Avaya Application Solutions Deployment Manual page 251

When the IP phone detects a failure with the signaling channel (H.225/Q.931), its recovery
algorithm depends on the call state of the endpoint:
If the user of the phone is on a call and the phone loses its call signaling channel, the new
IP Robustness algorithm will allow the phone to reestablish the link with its gatekeeper
without dropping the call. As a result, the call is preserved. Call features are not available
during the time the phone is trying to reestablish the connection.
If the user of the phone is not on a call, the phone closes its signaling channels and
searches for a gatekeeper using the algorithm defined below.
To reestablish the link, the phone tries to register with a C-LAN on its gatekeeper list. The new
C-LAN Load Balancing algorithm looks for the C-LAN on the list with the least number of
phones registered to it. As a result, the recovery time will be short, and there will be no
congestion due to too many phones trying to register to a single C-LAN board.
In the S8300/G700 or G350 configuration, the IP endpoint connects directly to the S8300 Server
(there is no C-LAN.) The connectivity path between the endpoint and the server is:
Endpoint IP network
To discover connectivity failure, keepalive messages are exchanged between the IP end point
and the server. When the endpoint discovers that it no longer has communication with its
primary gatekeeper, it looks at the next address on its list. If the next address is for an LSP, the
LSP accepts the registration and begins call processing.
While the LSP is not call preserving, the fail-over from primary gatekeeper to LSP is an
automatic process, and does not require human intervention. The fail-back from LSP to primary
gatekeeper, however, is not currently automatic, and requires a system reset on the LSP. During
the fail-back to the primary gatekeeper, all calls are dropped, with the exception of IP-to-IP calls.
To discover connectivity failure, two kinds of keepalive procedures are currently implemented
between the IP endpoints and the Avaya server:
RAS keepalive
TCP keepalive
RAS keepalive
This procedure is used for the H.225.0/RAS signaling channel, and depends on the exchange
of RAS keepalive messages between an IP endpoint and the server. The Avaya server uses
this procedure to determine if a currently registered endpoint is still available. The endpoint uses
the procedure to determine whether the server is reachable or not.
A registered H.323 endpoint periodically sends keepalive messages for the purpose of
performing lightweight re-registration with the switch.
S8300
Maintenance architecture
Issue 3.4.1 June 2005 251