802.11b 支持的速率有: 1Mbps, 2Mbps, 5.5Mbps以及11Mbps,但是这些速率 是如何计算出来的呢?
The different data rates have different chipping methods, with 1 and 2 Mbps using the Barker code and 5.5 and 11 using CCK. To calculate the total throughput, you have relate chipping method to the bits each data rate sends along with the symbol rate (modulation symbol changes per second) they use. Both run at 11 Mchips/s so the symbol rates look like this:
Barker Code - 11 chip code
Symbol Rate = 11,000,000 / 11 = 1 Msps
CCK - 8 chip code
Symbol Rate = 11,000,000 / 8 = 1.375 Msps
Then you take the Number of Bits each rate transmits and multiply is by the symbol rate:
1 Mbps = 1 bit x 1 Msps
2 Mbps = 2 bits x 1 Msps
5.5 Mbps = 4 bits x 1.375 Msps
11 Mbps = 8 bits x 1.375 Msps
The key here is that the symbol rate is only different between the code types, not the data rates themselves. It's those extra bits being sent over the same rate.