordance With a
`source port ?eld Within messages received by the
`transmitter, When the length of a message received by the
`transmitter means exceeds a predetermined threshold, When
`a continuation ?ag in a message received by the transmitter
`means is turned on, When a user buffer containing a plurality
`of messages to be sent via the transmitter means ?lls beyond
`predetermined level or in accordance With the type and
`nature of the softWare application that initiated a message to
`be transmitted via the transmitter means.
`The sWitching means comprises means for sWitching
`transmission from the second communication means to the
`?rst communication means either When a source port match
`ing a predetermined criteria ceases to transmit messages,
`When the length of a message received by the transmitter
`means fails to exceed a predetermined threshold, When a
`continuation ?ag in a message received by the transmitter
`means is turned off, When a user buffer containing a plurality
`of messages to be sent via the transmitter means empties or
`When the softWare application meeting a predetermined
`criteria that initiated a message to be transmitted via the
`transmitter means ceases to generate message data.
`In addition, the hub comprises collision detection means
`for determining When tWo frequency hops associated With
`tWo independent receivers are utiliZing the same frequency
`at the same time, thus improving decoding Within the
`receiver means.
`There is also provided in accordance With the present
`invention a multiple access communication system for use in
`a satellite communication netWork, the satellite communi
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`cation netWork including a plurality of user terminals and at
`least one hub, the system comprising ?rst transmitter means
`for transmitting data utiliZing a non synchronous frequency
`hopping code division multiple access communication
`scheme, second transmitter means for transmitting data
`utiliZing a frequency division multiple access communica
`tion scheme, sWitching means for sWitching transmission
`betWeen the ?rst transmitter means and the second trans
`mitter means in accordance With prede?ned criteria, ?rst
`receiver means for receiving data transmitted using utiliZing
`the non synchronous frequency hopping code division mul
`tiple access communication scheme, second receiver means
`for receiving data transmitted using the frequency division
`multiple access communication scheme and third receiver
`means for receiving preamble and synchroniZation data
`transmitted utiliZing the utiliZing a non synchronous fre
`quency hopping code division multiple access communica
`tion scheme.
`The sWitching means comprises means for sWitching
`transmission from the ?rst transmitter means to the second
`transmitter means either in accordance With a source port
`?eld Within messages received by the transmitter means,
`When the length of a message received by the transmitter
`means exceeds a predetermined threshold, When a continu
`ation ?ag in a message received by the transmitter means is
`turned on, When a user buffer containing a plurality of
`messages to be sent via the transmitter means ?lls beyond
`predetermined level or in accordance With the type and
`nature of the softWare application that initiated a message to
`be transmitted via the transmitter means.
`The sWitching means comprises means for sWitching
`transmission from the second transmitter means to the ?rst
`transmitter means either When a source port matching a
`predetermined criteria ceases to transmit messages, When
`the length of a message received by the transmitter means
`fails to exceed a predetermined threshold, When a continu
`ation ?ag in a message received by the transmitter means is
`turned off, When a user buffer containing a plurality of
`messages to be sent via the transmitter means empties or
`When the softWare application meeting a predetermined
`criteria that initiated a message to be transmitted via the
`transmitter means ceases to generate message data.
`The system further comprises interface means for inter
`facing the at least one hub to an external communications
`netWork. The external communications netWork can com
`prise the Internet, a packet sWitched telephone netWork
`(PSTN), an Integrated Services Digital Network (ISDN), a
`Community Antenna Television (CATV) netWork, a Digital
`Subscriber Loop (xDSL) or a Frame Relay netWork.
`The system further comprises collision detection means
`for determining When tWo frequency hops associated With
`tWo independent receivers Within the receiver means are
`utiliZing the same frequency at the same time, thus improv
`ing decoding Within the ?rst receiver means.
`There is also provided in accordance With the present
`invention a multiple access communications system for use
`in a satellite communication netWork, comprising a plurality
`of user terminals for transmitting and receiving data over the
`multiple access communication system, at least one hub for
`transmitting and receiving data over the multiple access
`communication system to and from the plurality of user
`terminals, a forWard communication link for transmitting
`data from the at least one hub to the plurality of user
`terminals, a return communication link for transmitting data
`from the plurality of user terminals to the at least one hub,
`the return communication link including a ?rst communi
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`US 6,240,073 B1
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`7
`cation means for transmitting short bursty data in combina
`tion with second communication means for continuous
`transmission of data, switching means within the plurality of
`user terminals for switching transmission between the ?rst
`communication means and the second communication
`means in accordance with prede?ned criteria and receiver
`means within the at least one hub adapted to receive data
`transmitted by the plurality of terminals utilizing either the
`?rst communication means or the second communication
`means.
`
`Further, each user terminal comprises means for generat
`ing a request to be sent over the return communications link
`in order to utiliZe the second communication means. The
`hub comprises means for polling each user terminal over the
`forward communication link as to whether the transmission
`of data should be switched to utiliZe the second communi
`cation means.
`
`BRIEF DESCRIPTION OF THE DRAWINGS
`
`The invention is herein described, by way of example
`only, with reference to the accompanying drawings,
`wherein:
`
`FIG. 1 is a high level block diagram illustrating two
`satellite communication networks coupled to the Internet
`utiliZing both a forward and a reverse link via satellite;
`
`FIG. 2 is a graph illustrating the relationship between time
`and frequency for data packets transmitted by two users;
`
`FIG. 3 is a diagram illustrating sequence of packets
`transmitted in time making up a message;
`
`FIG. 4 is a diagram illustra