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What is X.25 Protocol in Networking and its Architecture ?

May 31, 2018
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With the growth in corporate packet switching networks and an increasing awareness of the opportunities for data theft, therefore we focus on the need for data security in X.25 networks. X.25 was a standard suite of protocols used for packet-switched communications over a wide area network—a WAN. A protocol is an agreed-upon set of procedures and rules. Two devices that follow the same protocols can understand each other and exchange data. X.25 Protocol can be a cost-effective means of networking systems in a wide geographical area, compared to traditional dial-up (circuit switched) connections, or remote-bridged local area networks (LANs) connected by leased lines. It provides worldwide interconnection for international corporations.

Basic Overview of X.25 Protocol

X25 is developed by ITU-T and it is a packet switched wide area network. As shown in the figure below, it is a interface between DTE and DCE for operation on public data network in packet mode. DTE stands for Data Terminal Equipment and DCE stands for Data Circuit Terminating Equipment. X25 is an end to end protocol. User usually will not be aware of packets passing between these terminals.

X.25 defines procedures and protocols needed to establish, maintain and terminate the connections. It also defines services to provide functions such as reverse charge, call direct and delay control. It is also referred as subscriber networks protocol. It uses virtual circuit approach to packet switching and uses asynchronous TDM to multiplex the packets.


X.25 Architecture

The network is composed of DCEs and DSEs that route the packets of data through the network to the intended destination. The path that a user’s data takes might vary with every packet. In most cases the DTE is connected to a DCE in some form of network. In a few cases, two systems might be attached more directly, without an intermediate network. When this is the case, one system has to act as a DCE at the Data Link layer. This usage is not common because it restricts the flexibility of the protocol and degrades performance.

Operation and maintenance of DCEs and DSEs are the responsibilities of the network provider. If a link between two DSEs goes down, the provider must reroute traffic. X.25 protocol does not define the route through the network or the protocols employed within it.

The following diagram shows these elements of a packet-switched data network.

X.25 Architecture

Figure: X.25 Diagram

DTE (Data-Terminal Equipment.) A computer that uses a network for communications.

DCE (Data Circuit-Terminating Equipment.) A device at the point of access to a network.

DSE (Data-Switching Equipment.) A switching node in a packet-switched data network.

Uses of X.25

X.25 protocol is a mechanism for transferring transparent data. Standards exist for encapsulation of higher-level data such as TCP/IP or SNA traffic over X.25. One of the largest uses of X.25 is the transfer of asynchronous data streams such as those produced by simple terminals like credit card readers. These devices connect to a Packet Assembler/Disassembler (PAD) that organizes the asynch data streams into X.25 packets for transmission across the network. PAD design is based on ITU standards X.28, X.29 and X.3.

X.25 protocol remains important in Point-of-Sale credit card and debit card authorization. However, there is an enormous investment in X.25 infrastructure throughout the world, and in some regions, it continues to expand. Thus X.25 protocol will remain important for years to come.

Characteristics of X.25 Protocol

  • Maximum packet sizes vary from 64 bytes to 4096 bytes, with 128 bytes being a default on most networks. Both maximum packet size and packet level windowing may be negotiated between DTE’s on call set up.
  • 25 is optimized for what today would be considered quite low speed lines: 100kbps and below. At line speeds above 100 kbps the effects of latency, small packet sizes and small window sizes are such that the bandwidth cannot be efficiently utilized.
  • 25 has been the basis of the development of other packet switched protocols like TCP/IP and ATM. These protocols also have the ability to handle one-to-many connections and the ability to match DTE’s having different line speeds, both characteristics of X.25.


[1] Bradley Mitchell “A Guide to X.25 in Computer Networking”, available online at:

[2] “X.25 Network Communications Overview”, available online at:

[3] “X.25 Tutorial | X25 protocol layer basics | X25 frame | tutorials”, available online at:

[4] “X.25 Packet Switching Tutorial”, available online at:

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