And now, with the same AP, if I set the basic rate and limit it to a maximum of 11 Mbps. Will this degrade the throughput result?
Think of data rate as the width of a water pipe. Think of throughput as the volume of water that can flow through the pipe. The fatter the pipe, the greater the possible volume of water. Of course, the actual volume of water that exits the pipe depends on how much water is pumped into the pipe at the far end. If you have an unlimited supply of water at the far end, then a 4" pipe is going to deliver twice as much water as a 2" pipe. If you have a trickle of water, then both pipes pretty much do the same job.
Now apply this analogy to 802.11 wireless LANs. Setting your data rate to 11 Mbps instead 54 Mbps means that your WLAN will have about one-fifth the capacity (pipe width). Actual throughput (water flow) will depend on data to be sent/received. If station(s) only send about 1 Mbps of data, actual throughput will be the same in either case.
But what if all your stations combined have, say, 40 Mbps of data to send? In a WLAN with data rate set to 11 Mbps, throughput will max out around 5-6 Mbps. In a WLAN with data rate set to 54 Mbps, throughput will probably reach the mid-20's, best case. In both scenarios, the supply of data exceeds the capacity of the WLAN.
So why can't the 11 Mbps pipe carry 11 Mbps of data, or the 54 Mbps pipe carry 54 Mbps of data? Because 802.11 WLANs transmit and receive MUCH more than data -- there are management and control frames and headers and other overhead costs that reduce the "space" left for carrying data. This is also true in an Ethernet LAN -- a 10 Mbps half-duplex Ethernet connection can't actually deliver 10 Mbps of data -- but 802.11 LAN overhead is considerably higher.
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