Is it possible to use a Wireless LAN using 802.11a within a building in which we have more than 100 PCs - all are connected to a server that is operating at 100M bytes? Also the total output should not exceed 100 mW(20dbi)?
Let's start with power. Transmit power output depends upon the size and RF characteristics of the building you need to cover, because that will determine the number of APs, channels, and need for antennas. In the US, the maximum power permitted in the 5.15-5.25 frequency band is 40 mW. Between 5.25-5.35, maximum power is 200 mW. Limits in other regulatory domains vary. To reduce interference and satisfy different regulatory constraints, IEEE 802.11h specifies transmit power control procedures. In other words, you'll probably be able to configure your 802.11a APs to reduce power output - but whether the result will actually meet your 100 mW limit depends on your WLAN layout.
Next, let's consider bandwidth. If you have 100 PCs sharing one server with a 100M bit/sec. interface, you might say that each PC needs 1M bit/sec. Even an 802.11b WLAN can provide 1M bit/sec. to one station, but of course not to 100 stations at the same time. But all 100 PCs are probably not accessing the server at once. You really need to define your requirements for number of simultaneous users and per-user throughput. For example, suppose you decide that any single user requires 2M bit/sec. with 25 users needing simultaneous access at any time. That's an aggregate throughput of 50M bit/sec.
Now, how do you know whether your 802.11a WLAN can support this? Standard 802.11a data rates range from 6 to 54M bit/sec. But data rate varies with distance, so again the answer depends upon your actual WLAN layout. How far your stations are from your APs will directly impact throughput, and effective application throughput is about half the data rate. Conduct a site survey to determine the effective throughput delivered to each station, laying out APs so that every station receives the per-user throughput that you require. Finally, consider number of users per AP - stations using the same channel share the aggregate bandwidth provided by that channel.
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