| The PAD is a device
that collects data from a group of asynchronous terminals and
periodically outputs the data in X.25 packets, as represented in the
Figure. A PAD also takes data packets from a host and turns them
into a character stream that can be transmitted to the terminals.
The operation of the terminal-PAD interface, the services offered by
a PAD, and the PAD-host control interaction are defined by the
International Telecommunication Union Telecommunication
Standardization Sector (ITU-T) recommendations.
ITU-T Recommendations for PAD
Services
Some ITU-T recommendations defining
the PAD are as follows:
- X.3-Specifies
the parameters for terminal-handling functions (such as baud
rate, flow control, character echoing, and other functions) for a
connection to an X.25 host.
The X.3 parameters are similar in function to Telnet options or
attention (AT) command set for modems.
- X.28-Specifies
the user interface for locally controlling a PAD. X.28
identifies the keystrokes that you would enter at a terminal to
set up the PAD, similar to the AT command set for modems.
- X.29-Specifies a
protocol for setting the X.3 parameters via a network
connection. When a connection is established, the destination
host can request that the PAD or terminal change its parameters
by using the X.29 protocol. A PAD cannot tell the destination
host to change its X.3 parameters, but it can communicate that
its own parameters were changed.
- X.75-Specifies the gateway
between the clouds. It defines the signaling system between two
PDNs. X.75 is essentially a Network-to-Network Interface (NNI).
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