[Asterisk-Users] * point to point t1 solution? / alternatives

[Bill Michaelson]

This has been an interesting discussion for me (except for the 
sniping).  The last post led me, out of curiosity, to this wiki entry:

http://www.voip-info.org/wiki-Asterisk+TDMoE

I was unaware of this feature, and it looks pretty good.  I've been 
pondering replacing some T1's by leveraging IP capacity but of course 
have run up against the QoS issue.  My idea was different...

I don't have production experience with precisely this type of 
application, but I ask for validation from this list.  Pardon me for 
stating what is undoubtedly obvious to many...

The key to assuring adequate performance in replacing a TDM link with IP 
is to assure that adequate idle time is reserved for voice on the IP 
segment(s) involved in the route.  In this way, latency can be 
stabilized, and if maintained below a certain (arbitrary) threshold, 
performance can be deemed acceptable.

The first step, of course, is to assure that the virtual TDM allocation 
does not exceed the available IP bandwidth (so leave a margin, which is 
huge in the example given).  The next step is to use routers which 
respect the TOS field (however it is used; diffserv/whatever), and 
finally, to assure that no non-VoIP traffic can be injected into the 
path with higher routing priority.

On a point-to-point link, a pair of typical Linux boxes can do all 
this.  Given the original problem, I would place Asterisk boxes at 
either end of the link, and have them blend the ordinary traffic with 
the VoIP traffic (which would probably use IAX to relay calls between 
the T1s), while assuring (enforcing) that VoIP packets are marked as 
highest priority.  There are varied ways of accomplishing this, and a 
good reference which I've used in the past can be found at:

http://www.lartc.org/lartc.html

Additionally, I think one could use the tunneling  techniques described 
in that guide to encapsulate the non-VoIP traffic such that its packets' 
originally marked TOS values are preserved for transit outside the 
segment used for TDM emulation.  In this way, that part of the segment 
bandwidth not required for VoIP would function as a dedicated link, 
allowing other prioritization of traffic such as interactive vs. bulk 
(or even other voice!), with the added advantage that it could use the 
reserved VoIP bandwidth when it is otherwise not required (albeit in the 
case of a T-1 over 10Mb, that's insignificant).

Is this easier or harder than TDMoE as described?  Does the TDMoE shared 
idle bandwidth?  What about stability (I'm thinking of SW releases)?  
What other drawbacks or advantages are there?

>Date: Wed, 25 Jan 2006 23:53:59 -0700
>From: "Damon Estep" <damon at suburbanbroadband.net>
>Subject: [Asterisk-Users] * point to point t1 solution?
>To: "Asterisk Users Mailing List - Non-Commercial Discussion"
>	<asterisk-users at lists.digium.com>
>
>Can anyone point me to a reference or sample config for bypassing a
>nailed up (point to point) t1 between two PBXs with asterisk and a pair
>of t1 cards?
> 
>Right now I have 2 Nortel norstars connected to each other via a leased
>line t1. I also have a solid 10mbps low latency microwave link between
>the 2 sites.
> 
>My goal is to run an asterisk box at each end with a t1 card and
>Ethernet card to act as a TDM<>SIP gateway to bypass the nailed T1 in a
>relatively dumb configuration, with the goal of migrating off of the
>norstars eventually.
>
> In past situations I would have done this with a pair of Cisco routers
>with T1 interfaces in them but in this case I want to get asterisk into
>the picture as an eventual replacement for the norstars.
>
> 
>  
>