ISDN Connection Management
19
Protocol Awareness
For Connection Management to be effective, each of the P840s must be aware of the protocols used within the
data being transferred over the ISDN calls between them.
IP Client-Server sessions are established between devices located on the LANs that are routed by the P840
router. If the P840 is to manage the ISDN calls between the routed LANs, the P840s 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 P840 maintains a table for each ISDN call made to a partner ISDN P840 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 P840 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 P840 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 P840 initiating the suspension or by the partner P840
that was connected just prior to the call being suspended. This prevents other P840s from tying up the ISDN
calls and interfering with the suspended calls.
ISDN calls may be connected and disconnected between the two P840s 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 P840. When the connection to the partner ISDN P840 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 P840 determines when the ISDN call will
be suspended.
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