Q

Wireless duplex modes

Why is that the wireless link between a client and Access Point half-duplex? Since each DSSS channel is 22 Mhz

wide, what if one portion of it is used for sending and another for receiving to make the wireless link full-duplex? Wireless LAN radios operate in half-duplex mode, either transmitting or receiving at any point in time. A WLAN access point would need multiple radios to operate in full-duplex mode, transmitting and receiving simultaneously on two radios, on two different channels. You could build such an AP, but most vendors don't due to cost.

DSSS (ISM band) channels are indeed 22 MHz wide ? for example, the FCC-specified channel 1 occupies 2.412 GHz +/- 11 MHz. But only one radio can transmit on this channel at any point in time -- using the 802.11b standard, a chipping code spreads signal across the entire 22 MHz channel. All other radios MUST listen when another radio is transmitting to avoid interference.

Consider adjacent two radios, one listening to channel 1 (2.412 GHz) and another sending on channel 3 (2.422 GHz). These channels overlap from 2.412-2.422 GHz, so outgoing signal on channel 3 will collide with simultaneous incoming signal on channel 1. The result would be a high bit error rate and excessive retransmission, yielding low or even no effective throughput. For best results, adjacent radios should use non-overlapping channels ? for example, the minimum 3 MHz gap between channels 1 and 6 (2.437 GHz). Wider inter-channel gaps further reduce the probability of interference.

This was first published in October 2002

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