-

L,\]lor)

9.3 NETWORK ARCHITECTURES 347

in the case of a large network, or a fractional transponder lease for a medium to smallnel.work. The network architecture selected will depend on a number of factors that arediscussed in the following section"

9.3 NETWORK ARCHITECTURES'I'here

are tltree basic imp.lementations of any telecommunications service: one-way; two:way; and split-two-way (sometirnes referred to as split-IP, when referring to Internet traffic,since the outbound and inbound channels are routed over different systems). The two-wayiinplementation is further divided into two basic network architectures: Star and Mesh,'Ve will look first at the three basic implementations.

One-Way lmple;nentation

Tttis is the mode of a satellite used in the broadcast satellite service (BSS)" The intro-duction of digital technolcgy allows the provider a..^d user much greater flexibility inthc operation of a broadcast uetwork. By means of proprietary software in the user ter-minals, different parts of the downlink can be accessed by different subscribers accordingto the programs ordered from the supplier (and paid for by the user)" This form of chan-nel selectiotr is called narrowcusting. T'hete can be many narrowcasting groups withina larger hroaclcasting area. Figure 9.4 gives a schematic of this one-way (broadcast)applieation.

Split-Two-Way (Split lPl lmplementation

This implementation is used when there is no normal return ehannel as, for example, withKu-band broadcast satellite service (BSS) systems that carry' lnternet traf|rcT. The relatively

refer-r a i nrr i f l)ver

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atellite',ponder'rortders,

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FfGUiC 9"4 Schematic of a broadcast satel l i te service coverage region in which smaller, nar-rowcasting groups exist withrn the broader cove:age area (from Figure 2-1 of reference 3), Themaster control stat ion sends encoded signals within the broadcast stream that enables certainrlsers to have access to part icular channel groupings according to the subscriber's choice.

348 cHAPTER e vsAT sysrEMs

' high capacity downlink stream is not complemented by an uplink capability from the userterminal. If the BSS downlink is used as the download channel from an Internet serviceprovider, the only option the user has for a return link is via another telecommunicationschannel, such as a standard telephone line. The Internet. protocol (IP) is therefore splitbetween a satellite downlink (outbound) channel and a terrestrial telephone (inbound, o.return) channel; hence the term split IP fr-rr this implementation. The advantage of thisapproach is that the VSAT terminal does not require a transmit capability, which signifi-cantly reduces its cost and complexity. The disadvantage is that the telephone lineconnection must usually be through a tnodern, with a bit rate generally restricte6 to 56 kbpsor less.

TWo-iJUay lmpleinentation

tn itri, case, a return link is designed into the service so that rwo-way communicationscan be set up over the same satellite, from the hub to the user and from the user baek tothe hub. The VSAIMLL implementation illustrated in Figure 9.2 is a two-vzay servicebetween the hub (in this case the satellite gateway) and any VSAT terminal, The archi-tecture selected is the key to the economics of two-way connections; it can be either Meshor Star. These two architectures are illustrated in Figures 9.5a,b, with the topology asviewed by the satellite shorvn in Figures 9.6a,h.

Initially, the mosl comlnon VSAT architectures were St-r networks since the verylow receiv e G /T (gain-to-noise temperature ratio) of the VSATs, coupled with their limitedtransmit EIRP, was compensated for by using a large hub with high G/T and EIRp. The

Master ControlStation (the l,ub) VSATS

(b):if"i

FIOUiF :,5 {a} lllustration of a Star VSAT network. In this network arclitecture, all of the traffic is -':',$route ' l v ia the mastercont ro l s ta t ion, or hub. t f a VSATwishesto communicate wi th anotherVSAT, l i fthey have to go via the hub, thus necessitat ing a "double hop'" l ink via t ire satelf i te. Since al l of the itraff ic radiates at one t ime or another from the hub, this architecture is . 'eterred to as a Star nctwork. : l

{bl l l lustrat ion of a Mesh VSAT network. In this network architecturc, dach of the VSATs has the abi l i tY $to communicate directly with any of the other VSATs. Sinc* 'ilte traffic can go to or from any VSAT, ;,r;this architecture is referred to as a Mestr network. l t wi l l st i l l be necessary to have network controf andi

the duties of the hub can either be handled by one of the VSATs or the master control stat ion func' it ions can be shared among the VSATs,

wVS.

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9.4 ACCESS CONTROL PROTOCOLS 349

@{EK

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FIGURE 9.6 Jrl Topology of a Star VSAT newvork viewed from the satellite's perspective.Note how the VSAT communications l inks are routed via the satell i te to the huh in all cases.(bl Topology of a Mesh VSAT network from the satell i te's perspective. Note how all of theVSATs communicate .l irectly to each other via the satell i te without passing through a largermaster conrrol stat ion (hub).

cost of the hub was therefore quite high and, at least for the smaller VSAT networks,somewhat prohibitive. This led to the concept of a shared hub, where several netwrrksoperate through one main hub. The difficulty with this approach for large countries witlwidely dispersed communities is that the host computers for the small VSAT networksare larely close to the hub. A high-speed terrestrial data link is required between the hostcomputers of the networks and the hub, which increases the cost of the network" Ratherthan have one large hub for all of the VSAT networks sharing the same satellite, the over-all network evolved to allow each subnetwork to havo lts own hub as soon as the eco-nomics made it attractive, In this way, the host comluter of each VSAT network can beco-located with its own hub, thus eliminating the ;ost of the interconnection between thehub earth station and the computer controlling the service offered through the VSAT net-work. Whether the hub is shared or dedicated on the one hand or the VSAT is connectedto a single user or a local area network (LAN) with multiple users sharing access throughan Ethe.rnet connection on the other, ir^ every case there will need to be an access controlprotocol.

ACCESS COfTTROL PROTOCOT-S

The International Standards Organization (essentially a standards committee of the UnitedNations) has specified the open systems interconnection (IS0/OSI) that mandates a seven-layer model for a data communication system, as shown in Figure 9.?.

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35O cHAPTER e vsAT sysrEMs

(-;

Application

Presentation

Session

Transport

Network

Link

Physical

FIGURE 9,7 The ISO-OSI seven-layer "stack" for interconnecting data terminafs. In thisexample, user one and user two are conducting a two-way communications session witheach other. Each user interacts with their local device (e.9", d computer keyboard/visual dis-play unit) at the application layer of the ISO-OSI stack, Their transaction is then routed viat i te various layers, with suitable conversions, etc., unti l the content is ready to be trarrsmit-ted via the physical layer.

A satellite communications link occupies prinrarily the physical layer, which is whebits are carried tletween the terminals. A VSAT network must have terminal controllersieach end of the link and these occupy the network and link layers, the two layersthe physical layer. The network control center typieally controls the system and is risponsible for the remaining layers. Unfortunately, few communications systems conforin an easily identifiable way to the seven layers of the ISO-OSI,model. (For example,lP protocol stack of five layers simply puts the ftrst three laye:s of the ISO/OSI stackone layer). It is, however, very useful as a conceptual modei which identifies functiithat must be performed somewhere in every data communication network. Most datamunication networks use some form of packet transmission, in which blocks oftagged with an adclress, error control parity bits, and other useful information beforemission. The receiving end of a link checks arriving paekets for errors, and thenaeknowlerigement signal (ACK) that the packet was received correctly, or a notedge signal (NAK) that tells the transmit end to resend a particular packet becpacket had an elror. Some systems do not send acknowledgements, only NAK sirequest a retransmission of a packet with an error, since this speeds up data transm!This is the error control method used in the Intemet protocol TCP/IP. Genericallsystems are known as automatic repeat reqnest (ARQ systems. Chapter 7 discussestransmission systems and the.problem of error detection and correction in packetusing satellite links.

The ISO-OSI stack was init^rlly developed fbr terrestrial eommunicatinns s

Fbr ttris reason, the protocols tlrat implement tl:-e functions of'each hyer were-for ttse in terrestrial circuits with low delay and lcw bit error rale {BER), that is,performance levels. T,rese are key points when .rying to use such protocolslites, particularly those in geo(tationary earth orbit (GEO). Many of the ea'rlyhad a connection time-out of a few milliseconds. If po reply was received from the'

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s.4 ACCESS CONTROL PRCTOCOLS 351

in this interval, transmissions ceased. Similarly, an enored signal received fro.ur the sourceor an intervening node would trigger an automatic error recovery sequence. For exam-ple, the X.25 and X.75 packet systems use an ARQ approach, which, on detecting anerror in a packet, immediately requests a retransmission and halts further transmissionsuntil fhe corrected packet is received. Frame relay and ATM (asynehronous transfer mode)systems flag the elror but continue the flow of information (continuous transmissionARQ" In both cases, the errored transmission must be corrected and suitable buffers atthe receiver end (or internrecliate node) used fo restore the packets in their original or-der. The lnore enors that occur in the link, necessitating many retransmissions of pack-ets, tlte slower the effective data throughput rate of the link becomes. The potential fordelay and (propagation induced) errors are therefore critical design elements in digitalVSAf eonnections.

Delay Consideraticns

A typical slant range to a GEO satellite is 39,000 km. The one-way delay over such aGEO link (earth station to satellite to earth station) is 2 X (range/velocity) : 260 ms.J'he one-way delay in a typical 4000-km transcontinental link via fiber-optic cable isa l itt le over l3 ms. Neither example includes processing delay (e"g.,source codingand/or compression, chantrel cocling, baseband proce.ssing in the switehing elements,frarne length) which can add several tens of mill iseconds or even over'a hundredrnil l iseconds.

The time out elenrent of a protoccll is often referred to as the window of the con-neetiotl. As long as the window is "r)pen," communications can continue without inter-ruption. Figure 9.8 iilustrates a corrtinuous transmission ARQ system that has a 60-ms

1 0 m stransmission

delay AEKwindowA

TimeA 1

FlGt?RE 9.8 l l lus t ra t ion o f a communicat ions l ink wi th a 10-ms one-way de lay and ' i 60-mswincow, In this example, a packet or frame is sent at instant 41 from user 1 to use! ' 2: . .ser 2receivc: the irans;,", ission without error anrJ sends an aeknowledgement back, wh.c.r is re-ee ivee l a t ins tant A2,2A ms af ter the in i t ia l t ransmiss ion f rom user 1 . Th is is we! l w i th in thetime window of 60 ms, The t ime window rol ls forward afte, each successful ac<nowledge-ment. Thus the transmission from user 1 at instant B1 is received bry user 2, and the ac-knowledgernent received by user 2 at instant 82, within the new rol l ing t ime rr.r indow of 60ms, Each packet or f rame is successfu l ly rece ivec l in th is example.

Packets received by User 2

Packets transmitted bv User 1

35.2 cHAPTER e vsAT sYsrEMs

Packets transn,itted by User 1

FIGURE 9.9 l l lustrat ion of a communications l ink with a 260-ms one-way delay and a60-ms .ar indow. In th is example, a packet or f rame is sent a t ins tant A1 f rom user 1 touser 2 . User 2 rece ives the t ransmiss ion wi thout €r ror and sends an acknowledgement

back, which is rece ived at ins tant A2,260 ms af ter the in i t ia l t ransmiss ion f rom user 1 .Unfor tunate ly , ins tant A2 is wel l a f ter the ro l l ing window t ime-out o f 60 ms. Transmis-

s ions f rom user 1 are automat ica l ly shut down by the protoco l when the t in ie-out o f60 ms is exceeded. lgnor ing process ing de lays in th is example, user 1 is on ly t ransr . i , ' . -t ing for 60 ms in every 260 ms, thus drast ica l ly lower ing the throughpt t t . Again , nopropagation errors are assumed to occur,

I

window with a l0-rns one-way delay and Figure 9.9 illustrates a link with a 60-ms windowand a 260^ms one-way delay.

Clearly, satellite systems have to operate satisfactorily, and seamlessly (i.e., the userhas no idea whether the link is terrestrial or via a satellite), with existing terrcstrial net-works or their utility is severely compromised. This is particularly true for GEO systetnsand there are two ways to make terrestrial protocols work with a satellite linl:. First, theprotocols can be changed so that the time-out window is well in excess of 260 ms; sec-ond, the satellite element of the packet network can be configured to exist as'a separatesubnetwork within the global packet network. In practice, both solutions are adopted"Figtrre 9.10 illustrates the concepta.

The VSAT' and hub "protocol" equipment act as processing buffers to separate

the satellite (VSAT) network from the terrestrial network. This is sometimes known as

s:poofing hecause the terrestrial part of the system uses a- conventional protocol and is

unaware of the VSAT network's existence. The electronic processing and emulation

permit traffic to flow seamlessly between two very different networks without opera"

tor intervention. In essence, this is the interface through which the VSAT user is con-

rrected to the VSA:I'network via the physical layer (see Figureg.l l). Once the user's

traffic has moved from the terrestrial network through the interface and is inside tne

VSAT network, the facket header is reorganized, with the appropriate routing and ad'

dress of the traffic attached, so that the information can pass su...stfully over the sate l'

lite network to the correct recipient. Network management of the VSAT system' whrcn

incluc{es congestion qontrol, is also carried out in thls element of .the VSAT networN"

.ermerj the network kernel. In actdition, all rif the necessary protocol conversions are

carried <;ul so that the packet or frame ean suceessfuUy pass over a satel'lite connecti0n

with a long delay,, a typical iata link layer prclocol (layer 2 in the ISO-OSI stack) that is used in a

low delay, terrestrial link employs modulo-8 operation. That is, the proio.ol will transntrl

Packets received by user 2

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s.4 ACCESS CONTROL PROTOCOLS 353

Hubstation

VSAT network

----gffi[?,ii-FfGUBE 9.1O Protocol architecture of a Star VSAT network (Figure 2.2"1 of reference 4olTU, reproduced with permissionl. VSAT networks are normally maintained as independent,privatc networks, with the packetization handled at the user interface units of the VSATterminals. The satell i te access protocol (with a larger time-out window) is handled in theVSAT/hub network kernel, whieh also handles packet addressing, eongestion control, packetrouting and switching, and network managernent functions. Protocol conversion and, ifneeessary, emulation is handled by the gateway equipment.

only 7 unacknowledged frames before it stops transmissions; this leads to the lowthroughput demonstrated in Figure 9.9, particularly for GEO satellite links. The highlevel data link control (HDLC) protocol used in layer 2 for satellite systems thereforeusually employs a modulo-I28 operation. That is, 127 frames may he sent without re-ceiving any acknowledgements before the protocol shuts down transmissions. Movingfrom modulo-E to modulo-128 operation significantly increases the "window" size per-mitted for the link layer control. The concept, called protoccll emulation, is demonstratedin F igure 9 .114.

Another critical function performed in the VSAT' interfaee/kernel sections is torespond to polling activity from the terrestrial packet networks. [t is normal for packetttetworks to poll users to see if there are packets to be sent. The interface/kernelelements in the VSAT network respond to the polling signals of the terrestrial net-work immediately, thus avoiding the long delay that would oecur if the polling signalhad to be passed over the satellite link. Negative acknowledgements are made to thepolling signals until a request to send data is received over the satellite link. Siven thatttte eorrect protocols have been inserted at ISO-OSI layer 2 within the VSAT system,and the management functions have been carried out (i.e., polling, switehing, ro-uting,acldressing, and flow control) so that the link can operate suecessfully at a p;otocbllevel, there still remains ti:e major pafl ttf tl,e system design question t; rnswer: howis the physical connection to tre established r-rver the satellite? To ans,,ver this questionwe must move from protocol design/emulation to transmission engineering. First,we wil l cover some of the basic techniques involved in developing a transmissiondesign.

;l

Inbound narrowband VSAT channels

:q

s.s EAstc rEcHNtouEs 359

36-MHz satellite transponder

FIGURE 9.14 l l lustrat ion of a VSAT network frequency assignment in which the inboundand outbound channels share the same transponder in the satel l i te. In the exampfe here, 19MHz of spectrum is al located for each side of the system connection. On the upl ink to thesatellite, the collection of FDMA narrowband channels transmitted by the VSATs coexists inthe same transponder with the wideband TDM stream transmitted up by the hub. on thedownlink from the satel l i te, the hub receives the col lect ion of individual narrowband chan-nels while the wideband TDM downlink stream is received by each VSAT. The precise fre-quency assignment can vary to suit the capacity of the VSAT network.

that uses TDMA on the uplink is required to transmit ar rhe full burst rate of the TDMAscheme, and must therefore have a much more powerful transmitter than an SCPC-FDMAVSAT. If the average traffic for an individual VSAT is only one equivalent voice circuit(64 kbit/s), having to transmit at 5 Mbit/s, say, instead of 64 kbiVs can pose major diffi-culties" The VSAT transmit power must be increased by a factor of 500019{ = ?8 to main-tain the siunc uplink C/N, since the eafth station receiver must have a bandwidth that iswider by the same factor. VSAT economics and bandwidth efficiency trade-offs have ledto a hybrid TDMA-FDMA approach called MF-DMA (multifrequency TDMA). This isillustrated in Figure 9.15.

In the MF-DMA example shown in Figure 9.15, each of the VSATs has to ransmirat a btrnt rate that is approximately five times the normal single VSAT single+hannel rate.If each VSAT transmits at a message data rate of 64 kbit/s and there are five VSATs sharing

FIGURE 9.15 Examplc of a muftifrequency TDMA (MF-TDMA) scheme. ln this pafticular case,five VSAT terminafs (A, B, C, D, and E) share the same frequency assignmenu that is, they alltransmit at the same frequency, However, they each have a unique time slot in the TDMA framewhen they transmit, so that they do not interfere with each other. The bursts from each VSATare timed to arrive at the satellite in the correct sequence for onward transmission to the hub.

'errestrial channeltoUser equiPment

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