ls available in 802.11 a, the propagation loss is higher and the net throughput therefore falls faster as a function of distance (though this also means that access points can be positioned closer together SO channel reuse can be a bit more aggressive). In 802.11g, a request to send (RTS) and clear to send (CTS) message is needed if bandwidth is coshared with an 802.11 b transmitter (known as working in mixed mode). This produces a significant decrease in real throughput. The effect of distance in 802.11a and the impact of RTS/CTS overhead in 802.11g when used with 802.11 b is shown in Table 3.12. Real throughput rates in 802.11g and 802.11a quickly fall to levels that are not much higher and sometimes lower than standard 802.11b. RTS/CTS is a poll and response algorithm and therefore implies a scheduling delay in addition to introducing a significant protocol and time/bandwidth overhead. Note that 'mixed mode' actually implies two sets of MAC overheads. First, the way the contention MAC is managed in 802.11b is different from 802.11a and g. Both use time slot back off but 802.11 b uses a 20-microsecond slot width and a and g use 9 microseconds. If 11b devices are interoperating with 11g devices then the 20-microsecond slot length must be used. This means that contention overheads will be higher. Similarly with 11b devices, there is a choice of a long 192-microsecond and/or short 96-microsecond preamble. The OFDM preamble is 20 microseconds. In mixed mode, either the long or short 11b preamble will need to be used to support 11b devices. This means that preamble overheads will be higher. Table 3.11 Data rates, modulation and coding in 802.11 a and g Data rate 6/9 Mbps 12/18 Mbps 24/36 Mbps 48/54 Mbps Modulation 16 QAM 64 QAM Coding 1/2 or 2/3 or 3/4 Interrelationship of the Physical Layer with Other Layers of the OSI Model Table 3.12 Effect of distance on 802.11 a, b and g throughput, effect of mixed mode b and g signalling overhead on 802. .11g throughput Distance(ft) 802.11b Mbps 802.11a Mbps 802.11 g only 802.