18/08/2005

San Francisco to get blanket Wi-Fi coverage? - Broadband & ISPs - Breaking Business and Technology News at silicon.com

San Francisco to get blanket Wi-Fi coverage? - Broadband & ISPs - Breaking Business and Technology News at silicon.com

It all sounds nice, blanket coverage of a city using WiFi, but it's not really been done before and WiFi was never designed for it. The 802.11 standards are really based on Ethernet (which is a wired protocol) and Ethernet relies on every station that's connected to a LAN (local area network) being able to "hear" the traffic on the LAN and detect whether collisions have occurred. In WiFi terms it's Ethernet mapped on to radio where as Ethernet uses CSMA/CD (carrier sense mulitple access/collision detect), 802.11 uses CSMA/CA (carrier sense mulitple access/collision avoidance). CSMA/CA requires that every node on the network is able to "see/hear" every other node.

This leads to hidden node problems which can cause network meltdowns and the bigger your WiFi network, the more chance you have of hidden nodes.

There's also the problem of mobile users who can act as hidden nodes and can moves between access points - requiring some kind of roaming protocol which 802.11 does not as yet support (though there are planned enhancements and proprietry solutions).

So though it's a good idea, the practicalities make it very complicated.
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