SIP User's Manual 480 Document #: LTRT-12801
MP-500 MSBG
9.3 Stand-Alone Survivability (SAS) Feature
The device's Stand-Alone Survivability (SAS) feature ensures telephony communication
continuity (survivability) for enterprises using hosted IP services (such as IP Centrex) or IP-
PBX in cases of failure of these entities. In case of failure of the IP Centrex, IP-PBX servers
(or even WAN connection and access Internet modem), the enterprise typically loses its
internal telephony service at any branch, between its offices, as well as with the external
environment. In addition, typically these failures lead to the inability to make emergency
calls (e.g., 911 in North America). Despite these possible point of failures, the device's SAS
feature ensures that the Enterprise's telephony services (e.g., SIP IP phones or soft
phones) are maintained by routing calls to the PSTN (i.e., providing PSTN fallback).
The device supports up to 200 SASregistered users.
The SAS feature operates in one of two modes:
îš„ Normal: Initially, the device's SAS agent serves as a registrar (and outbound Proxy
server) to which every VoIP CPE (e.g., IP phones) within the Enterprise's LAN
registers. The SAS agent at the same time sends all these registration requests to the
Proxy server (e.g., IP-Centrex or IP-PBX). This ensures registration redundancy by the
SAS agent for all telephony devices. Therefore, SAS agent functions as a stateful
proxy, passing all SIP requests received from the Enterprise to the Proxy and vice
versa. In parallel, the SAS agent continuously maintains a keep-alive "handshake" with
the Proxy server using SIP OPTIONS or re-INVITE messages.
îš„ Emergency: The SAS agent switches to Emergency mode if it detects (from the keep-
alive responses) that the connection with the Proxy is lost. This can occur due to Proxy
server failure or WAN problems. In this mode, when the connection with the Proxy
server is down, the SAS agent controls all internal calls within the Enterprise. In the
case of outgoing calls, the SAS agent forwards them to a local VoIP gateway (this can
be the device itself or a separate analog or digital gateway). For PSTN fallback, the
local VoIP gateway should be equipped with analog (FXO) lines for PSTN connectivity.
In this way, the Enterprise preserves its capability for internal and outgoing calls.
The call routing rules for SAS in Emergency mode is configured in the 'IP2IP Routing
Table' page (refer to ''SAS Routing Table'' on page 176). Thi
s table provides enhanced
call routing capabilities (such as built-in ENUM queries and redundant SAS proxy
server load balancing) for routing received SIP INVITE messages in Emergency mode.
The received INVITE message is routed as depicted in the following flow chart:
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