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message security via satellite is discussed in Chap. 1 and earlier in this chapter.Thus, we examine other TCP=IP challenges in this section.Latency. Latency is a critical parameter of communication servicequality, particularly for interactive communications and for many standarddata protocols. TCP=IP performs poorly over high-latency or noisy channels.Bit error rates (BER) of 10 7 can be acceptable in telephone environments,but this level of performance will render TCP=IP almost unusable. Geostationarysatellite links are inherently high-latency and can be noisy. Forexample, we note in Chap. 1, Sec. 1.3, that if circuits are positioned ingeosynchronous orbits, they suffer a transmission delay of about 119 msecbetween an earth terminal and the satellite, resulting in a user-to-user delay of238 msec and an echo delay of 476 msec. Latency in voice communicationsbecomes noticeable with a round-trip delay over 100 msec. To overcome theslow-start this delay causes, an enhanced communications protocol over thesatellite link must be implemented which terminates and re-originates TCPconnections at both ends of the satellite link without sacrificing the discoveryprocess and still maintaining end-to-end standards compliance [16].Optimization. Window size and scheduling capacity are optimizationissues, which affect data throughput and network efficiency. Window sizingworks simply by allowing a transmitter to send a number of packets beforewaiting for an acknowledgment from the receiver. Typically, the window sizeis short in comparison to the 476 msec delay of a satellite link; however, itminimizes the amount of data that would need to be retransmitted whenpackets are dropped. This operation dramatically slows down a satelliteconnection.Scheduling capacity as well as the ‘‘burstiness’’ of subscriber traffic overthe satellite link is another area that can potentially reduce data throughput ornetwork efficiency. There are two common ways of scheduling capacity:connection-oriented and connectionless, which reflect two different philosophiesof link management. Traffic burstiness determines which schedulingmethod is appropriate for data transmission.The connection-oriented method assigns a fixed level of capacity prior totransmission and reassigns capacity at the completion of a transmission. Whatis remarkable about this method is that at least one round-trip delay is requiredto both allocate and de-allocate capacity. For long connections such as voicephone calls, the overhead to set up and tear down links is very small. For shortconnections, such as a URL request over a network, the setup and teardowntime can overwhelm any efficiency the existing protocol achieves. Thetraditional switched telephone network’s traffic is relatively smooth and isCopyright © 2002 by Marcel Dekker, Inc. All Rights Reserved.