[Asterisk-Users] Small office requirements - Can this be done ?

[Low, Adam]

I've done a fare amount of analysis on codec bandwidth requirements and you should remember that you typically will require more bandwidth over ADSL than you would over any other technology. I estimate a requirement of around 108Kb (on the wire) per G.711 channel rather than 86kb over straight PPP/HDLC based connections.

Why I hear you ask ?

The following calculations are based on G.711 PCM running at 20ms samples resulting in 200 byte packets (default for most codec implementations).

200 bytes G.711 packet + 8 bytes AAL5 overhead = 208 bytes
208 bytes fit in 5 cells of 48 bytes payload
5 cells are 265 bytes. VoIP over ATM AAL5MUX thus has an overhead of 21.51%
VoIP G.711 conversation sends 50 packets per second.  This uses 250 cells per second.
This causes approximately 10 OAM5 cells to be sent over the duration.

The total bitrate is thus (250 + 10) * 53 bytes * 8 bits = 110240 bits/second = 107.66Kbit/s


Steve Kennedy wrote:

>On Mon, Mar 01, 2004 at 07:08:29PM -0600, Michael Graves wrote:
>
>  
>
>>I have disagree about the ADSL. I have a1.5 Mbit/512kbit service from
>>Covad (in the US Southwest) and I have sustained 4 calls without a
>>problem.  I prefer to use GSM over G.711to squeeze it down, but that is
>>my choice. I don't feel that call quality is substandard.
>>    
>>
>
>That's the crunch (1.5/512) ... it's actually the 512 which is relevent.
>Virtually all DSL in the UK is a wholesale product from BT (they have
>about 2 million customers, Easynet who local loop unbundle may have
>20,000, the rest of the providers maybe another 10,000 between them).
>
>All BT ADSL is 256K upstream, all BT DSL is contented (in theory 20:1
>and 50:1, but actually a lot less than that), there are a few providers
>doing their own contention over BT's product.
>
>However the 256K upstream is still the limiting factor, so you can get
>one, and MAYBE two VoIP lines over it. If BT would up the upstream to
>512, you could probaly get 4 out of it ....
>
>
>Steve
>
>  
>
On the UK DSL using G.711 you should easily get 2 concurrect calls, 
G.711 uses about 84k(incl overhead) in each direction, so 2 calls would 
be 168K (of the 256k)

If you switch o GSM or iLBC you should get 6 concurrent calls, and if 
you were to use IAX2 trunking you could *maybe* squeeze another one..

Other codecs could offer even more but I haven't tested them..

Later..


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