Why has a VCS cleared an H.323 call involving a Lifesize endpoint?

Introduction

This article relates to Cisco TelePresence Video Communication Server Expressway.

Q. Why has a VCS cleared an H.323 call involving a Lifesize endpoint?

A. This issue may affect calls passing through a VCS that are between a Lifesize endpoint running software version 4.0 or earlier, and any other devices (including MCUs and gateways) that are not TANDBERG or Polycom endpoints.

The issue occurs due to a bug in the Lifesize code. The bug is due to be fixed in Lifesize version 4.1 (due out in April 2009).

The work-around is to increase the value of the Call time to live setting on the VCS, although this is not necessarily recommended because it will increase the amount of time taken by the VCS to recognize and clear any hung calls.

More information

A VCS will periodically query the endpoints in a call to ensure that they are still participating in that call. It does this by sending the endpoint an IRQ message, to which the endpoint must respond. If the VCS believes that one or more of the participants is no longer in the call, it will clear the call so that the call is not left "hanging". The frequency with which the VCS queries the endpoints is determined by the Call time to live setting in the Gatekeeper section of the H.323 page (VCS Configuration > Protocols > H.323). The default is 120 seconds.

Lifesize endpoints prior to version 4.1 respond to the IRQ query sent by the VCS, but the reply does not include any information about the call they are currently in (contrary to the H.323 V4 standard). The VCS therefore considers the endpoint to be no longer in the call, and so clears the call.

The work-around for this is to increase the H.323 Call time to live setting on the VCS (VCS Configuration > Protocols> H.323) to a value greater than the length of the majority of calls that take place in your network. This will stop the VCS from checking that the endpoints are in a call during this time.

Note: changing the H.323 Call time to live setting on the VCS will affect the frequency with which the VCS queries all H.323 devices in a call, not just the Lifesize endpoints, and will increase the amount of time taken by the VCS to recognize and clear any hung calls.

Related Information

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