MARITIME TACTICAL WIDE AREA NETWORKING (MTWAN ...

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Uncontrolled copy when printedUNCLASSIFIEDACP 200(B) Vol 2CHAPTER 2QUALITY OF SERVICEINTRODUCTION201. The "best-effort" nature of IP-based networking can result in significant performancedegradation when a network becomes congested. This is more likely to occur in bandwidthconstrained and high latency environments such as Wide Area Networks (WANs) than withinLocal Area Networks (LANs).202. To minimise the operational impact of congestion within WANs, it is highly desirableto implement Quality of Service (QoS) to optimise the traffic flow across WAN links and toensure that more important traffic is afforded an improved grade of service. This becomesessential when data, voice and video are converged onto a single network.AIM203. This chapter presents a framework for providing QoS in a wireless mobile tacticalnetwork.OVERVIEW204. IP only provides Best Effort Service in that traffic is processed as quickly as possible,but there is no guarantee as to timeliness or actual delivery. However, the fundamentalconcept behind QoS is that better service to certain traffic flows can be provided by eitherraising the priority of a flow or limiting the priority of another flow.205. This chapter concentrates on improving the performance of IP packet flow throughthe network(s). The challenge from a technical perspective is how to make the right trade-offbetween simplicity and control to ensure a predictable and manageable network.DEFINITION206. Quality of Service (QoS) is an encompassing term describing the collection ofactivities, management functions and strategies that aim at guaranteeing the end-to-end,predictable and consistent behaviour of network-dependent applications. This definition ofQoS highlights:a. Predictability and Consistency – A service response is predictable when theconditions for its delivery are known over a period of time. Consistency is thedifference between the nature of an expected response and the actual one;b. Guarantee – There needs to be some level of assurance in terms of predictabilityand consistency;2-1 OriginalUNCLASSIFIED
Uncontrolled copy when printedUNCLASSIFIEDACP 200(B) Vol 2c. Management – A management function is necessary in achieving predictabilityand consistency and includes such mechanisms as negotiation, admissioncontrol and monitoring; andd. End-to-end – QoS is really only achieved in a distributed application if it hasbeen addressed at every level of the OSI 7-layer model.LINK THROUGHPUT / BOTTLENECK207. The available bandwidth interconnecting shore-based IP routers can be very large(e.g. fiber). In an <strong>MTWAN</strong>, the available bandwidth is very limited, but the processing powerand storage capabilities – both increasing rapidly due to hardware advances – are relativelyunconstrained considering the router throughputs and network sizes that must be supported.Subsequently, the principal bottlenecks for any <strong>MTWAN</strong> is the throughput of its WAN links.Unmanaged congestion at speed-conversion bottlenecks on WAN-access links leads tounpredictable delays.208. The primary causes of slow throughput on WANs are well known: high delay (i.e.high latency), limited bandwidth, and ―chatty‖ application protocols. This chapter addresseshow to improve the performance of IP packets through WAN links. In doing so, a secondaryresult will likely be performance improvements within LANs.209. The temptation to define communication subnets using simplistic measures such asraw bandwidth alone should be avoided. Instead it is necessary to look for measures thatdirectly relate to the ability of the facilities to support the higher-level services, measures thatspecify QoS parameters such as bandwidth, delay, and loss characteristics.MAXIMIZING LINK THROUGHPUT210. If link throughput is sub-optimal, simply adding bandwidth or compression will notnecessarily solve the problem of application performance on WANs. One reason is becausecritical applications that suffer poor performance are not necessarily the applications that getaccess to extra capacity. Paradoxically, it usually is the less-urgent, bandwidth hungryapplications that monopolize increased bandwidth.LATENCY211. First, the ‗round-trip time‘ of a packet of data (i.e. the network latency), has a directeffect on the performance or throughput of a window-based protocol like TCP or any requestresponseprotocol. High round trip times slow down "chatty" applications, even if the actualamounts of data transmitted in each transaction are not large.212. Adding bandwidth (or compressing data) does not improve throughput when theround-trip time exceeds the critical point where throughput is bounded, or when the problem isprimarily latency and not bandwidth related. Once the latency exceeds the critical point,throughput decays quickly.2-2 OriginalUNCLASSIFIED
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