In order to allow fast detection of connectivity loss, particularly in low traffic periods, the SBC (Session Border Controller) must support receiving and sending SIP OPTIONS requests between survivable branch proxies and the Telephony Control Server. This allows the branch to switch to survivable mode and the Lotus Sametime Unified Telephony server to invoke automatic rerouting to the branch via the PSTN (Public Switched Telephone Network).
In order to allow the Telephony Control Server to differentiate between a branch users UA (User Agent) not responding and a WAN outage the SBC must send a SIP 504 response for SIP request timeouts occurring for requests sent to a branch proxy. At normal traffic levels connectivity failures are likely to be detected by SIP requests timeouts before they are detected by the SIP OPTIONS audit mechanism.
Parent topic: Additional SBC (Session Border Controller) Capabilities