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ISDN Connection Management
26
Protocol Awareness
For Connection Management to be effective, each of the routers must be aware of the protocols used within
the data being transferred over the ISDN calls between them.
IP and IPX Client-Server sessions are established between devices located on the LANs that are routed by the
Router. If the router is to manage the ISDN calls between the routed LANs, the routers on each WAN end of
the Client-Server session must be aware of the session and also must become actively involved in the
maintenance of the session. When an ISDN call is suspended, both the Client and the Server must still believe
that the session exists.
Suspension Process
The router maintains a table for each ISDN call made to a partner ISDN router in order to determine when the
ISDN call should be suspended during Connection Management.
Any number of LAN sessions may be currently using the established ISDN call to transfer data between the
client and the server. The router monitors the ISDN call for interesting traffic passed on each of the sessions
currently using the ISDN call. If no interesting traffic is observed on the ISDN call for a period of time greater
than the defined Idle Timer value, the ISDN call is suspended and disconnected. While the ISDN call is
suspended, the router will monitor the LAN sessions for interesting traffic and re-establish the ISDN call when
required.
A suspended ISDN call may only be re-established by the router initiating the suspension or by the partner
router that was connected just prior to the call being suspended. This prevents other routers from tying up the
ISDN calls and interfering with the suspended calls.
ISDN calls may be connected and disconnected between the two routers when required according to the
suspension and re-activation of the ISDN calls. When all of the sessions using the call in the table have been
closed, the call will be terminated and the ISDN B-channel becomes available for use to connect to a different
ISDN router. When the connection to the partner ISDN router is configured to use Auto-Call, the ISDN call
will be suspended when there are no sessions in the table.
Interesting Traffic
Interesting Traffic is defined as normal interactive user data for a session. Certain data exchanged during a
normal session is not considered to be interesting and usually is composed of keepalive messages, watchdog
messages, and routing messages. Non-interesting data is handled differently when Connection Management is
enabled.
Idle Timer
In order to determine the criteria for suspending an ISDN call, an Idle Timer is defined. The Idle Timer defines
the period of time that LAN traffic is monitored to determine when the ISDN call will be put in suspension and
disconnected. When Interesting Traffic is observed once again, the call is reconnected.
The Idle Timer is common to all ISDN calls and may be defined from 6 seconds to 5 minutes in 1 second
increments. The Idle Timer may be disabled so that only the partner router determines when the ISDN call will
be suspended.
Session Participation (Spoofing)
While an ISDN call is up and connected, all traffic within the sessions will be considered interesting and will be
transferred to the partner router across the ISDN call. When the router determines that the ISDN call is to be