Example for when I would need transcoding

[patrick.hurley]

When would I ever need transcoding?   Would it be if someone from the PSTN called me through my local gateway and both he and I would negotiate G711 and I wanted to add another person from another site where we would be set up for G729a across the WAN?  Would the ad hoc resources needed in that case automatically transcode so I wouldn't need transcoders?    Transcoding has always confused me so I have always added them using the DSP calculator.

[Nadeem Ahmed]

Hello Patrick,

It’s really depends on if your network needs a HW transcoder or conferencing. As your case having multi-party conferencing you can use AD-HOC conference as HW so that streaming of packetization from one stream another stream can happen using HW resources .if they using same Codec then there is no need. Transcoding can be really avoided altogether if either your system is completely G7.11 codec, Even both parties using G.729 then also there is no need for codec at all.
 

whenever you need anything which does not involve G.711 (e.g. G.729 to G.729ab or G.726 etc) aka Universal Transcoding.  Software Transcoders can only do transcoding between G.711 and any other Codec .

Just go through  once media resources sections

http://www.cisco.com/c/en/us/td/docs/voice_ip_comm/cucm/srnd/7x/uc7_0/media.html

Br,
Nadeem Ahmed

[Manish Gogna]

Hi Patrick,

The following cases determine when transcoder resources are needed:

•Single codec for the entire system

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

•Multiple codecs in use in the system, with all endpoints 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 Unified CM allows configuration of other low-bandwidth codecs between regions, some phone models do not support those codecs and therefore 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.

The software conference bridge can only handle G711 streams. if one or more parties in a conference  are talking on G729 then you will either need a transcoder or a hardware conference bridge, both needing DSP resources.

The software MTP can only handle G711 codec, transcoders are only hardware based.

You can refer the SRND for more details

http://www.cisco.com/c/en/us/td/docs/voice_ip_comm/cucm/srnd/8x/uc8x/media.html#wp1046264

HTH

Manish