X.25 is a standard by the ITU- T protocol family for large-scale computer networks ( WANs) via the telephone network. The standard also defines the physical layer, the data link layer and the network layer ( layers 1 to 3) of the OSI model. International X.25 packet switching network as a ( packet switched network ) is sold, mostly by telephone companies. Product name, both at Deutsche Telekom and at the Austrian Telekom, is Datex -P.


X.25 was started as a research project in England's National Physical Laboratory under the direction of Donald Watts Davies, who developed the concepts of packet- switched networks. In the late 1960s, a test network was built around 1974, a number of sites for SERCnet ( Science Engineering Research Council Network) was connected. After a large number of renovations and additions were incorporated into the system, and finally a series of technical books was published as a description. The differently colored bindings of these books made ​​aware of these descriptions as "Colour Books" and presented the first draft of which is what eventually evolved into the X.25 standard.

SERCnet later grew and was reorganized in 1984 as JANET, which still exists today, but in the meantime as a TCP / IP network.

The general concept of X.25 was to create a packet-switching network on the analog telephone network then unreliable. A large part of the X.25 system is a description of strict error correction, a procedure called LAPB. The X.25 model based on the fact that the entire network internally remained with the telephone companies, with data terminal equipment ( " data terminal equipment " DTE) as the end point for the user.

X.25 was developed in the period of " dumb terminals " that were tied to individual central computer. A random access from a point of many other such TCP / IP was not provided. Even that it was a network with X.25, was largely hidden from the users for which it essentially looked like a flawless modem connection.

As a result has the characteristics of a circuit switched X.25 network, although the packet-switched data internally. Tailed computers are described with a kind of telephone number in the X.121 Address Format - the Network User Address ( NUA ). The NUA is composed of a three-digit country code, a single-digit provider code and the ten-digit National Terminal Number. Worth mentioning is the digit provider code, which allows only ten provider in one country - at that time was that as sufficient. However, there were in the U.S. soon more than ten X.25 services - the country code has been shortened by one digit on the number " 31", thereby extending the provider code to two digits and allow up to 100 systems.

For a long time X.25 was as a kind of leased line between computers for permanent connections used (PVC for Permanent Virtual Circuit - permanent virtual connection). It was common for applications such as banking, where remote branch offices were connected to a central computer - usually cheaper than a permanent Telefonfernwählverbindung. X.25 was typically billed at a flat monthly fee and additional price per data packet. Typical speeds were 4,800, 9,600 and 19,200 bit / s

A number of publicly accessible X.25 networks ( Compuserve, Tymnet, Euronet ) were set up in the 1970s and 1980s to reduce the cost of access to the various online services. The user first had to interact with the network interface to start the connection. This type of connection is called SVC switched virtual circuit (such as switched virtual connection ) or PDN for public data networks ( public data network ). This use of X.25 was not widespread and disappeared almost as prices for Fernwählverbindungen in the 1990s, fell sharply.

In the time that it looked as if X.25 is the only universal power system would be, a number of methods have been developed to directly access the underlying packages. Many of these procedures were "private" applications, but also an X.400 e -mail method was based on X.25 as link layer.

With the availability of modems with high data transmission rate and built-in error correction method was easier to use the switching technology of the normal telephone network, and it was worth it for many users not more, more vorzuhalten which consists of special X.25 switches network infrastructure. The result was as Frame Relay, X.25 essentially with minimum error correction ( 6 bytes per packet) and for higher data throughput. Nevertheless in safety critical applications and authorities X.25 was still very long in operation.

X.25 networks are in the world still in use, but their use is drastically reduced. They are largely replaced by newer two-layer technologies such as Frame Relay, ISDN, ATM, POS or mainly by the ubiquitous Internet protocol family. However, they remain in many parts of the Third World as the cheapest and reliable connectivity to the Internet. Also at Deutsche Bahn is X.25 - since the 1980s - still for the transmission of messages within the danger alarm system MAS90 system in use.