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Wednesday, 2 July 2014

Making the most of Wifi

'Wires are not that bad (compared to wireless)' - Perry R. Cook 2001

Wireless performance is risky, lower, etc than wired, but dancers don't want to be cabled.

People use bluetooth, ZigBee and wifi. Everything is in the 2.4 gHz ISM band. All of these technologies use the same bands. Bluetooth has 79 narrowband channels. It will always collide, but always find a gap, leading a large variance in latency.

Zigbee has 16 channels, doesn't hop.

Wifi has 11 channels in the UK. Many of them overlap, but 1, 6, and 11 don't. It has broad bandwidth. It will swamp out zigbee and bluetooth.

the have seveloped XOSC, which sends OSC over wifi. It hosts ad-hoc networks. The presenter is rubbing a device and a fader is going up and down on a screen. The device is configured via a web browser.

You can further optimise on top of wifi. By using a high gain directional antenna. And by optimising router settings to minimise latency.

Normally, access points are omni directional, which will get signals from audiences, like mobile phone wifi or bluetooth. People's phones will try to connect with the network. A directional antenna does not include as much of the audience. They tested the antenna patterns of routers. Their custom antenna has three antennas in it, in a line. It is ugly, but solves many problems. the tested results show it's got very low gain at the rear, partly because it is mounted on a grounded copper plate.

Even commercial routers can have their settings optimised. This is detailed in their paper.

Packet size in routers is optimised for web browsing and is biased towards large packets, which has high latency. Tiny packets have huge throughput in musical applications.

Under ideal conditions, they can get 5ms of latency.

They found that channel 6 does overlap a bit with 1 and 11, so if you have two different devices, but them on the far outside channels.

Questions

UDP vs TCP - have you studied this wrt latency?

No, they only use UDP

How many drop packets do they get when there is interference?

that's what the graph showed.

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