THOMSON ST 2030 SIP, Administrator'S Manual

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ST2030s VoIP Business Phone
ST2030s SIP Administrator Guide v3.0
Page 269 of 308
ST2030 SIP Monitoring Extension Feature (BLF) v1.47
1 SUBSCRIBE/NOTIFY support for monitoring extension
states- Overview
The objective of this document is to present the way ST2030 SIP uses
backlit function keys to monitor the state of other extensions. The backlit
function keys will indicate if an extension is idle, in use, or ringing. This
feature is known as BLF or Monitoring Extension function.
ST2030 extension module can be used to have more programmable keys on
board. There are basically two scenarios:
-
Server-to-phone scenarios : there is a SIP proxy which controls the
state of each extension and sends the appropriate messages to the
supervisor phone. Asterisk is an example and the reference followed for
implementation.
-
Phone-to-phone scenarios : each phone is responsible for notifying its
own state to supervisor phones
2 Functionality
Mechanism used for BLF is SUBSCRIBE/NOTIFY messages described in RFC-
3265.
The device will subscribe to the state of the extension of interest and receive
status notifications from this extension or from the proxy in order to drive
the LEDs.
2.1 High level technical description
When the phone is initialized, it will send out a SUBSCRIBE message to the
proxy for each extension it would like to monitor. This is to subscribe to the
state of the other extension. The proxy (and in some cases the other
extension) needs to support these subscriptions. At this point 2 things can
happen depending on the Proxy, which correspond to the scenarios
listed above:
• Phone-to-phone scenarios : the proxy will forward the SUBSCRIBE to
the far phone. Then the phone needs to log all extensions which subscribed,
and send out NOTIFY messages when changes happens in its state.
• Server-to-phone scenarios : the proxy captures the SUBSCRIBE, does
not forward it and sends state changes as the body of a NOTIFY on behalf of
the phone. Asterisk Server does this.