Transcoding design with Call Manager and Local Voice Gateways

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Jan 9th, 2009
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When you have centralized UCM and 17 remote sites:

Do you need a local transcoder (ie local voice gateway) to handle the transcoding or can this be done

by a remote device (ie. UCM or remote voice gateway). How does the transcoding process work based on having to go back and forth across the WAN to the UCM to do the transcoding? Doesn't it make sense to have it locally?

Currently we have the UCM as the MTP transcoder. At what point do we want to use the local gateway and only use the UCM as a failover?

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Jaime Valencia Fri, 01/09/2009 - 12:48
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thumb rule is media resources as close to devices going to use them.

if you need xcoding for a local device but resources are across the WAN that means you'll have the original stream going to xcode across wan, then xcoded stream back to terminating device again across wan.

always use local resources as primary before trying to use remote resources



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CHRIS CHARLEBOIS Fri, 01/09/2009 - 12:49
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I would do whatever needs to be done to get away from transcoding. It is processor intensive, degrades voice quality, and is usually avoidable.

What is the scenario that requires transcoding in your network?

CHRIS KALETH Fri, 01/09/2009 - 13:01
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Our design is as follows: Office A to Office A = G711, Office A to Office B,C,D,E... is G.729. If site A is talking to site B across the WAN does it use transcoding?

CHRIS CHARLEBOIS Fri, 01/09/2009 - 13:16
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No, each Office in that scenario should be setup as a seperate region within CCM and you can define the maximuim codec between phones in different regions. That way, the phone, during setup, will use G.729 when the call is between offices.

The only reason I can think of to use transcoding is if a particular device did not support the codec that you need. It is always better to encode in one code end-to-end rather than transcode (decode/re-encode, losing quality and introducing delay at each step).

CHRIS KALETH Fri, 01/09/2009 - 13:23
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We do have our regions configure in CCM and they are configured for G711 (intra region) and G729 for all other regions.

In what scenarios would you need to transcode then?

1)Office A calls Office B and then conferences in a PSTN call. Will transcoding occur and by what device (assuming xcode is available local in each site)

2)PSTN call to user in Office B, will there be xcoding?

3) Intercluster call between Office B and Office X? Will there be xcoding

What other call scenarios would warrant local transcoding resources? In addition to the transcoding, what about conferening resources being local?

Jaime Valencia Fri, 01/09/2009 - 14:08
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you're proposing many scenarios which have many variables, start by reading this to get some more insight:

Media Resources

1. if PSTN call is set to use G729:

a) using HW CFB, no

b) using SF CFB, yes

what xcoder? the one the MRGL from the CFB has designated or available wherever it is

if call uses G711 NO, for a and b

2. no, phones support G711 or G729 natively

3. see above

scenarios: UCCX, UCCE, CUE, Unity and more. they only support one codec so if a call comes in with any other codec than the one configured, XCODER is necessary.

local conferencing depends on codec, DSP availability and available resources ($). if you have enough money, use DSPs on each remote site, or use the SW CFB using more BW over the WAN.



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STEFAN CHRISTEN Fri, 01/09/2009 - 15:25
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Hi Chris

As the other guys already mentioned. Try to avoid transcoding. Better use the money for more bandwidth. If you only rarely use conferencing, you could set up a specific region for the media resources on the CUCM (conference bridge, MoH, MTP, ...) that speaks G.711 with all other regions.

By the way MTP on the CUCM is not transcoding.




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