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frottle: Defeating the Wireless Hidden Node Problem
jasonjordan writes "The West Australian FreeNet Group was the first to go War Flying - and now we've released "frottle" (freenet throttle) - an open source project to control & manage traffic on fixed wireless networks. Such control eliminates the common hidden-node effect even on large scale wireless networks.
frottle works by scheduling client traffic by using a master node to co-ordinate - effectively eliminating collisions!
Developed and tested on the large community wireless network of WaFreeNet, We've found it has given us a significant improvement in network usability and throughput.
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Look up "slotted Aloha" for background on this class of idea.
In case the site (or routes to the site) get slashdotted. Here is a mirror. Sourceforge also has an annoying habit of downing themselves for maintenance. Enjoy!
Of course. Wireless access points generally aren't geared for large number of users OUTDOORS. The difference is that when you've got users 10-20km away collisions have a lot more effect. Individual clients don't see the traffic of other users, so it's very easy to cause collisions (this is the Hidden Node effect) - there is commercial software to solve this problem (ie. Karlnet), but the large expense and lack of Linux support (ie. use 2.4.2 kernel, Redhat 7.1 and their binary driver or else) put us off majorly.
So we rolled our own. Frottle is the result.
-- Wireless WaFreenet user since March 2002
It sounds like this is an attempt to change the topology of 802.11x to a polled topology without the true benefit of such topology without changing the hardware.
In a true polled topology client packets aren't sent until the AP says they can. The client equipment remains completely silent until they receive the right to broadcast packet. AP's are programmed to completely ignore packets that are sent out of turn anyway.
802.11x hardware is NOT designed that way. Sure you can control data flow that way but your AP is still open to the same problems as before. I wonder what happens when one of the client on one of the computers crashes and ceases to act as a polled client. Will it start hogging time slices from the AP again? Seems to me it would unless there was a radical hardware change to both AP and client adapter.
yes, check out nodedb.com
Most WaFreeNet nodes are listed here
Tannenbaum's best known work is Operating Systems, the Minix book.
Yes, I know he was critical of Linus on comp.os.minix, that is why I voted to create comp.os.linux. They are still excellent books.
This is not really a hidden node problem, as they make it out to be.
... data packet ...
This is more a problem with the inherent lack of scalability of a CSMA/CA architecture. Everyone is familiar with the way ethernet degrades under saturation: you only get about 70% of that 100Mbit throughput utilized. Ethernet is CSMA/CD - collision detection.
In wireless the problem is even more pronounced in infrastructure mode because you are using CSMA/CA -- collision avoidance. This means that for every packet to be sent, the clients must coordinate use of the medium before sending, using a RTS/CTS handshake.
(client) can I send now?
(AP) not your turn yet
(client) can I send now?
(AP) not your turn yet
(client) can I send now?
(AP) yes
(client)
When you put many clients (20+) on the same AP sharing the same medium, a large amount of bandwidth is spent simply coordinating contention free access to the wireless medium itself.
Traffic shaping (which is all frottle is doing) helps ease this problem by reducing the amount of data clients try to send/recv in a given period of time, and thus reduces some of the contention.
This is simply a band-aid on a more fundamental problem, however.
The only true way to prevent this kind of inefficiency for larger numbers of clients is to use a true wireless phased array switch, like vivato, which can effectively emulate a dedicated medium to each client, preventing any contention that arises in the broadcast CSMA/CA situation.
Also, it is important to note that communication between nodes on the wireless will NOT be shaped by the frottle queues unless you are using hostap or some other linux based access point. In such a scenario, two nodes talking to each other could use as much of the medium as they liked (as coordinated by the access point itself) without frottle seeing any of the traffic.