Will Cisco routers help my VoIP issue...


What I have is two office phone systems (2 locations) and 5 IP phones. The two office systems communicate with each other over IP and so do the 5 IP phones. We have a T1 at each office. The 5 IP phones are at branch offices that have DSL.

We are not current using Cisco routers and have in place some SonicWall routers. We are experiencing voice quality issues and are thinking about switching to Cisco routers because of this.

I have enabled QoS on the SonicWall routers as well as outgoing bandwdith management to limit the amount of bandwidth consumed by non-voice traffic. This made a huge improvement in voice quality although we are still not where we want to be.

I have an IP phone and I can always hear people just fine. This would mean that outgoing traffic from our main office to me works great and that the outgoing bandwidth limiting of non-voice traffic is doing its job. The problem is that people are unable to hear my voice going back to that location. I have a theory on this which is that people at that location are consuming the incoming bandwidth and there isn't enough leftover for the incoming voice traffic.

The question is - how do you limit incoming bandwidth? The SonicWall does have a feature to do this, but it makes the problem worse rather than better. I don't really see how you can throttle what is sent to you because once you have it, it has already been received.

I guess there must be some sort of technique for throttling outgoing requests that can throttle the incoming packets for that type of traffic. Does Cisco utilize some sort of techniques like this?


  1. Do you think replacing our routers with Cisco routers would solve this problem? In other worse, does Cisco have some sort of technique for properly sharing voice and data on the same WAN connection in such a way that the data traffic will not cause problems for the voice traffic?

  1. I thought about putting the voice on its on dedicated WAN connection, but this will complicate the network configuration as well as have a service cost to it.

  2. I found something called a "traffic shaping bridge". Would this be a possible solution worth looking at?



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Default User
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You obviously cannot influence the incoming internet traffic unless the ISP and all ISPs between the locations honour the QoS bits *AND* they are prepared to do QoS on your traffic. As far as I know there are no ISPs that do this but I am not sure.

There is however one trick that you might consider for controlling incoming traffic from the internet. Most internet traffic is TCP and TCP self regulates its use of bandwidth if there is a restriction in the path. So if you say have a 2M DSL you could in principle restrict the incoming non-voice to 1.5M leaving

512k for voice which might be enough for two simultaneous calls. You need to check the exact values yourself.

If you are doing other UDP (peer to peer systems seem to like UDP but will also use TCP) then this may not work so well but all web browsing is TCP.

It does not matter whether you restrict the non-voice traffic heading into the router from the outside or heading to the inside from the router.

This *does* work. I have implemented it myself. Of course you lose the 512k for non-voice traffic but that is something that you may be able to live with.

Cisco routers have the tools to implement the above scheme.

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1 way to improve on this is remove the sharing since QoS only matters when there is contention. it may be more cost effective to keep just your IP voice on 1 dedicated Internet link per site (depending on the cost of a 2nd broadband link in your area).

however it is worth remembering that IP voice will only be consistent and reliable under all conditions if there is QoS end to end - the reality is that doesnt exist on the Internet in the general case.

1 alternative here is alternative conventional voice providers - my UK calls to non mobiles cost connection charge, but no cost / min, and no monthly fee. USA calls run 0.5p / min.

I have given up on VoIP on Internet at home for now since the cost reduction just doesnt over the hassle.

So - maybe you need to think about how much this "cheap" voice service is really costing you in terms of frustration, repeat calls and possibly annoying your users and customers.

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It is a T1 connection and I have implemented outgoing bandwidth throttling for non-voice data at 50% of the connection. In theory this would split the

1.344mbps to 672kbps for voice and 672kbps for data.

Are you saying that because I've throttled the outgoing packets that this should do a decent enough job of throttling the incoming packets as well?

I *think* the problem is getting packets INTO this main location. I don't want to throttle the data more, but I wonder if I throttle it a bit more if that would help until we find a better solution...



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Default User

I think you need to work out the cause of your problems first, before throwing more hardware at it. Does the vendor of your phone system offer any guidance as to what kind of network configuration will work with your setup? Is there any debugging or diagnostics output from the PBX? What type of VoIP is it? One-way audio is a classic symptom of NAT issues with SIP. Do you have the SIP ALG enabled on the Sonicwall(s)? What kind of load is on your T1?

One thing that is worth trying would be to eliminate as much from your setup as possible until the problem disappears, then re-add things until the problem recurs. For example, if you think the one-way audio is load-related, disconnect everything from behind the Sonicwall bar the PBX and re-test.

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