7.1 X.25
7.1.4 The packet assembler/deassembler
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).