Congestion

Sunday morning, my dad ‘phones me for a chat.  As always.

Only this time, I hear just the merest snatches of his voice.  Likewise, he can’t hear me.  Somewhere on the line there is a problem, and it seems to be with me.  Specifically, a connection that’s inadequate for VOIP.  That shouldn’t happen: I have cable and pay for a 30Mb line in, and near 2Mb upload speed, which is an order of magnitude more than I should need.

While I figured out an alternative, my dad ‘phoned my mobile number.  Not something either of us like, as it costs (he gets weekend landline calls free).  I explained my alternative, which was to turn my Virgin router off and use VOIP over my O2 line.  Fortunately the weather was gorgeous and it was a pleasure to go and sit out on the front terrace for best 3G signal, though it appeared also to work in some parts of the house.

Later in the day I ran some more tests.  Performance was incredibly poor: on average rather worse than a 28K modem (remember those?) and much of the time a complete standstill.  It took a long time to get a speed test to run at all.  Whoops!

Experimentally I tried a wired connection to the router.  Now suddenly all is well: I’m getting the speeds I’m supposed to.  So it’s the wireless.  A bit of research later and I change the WIFI channel, whereupon all is well.  I also come upon a tool called Kismac, which tells me the problem was indeed congestion: my neighbour and I had both been using channel 11.

Is that a common problem in a cable area?  Lots of Virgin users, and they ship standard routers with a standard configuration that sets us up for conflict, rather than a more mixed area where routers are (a priori, at least) a random mix?

Credit where credit’s due.  Virgin’s help pages were genuinely helpful.  It’s a rare pleasure to find a help page that tells me anything that wasn’t already blindingly obvious.

BTW, I have a couple of DECT phones and an ATA adapter to use my old phone&fax on order.  Should be arriving tomorrow.  So I’ll have a well-equipped house and should hear the ‘phone from anywhere.

Posted on May 27, 2013, in telephony, virgin. Bookmark the permalink. 2 Comments.

  1. I know BT make a small aria out of their routers’ signal strengths which actually equates to channel-hopping, therefore I wouldn’t be surprised if some of the competition didn’t.

  2. In a former life, I actually worked in precisely this field. Which is how I know that big bandwidth is no guarantee of quality of service. The actual bandwidth requirements for VoIP are tiny – you only really need about 10-12K, way less than a 28K modem – what matters is the delay (imprecisely termed ‘latency’). Any delay more than about 125ms is, basically, equivalent to packet loss (except that because the router hasn’t actually dropped the packet yet, it still has to deal with it, thereby adding delay to all the others).

    You and your neighbour would get along just fine if you were both using just VoIP at the same time – then you’d each be using a tiny bandwidth – or if you were both using just TCP protocols (such as HTTP) at the same time – then you’d both settle to a reasonably equitable share of the available bandwidth. The problem arises because TCP is a very greedy protocol, which not only grabs all the available bandwidth for itself, but also fills buffers and expects packets not to be dropped. Which means that it tends not to notice a modest little UDP stream, and will smother it completely unless actively restrained.

    Virgin should know this, but they might reason that just giving people different channels would only transfer the congestion point upstream a little (into their network, where they might have to take responsiblity for it). So it may not be worth their while to do anything about it. (Unless you’re willing to pay through the nose for a commercial-grade ‘premium’ service, of course.)

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