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Written by: David Vossel
Initial version: 04-19-2010
Email: dvossel@digium.com

This document is designed to give an overview of how to configure and generate Advice of Charge along with a detailed explanation of how each option works.

Read This First

PLEASE REPORT ANY ISSUES ENCOUNTERED WHILE USING AOC. This feature has had very little community feedback so far. If you are using this feature please share with us any problems you are having and any improvements that could make this feature more useful. Thank you!

Terminology

AOC: Advice of Charge

AOC-S: Advice of Charge message sent at the beginning of a call during call setup. This message contains a list of rates associated with the call.

AOC-D: Advice of Charge message sent during the call. This message is typically used to update the endpoint with the current call charge.

AOC-E: Advice of Charge message sent at the end of a call. This message is used to indicate to the endpoint the final call charge.

AMI: Asterisk Manager Interface. This interface is used to generate AOC messages and listen for AOC events.

AOC in chan_dahdi

LibPRI Support:
ETSI, or euroisdn, is the only switchtype that LibPRI currently supports for AOC.

Enable AOC Pass-through in chan_dahdi
To enable AOC pass-through between the ISDN and Asterisk use the 'aoc_enable' config option. This option allows for any combination of AOC-S, AOC-D, and AOC-E to be enabled or disabled.

For example:

Since AOC messages are often transported on facility messages, the 'facilityenable' option must be enabled as well to fully support AOC pass-through.

Handling AOC-E in chan_dahdi
Whenever a dahdi channel receives an AOC-E message from Asterisk, it stores that message to deliver it at the appropriate time during call termination. This means that if two AOC-E messages are received on the same call, the last one will override the first one and only one AOC-E message will be sent during call termination.

There are some tricky situations involving the final AOC-E message. During a bridged call, if the endpoint receiving the AOC messages terminates the call before the endpoint delivering the AOC does, the final AOC-E message sent by the sending side during termination will never make it to the receiving end because Asterisk will have already torn down that channel.
This is where the chan_dahdi.conf 'aoce_delayhangup' option comes into play.

By enabling 'aoce_delayhangup', anytime a hangup is initiated by the ISDN side of an Asterisk channel, instead of hanging up the channel, the channel sends a unique internal AOC-E termination request to its bridge channel. This indicates it is about to hangup and wishes to receive the final AOC-E message from the bridged channel before completely tearing down. If the bridged channel knows what to do with this AOC-E termination request, it will do whatever is necessary to indicate to its endpoint that the call is being terminated without actually hanging up the Asterisk channel. This allows the final AOC-E message to come in and be sent across the bridge while both channels are still up. If the channel delaying its hangup for the final AOC-E message times out, the call will be torn down just as it normally would. In chan_dahdi the timeout period is 1/2 the T305 timer which by default is 15 seconds.

'aoce_delayhangup' currently only works when both bridged channels are dahdi_channels. If a SIP channel receives an AOC-E termination request, it just responds by immediately hanging up the channel. Using this option when bridged to any channel technology besides SIP or DAHDI will result in the 15 second timeout period before tearing down the call completely.

Requesting AOC services

AOC can be requested on a call by call basis using the DAHDI dialstring option, A(). The A() option takes in 's', 'd', and 'e' parameters which represent the three types of AOC messages, AOC-S, AOC-D, and AOC-E. By using this option Asterisk will indicate to the endpoint during call setup that it wishes to receive the specified forms of AOC during the call.

Example Usage in extensions.conf

AOC in chan_sip

Asterisk supports a very basic way of sending AOC on a SIP channel to Snom phones using an AOC specification designed by Snom. This support is limited to the sending of AOC-D and AOC-E pass-through messages. No support for AOC-E on call termination is present, so if the Snom endpoint receiving the AOC messages from Asterisk terminates the call, the channel will be torn down before the phone can receive the final AOC-E message.

To enable passthrough of AOC messages via the snom specification, use the 'snom_aoc_enabled' option in sip.conf.

Generate AOC Messages via AMI

Asterisk supports a way to generate AOC messages on a channel via the AMI action AOCMessage. At the moment the AOCMessage action is limited to AOC-D and AOC-E message generation. There are some limitations involved with delivering the final AOC-E message as well. The AOCMessage action has its own detailed parameter documentation so this discussion will focus on higher level use. When generating AOC messages on a Dahdi channel first make sure the appropriate chan_dahdi.conf options are enabled. Without enabling 'aoc_enable' correctly for pass-through the AOC messages will never make it out the pri. The same goes with SIP, the 'snom_aoc_enabled' option must be configured before messages can successfully be set to the endpoint.

AOC-D Message Generation
AOC-D message generation can happen anytime throughout the call. This message type is very straight forward.

Example: AOCMessage action generating AOC-D currency message with Success response.

AOC-E Message Generation

AOC-E messages are sent during call termination and represent the final charge total for the call. Since Asterisk call termination results in the channel being destroyed, it is currently not possible for the AOCMessage AMI action to be used to send the final AOC-E message on call hangup. There is however a work around for this issue that can be used for Dahdi channels. By default chan_dahdi saves any AOC-E message it receives from Asterisk during a call and waits to deliver that message during call termination. If multiple AOC-E messages are received from Asterisk on the same Dahdi channel, only the last message received is stored for delivery. This means that each new AOC-E message received on the channel overrides the previous one. Knowing this the final AOC-E message can be continually updated on a Dahdi channel until call termination occurs allowing the last update to be sent on hangup. This method is only as accurate as the intervals in which it is updated, but allows some form of AOC-E to be generated.

Example: AOCMessage action generating AOC-E unit message with Success response.

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