US006298052B1
`(10) Patent No.:
`a2) United States Patent
`US 6,298,052 B1
`Bousquetet al.
`(45) Date of Patent:
`Oct. 2, 2001
`
`
`(54) COMMUNICATION METHOD USED IN A
`SHARED RESOURCE TRANSMISSION
`SYSTEM
`
`(75)
`
`.
`:
`a.
`Inventors: Jacques Bousquet, Croissy sur Seine;
`Jean-Pierre Dehaene, Cormeilles en
`Parisis, both of (FR)
`;
`;
`(73) Assignee: Alcatel, Paris (FR)
`
`(*) Notice:
`
`Subject to any disclaimer, the term ofthis
`patent is extended or adjusted under 35
`US.C. 154(b) by 0 days.
`
`4,621,368 * 11/1986 Onoe et ale ieecsssecseecnserees 375/267
`4,841,527
`6/1989 Raychaudhurietal. .
`4,908,828 *
`3/1990 Tikalsky oo... sees 714/822
`:
`7/1991 Hochstein 2... cece 375/267
`5,034,966 *
`
`4/1992 Gilhousen et al. oe. 375/267
`5,109,390 *
`5138631
`8/1992 Taylor.
`5.142.533
`8/1992 Crisleret al
`5,168,510 * 12/1982 Hill
`ccessecsossssssseseessessssseeen 375/267
`5,457,678 * 10/1995 Goeldner ...cccsscsssescccsneeesen 370/395
`
`9/1997 Bolgiano et al. vss 375/138
`5,663,990 *
`3/1999 Dorenboschet al... 375/267
`5,883,581 *
`
`FOREIGN PATENT DOCUMENTS
`2029170 *
`3/1980 (GB)
`eessscccssssssscsssssseseseces 714/822
`61 043850
`3/1986 (JP).
`62 131636
`6/1987 (JP).
`(21) Appl. No.: 09/033,702
`08 032596—-2/1996 (JP).
`(22)
`Filed:
`Mar.3, 1998
`
`(30)
`
`Foreign Application Priority Data
`
`(FR) weeeceescecssssecesssesssesesesesesssenens 97 02491
`Mar. 3, 1997
`(SL)
`Tint, C07 cececcsssccsssscsssessssssnsesessesssnseseee HO4L 12/16
`
`...
`... 370/349; 375/267
`(52) U.S. Cl.
`(58) Field of Search 0... 375/267, 299,
`375/347, 138; 370/319-321, 344, 347, 349,
`440, 441, 474, 395; 714/822
`
`(56)
`
`References Cited
`U.S. PATENT DOCUMENTS
`
`3,906,445 *
`4,133,504 *
`4,400,811 *
`4,518,947 *
`
`9/1975 Beckmann et al. ccc 714/22
`1/1979 Dobleretal.
`wa 714/822
`.
`8/1983 Brownet al.
`«. 714/822
`5/1985 Poston et al. wees 375/267
`
`
`
`* cited by examiner
`
`Primary Examiner—Salvatore Cangialosi
`(74) Attorney, Agent, or Firm—Sughrue, Mion, Zinn,
`Macpeak & Seas, PLLC
`(57)
`ABSTRACT
`
`The invention concerns a method of communication used in
`a shared resource transmission system. In accordance with
`the inventionit consists in sendingat least two identical data
`packets without waiting for an acknowledgement of recep-
`tion between sending said data packets.
`
`‘The invention can reduce call set-up time compared to the
`prior art and applies in particular to satellite transmission.
`
`8 Claims, 1 Drawing Sheet
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`U.S. Patent
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`Oct. 2, 2001
`
`US 6,298,052 B1
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`US 6,298,052 B1
`
`1
`COMMUNICATION METHOD USED IN A
`SHARED RESOURCE TRANSMISSION
`SYSTEM
`
`The field of the invention is that of transmitting data in
`a shared resource transmission system. To be more precise,
`the present
`invention concerns an ALOHAtype shared
`resource transmission system that can be used to set up
`transmission between a calling station and a called station,
`for example via satellite. In the case of transmission via
`satellite the stations are typically earth stations.
`Considerfirst the context of transmitting data in packets
`via a shared resource for setting up a call between a calling
`station and a called station. Call set-up is a phase preceding
`communication proper and its function is to allocate a
`resource reserved to the call in question.
`The random ALOHAaccess system is used prior to call
`set-up. This system entails a calling station sending a data
`packet called the access packet for the attention of a called
`station using for this purpose a resource shared with other
`stations. The shared resourceis typically a carrier frequency.
`The various stations share the resource without any prior
`reservation of time slots and without any temporal synchro-
`nization of the stations. Because there is no reservation,
`collisions can occur between access packets sent by different
`stations. To compensate this risk of collision a station
`sending a first access packet
`that does not receive any
`acknowledgement message from the called station sends a
`second access packet via the shared resource after a par-
`ticular time-delay starting from sending the first access
`packet.
`Assuming that there is no correlation between the send-
`ing of access packets by the variousstations, in other words
`that the sending of the packets obeys Poisson’s law,
`the
`probability P of a collision between access packets occurring
`is equalto:
`P=1-e°R?
`
`where R is the average numberof packets transmitted per
`second and T is the duration of a packet. The digit 2 indicates
`that a collision occurs if two packets of duration T are
`transmitted within a time window of duration 2T.
`
`To overcomethis drawback it has been decidedto provide
`time windows of duration T during which stations are
`authorized to send access packets. In this case the various
`stations are temporally synchronized. This system is known
`as the slotted ALOHAsystem. In this case the probability P
`of a collision between access packets occurring is equalto:
`P=1-¢77?F
`
`the number r of access packets transmitted
`Accordingly,
`without repetition is generally written:
`r=R*eRF
`
`with k=1 for the slotted ALOHA system and k=2 for the
`random ALOHAsystem.
`The main drawback of these two types of access system
`is the high number of repetitions of access packets, which
`commensurately increases call set-up time. This problem is
`serious in the case of transmission viasatellite because of the
`
`distance traveled by the access packets between the sending
`station and the station to which the access packets are
`addressed.
`
`Packets of data can also be transmitted using ALOHA
`systems,slotted or otherwise. In this case the packets are not
`access packets but packets of data as such used for the call
`
`10
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`2
`between the calling station and the called station. The data
`packets convey alarm, supervisory, etc data. The same
`problem arises for these data packets as that which has
`previously arisen for access packets.
`One aim of the present invention is to solve these prob-
`lems.
`To be more precise, one aim of the invention is to reduce
`the numberof collisions between data packets transmitted
`by a station in a system where the senders of such packets
`use the same resource.
`This aim and others that will become apparent hereinafter
`are achieved by a method of communication used in a shared
`resource transmission system characterized in that it consists
`in sendingat least two identical data packets without waiting
`for an acknowledgementof reception between sending said
`data packets.
`The above method is advantageously used to set up a call
`and in this case the aforementioned data packets are access
`packets.
`The above method is advantageously employed in a
`satellite transmission system either for call set-up or for
`communication proper.
`The data packets preferably each include data for detect-
`ing a collision between the data packets transmitted on the
`shared resource.
`The packets can be transmitted with a temporal spacing
`and/or on different carrier frequencies and/or using code-
`division multiple access (CDMA).
`The invention applies to a slotted or otherwise ALOHA
`type shared resource transmission system.
`Other features and advantages of the invention will
`become apparent from a reading of the following description
`of one preferred embodiment given by way of non-limiting
`illustration and from the appended drawings in which:
`FIG. 1 showsthe probability of collision between data
`packets as a function of the load on the temporally shared
`resource for n=1 through 7 where n is the numberof times
`the same data packet is sent during a predetermined time
`period for a random ALOHAaccess system;
`FIG. 2 shows the probability of collision between data
`packets as a function of the load on the temporally shared
`resource for n=1 through 7 where n is the numberof times
`the same data packet is sent during a predetermined time
`period for a slotted ALOHAaccess system.
`The following description concerns a transmission system
`in whichthe data packets transmitted via the shared resource
`are access packets for setting up a call between a calling
`station and a called station.
`To limit call set-up time and therefore the probability of
`collision of access packets transmitted by a station the
`invention proposesthat each station requiring to set up a call
`should transmit at least two access packets without waiting
`for an acknowledgement between sending the access pack-
`ets. To be more precise, the invention proposes to send the
`same access packet n times (n>1) in a given time period
`whether an acknowledgement messageis received from the
`station to which these packets are sent or not.
`The transmission is done within a time period less than
`that required for a roundtrip of a packet betweenthe calling
`station and the called station.
`The probability that the called station has not received the
`transmitted message is equal to the probability that all the
`packets transmitted have collided with other packets. This
`probability is written:
`Paap'a(i-esry
`
`The total number R of packets transmitted via the shared
`resource is therefore equal to the sum of the n*r original
`3
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`3
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`

`

`US 6,298,052 B1
`
`3
`packets and the R/n messages repeated with the probability
`P,,, that is:
`
`R
`n
`R=n«rt+R«P, or r= —x[l—-(l—-e7)"]
`
`The effect of the invention can be seen in FIG. 1 which
`showsthe probability of collision between access packets as
`a function of the load on the temporally shared resource for
`n=1 through 7 where n is the number of times the same
`access package is sent during a predetermined time period
`for a random ALOHAaccess system. Here the packet error
`rate is 1%.
`The characteristic 10 corresponds to the prior art (n=1),
`Le. for a 4% load C on the shared resource a single access
`packet transmitted has a 10% probability of colliding with
`another packet. In this case it will be necessary to send the
`packet again.
`Characteristics 11 through 16 respectively correspond to
`transmission of n=2 through n=7 access packets in a given
`time period without waiting for an acknowledgement from
`the destination station. For n=2 and for the same 4% load C
`on the shared resource the probability P,, that the destination
`station has not received any of these packets correctly drops
`to around 2.5%, that is four times less than for n=1. The
`higher the value of n the greater the probability that at least
`one packet transmitted will be received correctly by the
`destination station. Note however that
`this process has
`limitations whenthe load C is high: in this case, the higher
`the number n of access packets transmitted the greater the
`risk of collisions.
`if the sending
`The invention is of increased benefit
`stations are temporally synchronized. The packets are then
`transmitted in accordance with the slotted ALOHAsystem.
`FIG. 2 shows the probability of collision between data
`packets as a function of the load on the temporally shared
`resource for n=1 through 7 where n is the numberof times
`the same data packet is sent during a predetermined time
`period for a slotted ALOHAaccess system. Here the packet
`error rate is also 1%.
`The characteristic 20 corresponds to the prior art (n=1),
`that is to say for an 8% load C onthe shared resource a single
`access packet transmitted has a 10% probability of colliding
`with another packet.
`Characteristics 21 through 26 respectively correspond to
`transmission of n=2 through n=7 access packets in a given
`time period without waiting for an acknowledgement from
`the destination station. It can be seen that for n=7 and for the
`
`same 8% load C on the shared resource the probability P,,
`that the destination station has not received any of these
`packets correctly drops to 0.3%. The phenomenon seen in
`FIG. 1 is no longer seen in FIG. 2 as it occurs for a load C
`greater than 10%.
`The systematic repetition of the access packets in the
`predefined time period therefore increases the probability
`that at least one of these packets will reach the destination
`station, which reduces the time necessary to set up a call.
`The n packets transmitted can be transmitted on the same
`carrier frequency and spaced in time, preferably at random.
`Another solution is to transmit these packets on different
`carrier frequencies, possibly in combination with a temporal
`distribution. Other implementations are possible,
`for
`example CDMA modetransmission in which the packets
`transmitted are by access codes,
`for example Walsh
`sequences.
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`4
`The invention is particularly beneficial whenit is applied
`to call set-up via satellite. In this case the probability of
`having to retransmit one or more access packets is much
`lowerthan in the prior art and it is not necessary to wait to
`confirm non-reception of a message acknowledging recep-
`tion before retransmitting. For a satellite link the time saving
`is that required for the radio signal to perform a roundtrip
`between the stations that are to enter into communication,
`ie. approximately 0.6 seconds for a geostationary orbit
`satellite.
`
`The invention applies equally to transmission systems in
`which access by sending stations, for example mobiles, to
`one or more destination stations, for example fixed stations,
`is in accordance with the ALOHA system as described
`above.
`
`The access packets transmitted advantageously each
`include a cyclic redundancy check (CRC) codeor other data
`for detecting whether there has been a collision between the
`packets transmitted.
`The aim of the CRCis to detect transmission errors. It
`
`therefore enables the addressee to determine if the message
`has been transmitted correctly.
`If n messages are sent
`simultaneously it indicates that at least one message has
`been transmitted correctly. Moreover,if the n messages were
`to arrive with different field values, it enables the receiver to
`tell which of the messages are good. It therefore provides
`additional security in terms of channel quality (not just in
`terms of collisions).
`Of course, the invention is not limited to call set-up and
`is equally directed to the sending of data packets
`(information packets) during a call between twostations.
`Whatis claimedis:
`
`1. Amethodof setting up a call between a calling station
`and a called station in a shared resource transmission
`
`system, the method comprising sending from said calling
`station to said called station at least two identical access data
`
`packets within a predetermined time period without waiting
`for an acknowledgementof reception between sending said
`access data packets, wherein said predetermined time period
`is less than a time period required for a roundtrip of a packet
`between the calling station and the called station.
`2. A method according to claim 1, wherein said shared
`resource transmission system is a satellite transmission
`system.
`3. A method according to claim 1, wherein said access
`data packets each include data for detecting collision
`between data packets transmitted over said shared resource.
`4. A method according to claim 1, wherein said access
`data packets are transmitted spaced in time.
`5. A method according to claim 1, wherein said data
`packets are transmitted on different carrier frequencies.
`6. A method according to claim 1, wherein said access
`data packets are transmitted by code-division multiple
`access.
`
`7. A method according to claim 1, wherein said shared
`resource transmission system is a non-slotted ALOHA
`shared resource transmission system.
`8. A method according to claim 1, wherein said shared
`resource transmission system is a slotted ALOHA shared
`resource transmission system.
`
`4
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