Introduction to Tandem X.25 Capabilities

PAD-Related Standards
What is X.25?
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PAD-Related
Standards
Many CCITT standards are associated with X.25, because they concern the way that
different types of user equipment connect to each other and to the network. Like X.25,
these standards (described below) are meant to ensure interoperability between the
various computers and devices that communicate with a PSDN.
PAD A Packet Assembler/Disassembler (PAD) is a device that allows an asynchronous
terminal or personal computer to access a PSDN. Several standards are related to the
functions of the PAD, as described below.
X.3
X.3 defines the functions of the PAD itself in a public data network in the same
country.
The PAD constructs packets from the single-character asynchronous data coming
from the terminal. It packetizes the data and adds header, address, and control
information for transmission over the PSDN.
The PAD processes the packets received from the PSDN for the terminal,
removing the surrounding packet information and transmitting the asynchronous
data in a form that is compatible with the particular terminal.
A PAD can be hardware or software. It can be at the DTE (a box next to a terminal or
software loaded into the terminal); or it can be at the DCE (a hardware or software
interface provided within the network). Figure 1-4 shows the possible placement and
functions of a PAD.