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Boffins teach Wi-Fi routers to dance to the same tune

'Wi-FM' listens to FM signals to sync access points

Research presented to this week's IEEE International Conference on Network Protocols suggests a fairly simple enhancement to Wi-Fi could help deal with the chronic congestion caused by its popularity.

It would be nice if twenty different base stations in twenty different apartments could coordinate their transmissions, but they can't: they're all serving different masters.

In this paper, the group from Northwestern University suggests adding a little bit of time-division multiplexing, using an interesting source of synch signals: FM radio transmissions.

Dubbed “Wi-FM”, the idea is that 802.11's original authors didn't envisage a world so crowded with wireless data communications. The venerable collision avoidance mechanisms that by now stretch back two decades are overwhelmed if there's a lot of access points and a lot of users fighting over the same airwaves.

To try and make things a bit more orderly, the researchers from Northwestern University in Chicago wanted a way to time-slice the signal to make collision avoidance work better.

To do that, they needed a time source that Wi-Fi base stations can see: “a common reference for neighbouring devices to harmonise their transmissions, yet without requiring any explicit communication among them”, the paper notes.

Of course, the Internet has a very good time signal – the network time protocol – but if a station's upstream connection is wireless (for example, it's a repeater), congestion would stop it from checking the time.

That's where FM radio comes in: a lot of baseband processors have FM receivers built-in, and in modern countries, FM broadcasters send song titles and other data over the Radio Data System (RDS).

The program information (PI) codes in the RDS provide a handy and predictable “heartbeat” that the researchers used in Wi-FM. In an apartment building, to take one example, the FM transmissions will be visible pretty much everywhere, and with it, the RDS – meaning all of the Wi-Fi base stations get the same clock signal at the same time.

Northwestern University associate professor (and lead author of the paper) Aleksandar Kuzmanovic told The Register's networking desk instead of wireless stations fighting over all the available spectrum, all of the time, using RDS to “time slice” the spectrum “reduces the contention for the medium by letting users select the quietest (RDS-enabled) time slots.”

To make sure a bunch of Wi-Fi devices all agree on which FM station (and therefore which RDS) they're listening to, Wi-FM proposes a very simple convergence strategy: everybody starts at the lowest frequency in its local FM band, and uses the first station the receiver locks to.

Kuzmanovic's paper explains that since FM has good propagation characteristics, there's a very good chance all of the base stations in a neigbourhood will lock on to the same station.

The paper claims as much as 1.5 times traffic gain for Wi-FM, compared to a pure unscheduled network – a non-trivial improvement given that all the scheme needs is some software to take advantage of an FM baseband receiver that's either lying unused in a device, or could be cheaply added to it. ®

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