[Asterisk-biz] T1 Technology and VoIP Gateway Primer

[alex at pilosoft.com]

On Thu, 28 Apr 2005, Matt Roth wrote:

> Following up on that suggestion, I began researching T-1 time division
> multiplexing in order to understand where the DSP load on an Asterisk
> server originates and the best options for purchasing a VoIP gateway to
> offload that processing onto. The results of that research can be found
> at the following links:
You are somewhat confused

> My understanding of the T-1 TDM and the PSTN side is pretty solid, as it
> is mainly based off of Intel Corporation's T1/E1 Technology Primer (see
> Sources), but the CPE side is largely deduced from what I knew about the
> PSTN side.  There may be holes or mistakes, so I would appreciate any
> corrections or additions that you can offer. Specifically, I would like
> a detail of the TDM - VoIP conversion process, similar to the basic T-1
> TDM one I provided.
You are severely confused, using wrong terminology, so it is very hard for 
us to understand what exactly you are trying to say and what you are 
asking. 

I do not understand what distinction you are trying to make between "PSTN" 
and "CPE". My gut feeling is you are trying to distinguish between "FXS" 
line (a desktop phone) and "FXO" line (to carrier). 

DS1 signalling does not really care whether it is carrying FXO or FXS 
signal. It is the endpoint equipment that needs to agree (one end needs to 
use FXS signaling and the other needs to use FXO signaling). 

http://home.comcast.net/~mroth01/T1-PSTN.gif
Generally, you don't refer to the ulaw codec as DSP.


> The differences between a T-1, DS-1, and ISDN are subtle and not
> universally agreed upon.  For a discussion of these issues see the
> following links:
They are not subtle and they are very clear.

> 
> What's the diff between a T1 and a DS1
> (http://pbxtech.info/showthread.php?t=1100) PRI setup
> (http://pbxtech.info/showthread.php?t=1250)
Don't try to gain knowledge from web forums, you'll only become dumber, it
is like learning about hosting by reading WHT. I feel dumber already after
reading those posts.

There's no difference. DS1 is a standard signaling with 1.54Mbps raw
capacity. T1 is a product name for DS1 by your carriers. PRI is "primary
rate ISDN" which is DS1 partitioned into 23 "Bearer" channels for calls,
and one "Data"  channel for ISUP call signaling.

> - Is my understanding of using the same codecs and signaling protocols
> on both sides of the Asterisk server in order to circumvent transcoding
> and conversions on the server correct?
Yes

> - Are there any other host-intensive processes that I should consider
> offloading to the gateway, such as echo cancellation?
Yes, echo cancellation.

> - What does the PCM µ-law codec used in T-1 multiplexing map to in terms
> of Asterisk codecs (G.711 µ-law, perhaps)?
Yes, PCM u-law codec is exactly the same as G.711 ulaw. 

> - What codec does the Monitor application use when digitally recording
> calls (if possible, I would like to avoid transcoding the streams when
> recording and let sox handle the conversions on a different box)?
I *believe* that it will write the data in G.711 format. Don't rely on 
this though.

-alex