18.4 ADDRESSING IN X.25 NETWORKS

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Because X.25 is used for WANs, addressing the different end systems is very important. Addressing in X.25 networks is performed based on the X.121 standard. Figure 18.5 shows the two formats for the X.121 addressing scheme.

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Figure 18.5: X.121 Address Formats.

X.121 addresses will have 14 digits. In one format, there will be 4 digits of data network identification code (DNIC) and 10 digits that can be specified by the VSAT network operator. In the second format, there will be 3 digits of data country code (DCC) and 11 digits that can be specified by the VSAT network operator. DNIC and DCC will be given by national/international authorities.

Generally, the address/subaddress portion is divided into logical fields. For instance, in the first format, out of the 11 digits, 2 digits can be given to the state, 3 digits to the district, and the remaining digits to the hosts. A hierarchy can be developed. This complete address directory has to be stored on the host at the central station.

Though X.25 has been used extensively in public data networks (PDNs), it is a highly complicated protocol because it has been developed to work on transmission media, which are highly susceptible to noise. At each hop (node to node), an exchange of data packet is followed by an acknowledgement packet. In addition, flow control and error control packets are exchanged. Such high overhead is suitable for links with high error rates. With the advent of optical fiber, which is highly immune to noise, such complicated protocols are not necessary. However, in satellite-based data networks, X.25 has been used widely to provide reliable data transfer.

In X.25 networks, the addressing of the various end systems is done per ITU-T Recommendation X.121. X.121 address will have 14 digits.

Summary

In this chapter, X.25-based wide area networking is discussed. X.25 provides a connection-oriented service. Hence, when two systems have to communicate with each other, a virtual circuit is established, data transferred, and then the circuit is disconnected. X.25 is a robust protocol and will work even if the transmission medium is not very reliable. X.25 addresses the first three layers of the OSI protocol architecture. The physical layer specifies the interface between the DTE and DCE. The datalink layer is called the LAPB (link access protocol balanced), which is derived from HDLC. The network layer (also called the packet layer) will take care of the virtual circuit establishment. Flow control is done at both layers 2 and 3.

When an asynchronous terminal has to communicate with an X.25 network, a packet assembler disassembler (PAD) is used that takes the data from the terminal and formats it into X.25 packets to send over the X.25 network. Similarly, it disassembles the packets from the X.25 network and presents them to the terminal. In X.25 networks, addressing is done based on the X.121 standard, which specifies a 14-digit address for each system.

Though X.25 is a robust protocol that can be used in networks with unreliable transmission media, as media have become reliable (for example, optical fiber), X.25 has lost its popularity. It is used extensively in satellite-based data networks.

References

W. Stallings. Computer and Data Communications. Prentice-Hall, Inc., 1999.
http://www.farsite.co.uk This site provides useful information on X.25 commercial products.

Questions

List the various issues in wide area networking.
Describe the architecture of an X.25-based wide area network.
Explain the call setup, data transfer, and call disconnection procedures in an X.25 network.
Describe the formats of X.121 addressing.
Explain the X.25 packet formats.

Exercises

1.	Study the X.25 switch, PAD, and X.25 add-on cards supplied by various vendors.
2.	Make a comparison of the protocols used in X.25 and Frame Relay. Explain why X.25 is called a heavyweight protocol.
3.	Design an addressing scheme based on X.121 for a nationwide network connecting various state governments, district offices, and central government departments.
4.	List the important PAD parameters.

Answers

1.	You can obtain the details of X.25 products from the site http://www.nationaldatamux.com.
2.	In X.25, there will be a number of packet transfers between the switches mainly for acknowledgements, flow control, and error control. Hence, the protocol overhead is very high as compared to Frame Relay, which is mainly used in optical fiber systems in which there are fewer errors.
3.	The addressing based on X.121 for a nationwide network should be done in a hierarchical fashion. X.121 address will have 14 digits, out of which 3 digits are for country code. Out of the remaining 11 digits, 2 digits can be assigned to the state, 2 digits for each district, and the remaining 7 digits for the different end systems.
4.	There are 18 PAD parameters per X.3 standard. The important parameters are baud rate, local echo mode, idle timer, line delete, line display, character editing, input flow control, and discard output.

Projects

Simulate a PAD on a PC. The PAD has to communicate with the PC through an RS232 link. From the PC, you should be able to set the PAD parameters.
For an X.25-based wide area network connecting all the universities in the country, you have been asked to design an addressing scheme. Design the addressing scheme based on X.121 formats. You can divide the address/ subaddress field into three portions. The first portion is for the state code, the second portion is for the district code, and the third portion is for the university ID. Think of an alternative that does not use the geographical location. (You can divide the universities into different categories such as technical universities, agricultural universities, and so on.)

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