AT&T 7400 series ISN Communication, Communication Between the LAN and Other Networks, X.25 Router

8-18LOCAL AREA NETWORK CONNECTIONS

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ISN Communication

The AT&T Information System Network (ISN) is a packet-switched LAN that links its nodes together through a data-only digital switch. Communications between ISN nodes and other DTEs is through an ISN concentrator and the communications system. The interface between the communications system and the ISN concentrator is usually through an EIA-port/ADU combination to an Asynchronous Interface Module (AIM) on the ISN concentrator, although other configurations are possible. See figure 8-9for the most common configuration. Refer to the AT&T Cross-Product Applications — Connectivity Reference manual, Issue 2 (555-025-901), for other possible configurations and for the ISN and communications system administration that must be performed to implement each configuration.

			OUTGOING
			FACILITIES
	SYSTEM 75
	OR
TERMINALS	SYSTEM 85	A	ISN	ISN	TERMINALS
PRINTERS	SWITCH	I	CONCENTRATOR	SWITCH	PRINTERS
AND		M			AND
COMPUTERS					COMPUTERS
			INCOMING
			FACILITIES
	Figure 8-9.	Common ISN Configuration

Communication Between the LAN and Other Networks

For communication between LANs over dedicated or dial-up circuit-switched facilities, bridges or routers and MPDMs are required at each end of the connection. The general configuration for this type of connection is shown in figure 8-10. Table 8-5lists some of the communications packages that provide this LAN-to-LAN communication.

Of the bridge/router packages shown above, only Gateway Communications provides synchronous communications between different types of LANs (for example, between token ring and Ethernet LANs). All other packages require that the communicating LANs be the same type.

Of the packages shown, AT&T offers two — the X.25 router and the SNA gateway.

The X.25 Router

The X.25 router enables communication across a System 75, System 85, Generic 1 or Generic 2 communications system for LANs that run with the StarGROUP operating system. (See figure 8-11.)

The router can be implemented on a 3B2 or 6386 PC. Although the router need not be dedicated to X.25 operations, it must be a node on a LAN operating with the StarGROUP software, as well as be connected to the System 75, System 85, or the Generic 1 or 2 communications system. At the LAN side of the router is