How many wireless base stations can connect to 802.11g access points? |
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EXPERT RESPONSE FROM: Lisa Phifer
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QUESTION POSED ON: 24 October 2009
How many wireless stations can I have connected to one 802.11g access point (AP) at a time? I'm using a Roving Networks WiFly GSX 802.11b/g transceiver that can only accept data from the microprocessor via UART at a max rate of 1 Mbps. So, although the 802.11g protocol can transfer data at rates up to 54 Mbps, it seems that the fastest rate at which one of these modules can actually transmit data is 1 Mbps. Thus, if you actually get 18-22 Mbps out of an 802.11g AP (when accounting for all the CSMA/CA overhead), and each module transmits simultaneously at 1 Mbps, can you support 18-22 of these modules on a network at a time?
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Your analysis makes sense, but you might be overlooking a couple of factors when calculating how many wireless base stations can connect to your 802.11g access point (AP). You have identified the max sustained data rate for each of your 802.11 clients and the max supported data rate for your 802.11g AP. You have correctly observed that the maximum aggregate TCP throughput ("goodput") for all clients connected to a single 802.11g AP tends to be roughly half of the max data rate – that could be a smidge higher than 18-22 Mbps but not much. If all clients are going full-bore, your capacity math (22 clients * 1 Mbps = 22 Mbps goodput) is sound.
However, you have not taken into account any external competition for the channel used by the AP. That's fine if you can find a clear unused channel for your wireless LAN (WLAN), but doing so is not always easy for 802.11g WLANs that operate in the crowded 2.4 GHz band. Furthermore, if you wanted to support additional clients, you might do so by installing a total of up to three APs, configured for Channels 1, 6, and 11 -- all using the same SSID. Finally, you don't say whether your client applications will actually have a sustained data flow of 1 Mbps to send. For example, if your client applications simply send brief bursts of short frames, you might support many more clients using a single AP. In short, I think you're on the right track with your calculations, but recommend you think not only about network hardware capacity but how you'll deploy and use that capacity.
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