VoIP, FXO, FXS, DSL and MTU (Can we all just get along?)

Posted on 2006-05-23
Last Modified: 2010-04-12
Hello experts!  Here is my question, but first my topology.

I have 7 remote facilities that are calling centers.  Their data networks connect via IPSec over DSL (6.0mb down | 512Kb up) back to corp utilizing a MTU of 1410 and yes it is mandatory to use such.

My question is what product or how would you recommend me doing the following?

I would like to have users at corporate be able to pick up a phone and either through the pbx OR through a device attached directly to the phone be able to connect to one FXS or similar device that would answer and send a off hook to the device that it is plugged into.

Here are the particulars:
The FXS device/remote device must be able to adjust it's MTU from 1500 to 1410 in order for all packets to not be fragmented and make it to it's destination.
Each branch office will have a maximum of two conversations. (Optional:one conversation at any time would be acceptable)
We would like to not have to buy equipment on a 1 to 1 ratio, meaning for every remote device we must have a corresponding device at corporate that works only with that associated fxs or similar device.  If it was possible we would prefer buying something that provided a multiport capacity for our analog equipment.

Ok I think that does it questions/suggestions?
Question by:thcit
    LVL 36

    Accepted Solution

    Can you give some more information on exactly what these 'devices' are and what you want to do with them. There is a considerable difference between voice and data as far as voip is concerned for example. You mention things like 'send off hook' which dont really mean anything.

    Voice packets are very small so MTU is not an issue as you will never get close to it. A 512kb bandwidth will normally be able to cope with 6 simultaneous calls but as you are running IPSEC aswell I would say 5 simultaneous calls maximum and that is assuming you are not sharing bandwidth with other DSL users.
    LVL 3

    Author Comment

    >>Voice packets are very small so MTU is not an issue as you will never get close to it.
    You may have answered my question.

    I trust you're familiar with FXO/FXS if not go here  I'm looking at buying a FXO to place the call and a FXS to receive the call.  We currently do not have any equipment in place.

    What I'm needing to do is make a phone call utilizing VoIP to one of the remote facilities where a device will answer.  This device (typically a FXS) will be connected to piece of equipment that is looking for an off-hook signal. Once it detects this signal then the application takes it from there.

    The overall application is to be able to monitor our call representatives.  Keyword is monitor because the conversation is just one way and there is virtually no traffic other than the initiation of the call being made, coming from the corporate office.


    LVL 36

    Expert Comment

    ok at the remote sites you will want FXS devices. There are various options for this such as the Sipura SPA-3000 which ha a FXO and a FXS port. Which option you go for will depend on how many FXS interfaces you require.

    At the corporate office can you explain why you need a FXO as this normally connects to a PSTN and I cannot understnd how this fits into what you want to do.
    Generally I would install Asterisk PBX ( on a PC and have all the VoIP devices connect to that and then you can define extension numbers so one device can phone any other.
    LVL 36

    Expert Comment

    The Linksys/Sipura SPA-2000 is a 2 port FXS device and is very commonly used and reasonably priced. If you need lots more than 2 interfaces have a look at :-
    The price does suddenly jump up though so often it is cheaper to get multiple SPA-2000's

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