The picture has become somewhat confused as service providers started using Wi-Fi to deliver services for which it was not originally designed. The two major examples of this are wireless ISPs and city-wide Wi-Fi mesh networks.
Wireless ISPs (WISPs):
One business that grew out of Wi-Fi was the Wireless ISP (WISP). This is the idea of selling an Internet access service using wireless LAN technology and a shared Internet connection in a public location designated a hot spot.
From a technical standpoint, access to the service is limited based on the transmission range of the WLAN technology. You have to be in the hot spot (i.e. within 100m of the access point) to use it. From a business standpoint, users either subscribe to a particular carrier’s service for a monthly fee or access the service on a demand basis at a fee per hour. While the monthly fee basis is most cost effective, there are few intercarrier access arrangements so you have to be in a hot spot operated by your carrier in order to access your service.
City-Wide Mesh Networks:
To address the limited range, vendors like Mesh Networks and Tropos Networks have developed mesh network capabilities using Wi-Fi’s radio technology.
The idea of a radio mesh network is that messages can be relayed through a number of access points to a central network control station. These networks can typically support mobility as connections are handed off from access point to access point as the mobile station moves.
Some municipalities are using Wi-Fi mesh networks to support public safety applications (i.e. terminals in police cruisers) and to provide Internet access to the community (i.e. the city-wide hot spot).
WiFi systems use two primary radio transmission techniques.
- 802.11b (<=11 Mbps): The 802.11b radio link uses a direct sequence spread spectrum technique called complementary coded keying (CCK). The bit stream is processed with a special coding and then modulated using Quadrature Phase Shift Keying (QPSK).
- 802.11a and g (<=54 Mbps): The 802.11a and g systems use 64-channel orthogonal frequency division multiplexing (OFDM). In an OFDM modulation system, the available radio band is divided into a number of sub-channels and some of the bits are sent on each. The transmitter encodes the bit streams on the 64 subcarriers using Binary Phase Shift Keying (BPSK), Quadrature Phase Shift Keying (QPSK), or one of two levels of Quadrature Amplitude Modulation (16, or 64-QAM). Some of the transmitted information is redundant, so the receiver does not have to receive all of the sub-carriers to reconstruct the information.
The original 802.11 specifications also included an option for frequency hopping spread spectrum (FHSS), but that has largely been abandoned.
WiFi make use of adaptive modulation and varying levels of forward error correction to optimize transmission rate and error performance.
As a radio signal loses power or encounters interference, the error rate will increase. Adaptive modulation means that the transmitter will automatically shift to a more robust, though less efficient, modulation technique in those adverse conditions.
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