X.25 Product Description

X.25 Configurations

The CCITT X.25 recommendations describe the packet interface between a DTE (Data Terminating Equipment) and a DCE (Data Communications Equipment). The DTE is a device (computer or terminal) which connects to an X.25 PSN. The DCE interface is supplied by the PSN to enable the DTE to connect to the network. This type of connection is usually made over telephone lines via a modem.

Figure 1-1 “Typical X.25 Network Connection” illustrates a typical X.25 network configuration between an HP 9000 and a remote host using an X.25 PSN.

Figure 1-1 Typical X.25 Network Connection

A direct connection can be made via a modem eliminator. A modem eliminator provides the clocking mechanism necessary to synchronize signals between a DTE and a DCE. Two X.25 interfaces may be connected in this way, but one of them must be configured as a DCE. This type of configuration is called a "back-to-back configuration".

Figure 1-1 “Typical X.25 Network Connection” illustrates a typical back-to-back configuration with a remote host. The remote host can be any type of processor, including another HP 9000 or any device capable of fulfilling the requirements of a DTE (or DCE in the back-to-back configuration).

Figure 1-2 Back-to -Back Configuration

The term "remote" is used (in the relative sense) to designate a distant DTE which can be only a few feet, or thousands of miles away from the local DTE.

X.25 allows for the creation of multiple virtual connections over the same connection to a PSN. Each connection is called a virtual circuit (VC). X.25 can multiplex up to 4095 VCs (CCITT) over a single physical connection to a DCE. The X.25 PSN can route each VC to a different remote host. From the point of view of the application, each VC is directly connected to the remote host.

	NOTE: In a back-to-back configuration, all VCs must be connected to the same remote host.