Carry on packet switching

X25 may be 24 years old but it still delivers good price performance, writes Nick Langley

X25 may be 24 years old but it still delivers good price performance, writes Nick Langley

In the SSP/Computer Weekly list of the top IT skills, X25 is number

  • What is it?

    X25 is a packet switching data network protocol approved by the CCITT in 1976 and revised many times since. It defines the physical, data link and network layers in the OSI Reference Model. Packets of data that contain the destination address are sent separately and are reassembled on their arrival.

    X25 packet switched networks allow remote devices to communicate with each other across high-speed digital links, without the expense of individual leased lines. X25 is used for switched virtual circuits, where a connection is dialled up, and permanent virtual circuits, where the connection is kept open permanently.

  • Where did it originate?

    In the early 1970s, the forerunner to the ITU became alarmed by the proliferation of proprietary data networks, and decided to establish a standard.

  • What's it for?

    It standardises the exchange of data, including control of its transmission from the user to a network node.

    It is part of a series which also includes the X21 standard for very fast digital links, the X400 e-mail standard, and X500 directory services for messaging. BT says X25 is for users for whom low bandwidth and low access speeds are suitable.

  • What makes it special?

    X25 may be looking a little dated, but it still delivers good price performance.

    "X25 is still the only protocol which is supported by virtually every carrier in the world - 202 networks in 131 countries," says a BT Syncordia spokesman. "It conforms to globally-agreed standards that make it easy to buy equipment, and it is inherently secure, with built-in error correction."

  • How difficult is it?

    BT Syncordia says that from a network management point of view, it is more straightforward than most.

  • Where is it used?

    X25 is widely established in legacy networks, but is unlikely to be chosen by a dotcom. Telecoms service providers such as BT and Cable & Wireless offer X25-based services. More than 60% of X25 users on BT's data network say they expect to still be using X25 in three years' time.

  • Not to be confused with

    A hardcore film certification.

  • What does it run on?

    Most network equipment manufacturers support X25 connectivity.

  • Few people know that

    Network users frustrated by slow line speeds often use Xpletives.

  • What's coming up?

    BT recently introduced host speeds of 128kbps and 256kbps, and enlarged the number of logical channels which could be supported on 64kbps lines from 100 to 250. However, users may need to migrate to a faster packet switched network, such as frame relay.

    Rates of pay and training

    As part of a package of wide area network (Wan) skills, X25 could help you become a network engineer, at a salary of between £25,000 and £40,000. As ever, you will get a lot more if your experience entitles you to call yourself a consultant.

    Courses are available from networking equipment suppliers, although usually these will be on-site at the customer organisation. QA Training and Learning Tree both provide courses on Wans, which include X25.

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