How H.323 devices selects which codec will be used within the media?

[Bahlul Guliyev]

Dear all,

I want to clarify one thing. In H.232 slow satart after Connect message what happens?

1.Capabilities exchange - Why we need it? Is this for Codec and DTMF relay Excahnge?

2. Master/Slave determination - Why we need it? Is this for Final word to say? for example which codec will be used within conversation

3. Open logical channel - This for one way audio

And how two side determines which codec will be used within the conversation?

Best Regards

Bahlul

[Manish Gogna]

Hi Bahlul,

This is explained in detail with an example cucm trace here

https://supportforums.cisco.com/document/44911/tracking-h323-calls-cucm-sdi-traces#Locate_the_problem_call

Manish

- Do rate helpful posts -

[Ayodeji Okanlawon]

Manish +5 ( Thats a great document. I am working on something similar based on cucm 9.1)

Bahlul,

To answer your questions you need to understand a few things about H.323 protocol

H.323 is an umbrella standard and it has two major components

1. H.225----Q.931

             -----RAS

2. H.245

H.225 is used for signalling the states of a call and has the ff elements. All of this are done using the q.931 component of H.225. The RAS conponent is used with gatekeepers

    Setup and Setup acknowledge
    Call Proceeding
    Connect
    Alerting
    Information
    Release Complete
    Facility
    Progress
    Status and Status Inquiry
    Notify

H.245

This is how media capabilities are negotiated within H.323 protocol.  A separate socket is used for this communication, which is negotiated in the H225 communication

Now H.245 uses several attributes to negotiate capabilites for a session. These are done in the obrder listed below

1. Terminal capabilites request (TerminalCapabilitesSetRequest): Each endpoint will send a TCS to indicate that capabilities it is capable of. Eg codecs, DTMF, packetization

2. Master slave determination (MasterSlaveDeterminationRequest): Both parites in a call will use this to determine which device will be the master and which will be the slave. This is used to resolve conflicts that might exist when two endpoints in a call request the same thing, but only one of the two can gain access to the resource at a time

3. Open Logical Channel (OpenLogicalChannelRequest): This is used to tell the remote endpoint that the originating endpoint wants to send media and needs to know where the media should be sent to. Ie what Ip address and port should I use. The remote endpoint will answer with a OLCACK which will include all the details the other endpoint needs to send media

Now lets talk about H.323 slow start and fast start.

In H.323 slow start, media negotiation begins at "connect" phase of H.225 signalling. It is in this phase that we know the H.245 address we will use for H.245 media negotiation.

In H.323 fast start, media information is sent before the capabilities are exchanged, by sending OLCs along with the H225 messages.

I hope its clearer

Have a little read here too..

http://docwiki.cisco.com/wiki/Cisco_IOS_Voice_Troubleshooting_and_Monitoring_--_H.323-Related_Standards