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VOIP Over DSL Lines

dayanandm asked
Last Modified: 2008-02-01
My company has its offices in 2 cities, both these offices are connected to the internet through their respective ISP, Internet is through DSL (512Kbps), both the offices have static ip's configured on their Linksys RV042 VPN Routers, I wanted to know can both my offices make calls to each other, and if they can, can this line be connected to the office PBX, what equipment is required and what is the configuration procedure.
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Yes it can be done but it will take a bit of work.

Firstly the RV042 will not be sufficient as you need to configure QOS so that the VoIP packets get priority. You will need to upgrade to something like a Cisco 1800 series router.

What hardware you will need will depend on how many simultaneous calls you want to support between offices and how many spare FXO, FXS and E1/T1 interfaces you have spare on your PBX's.


There would be just 3 simultaneous  calls between the offices,


For 3 calls you might aswell use an analogue connection to the PBX.

With analogue lines you will need a line for each outgoing connection to the remote site. Therefore at each site you will need 3 FXO and 3 FXS ports on the PBX and this will enable you to have a total of 6 simultaneous calls (3 originated from one site and 3 from the other).

At each site you will want something like http://www.voipsupply.com/product_info.php?products_id=1638 to connect to the FXS and FXO lines of the PBX.

A FXO port is like a telephone. It can go offhook and dial a number but it cannot receive calls to multiple number and differentiate between them.
A FXS port is like the telco. It can accept a call and listen to DTMF digits and hence can receive a call to multiple numbers but it can only ring a phone and not pass on information as to what number was dialed.
This is why you need a FXS and FXO ports on the PBX. One is used to dial a number and another is used to receive calls.


what if I use ip phones at both ends and plug these phones to my network switch.


Yes you can do that. Have IP phones at each site and configure them to talk to a VoIP server. You can then call each other by dialing their new individual phone number.

An alternative would be to setup Asterisk (or Trixbox) at one site and configure the phones to connect to it. This will have the advantage that you will be able to give each phone their own extension number of your choosing.
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