PAD

A PAD, or Packet Assembler/Disassembler, is a device which enables remote users to gain access to the X.25 packet switching systems PSE (Packet Switch Exchange) using the telephone network.

All PTTs or X.25 service providers throughout the world have public PADs available for subscribers to gain access to their X.25 service.

X.28 communications require the use of a PAD. The PAD converts the X.28 asynchronous protocol to X.25 and acts as a buffer between the two systems, converting X.28 data to packets and vice versa.

X.28

X.28 is a communications standard providing asynchronous connections to an X.25 network. It requires a modem connection and an X.28 dial-up subscription to the local PTT or preferred VAN.

Asynchronous transmission means that information is sent in a continuous stream of characters as opposed to the more reliable synchronous transmission used by the “native X.25” standard. X.28 is cheaper than native X.25 but rather more error-prone.

At the start of a communication session, the user’s software application will instruct the modem to make an outgoing telephone call to a PAD (Packet Assembler/Disassembler).

Once a connection to the PAD has been made, the PAD can be instructed to make an X.25 call to the trading partner of your choice. This is achieved in the same way as a native X.25 call would be made, by giving the destination’s NUA (Network User Address, the X.25 equivalent of a ‘phone number). Once the X.25 connection has been established then a two-way flow of data can be started.

The PAD acts as an interface between the asynchronous line, where data is transferred in a single stream, and the X.25 system, where data must be passed as packets. The PAD, as its full name suggests, takes data from the asynchronous phone line and assembles data packets to be sent and also takes packets from the X.25 service and disassembles them into a single asynchronous stream of data.

Unfortunately, PAD access is error prone as X.25 error correction standards do not cover this type of access because the X.25 connection terminates at the PAD (for X.25 over a dialup connection see X.32). Error correction on the line between the user and the PAD is the responsibility of applications at either end of the connection. (In the case of ODEX this is achieved by the use of OFTP Special Logic Extensions, which provides checksum functionality to check the integrity of received data and provides a mechanism for error recovery and data retransmission).

X.28 support in DI products

Owing to the relatively unreliable nature of X.28, DIP products no longer offer this method of communication.

X.29

X.29 is the CCITT procedure for the remote exchange of control information and user data between an X.25 host and a PAD. When an X.28 asynchronous user dials in to a PAD and establishes communication with a native X.25 host, the host may send X.3 parameters back to the PAD using the X.29 protocol. This lets the host system ensure that communications will be controlled by the correct forwarding characters etc., avoiding possible communications problems.

X.3

The X.3 standard deals with the first 18 parameters required for successful PAD operation.

Public PADs are used for many different functions, not just OFTP communications. They may, for example, be used as a logon to a mainframe service, which would have totally different communication attributes to the transfer of data. On an interactive terminal connection you would naturally wish to see what you were typing, and so the PAD would be required to echo all received characters back to you. This would be totally inappropriate for a file transfer system as it would double the data traffic and the cost of the connection.

For this reason a series of user-configurable settings known as the X.3 parameters (X.3 being the international standard), is set within the PAD to ensure that these communications attributes will be correct. Incorrect setting of these parameters will almost certainly result in communications problems as the two sides of an automatic conversation will not understand one another.

The X.3 standard defines the first 18 of these parameters but on some PADs there may be many more.