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Size of a Voip Call … and why VoIP can go ‘boom’

By June 17, 2012 Voip

I was at a meeting recently where the topic of VoIP, and the impact of the size of an individual voice call on a Wide Area Network or Internet circuit came up.

I thought I would share a few thoughts here.

First, the nature of the meeting was to review random occurrences of voice call quality.

First question, does this happen during busy periods? (the answer is often yes).

So, with surgical gloves in hand, we begin …. is it equipment, is it QoS, is it misconfiguration (duplex mismatch is our favorite – we lay bets on that one)….

Or, is it more obvious – all the above, covered by a lack of available bandwidth.

A data circuit is not a same as a data circuit that supports voice packets. I will keep this post simple, even with the right ‘pipe’ it can also be a case of appropriately ‘sizing’ the pipe and related bandwidth associated with this.

When you google (my weapon of choice), VoIP, you look at the area of ‘sizing’ or trying to properly determine the amount of bandwidth required for the total number of ‘peak’ calls that you would expect during the busiest hour, or busiest day. This is the number to remember.

I have always quoted two numbers, G711 (uncompressed) of 100 kbps and 50 kbps for G729 (compressed). Meaning for every 1 concurrent call you should budget 100 kbps or 50 kbps for each call. I am obviously rounding up – mostly because the math is easier (number x50 or number x100).

In fact the number may even be a bit higher when you run encryption on voice.

Planning for Voice over IP requires an understanding of the various headers added when transporting packetised voice, especially over an IPSec VPN- we call this Voice overhead as a result of encapsulation.

It means that the size of a call on a standard WAN link is not the same as a the same call when encrypted and encapsulated under and IPSec VPN.

Take a look:

Packet Size—IPSec Encrypted G.729

The Layer 3 data rate for a G.729 call 24 Kbps. Encrypting that packet using IPSec Tunnel mode for IP GRE increases that rate to approximately 56 Kbps (in each direction).

Packet Size—IPSec Encrypted G.711

The Layer 3 data rate for a G.711 call (50 pps) is 80 Kbps. Encrypting that packet using IPSec Tunnel mode for IP GRE increases that rate to approximately 112 Kbps (in each direction).

So, simple formula, if you are managing peak call volume of 40 calls:

Codec

State

# Calls

kbps

extended

ADSL (6mb / 800k)

ADSL2 (15 mb / 1 mb)

Bonded ADSL2 (30 mb / 2 mb)

T1 or 1.54 mbps

10 mbps

100 mbps

G729 Normal 40 24 960 No Yes Yes Yes Yes Yes
G729 VPN 40 56 2240 No No No No Yes Yes
G711 Normal 40 80 3200 No No No No Yes Yes
G711 VPN 40 112 4480 No No No No Yes Yes

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