On Fri, May 9, 2008 at 10:57 AM, Marcus Leech <[EMAIL PROTECTED]> wrote: > I'd be *very* interested to compare the distribution on a wired network. > It seems to me that given > the broadcast model, everybody should see everybody else in much > shorter time than the 55 seconds > shown in the outlying cluster on that graph. For example, if you > plugged all of those 65 Xos > into a wired network (100Mbits/sec), then if the "convergence time" > shrinks by roughly a factor > of two (100Mbits vs 54Mbits), then we know that the wireless > networking stuff on the XO is > basically functioning correctly. If, however, the convergence time > becomes *very* much shorter > on a wired network (let's say by a factor of 5 or more), then > something is likely wrong with > the 802.11 goop on the XO.
The common, but erroneous, assumption is often made that a wireless network is just like a wired network, but with the wires removed. In actual practice, wireless networks are fundamentally different beasts. Let's start with the broadcast speed: on a switched wired network, broadcasts happen at the same rate as all the rest of the traffic: if you've got a 100Mb ethernet, your broadcasts happen at 100Mb. Since a wireless network is a shared (not switched/routed) medium, broadcasts must occur at the fastest rate of the slowest connected machine. Since a wireless network doesn't actually provide any means of enumerating all the connected machines, broadcasts happen at 1Mb, regardless of the maximum speed of the sender. On a wired network, if you were successful in sending a packet the probability is very high that everyone in your local network received that packet. For older ethernet networks, this was because collision-detection told you with high likelihood whether anyone failed to hear it; for modern hi-speed ethernets this is because your 'switch' is much more akin to a router, and has all sorts of smarts under the hood to try to prevent your packets from being dropped. On a wireless network, broadcasts are successfully received with much lower probability. RF is mysterious and magical, and all sorts of connection asymmetries, near-field effects, and radiation lobe patterns conspire to make it unlikely that *everyone* can hear you equally at once -- and then you get into remote collisions and other mechanisms that make you unaware that not everyone heard you. And there is not 'ack' mechanism for 802.11 broadcast. On an access point network, this problem it mitigated somewhat: you're really not broadcasting directly to everyone else, you're sending your broadcast to the AP, and the access point is relaying it to everyone else. You still don't get any acks, so you've got less reliability than wireless unicast: you packet can get lost on the way to the AP, and then of course the packet can disappear on its way from the AP to everyone else. But at least the number of paths is limited. On a mesh, however, everyone who receives a broadcast then turns around and repeats it to everyone around it who might not have heard it the first time, for a particular TTL. In theory this improves reliability of broadcast in a dense network, since I've got three or so chances to hear the packet, but in practice the vast volume of traffic generated makes broadcast much much less bandwidth efficient and increases the probability of collisions a lot. On a sparse network, your probability of receiving the packet is even worse: if p is single-hop probability of reception, a node N hops away has something like a p^N chance of successfully receiving the packet. These are pretty much fundamental physical limitations of wireless networks. Attributing it to "802.11 goop" obscures the real issue. The question ought to be: are there algorithms capable of performing effective discovery and presence distribution giving the realities of wireless and mesh networks? Poly's work attempts to prove by example that the answer is "yes"; there may well be other algorithms that also work well on these networks. I'm not convinced that, say, DHCP is an algorithm that will work effectively on a wireless or mesh network. (Note that successful DHCP on 802.11abg often retries multiple times, which is an early sign that it's reaching its limits.) --scott -- ( http://cscott.net/ ) _______________________________________________ Devel mailing list Devel@lists.laptop.org http://lists.laptop.org/listinfo/devel