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(a) Internet groups; (b) World Wide Web groups.
The World Wide Web (WWW) is probably the most widely used application on the Internet. The evolution and growth of the WWW has been rather similar to that of the Internet itself, with no central authority but still with a structure that attempts to regulate what happens. The WWW Consortium, referred to as W3C, was founded in October 1994 (Jacobs, 2000). W3C oversees a number of special interest groups, as shown in Fig. 15.9b, and coordinates its efforts with the IETF and with other standards bodies. Details of the W3C will be found in Jacobs (2000).
Satellites in Networks
15.7 Internet Layers The uplink and downlink between satellite and earth stations forms the physical layer in a data communication system. By data communications is meant communications between computers and peripheral equipment. The signals are digital, and although digital signals are covered in Chap. 10, the satellite links must be able to accommodate the special requirements imposed by networks. The terminology used in networks is highly specialized, and some of these terms are explained here to provide the background needed to understand the satellite aspects (see also Sec. 15.3). The Internet, of course, is a data communication system (although there is presently a move to incorporate voice communications along with data in what is known as voice over Internet Protocol, or VoIP). The data are transmitted in packets. Many separate functions have to be performed in packet transmission, such as packet addressing, routing, and coping with packet congestion. The modern approach is to assign each function to a layer in what is termed the network architecture. This has already been encountered in connection with ATM, as shown in Fig. 15.1. The layers are conceptual in the sense that they may consist of software or some combination of software and hardware. In the case of the Internet, the network architecture is referred to the TCP/IP model, although there are protocols other than TCP/IP contained in the model. The layered structure is shown in Fig. 15.10. A brief description of these layers is included to familiarize the reader with some of the terms used in network communications, although the TCP layer is of most interest in this chapter.
Physical layer. This covers such items as the physical connectors, signal format, modulation, and the uplink and downlink in a satellite communications system. Data-link layer. The function of this layer is to organize the digital data into blocks as required by the physical layer. For example, if the physical layer uses ATM technology, as described in Sec. 15.3, the
Applications & Services TCP IP Data Link Physical
Figure 15.10 Layered structure for TCP/IP. (Courtesy of Feit, 1997.)
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data are organized into cells. Digital transmission by satellite frequently uses TDMA, as described in Chap. 14, and satellite systems are being developed which transmit Internet data over ATM. Thus the data-link layer has to organize the data into a suitable format to suit the physical-layer technology. In the terrestrial Internet, the data link converts the data into frames. The data-link layer and the physical layer are closely interrelated, and it can be difficult sometimes to identify the interface between these two layers (Mackenzie, 1998).
Network layer. This is strictly an IP layer. The packets are passed along the Internet from router to router and to the host stations. No exact path is laid out beforehand, and the IP layers in the routers must provide the destination address for the next leg of the journey so to speak. This destination address is part of the IP header attached to the packet. The source address is also included as part of the IP header. The problems of lost packets or packets arriving out of sequence are not a concern of the IP layer, and for this reason, the IP layer is called connectionless (i.e., it does not require a connection to be established before sending a packet on). These problems are taken care of by the transport layer. Transport layer. Two sets of protocol are provided in this layer. With the TCP, information is passed back and forth between transport layers, which controls the information flow. This includes such functions as the correct sequencing of packets, replacement of lost packets, and adjusting the transmission rate of packets to prevent congestion. In the early days of the Internet when traffic was comparatively light, these problems could be handled even where satellite transmissions were involved. With the enormous increase in traffic on the present-day Internet, these problems require special solutions where satellite systems are used, which are discussed in later sections. The TCP layer is termed connection-oriented (compared with the connectionless service mentioned above) because the sender and receiver must be in communication with each other to implement the protocol. There are situations where a simple standalone message may need to be sent which does not require the more complex TCP. For these types of message, another transport layer protocol called the user datagram protocol (UDP) is used. The UDP provides a connectionless service, similar to IP. The UDP header adds the port numbers for the source and destination applications.
The term packet has been used somewhat loosely up to this point. A more precise terminology is used for packets at the various layers, and this is shown in Fig. 15.11. At the application level the packet is simply
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