7
FUTURE COMBAT SYSTEMS WIRELESS NETWORK ARCHITECTURE CONSIDERATIONS Gary Pennett The Aerospace Corporation Chantilly, VA ABSTRACT This paper investigates how the ad hoc wireless network Throughput (T) in bits per second (bps) performance, as a function of capacity (W) bps and the number of nodes (n), is a dominate factor in determining the Future Combat Systems operational capability. The paper describes the large disparity between the theoretical Throughput of w bps compared to the best experimentally achieved of W bps and the inability of known approaches such as n TDMA, CDMA, relays, and mobility to approach the theo- reticalperformance. The large disparity between the theoreticalperformance and known approaches motivates need to answer the questions: What Throughput is re- quired?, What Throughput has been achieved?, and What will therefore be the architecture?, early in the system de- sign process. The appendix uses some simplifying assump- tions to provide a more intuitive approach that approxi- mates the proof of the theoretical limit of $1 bps, as the mathematically rigorous approach referenced in this pa- per involves a lengthy and complex proof The operational success of the Army's Future Combat Sys- tem (FCS) depends on the successful deployment and op- eration of a wireless network. This mobile, ad hoc', wire- less network will link together possibly thousands of nodes at the Brigade level and below whose data will be shared, correlated, and fused to create a picture of the tactical bat- tle space. Sharing data on this scale implies that large communications pipes are required. Yet as nodes are added to large wireless networks, the communications pipes2 become more restricted. Thus, there are tradeoffs between numbers of nodes, network capacity, mobility, and data sharing that have a direct impact on the architec- ture of the network. These trades are not fully understood, yet essential to creating a common tactical picture. The purposes of this paper are to explain the assertions made and Cynthia Dion-Schwarz The Institute for Defense Analyses Alexandria, VA here and suggest the following questions about the net- work design that should be asked by those designers and managers of the FCS system development. 1. What Throughput is required? 2. What Throughput has been achieved? 3. What will therefore be the architecture? Two terms are relevant to this discussion. Capacity (W), measured in bits per second (bps), is the measure of infor- mation flow between two directly connected nodes. It de- pends on the bandwidth available and the signal-to-noise ratio3 of the link, that is, it depends on the size of the pipes and the noisiness of the connection. The SINCGARs+ ra- dio has a data capacity of -16 Kbps, for example. Throughput4, again measured in bits per second, is the measure of the average information flow between two networked nodes (see Figure 1). In addition to depending on the capacity between any two nodes in the multi-hop chain, Throughput also depends on the number of nodes in the network. Throughput will be the rate of information transfer in a busy network, in which many nodes are com- peting to use communications channels and could interfere with one another. > Source A Destination A Destination B Source B Figure 1: Conceptual Diagram of Wireless Ad-Hoc Mobile Network 1 "Ad hoc" networks have no fixed gateways or infrastructure to sup- port connectivity. The other type of mobile network is cellular, which relies on a wired, fixed backbone to support the bulk of mes- sage routing. 2 In this case "pipes" refers to the wireless channel not physical fiber or cable. 3 Capacity = Bandwidth*log2(1+Signal-to-Noise Ratio), is the Hart- ley-Shannon Law, C.E. Shannon, Communications in the Presence ofNoise, " Proceedings of the IRE, pp. 10-21vol. 37, January 1949. 4 Throughput (capitalized) is to the total system throughput, whereas throughput (lower case) refers to node-to-node throughput. 1 of 7

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Page 1: [IEEE MILCOM 2005 - 2005 IEEE Military Communications Conference - Atlantic City, NJ, USA (17-20 Oct. 2005)] MILCOM 2005 - 2005 IEEE Military Communications Conference - Future Combat

FUTURE COMBAT SYSTEMS WIRELESS NETWORK ARCHITECTURE CONSIDERATIONS

Gary PennettThe Aerospace Corporation

Chantilly, VA

ABSTRACT

This paper investigates how the ad hoc wireless networkThroughput (T) in bits per second (bps) performance, as afunction ofcapacity (W) bps and the number ofnodes (n),is a dominate factor in determining the Future CombatSystems operational capability. The paper describes thelarge disparity between the theoretical Throughput ofw bps compared to the best experimentally achieved ofW bps and the inability ofknown approaches such asn

TDMA, CDMA, relays, and mobility to approach the theo-reticalperformance. The large disparity between thetheoreticalperformance and known approaches motivatesneed to answer the questions: What Throughput is re-quired?, What Throughput has been achieved?, and Whatwill therefore be the architecture?, early in the system de-sign process. The appendix uses some simplifying assump-tions to provide a more intuitive approach that approxi-mates the proofofthe theoretical limit of $1 bps, as the

mathematically rigorous approach referenced in this pa-per involves a lengthy and complex proof

The operational success of the Army's Future Combat Sys-tem (FCS) depends on the successful deployment and op-eration of a wireless network. This mobile, ad hoc', wire-less network will link together possibly thousands of nodesat the Brigade level and below whose data will be shared,correlated, and fused to create a picture of the tactical bat-tle space. Sharing data on this scale implies that largecommunications pipes are required. Yet as nodes areadded to large wireless networks, the communicationspipes2 become more restricted. Thus, there are tradeoffsbetween numbers of nodes, network capacity, mobility,and data sharing that have a direct impact on the architec-ture of the network. These trades are not fully understood,yet essential to creating a common tactical picture. Thepurposes of this paper are to explain the assertions made

andCynthia Dion-Schwarz

The Institute for Defense AnalysesAlexandria, VA

here and suggest the following questions about the net-work design that should be asked by those designers andmanagers of the FCS system development.

1. What Throughput is required?2. What Throughput has been achieved?3. What will therefore be the architecture?

Two terms are relevant to this discussion. Capacity (W),measured in bits per second (bps), is the measure of infor-mation flow between two directly connected nodes. It de-pends on the bandwidth available and the signal-to-noiseratio3 of the link, that is, it depends on the size of the pipesand the noisiness of the connection. The SINCGARs+ ra-dio has a data capacity of -16 Kbps, for example.Throughput4, again measured in bits per second, is themeasure of the average information flow between twonetworked nodes (see Figure 1). In addition to dependingon the capacity between any two nodes in the multi-hopchain, Throughput also depends on the number of nodes inthe network. Throughput will be the rate of informationtransfer in a busy network, in which many nodes are com-peting to use communications channels and could interferewith one another.

> Source A

Destination A

Destination BSource B

Figure 1: Conceptual Diagram of Wireless Ad-HocMobile Network

1 "Ad hoc" networks have no fixed gateways or infrastructure to sup-port connectivity. The other type of mobile network is cellular,which relies on a wired, fixed backbone to support the bulk of mes-sage routing.

2 In this case "pipes" refers to the wireless channel not physical fiberor cable.

3 Capacity = Bandwidth*log2(1+Signal-to-Noise Ratio), is the Hart-ley-Shannon Law, C.E. Shannon, Communications in the PresenceofNoise, " Proceedings of the IRE, pp. 10-21vol. 37, January 1949.

4 Throughput (capitalized) is to the total system throughput, whereasthroughput (lower case) refers to node-to-node throughput.

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In a busy network, where half the nodes transmit and halfreceive at any one time, the Throughput will be less thanthe point-to-point Capacity5. Upper bounds have been es-tablished for the Throughput of wireless ad hoc networksin several relevant geometries6. For a ground-based FCS-type wireless network, with optimal geometry, and with asingle point-to-point capacity hop ofW bps, the theoreticalupper bound on Throughput is proportional to w bps

-n

where n is the number of nodes. That is, on average, theThroughput that can be achieved decreases as the numberof nodes increases. Intuitively, this happens because as thenumber of nodes increases, a certain fraction of the nodesmust devote their time to relaying data from other nodes,thus reducing their own transmit/receive time. This prob-lem can be ameliorated somewhat by adding relays, but italso can be shown that the number of relays needed tomaintain a constant Throughput increases as n3/2 whichcan quickly grow untenable7. This formulation representsthe best that can be achieved by an ad hoc wireless net-work: other placements of nodes, noise, interference, andobstructions further reduce Throughput.

The Throughput that has been achieved experimentally ismuch more restricted than suggested by the theoreticallimit. For example, it has been shown that wireless net-works that adhere to the IEEE 802.11 (WiFi) standard pro-tocol can only achieve a Throughput proportional toW bps, which is significantly worse than the theoreticaln1.7limit8. This result is similar to Throughput results meas-ured by a researcher at the Army Engineering Researchand Development Center9. Other protocols, such as simplebroadcast with each node assigned time slots (Time Divi-

5 It is worth noting that wireless networks on a high-capacity wiredbackbone, as for example cell phone networks or wireless internetconnections, are not subject to the same constraints described here.This argument hinges on the limits of mobile, ad hoc networks thatdo not use high capacity fiber optics cable or T-3 lines.

6 P. Gupta and P.R. Kumar, The Capacity of Wireless Networkks,IEEE Transactions on Information Theory, vol. IT-46, no. 2, pp.388-404, March 2000.

7 This includes adding satellites or UAVs that act as communicationsrelays. In other words, for every k-fold increase in Throughput, oneneeds to add -(k2 -1)n relays with the same capacity.

8 The theoretical limit cannot yet be achieved primarily because rout-ing of messages (choosing the path by which a message hops to itsdestination) is inefficient and entails much overhead. Routing algo-rithms in dynamic wireless networks are the subject of vigorous re-search: we do not know when or if the technical problems will beovercome.

9 M.D. Ginsberg, Improved Analytical Tools for Predicting WirelessNetwork Performance, Proceedings of the 2002 Army Science Con-ference.

sion Multiplex, or, TDMA 10), can achieve W bps, bettern

than the IEEE standard, yet still far from the theoreticallimit. We shall see that this has significant implications forthe FCS wireless network.

The Throughput limitations suggest that networks with thefewest possible numbers of nodes will best support highdata rates close to the point-to-point Capacity. It is best tolimit the number of hops. Such a topology can beachieved, if, for example, the networks have to communi-cate via high-capacity links with other networks. But lim-ited numbers of hops implies limited or delayed sharing ofinformation, which would have a large impact on the con-struction of a common picture. For instance, some re-searchers" have suggested that by taking advantage ofmobility, Throughput can be increased primarily becausethe number of hops from source to destination can be lim-ited to two. However, we have estimated that the messagelatency cost would be high, and that messages would bedelayed on average for hours, while nodes carrying mes-sages wait for an opportunity to transmit.

A key waveform that will be used by FCS is the WidebandNetworking Waveform (WNW). Its maximum capacity is-5 Mbps. If 100 nodes are networked together using theWNW, and if the architecture designers can achieve thebest possible Throughput, then the Throughput will be-500 kbps12. For 1,000 nodes, more typical of a Unit ofAction, the Throughput will be -160 kbps. Although thelatter Throughput is about a factor 10 beyond what theSINCGARs+ radio can achieve point-to-point, it is never-theless much less than the point-to-point capacity forWNW.

However, if the protocols and architecture are more likethe IEEE 802.11 standard, then the Throughput for 1,000nodes will be a mere 0.1 kbps. That is clearly not desir-able, and would compel the designers to limit the networkto 100 nodes or fewer. Such a limitation will have a severe

10 In the simplest case Time Division Multiple Access assigns eachnode a distinct time slot to transmit. While one node is transmittingon its allotted time slot all of the other nodes are in receive mode.The performance stated in the paper assumes that all of the nodesare in radio contact with all of the other nodes, that every node is inperfect time synchronization with all of the other nodes and thatthere is no propagation delay between nodes. Since this is never thecase the actual performance of a TDMA network will always beless than W/n.M. Grossglauser and D. Tse, Mobility Increases the Capacity ofAd-hoc Wireless Networks, in Proc. INFOCOM, Anchorage, April2001.

12 The proportionality constant is of order 1, and depends on assump-tions about the geometry of the network. [private communicationfrom Kumar]

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impact on the architecture. A key question is therefore:"What Throughput is required, what Throughput has beenachieved, and what will therefore be the architecture?"

point-to-point Throughput, n Throughput ThroughputCapacity W = number of for n=100 for n=1000

Waveform (kbps) Protocol Nodes (kbps) (kbps) (kbps)

WNW 5,000 Theoretical w 500 158Time sharing W

5,000 (TDMA) n 50 5

Experimental 2.6 xW5,000 IEEE 802.11 n .7 5 0.103

EPLRS 100 Theoretical 10 3Time sharing W

100 (TDMA) n 1 01Experimental 2.6W

100 IEEE802E11 n1.7E 0103 0.0021

Table 1: Estimated Throughput for maximum channelcapacity of Wideband Networking Waveform and for

comparison, ELPRS. 13

If the WNW is not available, then the next largest capacitywaveform is EPLRS, with -100 kbps. Using this wave-form to network 1,000 nodes will reduce its available theo-retical Throughput to about 3 kbps (and the experimental,about 2 bits per second), which as we shall see, will noteasily sustain the data fusion required to achieve the com-mon tactical picture. Thus, a second key question concern-ing FCS is: "When the WNW will be available?"

Throughput requirements are largely determined by theamount of information necessary to maintain combat ef-fectiveness. A major component of combat effectiveness isthe creation and update of a common tactical picture. Tocreate the common tactical picture, FCS will use data cor-relation and fusion algorithms. Data from numeroussources will be shared and fused. However, shared data-bases are not necessarily good databases because interpre-tations of data can differ significantly. Here, the essentialproblem is simple: if two sensors see similar targets, arethey the same or different targets? Data fusion is intendedto answer this question more precisely than any one sensorcan alone. Yet to answer this question without confusionor mistakes requires the sharing of not only raw measure-ments, but also of the estimated uncertainties in the meas-urements. This implies that for every reported measure-ment of, say, position, at least 4 and possibly 8 additional14numbers must also be reported"5. Thus, correct data corre-

lation and fusion implies the sharing of many pieces ofdata.

In addition to sensor measurements, it is likely that video,or at least, video chips and pictures, will be transportedaround the wireless network. Even with data compression,it is well known that transporting such data consumes sig-nificant Throughput.

The Congressional Budget Office has estimated that thedemand for voice Throughput for the Unit of Action isfrom 1 to 3 kbps16, not including video and data rateswhich would increase throughput requirements. TheseThroughput rates cannot easily be sustained unless WNWis available. In addition, if the number of sensors and datasources increase, or the mobile, ad hoc wireless networkprotocol is designed poorly (so that, for instance, WNWcannot be used with its peak data rates), then there is a realdanger that the FCS network will never sustain the datarates needed to conduct data fusion.

Figure 2 shows a graph of the estimated Throughputachieved using WNW versus number of nodes in the net-work assuming three different protocols. It is important tonote that the best theoretical protocol has not been demon-strated experimentally. Also shown are several differentestimated Throughput requirements for voice, data,streaming video, compressed video, and a mixture of dif-ferent data sources. In a busy network, in which half thenodes are transmitting while half are receiving, the IEEE802.11 standard can support voice for up to about 200nodes. No protocol can support uncompressed simultane-ous streaming video, even for as few as five nodes. A realnetwork would support a mixture of voice, data, and com-pressed video. Also shown on the graph is the estimatedaverage Throughput required for a mixture of 30% voice,50% data, and 20% compressed video. This estimatewould change as the proportions change. In this examplemixture, TDMA would support up to about 300 nodes.

13 The channel capacity of both WNW and EPLRS will decrease asnecessary to maintain connectivity. Note the large variations in theestimates depending on assumptions about the protocols employed.

14 The measurement uncertainties are: uncertainty in x,y,z and time;and biases in x,y,z and time. The biases need to be reported sepa-rately because they are mathematically treated differently in datafusion.

15 The consequences of not reporting the uncertainties to support datafusion are well known. One of us has reported on the value of data

fusion elsewhere (Dion-Schwarz, et al., The Need for Sensor Fu-sion, Proceedings of the Fire Control Symposium, 2001)

16 The Army's Bandwidth Bottleneck, Congressional Budget Office,August, 2003.

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10000

1000

m 100

O-m

2 10

1

0.1

\ d

__ _Mixed Voice, Data & Compressed Video

10 100

Number of Nodes

-|-Theoretical -+TDMA IEEE 8

Figure 2: Estimated Throughput for a rnodes have a 5 Mbps Capacity. Variousshown as well as the estimated requirem(

data sources.

Tests of achieved Throughput can easilwith small numbers ( 20) nodes'7. Suchwork should show how Throughput depeof nodes so that estimates can be made Iwork. This, in turn, can be compared aThroughput requirements. Should the estiput be significantly less than the requirension-makers can in turn prioritize the impent kinds of data and the data fusionThroughput measurements can be used totecture designs, and in particular, assistsize and topology of the networks. Clearworks favor smaller numbers of nodes; bulnetworks rather than large, horizontally inhas many different implications for FCS.tantly, it is critical to FCS that WNW beated and deployed. Throughput measuremlight the importance.

REFERENCES

[1] P. Gupta and P. R. Kumar, "The Cap(Networks", based on work supported bysearch Office Contract No. DAAH 04-95-

[3] P. Gupta, R Gray, and P. R. Kumar, "An ExperimentalScaling Law for Ad hoc Networks", based on work sup-ported by U.S. Army Research Office Contract No.DAAD 19-00-1-0466.

[4] C.E. Shannon, "Communications in the Presence ofNoise," Proceedings of the IRE, pp. 10-21vol. 37, January1949.

[5] M.D. Ginsberg, "Improved Analytical Tools for Pre-1000 10000 dicting Wireless Network Performance", Proceedings of

the 2002 Army Science Conference.

>02.11 [6] M. Grossglauser and D. Tse, "Mobility Increases theietwork whose Capacity of Ad-hoc Wireless Networks", in Proc.protocols are INFOCOM, Anchorage, April 2001.

ents for various[7] "The Army's Bandwidth Bottleneck", CongressionalBudget Office, August, 2003.

ly be conductedtests of the net- Appendixnds on numbersfor the FCS net- Ad Hoc Wireless Network Throughput Simplified[gainst estimated This Appendix provides some additional detail and in-imated Through- sights to topics discussed in the paper. Papers by Gupta,nents, then deci- Kumar'8 and others have been written in an attempt toortance of differ- characterize the performance of wireless networks. Thisdemands. These appendix describes some of the basic concepts and sum-drive the archi- marizes the results. As in the paper, this Appendix focuses

in designing the on the relationship between channel capacity and the num-rly, wireless net- ber of wireless nodes19 in the network.t the use of smallitegrated systems Fundamentally, capacity is considered a function of theBut most impor- basic point-to-point link design and is subject to the con-successfully cre- straints of the channel such as noise, interference, connec-ents would high- tivity (terrain geometry), security, transmit frequency,

power and waveforms. In most military communicationenvironments the basic system capacity varies as a func-tion of the constraints, and channel capacity often con-

acity of Wireless stricts to maintain connectivity in stressful environments.U. S. Army Re- Throughput is computed using the instantaneous channel1-0090. capacity at the time of interest, and represents the average

[2] P. Gupta and P. R. Kumar, "Internets in the Sky: TheCapacity of Three Dimensional Wireless networks", basedon work supported by U.S. Army Research Office Con-tract No. W08333-04.

17 See, for example, Gupta, Gray and Kumar An Experimental ScalingLaw for Ad Hoc Networks, University of Illinois at Ur-bana/Champagne, May 2001 and Neely and Modiano, Capacity andDelay Tradeoffsfor Ad-Hoc Mobile Networks, Proceedings of IEEEInfocom 2004 (submitted).

18 P. Gupta and P.R. Kumar, The Capacity of Wireless Networks,IEEE Transactions on Information Theory, vol. IT-46, no. 2, pp.388-404, March 2000. and P. Gupta and P. R. Kumar, Internets inthe Sky: The Capacity of Three Dimensional Wireless networks,based on work supported by U.S. Army Research Office ContractNo. W08333-04.

19 For the purpose of the paper a node is defined as an informationsource, destination, or relay waypoints. Many of the FCS vehicleswill contain several nodes connecting to off board sensors, soldierradios, relays, and weapons control links. The average FCS vehiclewill have 8 nodes with the command and control vehicles support-ing about 12 nodes.

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- --- - -fEce.-I

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number of bits per second that can be sent in a busy net-work in which channels are shared among many nodes. Ina wireless, ad hoc network, the nodes cooperate with oneanother to send messages. If the protocol is TDMA, thenodes take turns sending messages. The Throughput isthus the node capacity divided by the number of nodes.However, if the nodes cooperate by acting as relays as wellas sources, the Throughput is theoretically larger. Thispoint is explained below.

Point to point capacity

-n

=Wbps .n

Figure A.1: 10 by 10 node arrangement

Consider a wireless network with n nodes. Each node canfunction as an information source (S), destination (D) orrouting waypoint (WP). Throughput is the measure of theaverage amount of information that moves from the sourceto the destination (S-D) over a period of time. The capacityof each point-to-point link is W bits-per-second.For the purpose of this discussion20, the n nodes are ar-ranged in a a/n by a/n square. Figure A.1 is a 10 by 10square used as an example for n = 100. Each node can ei-ther transmit or receive but cannot do both at the sametime on the same channel. The maximum Throughput fromone node to another is then limited to W or half the chan-

2nel capacity. We next assume that there is a way to opti-mally arrange the nodes such that half of the nodes cantransmit and half can receive simultaneously2l. In a realmobile network where the nodes are constantly rearranged,it will be difficult to achieve the optimum geometry, whichargues that this solution is a best case or upper bound22. In

20 This is not a mathematically rigorous proof. Within the bounds ofthe assumptions the results are comparable to the more detailedproof provided by Gupta and Kumar.

21 This is a rather optimistic assumption because it assumes that eachtransmitter has just enough power to reach the nearest node and nomore. In most cases the transmitting node will ether have morepower than is necessary and will reach not only the nearest node butalso several others or it will have insufficient power to reach anynode. The net result is that the transmitters will interfere with eachother reducing the channel capacity or the network will have to re-configure as nodes go out of contact.

22 A Paper by Grossglauser, M. and Tse, D, "Mobility Increases theCapacity of Ad-hoc Wireless Networks" shows that for mobile us-

the optimum configuration, each message has to transit- S" hops from source to destination on average. As in the

2

example if n = 100, a;n 10 so each message has to tran-sit 5 hops from source to destination. The resultingthroughput is the channel capacity divided by the averagenumber of hops per message: w . The term 1 can be

considered the capacity dilution factor based on the num-ber of nodes in a network and can be expressed as a per-cent of capacity.

Instantaneous Throughput for a single node can be higherby simply requiring that all nodes but the source maintainradio silence. In that case, the Throughput achieved by asource is equal to the channel capacity: all other nodes aresimply potential relays that pass along the message. How-ever, in a busy network environment, such as would befound during a battle, all nodes may need to transmit dataat once. To avoid collisions and interference, the networkprotocols algorithmically determine the order in whichsources can broadcast, the power (which determines thereach), and the routing path. The channel capacity pernode is thus diluted according to the factor shown.

Table A.1 below gives capacity dilution factors for somepossible configurations of a FCS network architecture as-suming a range of nodes at each echelon operating at thetheoretical maximum Throughput W bps.

Throughput for 5MbpsEchelon Nominal number of nodes (n) Capacity Dilution (%) Channel (kbps)Company 25 to 100 20.0% to 10.0% 1,000 to 500Battalion 300 to 500 5.8% to 4.5% 289 to 224Brigade 1,000 to 2,000 3.2% to 2.2% 158 to 112UE 5,000 to 10,000 1.4% to 1.0% 71 to 50

Table A.1: Theoretical Maximum Wireless NetworkThroughput (kbps)

A more recent experiment23 using 20 nodes networked us-ing Lucent Technologies IEEE 802.11 compliant hardwareexhibited a capacity to throughput dilution factor of 2.6

n1.68

Table A.2 is a recomputation of Table A.1 using the ex-perimental Throughput factor.

ers roaming a fixed area in a motion described by a uniformly dis-tributed stationary and ergodic random process that Throughput isnot constrained by the number of nodes. It is highly unlikely thatthe FCS UA will satisfy this random process or hold messages overthe period of time, days, which are not of interest in a fast pacedbattle environment.

23 P. Gupta, R Gray, and P. R. Kumar, "An Experimental Scaling Lawfor Ad hoc Networks", based on work supported by U.S. Army Re-search Office Contract No. DAAD 19-00-1-0466.

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Throughput for 5MbpsEchelon Nominal number of nodes (n) Capacity Dilution (%) Channel (kbps)Company 25 to 100 1.165% to 0.11% 58.26 to 5.67Battalion 300 to 500 0.018% to 0.01% 0.90 to 0.38Brigade 1,000 to 2,000 0.002% to 0.00% 0.12 to 0.04UE 5,000 to 10,000 0.000% to 0.00% 0.01 to 0.00

Table A.2: Experimental Wireless Network Through-put (kbps)

A simplistic approach to improve on the IEEE 802.11 ex-perimental protocol is to assign a unique time slot to eachof the nodes. Alternatively each node could be assignedone of n sub channels of the W bps capacity channel. Ineither case, a Throughput of 1 can be achieved. Table A.3

n

computes Throughput using the Time Division MultipleAccess (TDMA) factor of 1.

n

Throughput for 5MbpsEchelon Nominal number of nodes (n) Capacity Dilution (%) Channel (kbps)Company 25 to 100 4.0% to 1.0% 200 to 50Battalion 300 to 500 0.3% to 0.2% 17 to 10Brigade 1,000 to 2,000 0.1% to 0.1% 5 to 3UE 5,000 to 10,000 0.0% to 0.0% 1 to 1

Table A.3: TDMA Wireless Network Throughput(kbps)

Figure A.2 is a graph of the three Throughput factors for 5to 5,000 nodes (this is a repeat of Figure 2 in the paper).Lines have been plotted showing nominal data rates forsome data sources. In a busy network, in which half thenodes are transmitting while half are receiving, the IEEE802.11 standard can support voice for up to about 200nodes. No protocol can support uncompressed simultane-ous streaming video, even for as few as five nodes. A realnetwork would support a mixture of voice, data and com-pressed video. Also shown on the graph is the estimatedaverage Throughput required for a mixture of 30% voice,50% data and 20% compressed video. This estimate wouldchange as the proportions change. The point on the graphwhere the Throughput crosses the source data is the maxi-mum number of nodes that can be accommodated on thenetwork without degradation or loss of data.

10000

1000

m 100

2a

n

° 10

Copressd Vdeo

IMixed Voc,Dt &Cmrse Vdeo

..... ... ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~.....

10 100 1000 10000

0.1 I

Number of Nodes

-|-Theoretical {}TDMA = IEEE 802.11

Figure A.2: Throughputs for 5 Mbps Capacity Channel

As also discussed in the paper, results of the tables andgraphs suggest that smaller networks suffer less capacitydilution regardless of protocol. This could lead designersto construct smaller networks connected through gateways.Throughput to higher-level networks through gatewaysassumes that the backbone connecting subnet is of suffi-ciently large capacity to accommodate each subnet withoutdegradation. As an alternative, the data transfer require-ment between subnets could be reduced through data fu-sion or other mechanism.

Various approaches to the architecture of subnets and thedesire to minimize data passed to higher order nets raisesthe issue of where data fusion should occur. The optimumlocation for fusion of actionable intelligences and otherinformation remains open. In a system like FCS it is likelythat fusion will be required at several levels in the com-mand structure. The higher the point of data fusion in thecommand structure the larger the number of nodes in-cluded in a network. Figure A.3 shows notional fusionboundaries consistent with network configurations in thetables.

Figure A.3: Notional Data Fusion Boundaries

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Throughput constrains can also be ameliorated by assign-ing priority to data and then monitoring the network tolimit or delay the transmission of lower priority data. Forexample, cell phones that allow access to the Internet usetext only and special data formats to limit the amount ofdata stored and displayed. Similar constraints can be usedon the FCS network. The Throughput of the network hasto be known before decisions on data formats and protocolcan be made.

The dependence of Throughput on the number of nodes inthe network and the uncertainty of the exponential scalingfactor ranging from .5 in theory to 1.68 in experimentationsuggests that substantial uncertainty remains. Trades be-tween network architecture, data fusion, data prioritization,and sensor links will have to be considered simultaneouslyto determine the optimum design for the FCS network.

Acknowledgements

The authors are grateful to P.R. Kumar for his many help-ful and insightful conversations.

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