Northern UTA220/UTA220k - Manual (Page 77)

UTA220/UTA220k
12-1
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.