Survivable Remote Site Telephony Overview
Survivable Remote Site Telephony (SRST) is an optional feature for sites that depend on a Wide Area Network (WAN) connection to aUnified Communications Manager node. SRST references, which are configured in theUnified Communications Manager Administration interface, allow IP gateways to provide limited telephony service to IP phones at the remote site in the event of a WAN outage:
IP phones at the remote site can call each other
calls from the PSTN can reach the IP phones
calls from the IP phones can reach the external world through the PSTN
When phones at the remote site lose connectivity to all associatedUnified Communications Manager nodes, the phones connect to the SRST reference IP gateway. The status line indication on the IP phone shows the phone has failed over to the backup SRST gateway. When the connection toUnified Communications Manager is restored, the IP phones reregister withUnified Communications Manager and full telephony services are restored.
SRST supports remote sites that may have a mix of SCCP and SIP endpoints in addition to PSTN gateway access.
Connection Monitor Duration
An IP phone that connects to an SRST gateway over a Wide Area Network (WAN) reconnects itself to Unified Communications Manager as soon as it can establish a connection withUnified Communications Manager over the WAN link. However, if the WAN link is unstable, the IP phone switches back and forth between the SRST gateway andUnified Communications Manager. This situation causes temporary loss of phone service (no dial tone). These reconnect attempts, known as WAN link flapping issues, continue until the IP phone successfully reconnects itself toUnified Communications Manager.
To resolve the WAN link flapping issues betweenUnified Communications Manager and an SRST gateway, you can define the number of seconds (Connection Monitor Duration) that the IP Phone monitors its connection toUnified Communications Manager before it unregisters from the SRST gateway and reregisters toUnified Communications Manager. The IP phone receives the connection monitor duration value in the XML configuration file.