Northern UTA220/UTA220k manual Chapter Channel, Functional Description, Pad

D-Channel X.25

Chapter 12

D-Channel X.25

The TA provides the 1984 CCITT specified X.25 protocol for the D-channel with a Packet Assembler/ Disassembler (PAD) interface for asynchronous data terminals. The X.25 protocol supports the following X.25 parameters:

•Single channel operation

•Modulo 8

•Transmit packet size maximum of 256 byte

•Receive packet size maximum of 256 bytes

•Link level window size maximum of 7

•Packet level window size maximum of 7

FUNCTIONAL DESCRIPTION

X.25

X.25 is a synchronous error-correcting protocol for data transmission over public and private networks. The X.25 protocol is subdivided into three layers: physical, data link, and packet (or network). Each layer is responsible for specific parts of the data transmission, error checking, and error correcting. Data is transmitted in packets that contain additional information to facilitate the error checking and correcting.

PAD

The PAD acts as an intermediary between the local user's asynchronous terminal and the X.25 protocol. PAD functions are specified by CCITT X.3, X.28, and X.29 recommendations. These recommendations are as follows:

•X.3 defines 22 PAD parameters that govern how the PAD operates. These parameters specify how and when the data from the asynchronous DTE is forwarded to the remote host (assembly) and how the data from the remote host displays to the asynchronous terminal (disassembly).

•X.28 defines the PAD commands and responses that permit communication with the X.25 protocol via an asynchronous terminal.

•X.29 defines supervisory packets used for communication between the local PAD and the remote host's PAD.

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