Transcoders- When to use

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Sep 10th, 2008
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I have a question regarding Transcoders- when to use them

If you have a remote site with G711 in the local site but G729 between the sites


1. The handset advertises the possible Codecs it can use including G711& G729, G722 etc as some of the options. If the call is now between two sites which forces the call to use G729 - is there any transcoders involved since the phones can respond to G729 ?

2. If you now add Unity in to equation, I understand that you have configured it for either G711 or G729 or both but the Prompts are G711, Unity can do transcoding in S/W but is CPU intensive so you may need a transcoder correct ?

3. Can Unity play the prompts in G729 so in theory would there be no need for a transcoder as the prompts are already G729 fro both LAN and WAN based calls- what is the quality of the voice prompts like?

4. If you use conferencing between two phones on the same site i.e. G711 , If you now conference in an external PSTN user do you need a transcoder ?

5. If you repeat the above conference but now with another Site G729, G711-Internal and an external PSTN caller do you need a transcoder?

Lots of questions but many thanks

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Rajesh Revuru Wed, 09/10/2008 - 02:43
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Transcoder comes into picture when the "End points" does not support the code the stream is comprised of.

Here is a more explanation from Cisco SRND



A transcoder is a device that converts an input stream from one codec into an output stream that uses a different codec. It may also connect two streams that utilize the same codec but with a different sampling rate. In a Cisco Unified CallManager system, the typical use of a transcoder is to convert between a G.711 voice stream and the low bit-rate compressed voice stream G729a. The following cases determine when transcoder resources are needed:

•Single codec for the entire system

When a single codec is configured for all calls in the system, then no transcoder resources are required. The G.711 codec is supported by all vendors. A single-site deployment usually has no need for conserving bandwidth, and a single codec can be used. In this scenario, G.711 is the most common choice.

•Multiple codecs in use in the system, and all endpoints are capable of all codec types

The most common reason for multiple codecs is to use G.711 for LAN calls to maximize the call quality and to use a low-bandwidth codec to maximize bandwidth efficiency for calls that traverse a WAN with limited bandwidth. Cisco recommends using G.729a as the low-bandwidth codec because it is supported on all Cisco Unified IP Phone models as well as most other Cisco Unified Communications devices, therefore it can eliminate the need for transcoding. Although Cisco Unified CallManager allows configuration of other low-bandwidth codecs between regions, the current phone models do not support those codecs and would require transcoders. They would require one transcoder for a call to a gateway and two transcoders if the call is to another IP phone. The use of transcoders is avoided if all devices support and are configured for both G.711 and G.729 because the devices will use the appropriate codec on a call-by-call basis.

•Multiple codecs in use in the system, and some endpoints support or are configured for G.711 only

This condition exists when G.729a is used in the system but there are devices that do not support this codec, or a device with G.729a support may be configured to not use it. In this case, a transcoder is also required. Devices from some third-party vendors may not support G.729. Another example where G.729 is supported but might not be configured is with Cisco Unity. Cisco Unity has support for accepting calls with G.729a, but the codec is implemented in software and is CPU-intensive. Because as few as 10 simultaneous calls can cause significant CPU utilization, many deployments choose to disable G.729 on Cisco Unity and off-load the transcoding function to a dedicated transcoding resource external to the Unity server. If your system includes Cisco Unity, determine whether Unity will accept G.729a calls or whether it will be configured for G.711 only.

Note Cisco Unified MeetingPlace Express currently supports G.711 only. In an environment where G.729 is configured for a call into Cisco Unified MeetingPlace Express, transcoder resources are required.

To finalize the design, it is necessary to know how many transcoders are needed and where they will be placed. If multiple codecs are needed, it is necessary to know how many endpoints do not support all codecs, where those endpoints are located, what other groups will be accessing those resources, how many maximum simultaneous calls these device must support, and where those resources are located in the network.


Chris Deren Wed, 09/10/2008 - 06:30
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1. Phones can negotiate the codec and will not need a transcoder.

2. Unity can use native software transcoding, depending on the design (size) you may be better off using hardware transcoders.

3. Yes, during install you specify the codecs

4. Depending on the GW configuration, if the GW is in flex mode (default) then no you dont need a transcoder as it can negotiate any codec and will simply use more PVDM channles, if the GW is hard-conded to low complexity then yes you will need a transcoder.

5. same as above




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