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Physical architecture incorporating enhancer technology. (Courtesy of West and McCann, 2000.)
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Figure 15.14 shows the system developed at Roke Manor. The enhancers perform the same function as the IWUs in Fig. 15.5 in that they terminate the Internet connections and do not require any modifications to the TCP/IP. A propriety protocol is used over the satellite link. Figure 15.15 illustrates a situation where both setup and teardown are spoofed. In this illustration, host B refuses the connection, but host A receives the RESET signal too late. The spoofed FIN ACK tells host A that the data transfer was successful. Figure 15.16 shows a more appropriate strategy. In this case, the FIN sent by host A is not spoofed. Since host B has refused the connection, host A receives no FIN, ACK back from host B. Therefore, host A can infer from this that there was an error. While this is not as good as a regular TCP/IP connection, which would have reported a failure to connect on the first SYN, the system does adjust to the error and removes the spoofing on the setup on a second try.
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<DATA> FIN
RESET ACK FIN ACK RESET from Host B is too late
Connection refused by host
Connection establishment and closing. (Courtesy of West and McCann, 2000.)
Satellites in Networks
Host A SYN SYN ACK
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Host B Actual (end-to-end) message SYN Spoofed message
<DATA> FIN
RESET ACK Connection refused by host FIN RESET from Host B
(ignored)
Improved connection close. (Courtesy of West and McCann, 2000.)
15.13 Asymmetric Channels The term asymmetry applies in two senses to an Internet connection. It can refer to the data flow, which is often asymmetric in nature. A short request being sent for a Web page and the returned Web page may be a much larger document. Also, the acknowledgment packets sent on the return or reverse link are generally shorter than the TCP segments sent on the forward link. Values of 1500 bytes for data segments on the forward link and 40 bytes for ACKs on the reverse link are given in RFC-2760. Asymmetry is also used to describe the physical capacities of the links. For small earth stations (e.g., VSATs), transmit power and antenna size (in effect, the EIRP) limit the uplink data rate, which therefore may be much less than the downlink data rate. Such asymmetry can result in ACK congestion. Again, using some values given in RFC-2760, for a 1.5 Mb/s data link a reverse link of less than 20 kb/s can result in ACK congestion. The levels of asymmetry that lead to ACK congestion are readily encountered in VSAT networks that share the uplink through multiple access. In some situations, the reverse link may be completed through a terrestrial circuit, as shown in Fig. 15.17 (Ghani and Dixit, 1999). Here, the TCP source is connected to the satellite uplink through an IWU as before. The downlink signal feeds the small residential receiver, which is a receive-only earth station. An IWU on the receive side converts the data to the TCP format and sends them on to the destination. The ACK packets from the TCP destination are returned to the TCP source through a terrestrial network. As pointed out in RFC-2760, the reverse link capacity is limited not only by its bandwidth but also by queue lengths at routers, which again can result in ACK congestion. Some of the proposed
526 Satellite network Intersatellite links Data packets Forward uplink/downlink channels (Mb/s) Large (corporate) transmitter Terrestrial network Small residential receiver TCP destination IWU IWU ACK packets Reverse channels (kb/s)
TCP source
Asymmetric reverse ACK channel configuration. (Courtesy of Ghani and Dixit, 1999. Copyright, 1999 IEEE.)
Satellites in Networks
methods of handling asymmetry problems and ongoing research are described in Ghani and Dixit (1999). 15.14 Proposed Systems Most of the currently employed satellites operate in what is called a bent pipe mode, that is, they relay the data from one host to another without any onboard processing. Also, many of the problems with using geostationary satellites for Internet traffic arise because of the long propagation delay and the resulting high delay-bandwidth product. In some of the newer satellite systems, use is made of LEO and MEO satellites to cut down on the propagation delay time. Also, onboard signal processing is used in many instances that may result in the TCP/IP protocol being exchanged for propriety protocols over the satellite links. The satellite part of the network may carry the IP over ATM. The Ka band, which covers from 27 to 40 GHz, is used in many (but not all) of these newer systems. Wider bandwidths are available for carriers in the Ka band compared with those in the Ku band. A survey of some of these broadband systems will be found in Farserotu and Prasad (2000), but it should be pointed out that company changes in the form of mergers and takeovers occur that can drastically alter the services a company may offer. With this in mind, details of existing and proposed satellite internet services can be found using a search engine, such as Google. Some of the company names to search for are: Astrolink; Loral Cyberstar; Skybridge; Spaceway; iSky; and Teledesic. 15.15 Problems and Exercises
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