Original URL: http://www.theregister.co.uk/2006/09/26/blog_2609/
The haunting issue of latency
Unmasking The Ghost
Ionian Blog A storm forced us to spend two days here, on the island of Cephalonia.
This is the second day. It was the day we discovered the importance of the word "latency" in broadband communications. Also, the drawbacks of non-tidal waters. And, finally, the Ghost was revealed.
The Ghost has woken me up several times in the night. It walked across the deck above my bunk, and when I stuck my head out of the hatch to remonstrate, it wasn't there. I expected to find a human, and there was none: people do often find occasion to cross from one boat to the next. But normally, they do so quietly and tactfully: the Ghost was neither.
Latency was my Sermon for the day, and at first, I thought it had nothing to do with the Ghost. Latency is expressed at its simplest in the time taken for a message to be sent from your PC to another PC on the internet, and for the reply to be returned to you.
I was instructed to explain all this to the Irish rugby player. He seemed a little unhappy about something, and I sensed that he wasn't concentrating on my words. So I asked him whether he was healthy.
"Not sleeping well," he confessed.
It transpires that his bunk is the exact mirror image of mine. Both are at the stern of the boat. I have the port side stern cabin (it sleeps two, in comfort, as long as neither needs to sit upright in the night) and the rugby player has the starboard side stern cabin. He pulled his mattress up, and showed me the bunk under it.
"Look at that. It's been wet, and it bends!" he said indignantly.
That's a travesty of truth. What he should have said was that the eight-foot long, three-foot wide plywood timber was snapped nearly in half. And someone had tried a cosmetic repair job, covering the break (18 inches in length) in glass fibre repair gear.
"Every time I turn over in my sleep, it creaks!" he pointed out, demonstrating by pressing down on it. I had to concede that it did; it sounded like a Californian Redwood snapping in two, and it was, clearly, the source of the Ghost and his footsteps at night, I understood at once. The only thing I didn't understand was why it had not snapped in two. Our Irish rugby player is neither short, nor slender... and the cook's foot is still a colourful testimony to just how much weight is imposed on his bunk.
Anyway, I was explaining latency, because I was confined to an internet cafe. It's a very very nice internet cafe, in the port of Fiskardo. It is operated by young John, who is around 10 years old, and whose family are happy to leave him playing shoot-em-ups inbetween taking money from paying customers, while they run other arms of their dockside business; and a better trustee you could not get. And the Irish Rugby Player was impressed with how much faster the Internet Cafe appears to be, than my high speed broadband connection with the BGAN terminal.
The speed of BGAN is not illusory. If you have to download a large file, it comes down fast. But it is not quick at uploads, and even with a broadband connection in the internet cafe of either 64 or 128 kilobits per second, shared with four other customers, it "feels" much quicker. The reason is the long distance between the earth and a geostationary satellite; it takes a significant part of a second for the message to make its round trip. The latency is, in fact, as long as it is for a typical GPRS data connection - sometimes, almost a second per hop (see table on page two).
Since the typical web page requires you to load 20 or more items, each with a request and an acknowledgement, you can see that there are things best not attempted over a long-latency internet link.
And that explains why I had opted, for this occasion, to pay €3 an hour for the cafe: I had several rather nice files to upload, showing the inside of the local caves. The caves are hollowed out of the hillside about half a mile South of Fiskardo, and the work of digging them was, apparently, undertaken by occupying German forces in 1939-45 to provide a munitions dump that could not be destroyed by bombs from the air.
Uploading pictures requires a pretty quick link, and I judged that the latency of the BGAN coupled with its relatively slow upload speed, meant that I'd take longer than the battery would tolerate. Horses, as they say, for courses.
You're probably puzzled as to how the conversation about latency led to the exposure of the Ghost. It is simple: the Irish rugby player was so anxious to sit down and think about what I'd told him, that he rushed off into a bar (presumably hoping for silence and solitude) where eager fellow crew members plied him with beer until late in the night. He returned in the small hours of the morning complaining that he'd "got used to the motion of a ship at sea" and "this dock won't stay still" and dripping wet, because he'd fallen off the dock into the harbour.
And that led to his explaining about the creaking of his bed and why, despite having had a drink or two, or maybe three or four or so, he didn't manage to sleep well. It also explains why his cabin had a distinctly dodgy whiff; the water he was swimming in was not tidal. I doubt the Fiskardo Bay has been "refreshed" in a while.
The boat owner took advantage of his absence to start a barbie on the dockside, on which he cooked skewered prawns and a fresh blue-fin tuna that the flotilla leader hooked during the previous day. Apparently, in Greece, the sight of slightly inebriated flotilla boat owners sitting cross-legged on the dockside cooking meat over charcoal doesn't excite adverse comment.
I may try the trick next time I lunch at St Katharine Docks...
Tracing route to cix.co.uk [ 220.127.116.11] over a maximum of 30 hops:
- 849 ms 1039 ms 879 ms 18.104.22.168
- * * * Request timed out.
- 1753 ms 879 ms 1039 ms bura-rou-1001.NL.eurorings.net [22.214.171.124]
- 1039 ms 959 ms 1999 ms asd2-rou-1012.NL.eurorings.net [126.96.36.199 ]
- 1032 ms 880 ms 1038 ms asd2-rou-1001.NL.eurorings.net [ 188.8.131.52]
- 1038 ms 879 ms 1039 ms asd2-rou-1021.NL.eurorings.net [ 184.108.40.206]
- 1199 ms 879 ms 1039 ms ldn-s1-rou-1021.UK.eurorings.net [220.127.116.11]
- 1038 ms 880 ms 1679 ms ldn-s3-rou-1003.UK.eurorings.net [ 18.104.22.168]
- 1038 ms 879 ms 1039 ms te1-3.cr05.hx2.bb.pipex.net [ 22.214.171.124]
- 878 ms 879 ms 1279 ms g3-1-5.dr01.hx3.dco.pipex.net [ 126.96.36.199]
- 1600 ms 867 ms 879 ms v3957.ar41.hx4.dco.pipex.net [188.8.131.52]
- 4033 ms 879 ms 1279 ms technetium.cix.co.uk [184.108.40.206]