cvoice bandwidth question

Unanswered Question
Jul 6th, 2007

I found this question on one of the simulators that I have been using to study for the cvoice exam.

I want to fully understand the question. Can some one tell me why answer C is correct. Question with answers are below. How do i calculate this??

What is the minimum bandwidth that would be required to support three simultaneous Voip calls in this Network?

Network Characterisitics

- g.729 with a codec speed of 8kbps

- 20-byte sample size

- Frame Relay without cRTP

- FRF .12 with 6 bytes of overhead

a. 192000 bps

b. 51,600 bps

c. 79,200 bps

d. 247,200 bps

I have this problem too.
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Chris Deren Fri, 07/06/2007 - 05:47

None of the asnwers are correct, there is no way G729 call can take this much bandwidth. Here is the correct answer:

Total packet size (bytes) = (FRF header of 6 bytes) + ( IP/UDP/RTP header of 40 bytes) + (voice payload of 20 bytes) = 66 bytes

Total packet size (bits) = (66 bytes) * 8 bits per byte = 528 bits

PPS = (8 Kbps codec bit rate) / (160 bits) = 50 pps

Note: 160 bits = 20 bytes (voice payload) * 8 bits per byte

Bandwidth per call = voice packet size (528 bits) * 50 pps = 26.4 Kbps

HTH, please rate all posts!


compsolv Fri, 07/06/2007 - 06:04

NetMasterClass just released a CVOICE LEARNiT training module you might want to check out. I did not create this module - but I know the CCIE that did and he is awesome!

The reviews have been very positive so far. It consists of Study Notes, Videos that walkthrough the simulation material, and practice exams.

You can also e-mail experts directly regarding any questions or problems.

Check it out at:

I get so frustrated with incorrect exam prep materials - especially when there is no one you can speak with about it from the vendor.

Anthony J. Sequeira


srue Fri, 07/06/2007 - 06:27

a single G729 call won't use that much bandwidth but three calls will - which was a requisite of the question asked.

Chris Deren Fri, 07/06/2007 - 06:32

Yes, if you look at my claculations 26.4*3=79.2 kbps == 79,200 bps

Sorry, I missed the "three simultaneous Voip calls " part :-)


shirwaziri1_2 Fri, 07/06/2007 - 06:37

Chris, get the points. I needed to know how to answer this question which was more important, and you explained it thoroughly.

srue..thanks for pointing the details out

i appreciate everyone's input. Cvoice exam this Tuesday. Wish me luck.

srue Fri, 07/06/2007 - 07:09

good luck. if you've put in a good couple weeks, you'll do fine. this is the easiest of the VP exams btw (imho). from there, i did the gw/gk exam - as it was a natural follow up from cvoice (others might differ on this point). QoS was relatively easy, but mostly b/c Odom's book kicked @ss.

shirwaziri1_2 Fri, 07/06/2007 - 07:14

I have a new outlook from your statement. I can agree that once i pass the cvoice i should take the gw/gk exam. I am in agreement with you that this would be a natural flow.

So, i shall set my path that way as well. Initially I was gonna go for the qos, but i think your point is valid.

What do you recommend I use for gw/gk. I heard it is the toughest of all the exams..

thanks for input

srue Fri, 07/06/2007 - 07:33

it is definitely the toughest (although i have IPTT left still), partially b/c my experience in voice is minimal, and partially b/c it was just a [email protected] load of material. In the end though, everything the exam covered (from my recollection), is in the Cisco Press gw/gk text. Although it's not a certification oriented text, it's still a good book for the exam.

flash2200 Fri, 07/06/2007 - 10:23

Hey Steve,

What are you using to study for the IPTT? I know ciscopress has one book on Troubleshooting IP Telephony but it seems very dated. Working on GW/GK exam myself... kinda brutal :)


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