Request packet
`
`Multiaccess protocols for VSAT networks
`
`141
`
`Data Slots,
`
`Data Pkt for
`req
`in old fr
`
`Wa
`
`scenario uses slotted ALOHAfor access [34—36] (as shown in Fig. 7.12), so that
`the reservation overhead required is independent of the actual number of
`terminals supported. Although Fig. 7.12 shows a system based on a TDMA-like
`frame, an interleaved format may also be used to minimise framing latency
`effects. Note that, in principle, it is also possible to use other slotted contention
`mechanisms (such as the tree CRA) for the reservation messages. It is observed
`that, since contention access is characterised by a relatively low capacity, the
`allocation of channel time for reservations could becomesignificant unless the
`ratio of request message to data message length is quite large.
`Typical achievable capacities for VSAT traffic parameters are in the range of
`0-4 to 0-6, depending on thelength distribution of messagetraffic to be supported.
`Slotted ALOHAaccess generally leads to more effective coverage of a range of
`VSATtraffic profiles than the TDMA access case discussed above. As for all
`DAMAprotocols, this protocol is characterised by a high minimum delay of
`~ 0-65, offset to some extent by low delay variance. As for the TDMAaccess case
`discussed above, the throughput and delay variance advantages of DAMAareat
`the expense of higher implementation complexity and poorer robustness. Never-
`theless, since DAMA with slotted ALOHAaccess provides good overall perfor-
`mance and can handle mixed interactive/file-transfer traffic, it is an Important
`candidate for many VSAT applications.
`
`Channel
`propagation
`delay (+1 frame)
`Retx delay for R1(1)
`Retz delay for R2(1)
`
`Channel
`propagation
`delay (+1 frame)
`Retz delay for R5(1)
`Retx delay for R1(2)
`
`Legend:
`al Request
`4 Collision
`
`oO Data packet
`
`Fig. 7.12
`
`Illustration of DAMA with slotted ALOHA access
`
`7.2.3.3 Unslotted locally synchronous reservation Traditionally, controlled access has
`been associated with time slotted channels because of the need to identify and
`allocate channel time segments to stations on demand. In this subsection, a
`recently proposed alternative approach [37] 1s described which does not require
`
`39
`
`

`

`Multiaccess protocols for VSAT networks
`Asynch Access Mode (ARM)
`..LSRM..ARM, LSRM. ARM Locally Synchronous Reserved
`Reserv
`c
`Reserv
`Mode (LSRM)
`pKLA
`r
`pkt D
`
`RP-B & RP-C
`retransmitted In ARM
`with random delay
`
`Scheduled LSRM for
`Reserv PktsA&D
`
`TRANSMIT
`time
`
`Channel propagation delay
`
`
`
`RECEIVEtime
`
`Legend: | Data packet
`{L_] Reservation pkt
`Collision
`Locally synch
`timing marker
`
`4
`
`Fig. 7.13
`
` Lllustration of channel events in locally synchronous reservation with ALOHA
`access
`
`reservation schemes.
`
`TDMA-like timing, and is classified as ‘locally synchronous’or ‘self-synchronis-
`ing’. Protocols of this type were conceived as semi-compatible upgradesof unslot-
`ted random access protocols such as ALOHA, SREJ—-ALOHAortime-of-arrival
`CRA,for use in environments with a high proportion of long messages or mixed
`interactive/file-transfer traffic.
`The principle in locally synchronous reservation systems is to provideinitial
`access for request packets in an unslotted mode such as ALOHA. When a
`specified number ofsuccessful requests (A > 1) have been received, the channel
`switchesto the locally synchronous reserved message transmission mode. Asin the
`time-of-arrival CRA, scheduled transmissions are locally synchronised using
`channel event based timing. An exampleof the operation ofsuch a protocol using
`ALOHA access is shown in Fig. 7.13. For typical ratios of data packet
`to
`reservation packet length, achievable capacity is of the order of 0-6-0-7 with
`ALOHAaccess and 0-8-0-9 with time-of-arrival CRA access (applicableto fixed
`length packet formats only). Since partitioning of channel time is dynamically
`achieved in these protocols, they tend to have superior delay-throughput charac-
`teristics when compared with conventional TDMAbasedsystems. Of course, the
`irreducible reservation latency delay is also a characteristic of these protocols, as
`indicated in Fig. 7.2.
`
`
`
`“The demand assignment approachesdes-
`7.2.3.4 Hybrid reservation|random access
`cribed above are suitable for traffic scenarios with a relatively high proportion of
`long messages. However, for traffic profiles with a mix ofshort interactive and
`long transaction/batch messages,
`the use of pure DAMAresults is obviously
`inefficient for the short messages, and is associated with a high latency delay. On
`the other hand, random access systems, which are well suited to interactive traffic,
`are generally quite inefficient for mixed traffic environments. These considera-
`tions motivate hybrid reservation/random access schemes which combine the low
`delay advantages of random access with the high throughput properties of
`
`40
`
`

`

`7.3 Performance comparison of candidate VSAT
`protocols
`
`In this section, a more detailed quantitative performance comparison of can-
`didate VSAT multiaccess protocols is presented, chosen from those discussed in
`Section 7.2. Specifically, based on a variety of implementation and performance
`factors, unslotted ALOHA, SREJ-ALOHA,slotted ALOHA and DAMA with
`slotted ALOHAaccess have been chosen for the detailed comparison. Note that
`all the techniques considered in this section are ‘mature’ and have been validated
`by a variety of independent analytical and simulation studies. Althoughanalyti-
`cal tools for performance evaluation are available for each ofthe above four access
`methods, direct event simulation is used for the results presented here. This makes
`it possible to obtain delay distributions, which are important for data network
`design, as well as to incorporaterealistic traffic models which must generally be
`approximated in the analytical models. Detailed discussion of the simulation
`models used can be found in Reference 42,
`
`143
`Multiaccess protocols for VSAT networks
`Several approaches have been proposedforthis purpose [38-41]. The simplest
`strategy is to transmit short messages in the reservation slots directly, without
`reservation delays, and to transmit a reservation message (which could be ‘piggy-
`backed’ with short data packets) only if additional capacity is required. One
`could also establish procedures in which there is no explicit reservation of data
`slots, but where successful random access in a frameresults in an implicit reserva-
`tion of the sameslot in future frames. These protocols offer the possibility of high
`DAMAthroughput (> 0-5) along with low access delay for interactive messages,
`potentially leading to throughput-delay characteristics which transition grace-
`fully from the low-d/low-S random access curves to the high-d/high-S reservation
`curves in Fig. 7.2. However, optimisation of performance (while avoiding subtle
`instability and deadlock conditions) over a range of traffic profiles may be
`difficult to achieve in practice.
`
`Reproduced by permission of IEEE (© IEEE 1988
`
`7.3.1 VSAT traffic models
`Remote stations which support interactive data applications typically generate
`short, variable length messages with average data rate orders ofmagnitude lower
`than the multiaccess (terminal to hub) channel speed. Roughly speaking, the
`remote station traffic model is described by two attributes: (i) the averagerate at
`which new messages are generated, and (ii) parameters specifying the length
`distribution. Note that (i) and (ii) together can be combined to determine the
`average datarate per station in bits per second. In Fig. 7.14, approximate regions
`in the average data rate versus average message length plane that are occupied
`by some common VSATapplications are shown.It is observed that the impor-
`tant interactive data scenario is in the region near the origin, corresponding to
`low average data rate and short messages. Note that although, in Fig. 7.14, the
`length distribution has been approximated by a single parameter which is the
`average value, in general, an accurate evaluation requires exact determination of
`message length distribution for a particular application. It is often reasonable to
`Section 7.3 and Figs. 7.14-20 are from: RAYCHAUDHURI, D., and JOSEPH, K.: “Channel
`access protocols for VSAT networks:
`a
`comparative
`evaluation’, YEEE Communications
`Magazine, Special series on VSAT, May 1988, pp. 34-44
`
`41
`
`

`

`
`
`
`
`<
`
` Messagelength (chars)
`
`144 Multiaccess protocolsJor VSAT networks
`10*
`
`10
`
`=
`x
`5
`136
`as
`
`10°
`
`g
`gDa
`s
`=
`&
`o
`@o=
`
`3=
`
`
`
`Traffic source parameter regionsfor potential VSAT applications
`Fig. 7.14
`
`
`assume the distribution to be a truncated exponential with specified average and
`maximum length (typically 256 characters, based on present data communica-
`
`tion practice), Since the exponential distribution tends to produce pessimistic
`
` distribution is completely unknown. uld be noted that there are many
`
`subtleties to the station traffic model involving the detailed nature ofthe arrival
`Process (whichis, in general, not a Poisson source) that need to be considered in
`the evaluation. Typically, each remotestation is connected to a cluster controller
`that supports several interactive data terminals, which do not contribute traffic
`to the VSAT while they are waiting for a reply from the host computer. Detailed
`models for the general case, with Af > | terminals per VSAT and buffering at
`the VSAT,have been considered [43]. For simplicity, the results here are limited
`to the frequently used source traffic model with a single interactive terminal
`(Af = 1) per VSAT and exponential interarrival time between messages from
`The parameters used in this performance comparison correspond to a typical
`interactive transaction application from the parameter region identified in Fig,
`7.14, and are summarised for convenientreference in Table 7.3.
`7.3.2 Channel and protocol parameters
`The channel parameters (summarised in Table 7.4) used in these numerical
`examples are based on typical 56 kbit/s VSAT transmission speed, and are
`believed to reflect typical modem capabilities, Parameters for each of the four
`
`
`
`
`
`
`
`
`
`
`
`
`
`© TEEE 1988
`
`
`
`each station.
`
`_
`
`
`
`
`
`
`42
`
`

`

`%of56kbit/s
`
`o ay
`
`,
`
`|a
`
`re and
`munica-
`messtmistic
`the
`many
`Se arrival
`@ered in
`mitroller
`
`=054s
`
`protocols under consideration have been selected accordingto established design
`principles, and are given in Table 7.5. Note that, as discussed earlier, the key
`retransmission delay parameter is chosen for minimum stable delay, and is
`optimised at each level of load, V. For ALOHA,the retransmission delayis the
`only significant parameter of the protocol. However, for slotted ALOHA and
`SREJ—ALOHA,the choice ofappropriate packetsizes is also important. For slotted
`ALOHA,the convenient approach is adopted of selecting a message packetsize
`(i.e. slot size less guard time andall overheads) equal to the maximum possible
`message length. For the present example, referring to the traffic model in Table
`7.3, this is assumed equal to 256 characters, as might be the case in manypractical
`systems, For SREJ-ALOHA,subpacketsize is an extremely important choice: in
`general, optimum subpacket size depends upon the acquisition and addressing
`overhead, but is also a load dependent quantity. Fortunately, it can be shown
`that, for the present set of parameters, there is a fairly robust choice of subpacket
`size (50 characters for an average message length of 100 characters, as given in
`Table 7.5), which is close to optimum at moderate to heavy channel load. For
`DAMA/TDMA,several parameters such as the fraction of time allocated to
`reservation traffic,
`the average retransmission delay for colliding reservation
`messages and the message and reservation slot sizes need to be specified. The
`design procedure used here is based on optimally allocating a fraction of channel
`capacity to the reservation channel, noting that total delay is the sum ofreserva-
`tion access delay and message assignment delay. As discussed above for the
`ALOHAprotocols, the retransmission delay for the slotted ALOHA reservation
`subchannelis selected for minimum stable delay. As for the slotted ALOHAcase,
`the DAMA/TDMAsystem is based on a messageslot size equal to the maximum
`
`Multiaccess protocols for VSAT networks
`
`Summary of traffic source parameters used in performance
`Table 7.3
`comparison ©) IEEE 1988
`
`Interactive terminals per VSAT
`New messagerate per terminal
`(unblocked mode)
`New message rate per terminal
`(blocked mode)
`New message length distribution
`Average new message length
`Maximum new message length
`
`=
`=250msg/h = 0:07 msg/s
`
`=0
`
`— truncated exponential
`= 100 chars (800 bits)
`= 256 chars (2048 bits}
`
`Summary of channel parameters used in performance comparison
`Table 7.4
`© IEEE 1988
`
`Channel data rate
`
`Channel symbol rate (rate 1/2 coded)
`Minimum £,/N, required
`Modem acquisition preamble
`One way propagation delay
`Minimum ACK delay (2 hops)
`
`= 56 kbit/s
`
`=112kbit/s
`=8-0dB
`= 32 bits (4 characters)
`=0-27s
`
`43
`
`

`

`
`
` Maultiaccess protocolsfor VSAT networks
`146
`Table 7.5
`Summary ofprotocolparameters used inperformance comparison © IEEE 1988
`General
`Link level overhead per packet or subpacket (L2) = 7 chars
`Network level overhead per message (L8)
`= 8chars
`Minimum ALOHA moderetransmission delay
`= 067s
`
`Retransmission delay optimised for delay at eachtraffic load, under constraint of stable
`
`operation.
`
`
`
`ALOHA
`Single variable length packet per message
`ALOHA packet: data length + L2 + L3 + preamble
`
`SREJ-ALOHA
`Multiple fixed length subpackets per message
`Subpacketsize optimised for each average message length, L
`SREJ subpacket: fixed data length + L2 + preamble
`Subpacket size: S = 40 char for L = 50char
`50
`100
`60
`150
`
`Slotted ALOHA
`Single fixed length packet per message
`Slotted ALOHA packet: data length + filler data + L2 + L3 + preamble
`= 275 chars (fixed)
`Time slot size: packet length + guard time (4chars) = 279 chars
`
`
`
`DAMAITDMA (slotted ALOHA reservation)
`Reservation packet length 26 chars (including preamble)
`Reservation slot length: reservation packet length + guard time = 30chars
`DAMA message packet: data length + filler data + L2 + L3 + preamble
`= 275 chars(fixed)
`
`DAMA messageslot size: message packet length + guard time (4chars) = 279chars
`Assignment delay = 0675
`
`Fractional allocation of message and reservation slots optimised for minimum delay.
`
`Message and reservation slots interleaved
`
`message length of256 characters. In general, DAMAefficiency can be improved
`
`by using shorter message slots, but this is at the expense of some increase in
`
`implementation complexity.
`
`7.3.3 Numerweal results
`
`Fig. 7.15 showsthe familiar throughput-delay characteristics ofthe four protocols
`underconsideration, Note that throughput shown is the useful data throughput,
`
`after accounting for overheadssuch as acquisition preamble in ALOHA, guard
`time, acquisition time and wasted slot time (due to variable message length) in
`
`slotted ALOHA,subpacket overhead in SREJ-ALOBAandreservation channel
`
`44
`
`44
`
`

`

`Multiaccess protocols for VSAT networks
`
`147
`
`SREJ-ALOHA
`
`becomes overloaded. In Table 7.6, a comparison is presented of protocol capaci-
`
`overhead and wasted message slot time in DAMA/TDMA. The delay values
`include the effect of slot synchronisation latency delays for the slotted protocols.
`It is observed from the figure, that, for this message model, slotted ALOHA
`performsonly slightly better than ALOHA,while both are considerably outper-
`formed by SREJ-ALOHA. Obviously, the relative performanceis strongly de-
`pendent on message length, an effect which should be investigated at a later
`point. Fig. 7.15 also shows that the maximum throughput achieved by the
`DAMA approach is higher than that of the random access protocols, but is
`achieved with a high irreducible delay of about 0-75s.
`As discussed earlier, a more useful characterisation, as in Fig. 7.16, is provided
`by plotting curves ofaverage delay and peak (95th percentile) delay as a function
`of the number of VSAT stations supported on the same channel,
`.V. Since
`interactive network design specifications must include average as well as peak
`delay constraints, such a set of curves is a prerequisite for determining the
`*‘capacity’, ie. the number of remote stations (VSATs) supported on a channel
`while satisfying the performance requirements, V*. The operating value of .V*
`is determined by thetighter of the average and peak delay constraints. From Fig.
`7.16, it can be observed that peak delay for ALOHAtendstorise rather rapidly,
`while DAMAis characterised by high average delay and low delay variance.
`Slotted ALOHA and SREJ-ALOHAprovide better peak delay properties than
`ALOHA,butare also characterised by rapidly rising peak delay as the channel
`
`015
`
`Throughput
`
`Fig. 7.15 Delay-throughput characteristicsfor candidate multiaccess protocols © DEEE 1988
`
`45
`
`

`

`i
`
`ties with delay constraints of (0:75s average, 2-5s peak) and (1s average, 3s
`peak). Observe that the 0-75s average delay case is not achievable with DAMA,
`but when the average delay requirementis relaxed to 1s, DAMA supports many
`more terminals than the contention access techniques. Comparing the ALOHA
`
`80
`75
`ALOHA
`110
`105
`Slotted ALOHA
`140
`130
`SREJ-ALOHA
`
`
`*DAMA/TDMA 230ace
`* Objective not achievable
`
`
`
`148 Mudtiaccess protocolsfor VSAT networks
`10
`
`Delay(s)
`
`Slotted ALOHA
`SREJ-
`ALOHA
`
`95 percentile
`
`——--—-— Average
`
`Numberof VSATS
`
`Fig. 7.16 Average and peak delay versus number of VSATs for candidate multiaccess
`protocols. © IEEE 1988
`
`Table 7.6 Channel capacities for candidate protocols under alternative delay constraints
`© IEEE 1988
`
`Multiaccess
`protocol
`
`Channel capacity (number of VSATS supported, N*)
`
`Delay constraints:
`075s average
`25s peak (95%)
`
`Delay constraints:
`10s average
`30s peak (95%)
`
`ij
`
`46
`
`

`

`100
`Numberof remotestations
`Fig. 7.17 Variation of total traffic with number of VSATs for the different protocols
`© IEEE 1988
`
`125
`
`150
`
`Multiaccess protocolsfor VSAT networks
`L=100
`
`149
`
`S-ALOHA
`
`Pa DAMA
`7 (S-ALOHA
`access) e
`
`-
`
`™s/
`
`
`
`
`
`Totalchanneltraffic
`
`the delay penality experienced by long messages can be rather significant, par-
`
`protocols, it is observed from the table that, at both performancelevels, SREJ—
`ALOHAsupports about 25% and 75°, morestations than slotted ALOHA and
`ALOHA,respectively.
`Curves of total channel traffic (G) versus the number of remote stations per
`channel(.V’) are given in Fig. 7.17. These curves are ofsome ancillary importance
`because they relate to the ‘power efficiency’ of a protocol, which needs to be
`considered in power limited situations that frequently characterise VSAT
`networks. From the figures, observe that, at any value of VW, ALOHA has the
`lowest total channel traffic (and hence the best powerefficiency), followed by
`SREJ-ALOHA, DAMAandslotted ALOHA,in that order. Note also that
`random access systems display a rapid increase in G after reachingtheir respective
`congestion points, due to a non-linear increase in retransmission traffic.
`Further characterisation of the protocols in terms of delay distributions at
`specified load levels is given in Fig. 7.18. In the figure, delay distributions are
`plotted for ALOHA,slotted ALOHA, SREJ-ALOHAand DAMA,designed to
`operate at an V* corresponding to an average delay of 1s (as determined from
`curves in Fig. 7.16). Observe that, as expected, DAMA demonstrates relatively
`low delay variance, while ALOHAis characterised by very high delay variance,
`mainly dueto repeatedcollisions experienced by long messages. Slotted ALOHA
`and SREJ—ALOHAdisplay moderate delay variance, although substantially
`greater than that of DAMA. SREJ-ALOHAappears to have lower‘tails’ of the
`delay distribution than the other contention access techniques. A second detailed
`characterisation of interest is the plot of average delay versus message length
`shownin Fig. 7.19. Observe that DAMAandslotted ALOHA,for which theslot
`size has been chosen to equal the maximum message length, have delays which
`are independentofthe actual length. In contrast, ALOHA and SREJ-ALOHA
`accommodate variable length transmissions,so that longer messages tend to have
`a highercollision probability (and hence delay) than short messages. In ALOHA,
`
`47
`
`

`

`(charaaa
`
`oo
`
`| T
`
`
`
`Averagedelay(s)
`
`Fig. 7
`
`conside
`ACCESS
`ALOHA
`more cf
`ALO
`advants
`can be
`has also
`ately eq
`
`Table
`with ave
`
`Message:
`
`f
`
` 150
`
`AMultiaccess protocols for VSAT networks
`
`Mean delay = 1s
`
`CDF
`
`SREJ-ALOHA
`
`0
`
`2
`
`4
`
`6
`Delay (s)
`
`8
`
`10
`
`12
`
`Fig. 7.18 Delay distributions for candidate protocols at channel load corresponding to an
`average delay of 1-05. © IEEE 1988
`
`ticularly at high channel load. However, the SREJ mechanism inselective reject
`ALOHAovercomes this problem to a large extent, so that the curves show only
`a modest increase in delay with message length.
`The above results in Figs. 7.15—-7.19 were based on a specific VSAT traffic
`source model, with an average of 250 messages/hour and truncated exponential
`messages with maximum 256 characters and average 100 characters. In orderto
`study the sensitivity of protocol performance with respect to average message
`length, Table 7.7 is introduced which shows the number ofVSATssupported per
`channel (based on a 1-0s average delay criterion), for average message length
`equal to 50, 100 and 150 characters respectively (truncated exponential distribu-
`tion assumed). The results show that, as expected, slotted ALOHA and DAMA
`have capacities which areinsensitive to the message length, while both ALOHA
`and SREJ-ALOHAexhibit decreasing capacity as message length increases. In
`general, it is found that SREJ-ALOHAhasthe highest capacity for short to
`medium length, while for message lengths of 150 characters or higher, slotted
`ALOHAis the preferred protocol. Overall, Figs. 7.15-7.19 provide a detailed
`comparison of the candidate VSATprotocols considered. In general, contention
`techniques are characterised by curves of delay versus number of VSATs which
`start at the origin (i.e. low delay at low throughput), whereas DAMA systems
`exhibit significant latency(irreducible) delay. This may indicate the need to use
`random access for applications requiring very low access delay, even before
`
`ll i
`
`48
`
`

`

`Multiaccess protocolsfor VSAT networks
`
`15]
`
`——— 125 Remotestations
`
`—-—-—-— 75 Remotestations
`
`
`
`Averagedelay(s)
`
`: F
`
`Slotted ALOHA
`
`-— ~— ~~ —__ DAMA(100 Remotestations)
`SREJ-ALOHA
`
`
`
`0-01
`
`.
`
`0.04
`
`Length (s)
`Fig. 7.19 Variation of average delay with message length for the candidate protocols
`© IEEE 1988
`
`AA
`
`considering implementation complexity factors. When selecting between random
`access protocols, it has been shownthat, for practical parameter ranges, SREJ—
`ALOHAis superior to both ALOHAandslotted ALOHA (which is definitely
`more complex in terms of implementation). For short message environments,
`ALOHAmayalso be used,especially becauseit offers significant implementation
`advantages. Slotted ALOHAis generally outperformed by SREJ—ALOHA,but
`can be used in environments with a large fraction of long fixed size messages. It
`hasalso been observed that SREJ-ALOHA andslotted ALOHAoffer approxim-
`ately equivalent delay distribution effects, while ALOHAis generally charac-
`
`Table 7.7 Variation of channel capacity (with average delay constraint equal to 1-0s)
`with average message length. © IEEE 1988es
`Message length
`Channel capacity (number of VSATS supported, N*)
`(characters)
`
`DAMA/
`SREJ—
`Slotted
`ALOHA
`
`
`ALOHAALOHA TDMAga
`50
`130
`205
`250
`130
`160
`250
`
`130 11575 250re
`
`
`
`
`49
`
`

`

`152 Multiaccess protocols for VSAT networks
`
`
`
`the ‘peak’ delay
`terised by long delay tails. Depending on the application,
`performance such as those shownin Fig. 7.16 is also an important consideration
`
`for network design. In applications requiring very low peak delays, ALOHA may
`
`not be a suitable protocol, and either SREJ-ALOHAorslotted ALOHAshould
`
`be used. For scenarios in which relatively high (> 0-75s) average access delay is
`
`permissible, DAMA with slotted ALOHA access may provide capacity advan-
`
`tages. In addition, suitably designed DAMAprotocols tend to providerelatively
`
`low ratios of peak delay to average delay, which may be an advantage in some
`
`applications. However, the implementation complexity, which involvesslotting,
`
`framing, service queue management and control signalling,
`is considerably
`
`greater than for any of the contention access techniques. Thus, the requirement
`
`for low cost VSAT equipmentis likely to motivate the use of contention access
`
`for first generation VSAT systems, except in traffic scenarios for which the
`
`capacity penalty would be extremely severe. Moreover, as discussed in the earlier
`
`section, there are various approaches to semi-compatible upgrades of contention
`
`protocol equipmentfor future traffic scenarios which require a reservation mode.
`
`To complete the discussion, an attempt will be made to provide a qualitative
`
`comparison between multiaccess protocols over the range oftraffic source para-
`
`meters shown in the terminal-rate/message-length diagram shown in Fig. 7.14.
`
`Theresults presentedso far apply to the interactive (transaction or point-of-sale)
`
`environment shown near the origin of Fig. 7.14. Widening the scope of com-
`
`parison to the entire region shown in the Figure, it would be useful to specify
`
`optimum regions for the various alternative VSAT protocols. Accordingly, Fig.
`
`7.20 provides a qualitative assessment ofoptimumregions for the VSATprotocols
`
`discussed. An attempt has been madeto incorporatetheissue ofsatellite power
`
`as well as bandwidth, without entering into the details, which are discussed in
`
`Chapter 9. Specifically, Fig. 7.202 shows the optimum protocol regions” for a
`
`moderately power limited example, as might be encountered in a system with
`
`small antennasize (1-2 m), high ratio of hub outboundtraffic to inbound traffic
`
`and/or high availability requirements. Fig. 7.206 shows a similar plot for a
`
`moderately bandwidthlimited system, which mayoccurinasituation with larger
`
`antennas (1-8 m or greater), powerful forward error corrected modems, balanced
`
`outbound and inboundtraffic and/or low availability requirements.
`
`Observe from the two Figures that, in general, ALOHA-type protocols are
`
`optimum for scenarios with short messages and low average data rate per remote
`
`station, in agreement with conventional wisdom. DAMA is recommended for
`
`higher message lengths, but for low to medium station traffic volume, while
`
`TDMAisthe choice for systems with high station traffic volume, independent of
`
`message length. Note that there is considerable overlap between optimum regions
`
`for the powerlimited scenario since bandwidthuseis relatively non-critical. On
`
`the other hand,for the bandwidth limited case, thereislittle overlap between the
`
`optimum regions for each of the protocols, and this can be attributed to the
`
`greater importance of the multiaccess efficiency in determining system capacity.
`
`
`
`“Note also that TDMA,whichis a useful protocol for medium to high volumestations,is
`now included. Also the three ALOHAprotocols have been lumped together, with the
`understanding that the choice between these will depend upon the message length regime
`
`of interest along with implementation factors.
`
`
`
`il
`
`50
`
`50
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`

`

`
`
`
`
`
`
`s/sliq‘ayeieuiwe)eBeseays/siiq“eye!UE
`
`7.4 Conclusions
`
`Fig. 7.20 Optimum protocol regionsfor typical Ku-bandsatellite scenarios © IEEE 1988
`a moderately power limited case
`b moderately bandwidth limited case
`
`a&383$
`
`a,
`
`&2S=eB
`
`=
`
`3yiy38iteSbyBSSs
`
`153
`
`SAIDX9SJO%
`
`span99409%
`
`Messagelength (chars)
`
`
`
`'ujuuayabeiaAV
`
`A review of established and new approachesto satellite multiaccess for VSAT
`app
`lications has been presented. A variety-of contention and reservation based
`has been described and
`protocols for use on both slotted and unslotted channels
`
`51
`
`

`

`154 Multiaccess protocols for VSAT networks
`
`compared in terms of key attributes such as throughput, delay, stability, robust-
`ness, operational convenience and implementation complexity. After the survey,
`detailed performanceresults for four candidate‘first generation’ VSATprotocols
`were presented (ALOHA,selective reject ALOHA,slotted ALOHA and DAMA
`with slotted ALOHAaccess) applied to an example transaction application. It
`has been shown that, among the random class systems considered, SREJ—
`ALOHAgenerally outperforms both ALOHA andslotted ALOHA. DAMAis
`shown to achieve a higher capacity and lower delay variance than the random
`access alternatives, but this is at the expense of a 0-75 irreducible delay, poor
`robustness and higher implementation complexity. In view of the relatively low
`impact of VSAT to hub space segment cost on overall system economics, it is
`expected that delay, implementation complexity and robustness will be primary
`considerations in VSAT access protocolselection, with capacity (within reason-
`able ranges) being an important, but secondary issue.
`
`7.5 References
`
`
`
`METCALFE, R.M., and BOGGS,D.R.: ‘Ethernet: Distributed packet switching for
`local computer networks’, Communications ACM, July 1976, pp. 395-404
`BUX, W., et al.: ‘A reliable token system for local area communication’, National
`Telecommunications Conference, Dec. 1981, pp. A2.2.1-6
`ABRAMSON,N.: “The ALOHA systemAnother alternative for computer com-
`munications’, AFIPS Conf. Proc., 1970, 37, pp. 281-285
`TOBAGI, F.A.:
`‘Multiaccess protocols in packet communication systems’, EEE
`Trans. Commun., April 1980, pp. 468-488
`LAM,§.8.: ‘Satellite packet communication— Multiple access protocols and perfor-
`mance’, [EEE Trans. Commun., Oct. 1979, pp. 1456-1466
`SCHWARTZ, J.W., AEIN, J.M., and KAISER, J.: ‘Modulation techniques for
`multiple access to a hard limiting repeater’, Proc. IEEE, May 1966, pp. 763-777
`PURSLEY, M.B., and SARWATE, D.V.: ‘Performance evaluation for phase-coded
`spread spectrum multiple-access communication—Part I: System analysis’, [EEE
`Trans. Commun., Aug. 1977, pp. 795-799
`. GERANIOTIS,E.A., and PURSLEY, M.B.: ‘Error probabilities for slow-frequency-
`hopped spread-spectrum multiple-access communications over fading channels’, [EEF
`Trans. Commun., May 1982, pp. 996-1009
`SCHMIDT, W.G.: ‘Satellite time division multiple access systems: Past, present and
`future’, Telecommun., 1974, 7, pp. 21-24
`LAM,S.S.: ‘Delay analysis of time division multiple access (TDMA)channel’, JEEE
`Trans. Commun., Dec. 1977, pp. 1489-1494
`11 KOBAYASHI, H., ONOZATO,Y., and HUYNH,D.: ‘An approximate method for the
`design and analysis ofan ALOHAsystem’, IEEE Trans. Commun., Jan. 1977, pp. 148-157
`12 JENQ, Y.C.: ‘Onthestability of ALOHA systems’, ZEEE Trans. Commun., Nov. 1980,
`pp. 1936-1939
`13 BELLINT, S., and BORGONOVO,F.: ‘On the throughput of an ALOHA channel
`with variable length packets’, IEEE Trans. Commun., Nov, 1980, pp. 1932-1935
`14 RAYCHAUDHURI,D.: ‘ALOHA with multipacket messages and ARQ-typeretrans-
`mission protocols—Throughputanalysis’, IEEE Trans. Commun., Feb. 1984, pp. 148-154
`15 RAYCHAUDHURI,D.; ‘Stability, throughput and delay of asynchronousselective
`reject ALOHA’, IEEE Trans. Commun., July 1987, pp. 767-772
`16 MASSEY,J.L., and MATHYS,P.: ‘The collision channel without feedback’, /EEE
`Trans. Inf. Theory, March 1985, pp. 192-204
`17 ROBERTS, L.G.: ‘ALOHA packet system with and without slots and capture’.
`ARPANETSatellite System Note 8 (NIC11290), June 1972; reprinted in Computer Comm.
`
`52
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`

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`
`
`
`
`
`
`155
`Multiaccess protocolsfor VSAT networks
`channel: Performance evaluation’, JEEE Trans. Commun., April 1975, pp. 410-423
`—
`18 KLIENROCK,L., and LAM,S.5S.: ‘Packet switching in amultiaccess broadcast
`-
`
`
`19 CAPETANAKIS, J.1.: ‘Tree algorithms for packet broadcasting channels’, IEEE
`>
`
`
`
`Trans. Inf. Theory, Sept. 1979, pp. 505-515
`fancols
`
`
`DAMA
`
` 20 MASSEY, J.L.: ‘Collision resolution algorithms and random—access communica-
`
`tions’. UCLA Technical Report, ENG-8016, April 1980
`.
`
`
`
`
`
`21 GALLAGER, R.C.: ‘Conflict resolution in random access broadcast networks’, Proc.
`
`
`
`AFOSR Workskop in Comm. Theory and Applications, Sept. 1978, pp. 74-76 _.,
`
`
`22 MASSEY,J.-L. ‘Gollision-resolution algorithms and random-access communications’ ,
`
`
`in LONGO,J. (Ed.): Multi-user communication systems. CISM Courses and Lectures No.
`
`
`
`
`265 (Springer-Verlag, Vienna, 1981)
`
`
`23 LIU,T., and TOWSLEY,D.: ‘Window andtree protocolsfor satellite channels’, Proc,
`
`
`
`
`IEEE Infocom, April 1983, pp. 215-291
`
`
`24 RAYCHAUDHURLD.: ‘Announcedretransmission random access protocols’, JEEE
`
`
`
`Trans. Commun., Nov. 1985, pp. 1183-1190
`25 MERAKOS, L., and KAZAKOS,D.: *“Multiaccess ofa slotted c
`
`minislot’. Proc. Intl. Conf. on Communications, Boston, June 1983, pp. C-5.3.1-6
`hannel using a control
`
`26 RAYCHAUDHURL,D:: ‘High capacity