Subject: [Megaco] about the difference between H248 and MGCP

From: "Neal Zhu" <[email-masked]>
Date: Wed, 31 Aug 2005 13:52:46 +0000

In RFC 3435 

2.6 Use of Local Connection Options and Connection Descriptors 

   As indicated previously, the normal sequence in setting up a bi- 
   directional connection involves at least 3 steps: 

   1) The Call Agent asks the first gateway to "create a connection" on 
      an endpoint.  The gateway allocates resources to that connection, 
      and responds to the command by providing a "session description" 
      (referred to as its LocalConnectionDescriptor).  The session 
      description contains the information necessary for another party 
      to send packets towards the newly created connection. 

   2) The Call Agent then asks the second gateway to "create a 
      connection" on an endpoint.  The command carries the "session 
      description" provided by the first gateway (now referred to as the 
      RemoteConnectionDescriptor).  The gateway allocates resources to 
      that connection, and responds to the command by providing its own 
      "session description" (LocalConnectionDescriptor). 

   3) The Call Agent uses a "modify connection" command to provide this 
      second "session description" (now referred to as the 
      RemoteConnectionDescriptor ) to the first endpoint.  Once this is 
      done, communication can proceed in both directions. 

   When the Call Agent issues a Create or Modify Connection command, 
   there are thus three parameters that determine the media supported by 
   that connection: 

   * LocalConnectionOptions:  Supplied by the Call Agent to control the 
     media parameters used by the gateway for the connection. When 
     supplied, the gateway MUST conform to these media parameters until 
     either the connection is deleted, or a ModifyConnection command 
     with new media parameters (LocalConnectionOptions or 
     RemoteConnectionDescriptor) is received. 

   * RemoteConnectionDescriptor:  Supplied by the Call Agent to convey 
     the media parameters supported by the other side of the connection. 
     When supplied, the gateway MUST conform to these media parameters 
     until either the connection is deleted, or a ModifyConnection 
     command with new media parameters (LocalConnectionOptions or 
     RemoteConnectionDescriptor) is received. 

   * LocalConnectionDescriptor:  Supplied by the gateway to the Call 
     Agent to convey the media parameters it supports for the 
     connection. When supplied, the gateway MUST honor the media 
     parameters until either the connection is deleted, or the gateway 
     issues a new LocalConnectionDescriptor for that connection. 
     In determining which codec(s) to provide in the 
   LocalConnectionDescriptor, there are three lists of codecs that a 
   gateway needs to consider: 

   * A list of codecs allowed by the LocalConnectionOptions in the 
     current command (either explicitly by encoding method or implicitly 
     by bandwidth and/or packetization period). 

   * A list of codecs in the RemoteConnectionDescriptor in the current 
     command. 

   * An internal list of codecs that the gateway can support for the 
     connection. A gateway MAY support one or more codecs for a given 
     connection. 

   Codec selection (including all relevant media parameters) can then be 
   described by the following steps: 

   1. An approved list of codecs is formed by taking the intersection of 
      the internal list of codecs and codecs allowed by the 
      LocalConnectionOptions. If LocalConnectionOptions were not 
      provided in the current command, the approved list of codecs thus 
      contains the internal list of codecs. 

   2. If the approved list of codecs is empty, a codec negotiation 
      failure has occurred and an error response is generated (error 
      code 534 - codec negotiation failure, is RECOMMENDED). 

   3. Otherwise, a negotiated list of codecs is formed by taking the 
      intersection of the approved list of codecs and codecs allowed by 
      the RemoteConnectionDescriptor. If a RemoteConnectionDescriptor 
      was not provided in the current command, the negotiated list of 
      codecs thus contains the approved list of codecs. 

   4. If the negotiated list of codecs is empty, a codec negotiation 
      failure has occurred and an error response is generated (error 
      code 534 - codec negotiation failure, is RECOMMENDED). 

   5. Otherwise, codec negotiation has succeeded, and the negotiated 
      list of codecs is returned in the LocalConnectionDescriptor. 

i have a doubt that in step 5,the callee mg returns the negotiated  list of 
codecs.it is not a single codec .so in step 3),mdcx is only send to the 
first endpoint. but for the second endpoint ,how it decide to use which 
single codec ?for a call ,only a unique codec sould be choosed .

thanks,
Neal

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From: "Tom-PT Taylor" <[email-masked]>
Date: Wed, 31 Aug 2005 10:22:28 -0400

You can end up with multiple codecs quite reasonably -- for instance,
G.711, G.711 voice-band data, and RFC 2833 events, and perhaps RFC 2198
redundancy for good measure  (I've just been rewriting the data-related
part of RFC 2833bis).

> directional connection involves at least 3 steps:
...>
> 
> i have a doubt that in step 5,the callee mg returns the negotiated  list 
> of codecs.it is not a single codec .so in step 3),mdcx is only send to 
> the first endpoint. but for the second endpoint ,how it decide to use 
> which single codec ?for a call ,only a unique codec sould be choosed .
> 
> thanks,
> Neal
>

---

From: jijo <[email-masked]>
Date: Wed, 31 Aug 2005 20:57:41 +0530

Hi,

That true, But the Call Agent does a modify to first endpoint with media
info selected from the list of media parameter send by the second
endpoint. So the first endpoint will try to establish media connection
to the second endpoint using the media info rcieved in the MODIFY. so i
think still it works.

Thanks

Jijo