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ISP throttling of VOIP - it's for real

VOIP Security

As I have suspected for some time, it appears that some ISPs are now throttling (opens in new tab) customers' Internet telephony sessions to the point where the service is basically unusable.

I've been dabbling with using a couple of high-end VOIP phones (opens in new tab)over the weekend across a couple of broadband ISPs, and Pipex.

Both connections are 2 Mbps across BT's wholesale network and I've tried to use two main VOIP services: Skype (opens in new tab) and VOIPCHeap (opens in new tab).

Both services are being throttled by the ISPs concerned, resulting in dalek-style voices and packet jitter. The voice calls are, as a result, almost unusable.

The phones and service work just fine across my back-up 256Kbps full duplex (whoo-oeee -Ed) cable service from Telewest, so that's how I know the PCs, software and phones are working properly.

So how are the ISPs doing the throttling? My best guess is some sort of packet sniffer that looks for the distinctive traffic and headers that VOIP packets generate.

Is there a solution? Yes, and it's a bit sneaky (what's new? -Ed) . Most VOIP applications are set to work with specific ports.

If you use a non-standard port, usually at the other end of the numeric scale, then you should find your VOIP throttling problem easing considerably.

In theory, if you encrypt a VOIP session (Skype does this by default) then it should be more difficult for the ISPs to detect the VOIP traffic.

Not impossible though, so this leads me to be believe that the ISPs are using pattern recognition techniques on customer's broadband lines to detect VOIP traffic and then throttle the sessions.

What intrigues me, though, is that whilst VOIP - as well as BitTorrent (opens in new tab) - traffic is being throttle by a growing number of ISPs, standard Web browsing and downloads appear to be unaffected.

The technology being used is obviously quite sophisticated, which makes me wonder whether it is BT wholesale that is doing the packet throttling rather than the ISPs...