AudioCodes MediaPack 114 Bedienungsanleitung PDF herunterladen (Seite 315)

Version 5.6 315 November 2008

SIP User's Manual 7. IP Telephony Capabilities

7 IP Telephony Capabilities

This section describes the device's IP telephony capabilities.

7.1 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 SAS feature operates in one of two modes:

 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.

 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 SAS agent continuously attempts to communicate with the Proxy using the regular

keep-alive method. After the connection is re-established, the SAS agent switches to

pre-Normal mode. In this mode, the SAS agent 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 agent's database of

current active calls. After releasing all calls established during Emergency mode, the

SAS agent resumes operating in Normal mode.

For SAS implementation, the primary Proxy server for the VoIP CPE's (e.g., IP phones) is

the SAS agent (i.e., the device itself) while the IP Centrex or IP-PBX is defined as the

secondary Proxy server. For SAS configuration, the device is composed of two different

applications (SAS and Gateway), where each application has its own SIP interface

(UDP/TCP/TLS ports).

 Configuring the device to use and operate with the SAS capabilities (refer to

''Configuring SAS'' on page 316)

 Configuring SAS emergency c

all routing (refer to ''Configuring Emergency Calls'' on

page 316)

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