[asterisk-users] Re: Bandwidth requirements

[Dan Austin]

J. Oquendo wrote:
> Benny Amorsen wrote:
>>>>>>> "rJ" == raphael Jacquot <sxpert at sxpert.org> writes:
>>>>>>>             
>>
>> rJ> ATM cell tax is actually 10% as there's 5 header bytes for each
53
>> rJ> bytes cell,
>>
>> For VoIP the cell tax is much larger. In the example, each RTP packet
>> contains 20 useful bytes and 40 bytes IP overhead. 60 bytes doesn't
>> fit in one cell, so you end up with 106 bytes at the ATM layer to
>> transport 20 bytes of G.729. The ATM-caused overhead is thus 46 bytes
>> per voice packet, thereby making the needed bandwidth 77% larger.
>>
>>   

> CRTP solves this issue (40byte waste)
cRTP is a great idea that is not widely implimented.  Not very
many endpoint support it, and Asterisk currently does not.

In version 1.4, Asterisk will support configurable RTP packetization.
So you could use 40ms for your G729 payload and reach a 50/50
split between payload and overhead (before ATM encapsulation for
DSL).  At 50ms for g729, you get a nice fit into two ATM cells, while
a small bump to 60ms will force you into three ATM cells.

Of course you need to be mindful of the latency between endpoints
when you start increasing the payload, but a healthy network and 
tuning the payload can greatly reduce bandwidth requirements.

AND most endpoints do support configurable RTP packetization.
If Asterisk and more endpoints supported cRTP, then combining that
feature with larger RTP payloads would result in very low 
overhead numbers.

Dan