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Chapter 18: Wide Area Networks and X.25 Protocols

To connect computers or LANs spread over a large geographical area is now the order of the day. These wide area networks (WANs) may be private networks connecting corporate offices spread across the country or the globe, or they may be public networks offering data services to the public. In this chapter, we will study the issues involved in wide area networking, with special emphasis on X.25 protocols used extensively in wide area networks.

18.1 ISSUES IN WIDE AREA NETWORKING

When a computer network is spread over a large geographical area, some special problems are encountered that are not present in the LANs.

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  WANs generally do not support very high speeds. Due to lower transmission rates, delays are likely to be higher. If satellite radio is used as the medium, then the delay is much higher and, as a result, special care must be taken in terms of flow control protocols.

  
  


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  Because of the delay, the response time also will be high. To the extent possible, protocol overheads need to be minimized.
  

  
  


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  The communication medium in a WAN environment may not be as reliable as in LANs, and hence the error rate is likely to be higher. This may lead to more retransmissions and more delay.

  
  


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  Lower transmission rates and higher delays pose problems for real-time voice and video communication. Higher delay causes gaps in voice communication and jerky images in video communication.

  
  


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  Network management is more involved and complex as network elements are spread over large geographical areas.

  
  

Wide area networks are characterized by low transmission speeds, high propagation delay, and complex network management.

The options in transmission media for WANs are dialup/ leased lines, optical fiber, and satellite radio. X.25, Frame Relay, and Asynchronous Transfer Mode (ATM) protocols are used in WANs.

The various options for developing WANs are:

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  Dial up lines

  
  


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  Point-to-point leased lines

  
  


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  Switched digital networks based on Integrated Services Digital Network (ISDN)

  
  


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  Switched digital networks based on X.25 standard protocols

  
  


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  Optical fiber networks based on Frame Relay and Asynchronous Transfer Mode (ATM)

  
  

For WANs, X.25 is an important standard. X.25-based WANs have been in place since the 1980s. An overview of the X.25 standard is presented in the following sections.

Note

X.25 protocols are still used extensively in satellite-based wide area networks. However, the protocol overhead is very high in X.25 as compared to Frame Relay.