rol in place to enable the smooth and efficient transfer of data. Transparency of the user data. Speed and code conversion capabilities. Protection from an intermediate node or link failure. Downloaded from Digital Engineering Library @ McGraw-Hill (www.digitalengineeringlibrary.com) Copyright © 2004 The McGraw-Hill Companies. All rights reserved. Any use is subject to the Terms of Use as given at the website. X.25, Internets, Intranets, and Extranets Chapter 7 Pricing advantages: because many use the network, the prices are on a per-packet, rather than per-minute, basis. Other Components of Packet Switching Although the primary components of packet switching have already been discussed, others are also available. As the evolution of the X.25 standard continued in the early 1970s, several different implementations were enhanced. These include devices or access methods that you should under- stand. They are summarized SO that you can gain an appreciation of how these pieces all work together. Some of the added pieces include The ability to dial into the packet-switching network from an asynchronous modem communication. Although packet switching (a la X.25) is a synchronous transfer system, the need for remote dial-up communications exists. Therefore, the standards bodies included this capability in one of the enhanced versions of the network. As shown in Figure 7-9, a dial-up connection can be made from a user. In this case, the asynchronous communication is a serial data transfer to a network-based PAD. The PAD accepts the serial asynchronous data, collects it, segments it into packets, and establishes the connection to the remote host desired. In this case, the connection is now across the X.25 world, synchronously moving packets across the network. This uses an X.28 protocol to establish the connection between the asynchronous terminal and the PAD. Figure 7-9 Dial access to X.25 Packets packet switching. 10011101110010100101 00000000 Modem Serial Data Downloaded from Digital Engineering Library @ McGraw-Hill (w