Asterisk Feature Busy Lamp Field (BLF)

The Wiki of Unify contains information on clients and devices, communications systems and unified communications. - Unify GmbH & Co. KG is a Trademark Licensee of Siemens AG.

Revision as of 07:40, 22 August 2008 by Hans.zeiner (talk | contribs) (Technical Backround)
Jump to: navigation, search


The busy lamb feature allows users to monitor the dialog state of another phone/user extension. This is indicated by the LEDs adjacent to an FPK. This feature is called "Busy Lamp Field" in the Asterisk community. Sometime the term 'Direct Station Selection' is used for the same functionality.

For Users

The basic Busy Lamp field feature allows the user to monitor the dialog state of another phone/user extension. The dialog state is shown by a distinct LED behavior:

  • LED off -> Watched extension is idle
  • LED blinking -> Watched extension is ringing
  • LED on -> Watched extension is busy/connected

In the basic configuration pressing the FPK will call the monitored persion in any call state


There are four users: Leo, Alice, Joe and Bob. Leo is watching the extensions of Alice, Bob and Joe. Each of the extensions are configured on an specific FPK. The example shows the three different extension status. Joe is talking to annother persion, Alice gets a call, her phone is ringing and Bob is in idle like Leo is. The following picture shows this example:

Joe is in busy, Alice is in ringing and Bob is in idle state

For Administrators

The FPK are administrated as Server Key. This is one option of the FPK programming options. The configuration can be found in

Administrator Pages -> System -> Features -> Program Keys:

The FPK function is ‘Server Feature’. Select this option and press ‘Edit

Basic BLF Configuration with Server Key

Within the server feature four options are given:

Basic BLF Configuration Parameters

Key label - This is the label of the selected FPK
Feature Code - This code is sent within an INVITE message, if the key is pressed in idle status
DTMF Digits - These Digits are sent as DTMF signals, if the key is pressed during a call
LED control - A Subscribtion is sent to the given URL


If Bob has the extension numer 222, the parameters will be:

Key label - Bob
Feature Code - 222, pressing the FPK will always call Bob independant of his line status..
DTMF Digits - n.a.., not neccessary in this feature
LED control - 222@, a subscription for the LED will be sent to the server at the IP address If no IP address is given, the request will be automatically sent to the configured SIP proxy server.

Technical Backround

The BLF features give the user the possiblity to watch one other users dialog state. To support the feature RFC 4235 is used.

This document defines a dialog event package for the SIP Events architecture, along with a data format used in notifications for this package. The dialog package allows users to subscribe to another user and to receive notification of the changes in state of INVITE -initiated dialog usages in which the subscribed- to user is involved.

The framework is startet with an initial subscribtion from the watching phone.
Request: SUBSCRIBE SIP: Request: SUBSCRIBE sip:333@ ------------------>
Status: 401 Unauthorized SIP: Status: 401 Unauthorized <------------------
Request: SUBSCRIBE SIP: Request: SUBSCRIBE sip:333@ ------------------>
Status: 200 OK SIP: Status: 200 OK

See also

Preview:OpenStage and Asterisk