User`s manual

SIP User's Manual 188 Document #: LTRT-83303
Mediant 1000 & Mediant 600
3.4.5.2.6 Stand-Alone Survivability
The 'SAS Configuration' page allows you to configure the device's Stand-Alone
Survivability (SAS) feature. This feature is useful for providing a local backup via the PSTN
in Small or Medium Enterprises (SME) that are serviced by IP Centrex services. In such
environments, the enterprise's incoming and outgoing telephone calls (external and
internal) are controlled by the Proxy, which communicates with the enterprise through the
WAN interface. SAS ensures that incoming, outgoing, and internal calls service is
maintained in case of a WAN or Proxy failure using a PSTN (or an alternate VoIP) backup
connection and the device's built-in internal routing. To utilize the SAS feature, the VoIP
CPEs such as IP phones or residential gateways, need to be defined so that their Proxy
and Registrar destination addresses and UDP port equal the SAS feature's IP address and
SAS local SIP UDP port.
Note: The SAS feature is applicable only to Mediant 1000. The 'SAS Configuration'
page is Feature Key dependant and therefore is available only if included in
the device's Feature Key (refer to 'Updating the Software Upgrade Key' on
page 268).
The SAS feature operates in one of two modes:
 Normal Mode: Initially, the SAS feature acts as a registrar so that every VoIP CPE
within the SME's LAN registers to it, while the SAS feature passes all registration
requests to the Proxy. In this mode, the SAS functions as a stateful proxy, passing all
SIP requests received from the enterprise to the Proxy, and vice versa. In parallel, the
SAS feature continuously maintains a ‘Keep-Alive’ handshake with the Proxy using
SIP OPTIONS or re-INVITE messages.
 Emergency Mode: The SAS feature switches to Emergency mode if it determines
(from the ‘Keepalive’ 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 feature controls all internal calls
within the enterprise. In the case of outgoing calls, the SAS feature forwards them to a
local VoIP gateway (this can be the same gateway where the SAS feature resides or a
separate analog or digital gateway). For PSTN fallback, the local VoIP gateway should
be equipped with analog (FXO) lines or digital (E1/T1) trunk(s) for PSTN connectivity.
In this way, the enterprise preserves its capability for internal and outgoing calls.
When Emergency mode is active, the SAS feature continuously attempts to access
the Proxy using the regular ‘Keepalive’ method. After the connection is re-established,
the SAS feature switches to pre-Normal mode. In this mode, the SAS feature
maintains all terminations of existing calls while any new SIP call signaling (issued by
new INVITE sessions) is transacted to/from the Proxy server. This is accomplished
using the SAS feature 's database of current active calls. After releasing all calls
established during Emergency mode, the SAS feature can continue operating in
Normal mode.