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Configuring Transcoding in CUCME

Transcoding is one of those features that isn't considered all the time and can easily become an after thought.  I suggest taking the time to read about the features available within CUCME and when you'll need to use them before you jump into configuration with both feet.  Sometimes, the pool is deeper than you expected!

This topic is pretty large and I won't be able to cover it all in this blog entry.  Instead, I'm going to emphasize the configuration snippets that are related to transcoding within CUCME.  A previous blog entry on A previous blog entry on DSP Farm Configuration will also reinforce the content on this page.

Please refer to the "More Information" document at the bottom of the page for ALL of the details. 

Transcoding Functionality

Transcoding compresses and decompresses voice streams to match differing endpoint-device capabilities. For example, transcoding is used when an incoming voice stream uses a codec to save bandwidth, but the local phone does not support that type codec. 

Cisco CME 3.2 and later versions support transcoding between G.711 and G.729 codecs for the following features:
Ad hoc conferencing—One or more remote conferencing parties uses G.729.
Call transfer and forward—One leg of a Voice over IP (VoIP)-to-VoIP hairpin call uses G.711 and the other leg uses G.729. 
Cisco Unity Express—An H.323 or SIP call using G.729 is forwarded to Cisco Unity Express. Cisco Unity Express supports only G.711, so G.729 must be transcoded.
Music on hold (MOH)—The phone receiving MOH is part of a system that uses G.729. The G.711 MOH is transcoded into G.729 resulting in a poorer quality sound due to the lower compression of G.729.

How Does Transcoding Happen?

Transcoding is facilitated through digital signal processors, which are located in network modules. All network modules have single inline memory module (SIMM) sockets or packet voice/data modules (PVDM) slots that each hold a Packet Voice DSP Module (PVDM). Each PVDM holds DSPs. A router can have multiple network modules. 

Cisco Unified CME routers and external voice routers on the same LAN must be configured with digital signal processors (DSPs) that support transcoding. DSPs reside either directly on a voice network module, such as the NM-HD-2VE, on PVDM2s that are installed in a voice network module, such as the NM-HDV2, or on PVDM2s that are installed directly onto the motherboard, such as on the Cisco 2800 and 3800 series voice gateway routers. 

DSPs on the NM-HDV, NM-HDV2, NM-HD-1V, NM-HD-2V, and NM-HD-2VE can be configured for transcoding.
PVDM2-xx on the Cisco 2800 series and the Cisco 3800 series motherboards can also be configured for transcoding.

Transcoding of G.729 calls to G.711 allows G.729 calls to participate in existing G.711 software-based, three-party conferencing, thus eliminating the need to divide DSPs between transcoding and conferencing. 

Reviewing DSP Resources Available for Transcoding

show voice dsp - display current status of digital signal processor (DSP) voice channels
show sdspfarm sessions - display the number of transcoder sessions that are active
show sdspfarm units - display the number of DSP farms that are configured


Configuring DSP Farms for NM-HDs and NM-HDV2s

1. enable
2. configure terminal
3. voice-card slot
4. dsp services dspfarm
5. exit
6. sccp local interface-type interface-number
7. sccp ccm ip-address identifier identifier-number
8. sccp
9. sccp ccm group group-number
10. bind interface interface-type interface-number
11. associate ccm identifier-number priority priority-number
12. associate profile profile-identifier register device-name
13. keepalive retries number
14. switchover method {graceful | immediate}
15. switchback method {graceful | guard timeout-guard-value | immediate | uptime uptime-timeout-value}
16. switchback interval seconds
17. exit
18. dspfarm profile profile-identifier transcode [security]
19. trustpoint trustpoint-label
20. codec codec-type
21. maximum sessions number
22. associate application sccp
23. end

Configuring DSP Farms for NM-HDVs

1. enable
2. configure terminal
3. voice-card slot
4. dsp services dspfarm
5. exit
6. sccp local interface-type interface-number
7. sccp ccm ip-address priority priority-number
8. sccp
9. dspfarm transcoder maximum sessions number
10. dspfarm
11. end 

Acting as the DSP Farm Host

1. enable
2. configure terminal
3. telephony-service
4. ip source-address ip-address [port port] [any-match | strict-match]
5. sdspfarm units number
6. sdspfarm transcode sessions number
7. sdspfarm tag number device-number
8. end 

Verifying DSP Farm Operation

1. show sccp [statistics | connections]
2. show sdspfarm units
3. show sdspfarm sessions
4. show sdspfarm sessions summary
5. show sdspfarm sessions active
6. show sccp connections details
7. debug sccp {all | errors | events | packets | parser}
8. debug dspfarm {all | errors | events | packets}
9. debug ephone mtp

More Information

Cisco Unified Communications Manager Express System Administrator's Guide

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