[asterisk-users] Benefit of PRI vs SIP trunk calls

[Tom Rymes]

On Jan 6, 2011, at 8:08 PM, Joel Maslak wrote:

> On Thu, Jan 6, 2011 at 7:06 AM, Jim Dickenson <dickenson at cfmc.com> wrote:
>> Are there reasons to prefer the use of PRI over SIP or SIP over PRI?

[snip]

> I run the PBX for my organization which has about 160 extensions.  I
> wouldn't even think of doing anything but PRI for the main lines
> because (A) for our size organization where we are located, we're
> talking a couple hundred dollars a month difference between PRI and
> SIP in cost so it's nearly break-even in cost which means cost
> difference isn't a huge motivator, (B) it supports FAX, modems, and
> TTYs - perfectly, (C) Quality is 100% consistent.  In addition, the
> reliability is good enough that I'm willing to use it for 911.

[snip]

I have to agree with most of what Joel said in his message. For me, the main problem with many sip implementations is that your phone service will be only as reliable as your internet service. If you have a dedicated internet line that is highly reliable, that's not a big deal, but DSL, Cable, and the like aren't reliable enough for our needs.

Having said that, one downside of a PRI is that you are paying for all of those channels, even when you aren't using them. Companies like Paetec and most other large telcos are offering SIP trunks over an MPLS circuit, running on a T1 loop. This covers the reliability problem, as you are running over the same type of circuit as your PRI, and it allows you to take advantage of unused channels as data bandwidth. This is especially helpful for folks who have a data T1 and a PRI, as they can get higher bandwidth for data when there isn't much voice traffic. Because they use G.729, you can also fit more calls on the same circuit. That choice of codec eliminates the ability to send/receive faxes, though, and it's likely expensive when compared to other SIP solutions, but it does appear to be pretty slick. 

Another benefit of SIP is that it doesn't require a Digium, Sangoma, or similar interface card in the server, simplifying migrations and reducing cost in many scenarios. 

Tom