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Busy Lamp Field (BLF)

Author: Robert Abela
Date: 14-03-2008

Busy lamp field, BLF for short, is a light on an IP phone which tells you whether another extension connected to the same PBX is busy or not. This has to be configured manually from the phone user and it is usually done by making use of the web interface. When configured, the phone subscribes to a resource list available on an IP PBX to be notified with such information about other extensions. BLF works through the SIP protocol by making use of SUBSCRIBE and NOTIFY messages. In a normal scenario, the phone is the subscriber and the IP PBX is the notifier.

How does it work?

When an IP phone is configured to monitor an extension it sends a SUBSCRIBE SIP message to the PBX. If authentication is configured, authentication takes place and if the subscriber is successfully authenticated a 200 OK SIP message response is sent back to the subscriber. A NOTIFY SIP message which includes XML in the message body is sent to the subscriber (in this case the phone) to advise the subscriber the current state of the extension being monitored. Once status of monitored extension is changed from idle to busy or vice versa, the subscriber is notified from the PBX with a NOTIFY SIP message. The subscriber must acknowledge the NOTIFY SIP message by responding back with a 200 OK SIP message. The subscriber can unsubscribe from this resource by sending a SUBSCRIBE SIP message to the PBX with Expires header set to 0. The way this works is a three step process.

The Subscribe Process

In this scenario we have a phone that is configured to be registered with a PBX on extension 106. Below is a sample capture of a SUBSCRIBE SIP message of this phone when subscribing to be notified about the status of extension 136. Note that the sample has been shortened for documentation purposes.


From the sample above we can see that:

• SUBSCRIBE sip: 136@10.130.130.4;user=phone SIP/2.0
  The SUBSCRIBE SIP message sent to monitor extension 136 registered on IP PBX 10.130.130.4.
• From: <sip: 106@10.130.130.4>
  The subscriber, in this case extension 106 registered on IP PBX 10.130.130.4.
• Accept: application/dialog-info+xml
  The application will accept dialog-info which is in XML format.
• Expires: 3600
  The Expires header, where the duration of the subscription in seconds is stated. Therefore any time before 3600 seconds pass, the subscriber should send a SUBSCRIBE SIP message using the same event header and ID parameter to refresh the subscription. Note that the IP PBX server can reply back with a shorter expiration timeout but cannot lengthen the timeout.
• Content-Length: 0
  This specifies that there is no XML message body attached to this SIP packet.

Once the SUBSCRIBE SIP message is received from the IP PBX server and successfully processed, a 200 SIP OK message is sent back to the subscriber, as shown in the sample below.

The Notify process

The below sample capture shows how the monitored extension changes status. A NOTFIY SIP message including XML body is sent to the subscriber informing the subscriber of the status of the monitored extension. Note that the sample has been shortened for documentation purposes.


From the sample above we can see that:

• NOTIFY sip:106@10.130.130.58:2051;line=1ofdyu7v SIP/2.0
  From this line we can see that this is a SIP notify message sent to extension 106 which can be found on IP 10.130.130.58.
• Subscription-State: active;expires=3521
  Here the subscription state and how many seconds are left for subscription to expire are stated.
• Content-Length: 507
  The XML message body size in bytes attached to this NOTIFY SIP message.
• entity="sip:136@10.130.130.4;user=phone"
  The monitored extension is specified in the XML message body.
• direction="initiator"
  The monitored extension is starting the call in this example. If the monitored extension is receiving a call, direction will be set to 'recipient'.
• <state>confirmed</state>
  The status of the call, in this example the call has been established. If the phone is still ringing, state will be set to 'early' and if the call is terminated the sate will be set to 'void'.
• in <remote>, <target uri="sip:117@10.130.130.4"/>
  The remote SIP entity to which extension 136 is calling, in this example extension 117. If the monitored extension is receiving the call, the remote extension would be the caller.

Once the NOTIFY SIP message is received from the subscriber, the subscriber sends back a 200 OK SIP message to the notifier as shown in the sample below.

The Unsubscribe process

Below is a sample SUBSCRIBE SIP message sent from the subscriber to the notifier to unsubscribe from the resource. The only difference between a SUBSCRIBE and UNSUBSCRIBE SIP message is that when unsubscribing the Expire header is set to 0.


From the sample above we can see that:

• SUBSCRIBE sip:136@10.130.130.4;user=phone SIP/2.0
  In this line we can see that the SIP message is a SUBSCRIBE message and that the extension being monitored is 136 registered to IP PBX 10.130.130.4.
• From: <sip:106@10.130.130.4>;tag=4fog9tlb9d
  The subscriber, in this example extension 106 registered to IP PBX 10.130.130.4.
• Expires: 0
  This specifies the IP PBX that the subscriber is unsubscribing from the resource.

Once the SUBSCRIBE SIP message is received and successfully processed from the IP PBX, a 200 SIP message is sent back to the subscriber, as shown in the sample below.

Common error codes of subscription events

403 - Forbidden. Server did not authorize the request.
423 - Interval to small. The Expires value specified in the SIP message is too short.
481 - Subscription does not exist.
489 - Bad event. The event package designated is not supported.

During these tests we made use of the following:

• 3CX Phone System ( http://www.3cx.com )
• Snom 320 IP Phone ( http://www.snom.com )
• Grandstream GXP 2000 IP Phone ( http://www.grandstream.com )
• WireShark network protocol analyzer ( http://www.wireshark.org )

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