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X.25 Interface and related

February 10th, 2011 Remo Leave a comment Go to comments

What is X.25 interface? Define DTE & DCE

The X.25 software and hardware allow the router to communicate over a public X.25 network. The X.25 network interface complies with CCITT 1980 and 1984 specifications for X.25 interfaces by offering multiplexed channels and reliable end-to-end data transfer across a WAN. Layer 1 X.25 defines the electrical and mechanical procedures for activating and deactivating the physical medium connecting the DTE and the DCE. This relationship is shown below:


DTE: DTE or Data terminal equipment devices are end systems that communicate across the X.25 network. They are usually terminals, personal computers, or network hosts, and are located on the premises of individual subscribers.

DCE: DCE or Data circuit-terminating equipment are communications devices, such as modems and packet switches that provide the interface between DTE devices and a PSE (Packet-switching exchange), and are generally located in the carrier’s facilities.

Briefly describe multiplexing service of X.25. How flow control & error control is being achieved in X.25

Multiplexing of virtual circuits takes place at layer 3.The traditional approach to packet switching makes use of X.25, which not only determine the user network interface but also influences the internal design of the network. Today’s networks employ reliable digital transmission technology over high quality, reliable transmission links, many of which are optical fiber. Thus multiplexing of X.25 makes its more reliable & easily operable.

Flow control is generally based on the start & waits technique and is achieved by using an acknowledgement (ACK) for error free frames & a negative acknowledgement (Nak) for frames containing error. Ack0 & Ack1 are usually used alternately for detecting sequence errors, which are caused by either, duplicated or lost frames.

Error control is achieved by 16-bit CRC or FCS algorithm. In the event of an error frame, the sender retransmits the frame to the receiver.

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