BW Calc Question

I've been reviewing study questions for CVOICE. I haven't figured out how to create the equation for the question below.

Assuming no cRTP or header compression, how many VoIP G729 calls can be made simultaneously over a 64kbps Frame Relay circuit (Layer 3)

A. 1
B. 2
C. 3
D. 4
E. 5

It seems the Voice_Payload_Size variable is missing (not given). How do i find it?

Here's what I know:
codec bitrate = 8000 (G729)
FRF= 6bytes
IP/UDP/RTP=40bytes
Total_Frame_Size= 40+6+Voice_Payload_Size
PPS=codec_bitrate/Voice_Payload_Size

The equation for bandwith is
BW= Total_Frame_Size x PPS

Thanks,

RCHASE

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Comments

jahsoul

The voice payload for G.729 defaults at 20 bytes, so you you voice payload for frame size would be 20 and you voice payload size for PPS calculation would be 160. (Voice Payload Size is PPS calculation is payload size * 8 bits per byte). HTH.

rchase

Thanks jahsoul! I actually realized this earlier today while my coworker was driving us to a jobsite and I was reviewing Cisco's Bandwith Calc document that I got from shawn's post. I printed it out and have been reading over it each time I get a few free minutes during the day, highly recommend:

Voice Over IP - Per Call Bandwidth Consumption - Cisco Systems

The first table breaks down the default Voice Payload Size, PPS, Codec Sample Interval etc.

So, my fellow CVOICE students, this means in addition to memorizing the bandwith calc FORMULAs, we also need to memorize all the codec default VARIABLES, in order to answer questions that do not provide the variables.

jahsoul

That bandwidth calculator is a life saver. The variables is what killed me because I would always forget to calculate the PPS and always pick 50...lol