Avoiding Wi-Fi surprises

Enterprise firms won't invest in 802.11g  products before the standard is ratified this summer. But small offices/home offices and consumers are buying products by the bucket load. Most vendors say prestandard gear will require only a firmware upgrade to interoperate with products built using the final specification. But there's no way to know for sure until the standard is ratified and interoperability is tested. Until then, here's what you need to know to make smart buying decisions today.

Overall, the SOHO hardware companies that offer pre-g are producing comparable products. But differences in chipset designs and iterations of the 802.11g specification vary, so it helps to know what's under the hood.

The two concerns are interoperability and performance. How well an 802.11g product works with 802.11b products from the same and other manufacturers, and how well it works with 802.11g products from other vendors, will also vary - today and later on when the standard is ratified.

Experts agree most products will work together on the same network; the question is how well. There might be problems with encryption standards and processing that need more power than some chips can support, says Alan Nogee, senior analyst at In-Stat/MDR.

And on ad hoc networks, where clients communicate without using an access point, early 802.11g products might block transmissions from 802.11b clients, says Rich Redelfs, of Atheros Communications.

The bigger question is performance. Before buying, consider your network needs. The maximum speed you can get on a pure 802.11g network is 24M bit/sec. However, once you add 802.11b clients to the network, speeds will suffer.

Until the specification is ratified, it helps to know which vendor's chipset is in your gear. Broadcom stresses performance over interoperability; Intersil stresses interoperability and Wi-Fi compliance over performance. Broadcom argues that some aspects of the final standard impede performance unnecessarily.

At issue is how 802.11b and 802.11g clients communicate. While each transmits data over the same 2.4-GHz frequency, each speaks a different language (802.11b speaks CCK, or complimentary code keying, and 802.11g speaks OFDM, Orthogonal Frequency Division Multiplexing). Because 802.11g is backward-compatible, it speaks both CCK and OFDM. However, 802.11b clients can't speak OFDM, nor can they recognize when an 802.11g client is transmitting data. As a result, 802.11b clients can "step on" 802.11g clients by transmitting data simultaneously, creating packet collisions that slow the network.

To address this, the IEEE added a mechanism called protective mode to the final specification. This ensures that on a mixed 802.11b and 802.11g network, all clients check with the access point before transmitting data, to ensure the channel is clear. But protective mode also slows the network to a maximum 14.4M bit/sec, Redelfs says.