802.11a

Technology Overview

802.11a operates at the 5 GHz UNII (Unlicensed National Inforamtion Infrastructure) band. The total bandwidth is broken into three "domains" for a total of 300 MHz.

Frequency	Max Power Output Allowed
5.15 MHz - 5.25 MHz	50mW
5.25 MHz - 5.35 MHz	250mW
5.725 MHz - 5.825 MHz	1W (outdoor only)

The encoding is done with "coded OFDM", which was developed specifically for indoor wireless use. COFDM breaks a single 20 MHz channel of a high-speed data carrier into 52 lower-speed subcarriers. Each subchannel is approx 300 KHz wide. These subchannels are then transmitted in parallel. COFDM uses 48 of these subchannels for data and the remaining four for error correction.

The 802.11a standard specifies that all vendor products must support 6Mbps, 12Mbps, and 24Mbps. Higher data rates are allowed, but not explicitly discussed in the standard. However, the de-facto vendor standard is turning out to be 54 Mbps.

The 54Mbps data rate is achieved by using 64QAM (64-level quadrature amplitude modulation ). This allows for up to 1.125Mbps per 300 KHz subchannel. With 48 channels the result is a 54Mbps data rate.

At lower speeds, binary phase shift keying (BPSK) is used to encode 125Kbps per subchannel, resulting in 6Mbps. Using quadrature phase shift keying, doubles it to 12Mbps. Using 16-level quadrature amplitude modulation (encoding 4 bits per hertz), yields 24Mbps.

Current Status and Issues

However, there are still some issues to consider before worldwide adoption of this new standard. While the UNII band is relatively unpopulated in the United States, the same is not true in other parts of the world.

In Europe, the ETSI (European Telecommunicaionts Standards Institue) is requiring DFS (Dynamic Frequency Selection ) and TPC (Trasmit Power Control) functionality before allowing unlicensed applications to use the 5 GHz band. These two protocols will allow a client to dynamically change channels and/or use lower power modulation if it sees interference, thus giving existing signals on the band first priority.

In Japan, only the lower 100 MHz of the FCC's UNII band is available. This means that users in Japan will only have 5 channels to chose from instead of the 10 that will be availabe in the United States and Europe.

And finally, the HiperLAN/2 protocol already exists in the 5 GHz band.

Update 8/10/01

Card Access has released PCMCIA cards that combine two of the 12 channels available in 802.11a to give 72Mbps of true throughput. Their product if based on the Atheros chipset.

Intermec plans on releasing an AP based on the same chipset for about $1000 at the end of September 2001. Proxim, TDK, and Xircom have announced cards and APs by the end of the year.

Still no word from the Radiata chipset that Cisco bought.