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The meaning of 'Q931_FAST_START' in the output of h.323 debug

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Article ID: KB22416 KB Last Updated: 13 Jun 2012Version: 1.0
Summary:
This article provides information about the meaning of Q931_FAST_START in the output of h.323 debug.
Symptoms:
  • What does Fast Start mean in the H.323 output?

  • Do ScreenOS devices support it?
Cause:

Solution:
The output of debug h.323 all is as follows:
FS machine - S:FS_S_INIT E:FS_E_SETUP_NO_FS A:FS_A_NO_FAST_START
## 2011-04-16 21:08:31 : FS machine - state change: FS_S_INIT -->FS_S_DECLINED
## 2011-04-16 21:08:31 : FS machine - media action = MEDIA_NO_ACTION
## 2011-04-16 21:08:31 : H.323 HA: building sync cookie
## 2011-04-16 21:08:31 : H.323 HA: H323_HA_TYPE_Q931_FAST_START
## 2011-04-16 21:08:31 : H.323 HA active sync successful: context id 127 fast_start
## 2011-04-16 21:08:31 : H.323 HA active sync successful: context id 127 q931_tpkt

With the standard H.245 negotiation, the two endpoints need three round-trips, before they agree on the parameters of the audio/video channels:

  • Master/slave voting

  • Terminal capability set exchange

  • Opening the logical channels

In certain situations and especially with high-latency network links, this can last for too long and users will notice the delay. To overcome this, the Fast Connect procedure was designed (also known as Fast Start). The endpoint will prepare several variants of the H.245 requestopenLogicalChannel, based on how many codecs it supports. After that, the endpoint encodes each variant of the message to the binary form and the resulting array of binary strings is inserted into a H.225.0/Q.931 message (usually the Setup message).

The called party will pick one of the variants and confirm it in the next H.225.0/Q.931 message, together with its own list of logical channel variants. The rest of the H.245 negotiation will be performed the standard way. Using Fast Connect, the parameters of logical channels (codecs, IP addresses, and ports) are negotiated early in the message exchange; before the called user accepts the call.
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