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8-18 LOCAL 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 8-9 for 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.
CONCENTRATOR
ISN
SWITCH
PRINTERS
ISN
COMPUTERS
FACILITIES
I
TERMINALS
TERMINALS
A
SWITCH
OR
SYSTEM 85
COMPUTERS
PRINTERS
OUTGOING
FACILITIES
SYSTEM 75
AND AND
M
INCOMING
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 8-5 lists 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