Monday, May 7, 2007

X.25 - I’ve got to say, this is definitely not an article on a X-Men movie sequel.

I’ve got to say, this is definitely not an article on a X-Men movie sequel.

X.25 is an International Telecommunication Union-Telecommunication Standardization Sector (ITU-T) standard protocol suite for wide area networks (WAN) communications that defines how connections between user devices and network devices are established and maintained. Designed to operate effectively regardless no matter what kind of systems are connected to it, X.25 is typically used in the packet-switched networks (PSNs) of common carriers, such as the telephone and ISDN companies.

The need for WAN protocols capable of providing connectivity across public data networks (PDNs) in the ‘70s helped the standardization of X.25 by common carriers. The result is the international standard managed by the ITU-T. “Packet Switched Network” was the recognized name of the international collection of X.25 providers, mainly the various national telephone companies. Their combined network provided a huge global coverage for the following decades, and is still used in limited use.


First off, X.25 devices have to be explained before moving on. These network devices fall into three general categories:
Data terminal equipment (DTE) - End systems that communicate across the X.25 network. They are usually terminals, personal computers, or network hosts. Located on the property of individual subscribers.
Data circuit-terminating equipment (DCE) - Communications devices, such as modems and packet switches, which provide the interchange between DTE devices and a PSE. Generally located in the carrier's facilities.
Packet-Switching Exchange (PSE) - Switches that make up most of the carrier's network. They transfer data from one DTE device to another through the X.25 PSN.

X.25 was based on the concept of establishing "virtual calls," or switched virtual circuits (SVC), through the network with "data terminal equipment" (DTE) providing endpoints to users, which looked like point-to-point connections. See, X.25 was developed in the era of dumb terminals connecting to host computers. Dialing through a host computer would require a set of modems and phone lines for the computer, and require non-local callers make long-distance calls. Instead of dialing directly through the host computer, the host could have an X.25 connection to a network service provider. Subscribers, such as banks, are charged based on their use of the network. X.25 was typically billed as a flat monthly service fee, and then a price-per-packet on top of this.

Thanks to X.25, dumb-terminal users could dial into the network's local “PAD” (Packet Assembly/Disassembly facility). The PAD, a device commonly found in X.25 networks, is used when a DTE device, such as a character-mode terminal, is too simple to implement the full X.25 functionality.

The PAD is located between a DTE device and a DCE device, and it performs three primary functions:
Buffering (storing data until a device is ready to process it) – Buffers data sent to or from the DTE device.
Packet assembly - Assembles outgoing data into packets and forwards them to the DCE device. (This includes adding an X.25 header.)
Packet disassembly - disassembles incoming packets before forwarding the data to the DTE. (This includes removing the X.25 header.)

Virtual Circuits in X.25

A virtual circuit is a logical connected created to ensure two network devices communicate reliably. It denotes the existence of a logical, bi-directional path from one DTE device to another through an X.25 network, even though the connection can pass through any number of nodes/devices like DCE and PSEs. Multiple virtual circuits (logical connections) can be multiplexed into a single physical circuit (a physical connection). Virtual circuits are de-multiplexed at the remote end, and data is sent to the appropriate destinations.

There are two types of X.25 virtual circuits:
Switched Virtual Circuits (SVCs) - Temporary connections that require a request session connection each and every time the devices need to communicate. If communication occurs over an SVC and neither device has additional data to transfer, the virtual circuit is terminated.
Permanent virtual circuits (PVCs) - Permanently established connections used for frequent and consistent data transfers that do not require that sessions be established and terminated. Data transfer can be done whenever necessary because the session is always active.

X.25 Today

Thanks to the advent of "perfect" quality digital phone services and error correction in modems, the operating cost of X.25 was no longer worthwhile. This brought forth Frame relay, which is essentially the X.25 protocol with the error correction systems removed. The concept of virtual circuits is still used within ATM to allow for traffic engineering and network multiplexing.

X.25 networks are still in limited use around the world. It remains one of the only available reliable links in many portions of the developing world, where access to a PDN may be the most reliable and low cost way to access the Internet.