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the bandwidth of let say 802.11g is 54mbps ,when 2 clients join the same ap and the same channel becomes 54/2 and when a third client comes it becomes (54/2)/2 and so on,how lets say the 7762 supports 256 clients ?what bandwidth 256 clients will have?
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f f

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Posted 4 years ago

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David Moore, Employee

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Official Response
There are multiple factors that affect the capacity of an AP.
- Protocol used (802.11b vs 802.11a/g vs 802.11n)
- Channelization (20 MHz vs 40 MHz)
- RF conditions (multi-path, noise, adjacent channel and/or co-channel interference, etc)
- AP type (1x1:1 vs 2x2:2 vs 3x3:2 vs 3x3:3, etc)
- Client type (1x1:1 vs 2x2:2 vs 3x3:3, etc)
- Air time fairness (this is a big one)
- Processor power and design of the AP

I'm sure someone could come up with others. But it is the last one to which I am referring. Simply because an AP can provide 50 Mb/s of throughput does not mean that it can support 50 clients simultaneously. For any AP, there will be a point where additional clients will greatly affect performance beyond what is explained by the data transmitted. For a "cheap" AP that is designed for the home or a small office, the AP design is not typically intended to support 50 clients and performance will be limited as much or more by the number of clients than by the protocol. Don't assume that any AP can provide 50 Mb/s aggregate for 50 clients.

Now that I've opened up the can of worms a bit more and mentioned air time fairness, this deserves some discussion as well, especially since a "cheap" AP is unlikely to provide this feature. With air time fairness, each client is provided equal air time independent of the protocol used. That is, 802.11b clients, 802.11g clients, and 802.11n clients are all given equal air time as long as they all have packets to transmit. Without this, the statistical nature of WiFi is such that slower clients will take up more air time communicating with the AP. And, without air time fairness, slow clients will end up dragging down the throughput of all clients to their speed. This is mitigated somewhat by block acknowledgments for 802.11n clients, but it is still an issue. Also consider that even if you had all 802.11n clients, some will be faster than others, but all will be limited by the slowest client if you do not have air time fairness. With air time fairness, faster clients have equal time access, but will be able to transmit more frames in that amount of time than slower clients will. Faster clients will be able to achieve higher throughput than slower clients. Overall, the capacity of the AP increases.

Let's take your example of 50 clients on an AP. It is conceivable that all clients will operate at the same data rate, but in reality this is pretty unlikely. What is more realistic is to assume that you have a variety of data rates. With air time fairness, the AP can maximize capacity rather than limiting all client rates to the slowest one. Again, a "cheap" AP is unlikely to support air time fairness and will be subjected to this problem. With all clients taking longer to transmit data, they are on the air more and there is more contention, further reducing capacity for the AP. The problem builds on itself. You can see the affect of this in some of the tests performed by organizations such as WLAN Pros and Tom's Hardware. APs that don't provide air time fairness were not able to support 50 or, in some cases, even 10 concurrent clients streaming data. APs that did provide air time fairness supported the greatest number of concurrent clients.