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VoIP White Paper

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Revised: February 8, 2007

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11 Key System Feature – Allworx BLF Protocol

The Allworx server and IP phones (local or remote) have several differentiating features that enable emulation

of classic Key system-type capabilities – such as line appearance, busy lamp field monitoring, and direct

station selection. This is all done on an industry standards-compatible SIP VoIP platform via the addition of

some Allworx advanced mechanisms built on top of SIP.

Specifically, InSciTek has added some SIP specification-compliant private headers to the Allworx

implementations of SIP to activate the advanced features. These features include support for NAT-enabled

remote phones and operations like automatic off-hook for direction station selection. However, the live system

status monitoring required for things like busy lamp field indicators and line appearances goes beyond what

SIP was designed for. As a result, InSciTek developed a companion protocol for SIP to offer some of these

advanced features. This is referred to as Allworx Busy Lamp Field (ABLF) Protocol.

The internal syntax of the ABLF is not important here, since Allworx takes care of the details. But it is helpful

for the system administrator to have some understanding of how ABLF operates at the IP level. This will assist

with troubleshooting specific problems where ABLF or line appearance lights do not seem to operate properly

or consistently on a particular phone or set of phones.

11.1 ABLF Protocol Background

The ABLF protocol is an event-driven, peer-to-peer protocol that is implemented by both the Allworx server and

Allworx IP phones. Because ABLF is a peer-to-peer protocol that uses sub-net broadcasts to reach all devices

simultaneously, all ABLF devices associated with a particular Allworx server site must transmit and receive on

the same UDP port number. By default, this UDP port number is 2088, but is configurable on a site-by-site

basis using the Servers / VOIP Server page of the Allworx administrative Web site to change the ABLF port

option. When this setting is changed, all Allworx devices (IP phones and server) must be restarted to acquire

the new configuration information and keep all devices monitoring and sending with the same correct port

number.

11.2 ABLF Protocol Operation

Each time an ABLF peer has a change in status, it broadcasts this information to all peers on its subnet. If a

peer is not located on the same LAN subnet as the Allworx server, the ABLF peer also sends a directed packet

with the same content to the Allworx server so the server can forward that packet to all other subnets with

phones attached. If the phone is an Allworx remote phone, this directed packet goes to Allworx’s WAN

address, otherwise this is typically directed at Allworx’s LAN port, since the device is on the private side of

Allworx’s firewall. The ABLF device automatically determines the interface to use when it is configured during

startup.

The Allworx server keeps track of a list of all local subnets that contain ABLF devices and also contains a list of

each remote device participating in the ABLF protocol. When the server receives a packet from any device, it

automatically forwards the packet to a single device on every subnet (other than the originating one) and

requests that one device to broadcast the packet on its own local subnet. This mechanism lets the subnets

learn the topology of all ABLF devices and manages the traffic so that every device gets each ABLF update

notification in as efficiently a manner as possible.

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