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IP Telephony - bandwith issues

IP Telephony - bandwith issues

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Issues re bandwidth for IP telephony

We've been lead a merry dance by BT over the last 8 months with a "integrated phone and connectivity system".

There are seven of us here, working "lifestyle" hours. Only myself and my PA in the main office, everyone else works from home.

We had a phone system specified by BT to provide two ISDN lines into the office, two business ADSL lines, and three homeworkers via VOIP.

At the time I questioned them at some length over bandwith as we are in the middle of nowhere, with corresponding broadband speeds.

We get 0.5Mbps up, 3Mbps down, line quality A, on each of two ADSL lines. Those lines then service, via load balancing router ( professionally configured), the VOIP x 3 homeworkers, exchange, web browsing and up to 5, but normally less, concurrent RDP sessions.

I was told, initially, the bandwith requirement was around 70k for a IP call, 50k for a RDP session. That seemed to give us ample capacity.

Incidentally, our local exchange gets fibre next month, and we have a date for fibre to cabinet The cabinet is a mile away over old copper No satisfactory WiMax in area.

The VOIP aspects of the system have never worked well - call quality very poor and call drop outs -and trying to get sense out of BT has been frustrating over 6 months. Engineers kept saying there "not enough bandwidth", we responded "thats not what you told us at the outset and you sold us lines plus phones as a integrated solution". Repeat to fade.

Anyway, after I took to venting my fury about BT on twitter they've perked their ears up, and we had area sales director out today, offering to upgrade the system.

But I couldn't get anything definitive about bandwith requirements.

I appreciate configuration and compression makes a difference, but assuming a competent set up, what is the bandwith requirement for VOIP?

If we assume up to 5 RDP connections and up to 3 voip calls, worst case, what should our requirements be?

We've got good local IT support, and they have tweaked as much as they can on the RDP connections, router balancing, etc.

I've had one supplier telling me they could run a call centre with 50 VOIP users off our line, claiming 5k per call, whereas BT were muttering something about 400k per call this morning. A big difference.

Any thoughts from an independent source would be welcome.

Incidentally we have a BT Quantum system at the moment (£2,000), they are offering us a free upgrade to a Avaya IP office (£3,500)

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By Cube
08th Feb 2013 16:24


I see no-one has responded, so I will.


I know little about this, BUT I do know that these people do:


I have heard good things about them, but otherwise have nothing to do with them.

I hope that they are able to point you in the right direction (to one of their products prolly :))



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By merlyn
08th Feb 2013 21:23


For a decent quality VOIP call you need around 70k, for high quality it's more like 120k.

RDP Sessions vary depending on the colour depth, printing etc. But you should be ok with 50-60k for a stable 16bit session.

It's more than likely your upstream bandwidth is whats causing the issues, but if you perform some traffic logging on your routers you will be able to see exactly whats causing the issue and the use some sort of traffic shaping to ensure your VOIP calls always get enough bandwidth.

It could also be if you are hosting Exchange "in house" that you get a lot of big attachments which are killing the line, or it could just as easily be someone downloading a lot of data.  Really without seeing detailed traffic logs it's impossible to tell.



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