How Many Users Per Wireless Base Station?

The popularity of wireless ISP solutions is growing by leaps and bounds, but there seem to be some fundamental info gaps in the ranks of would-be implementers.

[February 11, 2000]

On the ISP-Wireless list in February 2000, CW posted this question:

"Does anyone have any advice about the number of concurrent users a BreezeCom WBS-DS.11 can support?"

A number of respondents cited what seemed to be a "magic number."

[RD replied] "256 is the table limit. Half that is a reasonable place to start."

Several respondents reinterpreted the question:

[DD wrote] "I think the question was more along the lines of how many average clients can one WBS-DS.11 serve. Keeping in mind that using a single unit as a relay hub reduces that number by about 50 percent, I would guess 20 or 30."

[CE posited] "I believe system overhead consumes about 20 percent of bandwidth for DSSS and 50 percent for FHSS. If you take the remaining available bandwidth and divide it by the data rates of the subscribers that should tell you how many simultaneous connections you can have. Then multiply that number by an oversell ratio of 5 since all subscribers are not on-line at the same time. This means an 11 Mbps DSSS AP talking to 2 Mbps subscriber cards could support 22 subscribers, half that if using it as a relay hub."

[RD refined the calculation] "Raw RF overhead for 802.11 DSSS systems is between 30 and 35 percent. That should give you about 1.6 Mbps actual throughput. The rest sounds about right, but you might get a few more users."

Another respondent questioned both scenarios:

[BJ wrote] "If you have an 11Mbps radio talking to a 2Mbps radio that is sending data at 2Mbps, it is using 100 percent of the available bandwidth, as the 11Mbps radio is actually operating at 2Mbps."

[JS disagreed -- in detail] "As with modem port usage, not all ports are active at the same time even if they are busy. The number of subscribers per access point is a factor of the physical limits of the radio's ability to associate with clients and the usage pattern of the clients.

"A DS AP can associate with 256 clients. As someone else said, a safe place to start is probably half that many, but then again it might be able to support more depending on the usage pattern or perhaps fewer if the clients are all BW pigs.

"An FH AP system can associate with 64 clients. Our usage indicates we should be able to push the ratio to 40:1 or higher without the clients taking a hit, but again the real world will dictate. I think it is important to allow for slop in any estimate -- and in any 'promise' to the customer. We have around 750 dialup ports in the system, and they support over 4,700 customers without creating a busy problem. That is a 7:1 ratio, but modems make a bad comparison."

[CD flatly contradicted this] "There will not be any difference in total system throughput between using an 11mbps base and a 2mbps base if all your remote clients are 2mbs radios. The 11mb base talks to a 2mb radio using a 2mb algorithm, So, 2mb minus 20 percent equals 1.6mb throughput divided among all your 2mb remote customers. The base cannot transmit to more than one client at a time, so it would be impossible for an 11mb base station to have more than 2mb worth of throughput total when talking to 2mb client cards. The efficiency of the whole radio system will be much better if all radios clients use 11mbs radios. The more data a radio can transfer in a set amount of time the better, obviously."

Another respondent wondered about the assumptions used in arriving at these conflicting answers:

[DD wrote] "The assumption here is that the AP will spend equal amounts of time servicing an ongoing request from a 2 mb/sec client and an 11 mb/sec client. I wonder if this is true, and does it hold true regardless of who makes the client adapters, provided they are 802.11 compliant?"

Which prompted a call for further investigation:

[JS replied] "If they are fully 802.11 compliant, they will follow the rules of communication. Given the right circumstance the 2Mb has as much opportunity as the 11Mb to get access. Everyone here should read the 802.11 standard. There is no excuse for being -- or thinking of being -- in this business and not being familiar with the standard.

Online Resources:
   A Short Tutorial on Wireless LANs and IEEE 802.11
   BreezeCom's IEEE 802.11 Technical Tutorial (PDF)
     IEEE 802 Standards Committee General Information

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