[Asterisk-Users] Calculating required bandwidth

[Jim Van Meggelen]

asterisk-users-bounces at lists.digium.com wrote:
> http://www.voip-info.org/tiki-index.php?page=Bandwidth%20consumption
> 
> A T1 set up for voice uses G711 at 64Kbps which at 1.544Mbps
> equals 24 channels. This is also known as a PRI.

No, that is incorrect. PRI is a type of ISDN service that is typically
carried across one (or more) T1s. The PRI-ISDN service uses one of the
24 channels in the T1 as a signalling channel (or D-channel), the other
23 channels become the bearer-channels (or B-channels).

PRI is sometimes referred to as 23B+D, but in reality can be built far
more creatively than that. Many large PBXs will have a single PRI
circuit that is delivered on two T1s, with a primary and backup
D-channel in each T1 (46B+2D). If you wanted, you could save the backup
D-channel and have a 47B+D (bad idea, but technically possible). It is
even possible to have a D-channel running across a serial link,
completely out of the T1 altogether; although I haven't seen one of
these in a long time . . .

To summarize:
A T1 is a *circuit*, that can be used to carry all kinds of different
services.
PRI is one kind of *service* that is commonly carried over a T1.

Cheers,

Jim.



> 
> -Matthew
> 
> ----- Original Message -----
> From: "Ed Greenberg" <edg at greenberg.org>
> To: "Asterisk Users Mailing List - Non-Commercial Discussion"
> <asterisk-users at lists.digium.com>
> Sent: Thursday, December 16, 2004 11:45 AM
> Subject: [Asterisk-Users] Calculating required bandwidth
> 
> 
>> I was posed this question:
>> 
>> A T1 set up for voice carries 24 conversations on a circuit that is
>> 1.544 megabits/second. Right?
>> 
>> Well, if you set that T1 up to carry data and run a link between two
>> IP networks over it, how many SIP conversations could it be expected
>> to
> carry?
>> How about IAX?
>> 
>> How would one extend this calculation to varying bandwidth circuits
>> and various VOIP protocols (MGCP, SCCP and H323 come to mind)?
>> 
>> Rather than asking for a full education here, can somebody point me
>> at a suitable practical reference? Of course, if somebody wants to
>> actually
> post
>> the answer that'd be fine too :)
>> 
>> THanks,
>> </edg>
>> 
>> 
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