From Academic Kids

X.25 is an ITU-T standard protocol suite for WAN networks using the phone or ISDN system as the networking hardware. It defines standard physical layer, data link layer and network layers (layers 1 through 3) of the OSI model. The packet switching network was the common name given to the international collection of X.25 providers, typically the various national telephone companies. Their combined network had large global coverage during the 1980s and into the '90s, and it is still in use mainly in transaction systems.

X.25 was developed in the ITU Study Group VII based upon a number of emerging data network projects, such as the research project at the UK's National Physical Laboratory under the direction of Donald Davies who developed the concepts of packet switched networks. In the late 1960s a test network was started, and by 1974 a number of sites had been linked together to form SERCnet (Science and Engineering Research Council Network). SERCnet would later grow and be re-organized as JANET in 1984, which continues in service today, but as a TCP/IP network. Other contributions to the standardising process came from the ARPA project as well as French, Canadian, Japanese and Scandinavian projects emerging in the early 1970s. Various updates and additions were worked into the standard, eventually recorded in the ITU series of technical books describing the telecoms systems. These books were published every fourth year with different colored covers.

The general concept of X.25 was to create a universal and global packet-switched network on what was then the bit-error prone analog phone system. Much of the X.25 system is a description of the rigorous error correction needed to achieve this, a system known as LAP-B. The X.25 model was based on the concept of establishing "virtual calls" through the network, with "data terminating equipment" (DTE's) providing endpoints to users that looked like point-to-connections.

X.25 was developed in the era of dumb terminals connecting to host computers. Instead of dialing directly “into” the host computer — which would require the host to have its own pool of modems and phone lines, and require non-local callers to make long-distance calls — the host could have an X.25 connection to a network service provider. Now dumb-terminal users could dial into the network's local “PAD” (Packet Assembly/Disassembly facility), which, in essence, is a gadget with a bunch of modems and serial ports on one end and an X.25 connection on the other end, defined by the ITU-T X.29 and X.3 standards.

Having connected to the PAD, the dumb-terminal user tells the PAD which host to connect to, by giving a phone-number-like address in the X.121 address format (or by giving a host name, if the service provider allows for names that map to X.121 addresses). The PAD then places an X.25 call to the host, establishing a virtual circuit. Note that X.25 provides for virtual circuits, so appears to be a circuit switched network, even though in fact the data itself is packet switched internally, similar to the way TCP provides virtual circuits even though the underlying data is packet switched. Two X.25 hosts could, of course, call one another directly; no PAD is involved in this case. In theory, it doesn't matter whether the X.25 caller and X.25 destination are both connected to the same carrier, but in practice it was not always possible to make calls from one carrier to another.

The X.121 address consists of a three-digit Data Country Code (DCC) plus a network digit, together forming the four-digit Data Network Identification Code (DNIC), followed by the Network Terminal Number (NTN) of at most ten digits. Note the use of a single network digit, seemingly allowing for only 10 network carriers per country, but some countries are assigned more than one DCC to avoid this limitation.

For much of its history X.25 was used for permanent virtual circuits (PVCs) to connect two host computers in a dedicated link. This was common for applications such as banking, where distant branch offices could be connected to central hosts for a cost that was considerably lower than a permanent long distance telephone call. X.25 was typically billed as a flat monthly service fee, and then a price-per-packet on top of this. Speeds increased during the years, typically up to 48 or 96 kbit/s.

Publicly-accessible X.25 networks (Compuserve, Tymnet, Euronet) were set up in most countries during the 1970s and 80s, to lower the cost of accessing various online services, in which the user would first interact with the network interface to set up the connection. Known as switched virtual circuits (SVCs) or "virtual calls" in public data networks', this use of X.25 disappeared from most places fairly rapidly as long distance charges fell in the 1990s and today's Internet started to emerge.

A number of systems were developed to directly use the underlying packet nature of X.25, back when it appeared that X.25 would become the single universal networking system. Many of these were "private" applications, but the X.400 e-mail system was also based on X.25 as a transmission layer. The basic idea was to develop a universal set of standards for "Open System Interconnect (OSI)", however, industry developments eventually led down the path to Internet.

With the widespread introduction of "perfect" quality digital phone services and error correction in modems, the overhead of X.25 was no longer worthwhile. The result was Frame Relay, essentially the X.25 protocol with the error correction systems removed, and somewhat better throughput as a result.

X.25 networks are still in use throughout the world, although in dramatic decline, being largely supplanted by newer layer 2 technologies such as frame relay, ISDN, ATM, POS, and the ubiquitous layer 3 Internet Protocol. They remain some of the only available reliable links in many portions of the third world however, where access to a PDN may be the most reliable and low cost way to access the internet. A variant called AX.25 is also used widely by amateur packet radio, though there has been some movement in recent years to replace it with TCP/IP.

External link

de:X.25 it:X.25 pl:X.25 pt:X.25 tr:X.25


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