By Craig Ellison  |  Posted 2003-03-25 Print this article Print

"G" to Work"> Getting "G" to Work

The 802.11g task force had to overcome significant engineering challenges when it was establishing the standard. The biggest challenge was to ensure high-speed performance while providing backward compatibility with the large installed base of 802.11b networking products, which share the same radio spectrum.

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Indeed, the major issues with 802.11g products concern interoperability, both with legacy 802.11b clients and among different makes of equipment. To deal with legacy 802.11b products, the 802.11g draft specifies a protected mode. 802.11g, as previously noted, uses OFDM, whereas 802.11b uses DSSS (direct sequence spread spectrum). Unfortunately, radios using these different transmission methods dont "hear" one another.

To solve this problem, the IEEE is relying on a mechanism that was part of the original 802.11 specification: CTS/RTS (clear to send/request to send). Think of it as a wireless handshaking mechanism, much as in the old RS-232 serial days. In protected mode (or mixed mode, as its also called), the access point uses CTS/RTS to give clients access to the airwaves.

Other changes also have to take place to ensure interoperability between "b" and "g" clients. The slot time, the time between packets, is increased from 9 microseconds (used by 802.11g clients operating in a pure "g" environment) to 20 microseconds (the slot time used by 802.11b clients). This means that 802.11g clients operating in a mixed-mode environment (with its associated overhead) will have poorer throughput than those operating in a "g"-only mode—even if the 802.11b clients present arent sending any traffic.

Craig Ellison is PC Magazine Labs' director of operations. The Labs staff, in consultation with PC industry experts, develops procedures and scripts for the independent and impartial testing underlying all PC Magazine reviews.

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