X.25 is a packet-switched network technology used long before ago. X.25 is one of the first packet-switched network used to transmit data between multiple hosts. In this tutorial, we will define the X.25 protocol as well as the X.25 standard, history, network equipment, and components.

X.25 Packet Switching

As its name suggests X.25 is a packet-switching technology that is used to transmit data between different host locations. X.25 uses HDLC packets which also contains some information about the routing like source and destination address.

X.25 Standard

X.25 protocol is created by the International Telecommunication Union (ITU) in 1976. Originally X.25 is created to carry voice signals over the telephone lines. But later the capability of the X.25 is used for network and internet communication. X.25 encompasses the first 3 layers of the OSI Reference model which are Physical Layer, Data Link Layer, Network Layer. X.25 was a very popular late 1970s and 1980s among the telecommunication and IT world. The latest version of the X.25 standard is released in 1996.

Characteristics of X.25

X.25 network protocol provides some characteristics.

• The packet size varies from 64 bytes to 4096 bytes where 128 bytes is the default packet size. If the packet size is different for multiple DTE and DCE the packets will be fragmented.
• X.25 is a low-speed network for today’s internet speeds. X.25 provides speed around 100kbps and below.
• X.25 is used to create faster and newer network communication protocols like ATM and Frame Relay.
• X.25 is a very robust protocol because it is debugged and inspected heavily from its creation from the 1970s. Literally there is no data error on modern X.25 networks.
• Another problematic attribute of the X.25 protocol is its delay because of the DCE’s store and forward mechanism.

X.25 Network Equipment and Components

Some equipment and terms are used to fully implement the X.25 network. Here are some of the most important of them.

• `Data Terminal Equipment (DTE)` is used on the end-user, device or host part in order to terminate the X.25 network.
• `Data Circuit-Terminating Equipment (DCE)` is used to route the X.25 packets between DTE devices. Multiple DCE devices are used to connect the DTE’s and other DCE devices together.
• `Switched Virtual Circuits (SVC’s)` are like virtual connections between different DTE’s. They can be created by using different DCE’s to connect the same two DTE’s. Every time a transmission session will occur a new SVC is created.
• `Permanent Virtual Circuits (PVC’s)` are similar to the leased line where the connection is always established between two DTE’s. There is no need to set-up phase.

X.25 Physical Layer

The physical layer of the X.25 provides some communication lines which transmit electrical signals according to V.35, RS232, and X.21 standards. The physical layer is simply copper lines that are used for telephone communication.

X.25 Data Link Layer

Data Link layer is one of the most important parts of the X.25 protocol. Data Link Layer provides error-free communication and transmission between two X.25 nodes. X.25 provides the error checking mechanism in each hop during transmission. This can create some delay. X.25 packets are generally shorter than other protocol packets which are generally 128 or 256 bytes.

X.25 Network Layer

X.25 network layer simply used to manage end-to-end communication between different DTE devices. The network layer also governs set-up and teardown and flow control between the DTE devices as well as routing function with multiplexing multiple logical or virtual connections ver single physical connection.