PBX T1 Channel behavior

Posted on 2011-05-10
Last Modified: 2013-12-27
We are working on a "learn-as-you-go" project and we are running into a roadblock. I was hoping to get some clarification on a point.

We have 2 NEC Aspire PBX systems. They exist in seperate buildings and are currently connected via a point-to-point t1 circuit via a kentronix conditioner. We have an IP Network running between the buildings. So, our idea is to connect the PBXs to cisco routers (via t1 controllers). One router would convert the voice traffic to voip, pass it through the ip network, and the other router would pass it back. The idea is that the routers would simulate the t1, the PBXs wouldn't be the wiser, and we would save hundreds of dollars a month.

However, I need to get DSP cards for our routers to convert the voice to data. I can get 8,16, 32, and 64 channel DSPs. Based on call volume, I don't believe I need more than 16 channels and I may not need more than 8. So, let's say I go with the 16 channel DSP. However, I am worried that, becuase the PBX is connected to a T1, it's going go assume that it can use 23 channels and may use the 17th channel, even when there aren't 16 concurrent calls. And, that call would be dropped.

So, my questions are...
1. What is the behavior of the PBX? Would it always use the lowest available channel or would it use higher channels, even if the lower channels aren't in use?
If you can't speak to the NEC product, can you speak to PBX systems & t1 technologies in general?
2. Is this even a concern? Maybe I am overstating the necessity of having the channels "line up". Maybe when the PBX uses channel 17, the router is still going to connect it, as long as the concurrent calls don't exceed 16.

Any other comments or suggestions would be appreciated.

Question by:Jack5Back
    LVL 32

    Expert Comment

    Why not put voip cards on the PBX, then you don't have to worry about carving channels out of the T1?

    The data routers can stay data routers, and you implement QoS to prioritize the VoIP traffic.

    That also leaves an easy upgrade or replacement path, as you can drop in any VoIP PBX system in-house, or use an outside vendor.

    Just talking off the top of my head.  We have multi T1 carving out channels and handing off via PRI to our PBX.  But, I also have 2x VoIP channels for backup calls (yes, the T1 did go down for a significant amount of time), as well as VoIP in-house for desk phones and software clients.
    LVL 15

    Expert Comment

    I think you are making the problem too complex as well.
    you will waste all the bandwidth of your t1 on just voip when it could also carry data.

    I dont think you would be able to hear the difference if you use voip as above.


    LVL 32

    Accepted Solution

    The PBX should manage each interface/provider separately.  For the T1/PRI, I can spec that it has 50 channels, or 19, or 4, or 1.  I don't see why it would "assume" how many channels are available.

    You'd be running into constant problems if 50 people where trying to originate calls on a single T1.  The PBX should return back an error that no lines are available.

    Author Closing Comment

    I never fully got my question answered.

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