Advice on multi-branch Asterisk deployment


I am currently in the planning stages of replacing our branch office POTS lines with a voip solution using an Asterisk server in the corporate office. Since I am new to Asterisk, I am working with an Asterisk consultant but I would like to run a few questions by you guys just to make sure this guy knows what he’s talking about.

1.) We currently have VPN connectivity between the branches and corporate. Will these connections suffice, or would I be better served using a VOIP provider to deliver the connections?

2.) Can anyone point me to hardware requirements that would cover up to 100 simultaneous connections as well as a handful of those being recorded and voice mail? (I apologize if this is obvious, but I have been unable to find any specs).

3.) Can anyone provide real-world examples of where they have tried this and succeeded or failed? What were some of the problems you ran into? Did you have any branches that were able to successfully utilize existing bandwidth (DSL, cable)?

I have read a ton and these are the issues that I can’t seem to find specific answers for. If there are resources out there that I should absorb (rather than bugging you guys), feel free to point me there.


A VPN connection can work, although you will not have any quality of service (QoS) for your voice traffic. This means that the voice traffic will be subjected to the vagaries of the internet. It will work most of the time, depending on what kind of connections yuou have at each location.

I am more concerned about the server located at corporate. Having a physical server able to handle 100 simultaneous connections isn’t the issue but, rather, having a large enough pipe to the internet to service them. Using G.729, you will consume about 35 Kbps per channel, so 100 conversations is 3.5 Mbps. If you use G.711, you will need 8.0 Mpbs. Additionally, if 2 people in the same remote office are talking extension to extension, the traffic still goes to the server and then comes back. Checking voicemail takes as much bandwidth as a phone call. You might consider collocating your voice server with an ISP that can provide the bandwidth necessary.

DSL can handle VoIP but again, you won’t have any QoS. Additionally, the upstream connections on DSL are usually rather low and you may find that you don’t have enough bandwidth if you have a number of people at the remote office.

The good news is that we will be no where near 100 connections to start. I just wanted to put a server in place that would scale to that if required. The corporate office location would simply be a starting point for this project. Once it appears that the plan is sound, we would then move the server to our co-lo facility which has all the bandwidth we will need.

The biggest concern, that you thankfully brought to my attention, was the issue with inter-office communication. That little item totally slipped off of my radar. What is the best practice for a situation like that? Is there where I should consider a separate provider? Server at each location?

This is where a couple of good case studies would come in handy. How do others use voip in conjunction with remote branches?

Seems that I still have quite a few kinks to work out…


Best practice is to have enough bandwidth, with QoS, that will serve the voice and data requirements for each office. Without that, you will likely have unhappy users.

You can put a server at each location or add a server to a location that begins to have more traffic than the circuit in place can handle. Add up the locations and the number of users at each one and do the math. Just remember that you won’t have QoS.

We are an ISP and provide hosted PBX solutions for our customers, so in a way, we have lots of remote locations. We use our own circuits and provision them with QoS. Everything is on the same network, so we provide QoS all the way to our asterisk servers. We also provision QoS on the customer’s LAN. We use ADSL for smaller customers because we have the ability to provision our DSL with 2 PVCs, thus giving one of them precedence for the voice traffic. Most of our voice customers have T1s.