G.711 vs G.729 w/ CallManager

[NPT_2]

I am looking at installing IP Phones at a remote stie that only has a 768K Frame relay connection back to our host site. We have been running G711 codecs for all our other IP phones for the increased voice quality but since this remote site has only a frame relay connection I am pondering using G729 to lower the call bandwidth and increase call density. In testing on our local network I have tried G729 but am not too happy with voice quality overall compared to G711. Any recomendations regarding using G729 codec for IP Phones in this situation? Also realisticly what do you think our maximum call density would be over this WAN link?

[allan.wells]

In the first instance You may want to look at running ppp multilink fragment-delay 10 to avoid some inconsistent serialization delay which will cause jitter.

Realalisticallay there are many factors such as queue depth of other class traffic how much of CDR the circuit is shaped to use all this effects VoIP

With g.729 over frame the BW used is approx 27kbps per call.

So whith that figure and the data traffic your pushing you should be able to dimension your link accordingly,

http://www.cisco.com/en/US/tech/tk652/tk698/technologies_tech_note09186a0080094660.shtml#link_frag

[NPT_2]

Thanks for the info, lots of things to consider on this project.

Any thoughts on whether it would be prudent to run callmanager express on the local router at the remote site to reduce the signalling sent back and forth to the host site call managers? Or would the signalling traffic bandwidth saved be insignificant?

[rbradfield]

Number of things to consider.

1) how many active voice calls are there going to be over the WAN link , remember g711 takes about 88k of bandwidth per call.

2) does the telco support FRF12 on the frame relay link ( fragmentation need to fragment the packets to 840 bytes)

3) Other realtime traffic on the WAN link

I would say you could have a max of about 5 G711 calls on that link, which would still leave over 30k for other traffic.but make sure you have frf12 and qos inplace on the link.

on the other hand:

I have multiple sites running g729 on low bw links and the voice quality is good, in fact I cannot tell the difference between a g729 call or a g711 call, so i suspect if the voice quality is noticably different then you might have another problem

[jolo07310]

Can you post your config??

Coz I agree with Rbradfield, I couldnt really tell the different between G711 & G729.

[CCampbell_2]

Where did you get that 840 bytes? I've always used speed * 1.25. Which works out to 960 bytes on a 768k circuit. I'm wondering if this could be part of my problems.. :)

Cliff

[rsamtani]

With G729 codec, you would take aproximately 30 Kbps per call. With half of the 768 K reserved for voice (384k), you can get approximately 13 simultaneous calls. Of course the question which needs to be answered is that how many phones do you have @ this remote site? You can also Frame-relay fragmentation and apply 10 ms delay. Fragmentation is recommended for ckts with bandwidth less than 768 k. To get more call density, you can run CRTP on the link, this should reduce the bandwidth to aproximately 13 KBPS.

This could give you increased Call Density.

[NPT_2]

I am looking at about 24 Phones maximum at this remote site. So 13 Simultaneous should be sufficiant. I am waiting on some pricing for local circuits to see if it would be cost effective to drop a partial T1 or PRI of dialtone in locally to keep that traffic off the WAN.

[aalejo]

What Ip Phones are you using?

[NPT_2]

Likely it will be mostly 7940's for this remote site,