Practical TCP/IP and Ethernet Networking- P56

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Practical TCP/IP and Ethernet Networking- P56: The transmitter encodes the information into a suitable form to be transmitted over the communications channel. The communications channel moves this signal as electromagnetic energy from the source to one or more destination receivers. The channel may convert this energy from one form to another, such as electrical to optical signals, whilst maintaining the integrity of the information so the recipient can understand the message sent by the transmitter....

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Satellites and TCP/IP 257 Figure 18.1 Satellite classifications (courtesy of Byte Publications ref. (1)) The important issues with the satellite system are the distance in which it orbits the earth and the radio frequency it uses. This impacts on the delay in data transfer (the latency) and the power of the signal and the data transfer rate. The various satellite bands can be listed as per the tables below. These tables have been sourced from reference (1).
258 Practical TCP/IP and Ethernet Networking Table 18.1 Frequency allocation Table 18.2 Satellite classification 18.3 Advantages of satellite networks Most TCP/IP traffic occurs over terrestrial networks (such as land lines, cable, telephone, fiber) with bandwidths ranging from 9600 bps to OC-12 with 622 Mbps. There are a number of opportunities for using satellite networks as a useful supplement to these
Satellites and TCP/IP 259 terrestrial services. It is unlikely that the satellite will ever replace landline-based systems. But they will form a useful supplement to landline-based systems. According to Satellite Communications in the Global Internet – Issues, Pitfalls and Potential, (see References at the end of this chapter) using satellites for the Internet (and by default the TCP/IP protocol) has the following advantages: • The high bandwidth capability means that large amounts of data can be transferred. A Ka-band (20–30 GHz) satellite can deliver many gigabits/second • Inexpensive means of transmission. There are no land line laying costs and the satellite can cover a huge area. Indeed, in remote areas the high costs may preclude using any land line with significant bandwidth • Portability in communications in the satellite’s range. The ability to move around may have great use for mobile applications • Simplicity in network topology. The satellite has a very simple star type network structure. This is far easier to handle (especially for network management programs) than the complex interconnected mesh topology of land line-based Internet systems • Broadcast and multicast. The satellite due to its star connection structure is easy to use in a broadcast capability. The typical mesh interconnection of land line-based systems is far more difficult to implement in a broadcast system 18.4 Applications of satellite systems According to Satellite Communications in the Global Internet – Issues, Pitfalls, and Potential (see References at the end of this chapter), the following typical applications have the following features when running on a satellite system. 18.4.1 Remote control and login These applications would be very sensitive to any delays in communications. If the delay extended beyond 30 to 40 ms the user would notice and not be very happy. Interestingly enough, one advantage that the satellite system does have over land-based systems is the fact that although the delays can be significant they are predictable and constant. This can be compared to the terrestrial Internet where response times can vary dramatically from one moment to the next. 18.4.2 Video conferencing Assuming that the video conferencing application can tolerate a certain amount of loss (i.e. caused by transmission errors), the UDP protocol (see later) can be used. This has far less overhead than the TCP protocol as it does not require any handshaking to transfer data and is more compact. Hence satellites would provide an improvement over normal terrestrial communications with a better quality picture due to a greater bandwidth and a simpler topology. Another benefit that satellites would provide for video transmission is the ability to provide isochronous transmission of frames (i.e. a fixed time relationship between frames and thus no jerky pictures).
260 Practical TCP/IP and Ethernet Networking 18.4.3 Electronic mail This does not require instantaneous responses from the recipient and hence satellite communications would be well suited to this form of communications. 18.4.4 Information retrieval The transmission of computer files requires a considerable level of integrity (i.e. no errors) and hence a reliable guaranteed protocol such as TCP has to be used on top of the IP protocol. This means that if a particularly fast response is required and a number of small transfers are used to communicate the data; satellite communications will not be a very effective means of operation. 18.4.5 Bulk information broadcasting Bulk data (such as from stock market databases/medical data/web casting/TV programs) can effectively be distributed by satellite with a vast improvement over the typically mesh-like land line based systems. 18.4.6 Interactive gaming Computer games played on an interactive basis often require instantaneous reaction times. The inherent latency in a satellite system would mean that this is not particularly effective. Games that require some thought before a response is transmitted (e.g. chess and card games) would however be effective using satellite transmission. The following diagrams summarize the discussion above. (a) By application demands Figure 18.2 Summary of different satellite applications (courtesy of Satellite Communications in the Global Internet – Issues, Pitfalls, and Potential, see References at the end of this chapter)
Satellites and TCP/IP 261 18.5 Review of TCP/IP As discussed earlier the TCP/IP protocol suite is the one on which the Internet is based. When it was developed very little consideration was taken of its performance over very high-speed fiber optic lines or very high latency satellite links. Some initiatives have been taken (see later) to address these shortcomings. A quick revision of the main structure of the TCP/IP protocol is found below. The way in which the different protocols fit together is shown in the figure below. Figure 18.3 Typical structure of Ethernet, TCP/IP and application header 18.5.1 Internet protocol (or IP protocol) The focus of the IP protocol is on routing data through multiple interconnected networks. The actual IP protocol (containing the data it is routing) can be transported on a number of different mechanisms such as Ethernet, token ring, Arcnet, ATM and frame relay. As each packet arrives at a router (also popularly referred to as a gateway) it is examined for the appropriate destination address and then sent onto the appropriate network. It is important to realize that the actual transport of the IP protocol is done with Ethernet using the Ethernet 48-bit hardware addressing. Ethernet (or any of the other mechanisms listed above) do not see the IP addressing. The IP address provides a hierarchical and unique way of identifying each of the different interconnected networks world-wide. The IP address comprises a host ID (station or PC address) and a net ID (or network address). A typical protocol stack is shown in the figure below. Figure 18.4 Typical protocol stack for TCP/IP (courtesy of Stallings 1997)