Re: [OLPC-AU] limits on ad-hoc connections
On Wed, Feb 8, 2012 at 10:59 PM, James Cameron qu...@laptop.org wrote: No, that's not how ad-hoc works. I'll simplify and translate for you. Your explanation is correct but doesn't exactly match the buggy behaviour of our wireless hardware/firmware. As far as I can tell, the ad-hoc nodes in our setup make no attempt to synchronize clocks/beacons, nor do they stop transmitting their own beacons in the presence of another node transmitting beacons too. I guess this is why our ad-hoc networks perform slower and less reliably than the XO-1 mesh, which at least had synchronization. Daniel ___ Devel mailing list Devel@lists.laptop.org http://lists.laptop.org/listinfo/devel
Re: [OLPC-AU] limits on ad-hoc connections
On Wed, Feb 08, 2012 at 10:16:53PM +1100, Sridhar Dhanapalan wrote: Ad-hoc connections only scale to a limited number of participants before problems begin to occur. What are the problems you observe? It may be that the problems you observe are not due to the ad-hoc network, but due to something else as well. What would be the maximum number of participants that an ad-hoc network can reliably handle? There's no maximum that I know of. A well placed set of laptops that can hear each other, with no outside noise, can operate an ad-hoc network to a quite large size. One node will be the beacon. Once you place traffic on the network, things will slow down. Once the slow down is enough, certain applications may fail. Can we impose a hard limit on the number of clients to prevent too many XOs connecting to a single ad-hoc session? No, there is no control for that as far as I know. -- James Cameron http://quozl.linux.org.au/ ___ Devel mailing list Devel@lists.laptop.org http://lists.laptop.org/listinfo/devel
Re: [OLPC-AU] limits on ad-hoc connections
On Thu, Feb 09, 2012 at 03:11:42PM +1100, Sridhar Dhanapalan wrote: Hmm I am thinking that my understanding of the ad-hoc implementation might be incorrect. I was under the assumption that one XO acts as the ad-hoc host, and the others connect to it. That made me wonder whether that host could limit how many clients connect to it. No, that's not how ad-hoc works. I'll simplify and translate for you. In an 802.11 wireless ad-hoc network, each node has the duty and right to be the beacon, especially if there is no other beacon heard. The beacon is used for timing the transmissions, so that they occur in empty time. Transmissions that occur simultaneously would interfere with each other, and the receivers would be more likely to miss them. Always, the first node to begin an ad-hoc network begins by being the beacon. If a node cannot hear a beacon, then after a very short while it will try to become the beacon. In effect, they compete for the job, in a psuedo-random fashion. This is implemented in the wireless device firmware, not in the host, not in the CPU, not in the kernel, not in the user-space networking tools. If a cluster of XOs that have formed an ad-hoc network, are slowly spread out physically, then eventually the responsibility for beacon should tend to be in the centre of the cluster. If a cluster of XOs is split in two, and the two groups slowly moved apart from each other, then eventually two beacons will be active and there will be two networks. References: http://en.wikipedia.org/wiki/Timing_Synchronization_Function_%28TSF%29 http://en.wikipedia.org/wiki/Beacon_frame http://en.wikipedia.org/wiki/Traffic_indication_map (ad-hoc is IBSS) -- James Cameron http://quozl.linux.org.au/ ___ Devel mailing list Devel@lists.laptop.org http://lists.laptop.org/listinfo/devel
Re: [OLPC-AU] limits on ad-hoc connections
To expand on James' excellent notes... On Wed, Feb 8, 2012 at 11:59 PM, James Cameron qu...@laptop.org wrote: The beacon is used for timing the transmissions, so that they occur in And that is _all_ it can do. It just broadcasts a beacon, like a metronome for a band recording in a studio. This is implemented in the wireless device firmware, not in the host, not in the CPU, not in the kernel, not in the user-space networking tools. In other words, in practice we can't patch it to be smarter. m -- martin.langh...@gmail.com mar...@laptop.org -- Software Architect - OLPC - ask interesting questions - don't get distracted with shiny stuff - working code first - http://wiki.laptop.org/go/User:Martinlanghoff ___ Devel mailing list Devel@lists.laptop.org http://lists.laptop.org/listinfo/devel
