• X.25 Packet Switched networks allow remote devices to communicate with each other across high speed digital links without the expense of individual leased lines.
• Packet Switching is a technique whereby the network routes individual packets of HDLC data between different destinations based on addressing within each packet.
• The protocol known as X.25 encompasses the first three layers of the OSI 7-layered architecture as defined by the International Standards Organization(ISO) as follows:

• The user end of the network is known as Data Terminal Equipment (DTE) and the carrier's equipment is Data Circuit-terminating Equipment (DCE). The X.25 PLP permits a DTE user on an X.25 network to communicate with a number of remote DTE's simultaneously.
• Connections occur on logical channels of two types:

• How does a DTE set up a SVC with another DTE?

1. The calling DTE sends a Call Request Packet, which includes the address of the remote DTE to be contacted. The destination DTE decides whether or not to accept the call (the Call Request packet includes the sender's DTE address, as well as other information that the called DTE can use to decide whether or not to accept the call).
2. A call is accepted by issuing a Call Accepted Packet, or cleared by issuing a Clear Request Packet.
3. Once the originating DTE receives the Call Accepted packet, the virtual circuit is established and data transfer may take place. When either DTE wishes to terminate the call, a Clear Request Racket is sent to the remote DTE, which responds with a Clear Confirmation Packet.

• The destination for each packet is identified by means of the Logical Channel Identifier (LCI) or Logical Channel Number (LCN). This allows the PSN to route the each packet to its intended DTE.
• X.25 relies on the underlying robustness of HDLC LAPB to get data from node to node through the X.25 network. An X.25 packet makes up the data field of an HDLC frame.
• Additional flow control and windowing is provided for each Logical Channel at the X.25 level.
• Maximum packet sizes vary from 64 bytes to 4096 bytes, with 128 bytes being default on most networks. Both maximum packet size and packet level windowing may be negotiated between DTE's on call set up.
• X.25 gives you a virtual high quality digital network at low cost. It is economical for the same reason that it is usually cheaper to use the mail than to run your own postal service: there are tremendous savings to be made if multiple parties share the same infrastructure.
• In most parts of the world, X.25 is paid for by a monthly connect fee plus packet charges. There is usually no holding charge, making X.25 ideal for organizations that need to be on line all the time.
• Another useful feature is speed matching: because of the store-and-forward nature of Packet Switching, plus excellent flow control, DTE's do not have to use the same line speed. So you can have, for instance, a host connected at 56 KBPS communicating with numerous remote sites connected with cheaper 19.2 KBPS lines.
• X.25 has been around since mid 1970's and so is pretty well debugged and stable. There are literally, no data errors on modern X.25 networks.

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