Channel Listing 2.4 GHz

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Channels available, their frequency, legality in Ireland, Atheros Turbo mode supported (108 MBit) and eventually comments.


802.11b/g

Channel Frequency IE Legal Turbo mode ? Comment
2312 x
2317
2322
2327
2332
2337
2342
2347
2352 x
2357
2362
2367
2372
12412 x
22417 x
32422 x
42427 x
52432 x
62437 x x
72442 x
82447 x
92452 x
102457 x
112462 x
122467 x
132472 x
142484 japanese only, but support by a couple of routers like Linksys WRT54's
152512 x
162532 x
172552 x
182572 x
192592 x
202612 x
212632 x
222652 x
232672 x
242692 x
252712 x
262732 x


In 802.11b each channel uses a 22 MHz wide band, 11 MHz in each direction from the center frequency. It has a maximum raw data rate of 11 Mbit/s and uses CCK (Complementary code keying), which is a variation of CSMA/CA (Carrier sense multiple access with collision avoidance). Unfortunatly the overhead that CSMA/CA produces means, that a maximum of 5.9 Mbit/s over TCP and 7.1 Mbit/s over UDP can be archieved. Some of the implementations like most Atheros drivers make up for that by providing compression etc. For speeds at 1 and 2 MBit/s DSSS (Direct-sequence spread spectrum) is used, instead of CCK.


802.11b can operate at speeds of 11, 5.5, 2 and 1 Mbit/s, lower speeds might help when signal quality issues exist. Most implementation will automatically scale back, this is called Adapative Rate Selection. Bastardised versions of 802.11b exist (802.11b+ etc.) that support speeds of 22, 33 or even 44 MHz, these have however never been standardised and are usually proprietary to certain manufacturers.


The channels will overlap and so channels used for a node will have to be wisely choosen. Each channel will range +/- 11 MHz from it's centerfrequency at 30 dB, however with increased strengh the band used widens as for example +/- 22 MHz at 50 dB.


Graphical view of the channel overlaps in 802.11b Image:80211bchannels.gif


The regulatory limitation is to use no more than a 22 MHz wide band, the US allows only 11 channels (1-11), Europe 13 (1-13) and Japan 14 channels (1-14). On top of that Europe has a 100mW power restriction on 802.11b wireless links.


802.11g operates in the same frequency band as 802.11b, but tried to implement the best of the 2.4 GHz world (802.11b) and the 5 GHz world (802.11a) to gain more speed. 802.11a suffers from, that you need power to gain the same distance in comparison to 802.11b/g, because of it's wider wavepattern. 802.11g uses OFDM (orthogonal frequency-division multiplexing) for it's modulation and is backwards compatible to 802.11b.


One of the problems with 802.11g is, that it is more affected by interference than 802.11b. This can be caused by 802.11b only devices, microovens, radar and the likes. You would usually see a drop in the Signal/Noise Ratio (SNR) when you switch a link from 802.11b to 802.11g on cards, that support this. It also suffers by the same overlap problems that 802.11b has.


Atheros introduced a proprietary extension providing up to 108 MBit/s called SuperG or Turbo mode. The embedded boards used for our wireless application (Wrap, Routerboard, Soekris) often don't allow to even pass 54 MBit over the internal bus in the board, so it's not usefull in the Wan, beyond that the frequency band would be even more poluted. It causes interference for other channels.

Source: Marlow