Re: [codec] #1: Application: 2.1. Point to point calls supporting transcoding?

"Christian Hoene" <> Wed, 24 March 2010 16:08 UTC

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From: Christian Hoene <>
To: 'James Rafferty' <>,
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Date: Wed, 24 Mar 2010 09:08:09 -0700
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Subject: Re: [codec] #1: Application: 2.1. Point to point calls supporting transcoding?
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Hi all,

the well known reasons for avoiding transcoding/tandem-coding are:

1) It decreases the conversational quality by decreasing speech quality and increasing transmission delay. Thus, it is in the interest of users/customers to avoid transcoding.
2) It violates Internet's end-to-end principle and thus increasing system complexity and costs. Thus, an Internet codec shall not consider operation modes of transcoding, typically.
3) Codecs must be optimized towards transcoding, which is a time consuming tasks. Thus, it is in the interest of codec developers and tester to avoid the additional cost of verifying the transcoding scenarios.

Of course, we are not living in an perfect world and there are many valid reasons to install transcoding gateways to manage proper interaction between two remote sides/operators. If it is required to do so, there are multiple options:

1) The gateway tries to negotiate a proper codec that is supported by both sides.
2) Two ITU-T codecs, which are very well optimized towards tandem-coding conditions, are selected.
3) Only, as a last resort, it might be needed to transcode between an Internet-codec and other codecs.

I am not yet convinced that option 3) will happen frequently, especially considering the facts that the Internet codec is intended towards IP-to-IP use cases.  Also, it is quite easy to include additional traditional codecs beside the Internet codec into a phone or gateways. In addition, the costs of updating the software of a phone or gateway to support the Internet codec might not be very high because of the lack of IPR fees.

Thus, I would recommend to consider transcoding as a requirement of very low priority, as it is going to be used seldom. It shall be ensured that the Internet codecs does not "fail" during tandeming but that not particular effort is invested to make the Internet codec work particular well for transcoding.

Having this say, it is appreciate if volunteers will test the Internet codec in tandem conditions so that the codec design can be influenced.

With best regards,


Dr.-Ing. Christian Hoene
Interactive Communication Systems (ICS), University of Tübingen 
Sand 13, 72076 Tübingen, Germany, Phone +49 7071 2970532

>-----Original Message-----
>From: James Rafferty []
>Sent: Tuesday, March 23, 2010 4:14 PM
>To: Christian Hoene
>Subject: RE: [codec] #1: Application: 2.1. Point to point calls supporting transcoding?
>Please see my comments below.
>-----Original Message-----
>From: Christian Hoene []
>Sent: Tuesday, March 23, 2010 2:53 PM
>To: James Rafferty
>Subject: RE: [codec] #1: Application: 2.1. Point to point calls supporting transcoding?
>Hi James,
>>In my view, this is dodging the real issue.  I believe there WILL be transcoding for point to point
>>calls between wireline and wireless networks, where the connection is IP on both sides and multiple
>>wide band codecs are used.
>Could you elaborate a bit more on this scenario? Do you mean that the wireline has -for example- Skype
>phone with the Internet codec and the wireless end will have an LTE codec? Of course, both sides will
>use IP.
>JR - Yes, the example of Skype from the desktop connecting to a mobile phone with wideband audio (say
>G.722.2) is an example.  In order for that to happen, there would likely be a border element at the
>network edge (what the Speermint architecture calls a Data Border Element) which negotiates the
>transcoding to take place in real time.  Another use case example would be where an enterprise IP
>phone uses a wideband codec when calling to a mobile phone, again requiring transcoding to take place
>to complete the call, if the two handsets do not support a common HD codec.
>>For that matter, there will also be cases where a PSTN - IP gateway is needed, but that's not usually
>>defined as transcoding.
>As a manufacturer of IP-to-IP gateways your insight knowledge is highly appreciated.
>With best regards,
>  Christian Hoene
>>James Rafferty
>>Product Line Director, Integrated Media Gateways
>>Dialogic, Inc.
>>15 Crawford Street
>>Needham, MA 02494
>>Tel:    781 433 9462
>>Mobile: 781 929 3895
>>Fax:    781 433 9268
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>>-----Original Message-----
>>From: [] On Behalf Of codec issue tracker
>>Sent: Tuesday, March 23, 2010 10:20 AM
>>Subject: [codec] #1: Application: 2.1. Point to point calls supporting transcoding?
>>#1: Application: 2.1.  Point to point calls supporting transcoding?
>> Reporter:  hoene@…                 |       Owner:
>>     Type:  enhancement             |      Status:  new
>> Priority:  major                   |   Milestone:
>>Component:  requirements            |     Version:
>> Severity:  Active WG Document      |    Keywords:
>> Question: Shall the point-to-point call support transcoding?
>> If not, please add something like
>> "This scenario does not include the use case of using a VoIP-PSTN gateway
>> to connected to legacy telephone systems. In those cases, the gateway
>> would make an audio conversion from broadband Internet voice to the frugal
>> 1930’s 3.1 kHz audio bandwidth. Interconnections to the PSTN will most
>> likely stick with its legacy codecs to avoid transcoding."
>>Ticket URL: <>
>>codec <>
>>codec mailing list