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Interconnecting two pbx over satellite

My client wants to interconnect two different types of pbx over satellite link.

One PBX in London, the other one in south Africa. They have 256k satellite link with 500ms RTD. My client wants to be able to make 2~4 concurrent calls to each locations as the pstn in south africa isn't reliable. Those two pbxs are different pbx brand, and not voip enabled (the client wants simple, and cost effective solution for now). so i guess the solution would be with e&m both ends to interconnect pbx as below

PBX --> e&m on cisco router --> {Satellite} --> e&m on cisco router --> PBX

As I?m fairly new to interconnecting two pbx, and e&m stuff? I have the following questions

1. With one e&m interface on each end of the router, how many calls can you make?

2. So when interconnecting two pbx(tie lines), the section between cisco router to cisco router becomes voip? Which I can compress the voice packets to g729 to save bandwith?

3. Do you have any sample config?

4. any other design consideration or caveats?

5. any other general information would be helpful!


Re: Interconnecting two pbx over satellite

You can accomplish this, but you will have consider the bandwidth and latency first. 500ms rtd may be ok to use.

Basically, each PBX will need some sort digital or analog port into the router. The router will have to able to be convert this signal to a an IP voice gateway and be able to route it the remote router.

In your PBX, you are configuring a digital translation or "site code" If someone at Site A wanted to call Site B, regardless of the connection, the PBX has to know where to route it to. In simple terms for this example:

Site A Extensions are 2000-2999

Site B Extensions are 3000-3999

IF a person at Site dials 3xxx, the PBX is configured to push the 3xxx to the trunk that is connected to the router. The router will look at the 4 digits and route it based upon it's configured parmeters.

At Site B, the router receives the 3xxx and pushes through the voice port connected to the local PBX. The local PBX picks up the 3xxx and rings this phone.

You will also want some sort of QOS between the routers if possible or RSVP configured for bandwidth for the voice calls. Don't let the data streams hog the bandwidth. Reserve a few kb's for voice only.

I think the routers are well documented on Cisco for voice gateway configurations and routing. Your biggest challenge, from what it sounds like, is how to configure the PBXs for Site to Site calling.

If you have overlapping Dial plan, meaning you have Site A with extensions 2000-2999 and Site B with 2000-2999, you will need to use an "access code" to access the trunk.

Site A wants to dial a person at Site B. Site A dials "6" then 2xxx The 6 tells the PBX to use the Tie line trunk to the router. In the example above, the "3" is telling the PBX to use the Tie line trunk to the router. You can use any number you want. It's all just a matter of training.


Hall of Fame Super Red

Re: Interconnecting two pbx over satellite

Hi Tommer,

Really an excellent answer here! Impressive. You get 5 points and my vote for "post of the day"!

Take care,


Re: Interconnecting two pbx over satellite

ha... thanks Rob! I was a little slow yesterday at work and had time to answer a few questions.....