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Walkthrough Wednesdays

Transcode in SRST

is it possible to transcode in SRST mode?

thank you all,


Tommer Catlin

When you are in SRST mode, why would you transcode? Typically, all calls will go in/out the local PSTN connection on the router.

If you can give us a clearer picture on what you are trying to accomplish, maybe we can help


my srst router is connected to a softswitch that routes calls in G729r8. that's the reason why...

I guess I dont understand the purpose of SRST then for your configuration.

Sounds like you have:

CUCM Pub and Sub at hub location.

Remote Site with SRST router and IP phones.

The remote site looses connection with CUCM Pub and Sub, the phones will also stop "keep alives" to the Pub and Sub. The phones are then pre-configured (based on Device Pool and SRST configs) to fallback in SRST mode and register with the designated SRST router.

This router then has control of all phones, PRI, etc.

If you need to transcode calls, where are you going to transcode the call to/from if you WAN link is down??

I could be missing something here

my wan link is not down. i have mpls to reach my softswitch, it's all ip. i don't have any T1 or E1.

my router goes to srst mode when it loses connection with CCM or CCME (this case), but does not loses connectivity with my softswitch!

I think you are out of luck on that.

It's an unsupported option when in SRST mode. I dont SRST was really meant for this anyways. Your WAN is your MPLS. So if MPLS is down your phones fall into SRST mode. Once in SRST mode, and since you have no PSTN connection, the phones will basically be able to call each other and no-one else. If you had a secondary backup WAN connection back to CUCM (which I thinks is what you are calling "softswitch?") then the router and phones would not be in SRST mode.

You can configure CME as SRST (supported starting with CME 4.0) which will allow you to register local PVDMs with CME and use for transcoding.


But I dont think you can do this when you use CUCM correct?

Sure you can. Starting with CME 4.0 this is supported, licensing is the same with CME and SRST. Take a look at this doc.



ok, but I have a CUCM and a router/phone registered to it, there is a way to do transcoding in SRST?? I thought when in CUCM, all transcodes are registered to CUCM. If the router is disconnected from CUCM, the transcodes/conferences are down. SRST has basic options which were listed in the doc I posted.

What I dont understand is where you would use transcoding in SRST mode. If the phones can not connect to CUCM and fallback to the local SRST router, where is the router going to transcode to/from??

As I said reconfigure SRST as CME as SRST as described in the document.


Sorry, didnt realize you could have SRST and CME as a fallback. I thought it was one or the other.

CUCM + SRST router



But what the doc says, you can have the SRST router be connected to the CUCM cluster, in the event the SRST router loses connection, it fallsback into SRST mode, but then functions as an CME router to the local phones.

Excatly, with all CME features including Xcoding.


Thanks for the info. Must be a feature that's either new or I have never used. All of the typical SRST sites I have deployed over the years never had to use CME as backup or needed. It's always basic SRST until MPLS or WAN is backup. Make phone calls, receive phone calls, roll to Unity through PSTN.

Yeah, it was introduced with CME 4.0 1-2 years back, most people dont realize it's there and are not aware of it. I've done few projects with it, recently I did one with B-ACD for remote contact center sites. The sites normally utilize CVP, but now when they fall into SRST/CME B-ACD kicks in with basic call treatment with couple of sales queues, AA, etc.


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