Mike Bresnahan wrote: > [...] > What little I have read about 802.11b and its counterparts (e.g. bluetooth > wireless) discusses effective ranges of 10-100 meters. My limited > experience with my Linksys Prism2.5 card confirms this range. How is it > that people on this list are discussing ranges upwards of 23 miles? Perhaps > the stuff I have been reading assumes very small antennae? There are some limits imposed by the 802.11b spec that will kick in, even if you have an rf link that defies the laws of physics. The 802.11b protocol uses link-layer acks sent by the MAC to acknowledge packet reception. All receivers are required to respect a quiet window after packet reception (even if it's not for them) to permit the STA a chance to transmit it's ack. If my memory serves me correctly, this window combined with the flight time of the packet effectively puts an upper bound on a vanilla 802.11b link. The window is something like 156uS, which implies ~47Km (29 Miles) (assuming 300m per uS.) Some MACs have special modes where these timers are disabled, but then it's not 802.11b any more (see below.) > After seeing the coverage maps of the Twin Cities displayed at the meeting, > I'm left with some confusion. What good is it to put up a big antennae > backed by a big amplifier and coat a very large area with 802.11b? Can > people with ordinary WLAN cards in their laptops actually communicate with > the access point? I can understand how they might be able to recieve data, > but how can they hope to transmit over such a large distance with their > wimpy amplifier and antennae? Do you need a big antennae only on one end to > make it work? If you're planning a point-to-multipoint setup, remember that hidden-tranmsitter syndrome will start to hurt you. This can be alleviated by using PCF (Point Control Function), where the AP effectively polls the STAs (problem is, most APs don't support PCF), or by enforcing RTS/CTS values in the STAs (most default settings ensure that RTS/CTS will never be used.) The proprietary solutions in this space (e.g. KarlNet) use all kinds of techniques to address issues like hidden transmitter and improve their performance over vanilla 802.11b. Remember also that you'll be talking 1 & 2 Mbit links for the majority of any given coverage area. -- andyw at pobox.com Andy Warner Voice: (612) 801-8549 Fax: (208) 575-5634