802.11a ups the ante on data rates

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Broadband wireless LANs are about to make their debut. Wi-LAN and Philips Semiconductors just released the first application-specific integrated circuit (ASIC) chip based on the high-speed 802.11a standard. Wi-LAN will integrate the chip into the I.Will products it will deliver this fall, and vendors such as Radiata Communications and Clarity Wireless are right on Wi-LAN's heels. Does this mean you should hold off for the higher data rate products before implementing a wireless LAN? Let's take a closer look.

The IEEE finalized the initial 802.11 standard for wireless LANs in June 1997. The standard specifies a 2.4 GHz operating frequency using frequency hopping or direct sequence modulation with data rates of 1 or 2 Mbps. In late 1999, the IEEE published two supplements to the 802.11 standard: 802.11a and 802.11b. One of my previous columns discussed the 11 Mbps direct sequence 802.11b standard, for which many vendors have compliant products on the market.

The 802.11a standard is quite different from its 802.11b counterpart. Among 802.11a's characteristics:

  • A wide variety of high data rates are available: 6, 9, 12, 18, 24, 36, 48, and 54 Mbps. 6, 12, and 24 Mbps are mandatory for all products.

  • Operating frequencies fall into unlicensed national information structure (U- NII) bands at 5.15 to 5.25 GHz, 5.25 to 5.35 GHz, and 5.725 to 5.825 GHz. This provides plenty of bandwidth to support the higher speeds. In the US, the Code of Federal Regulations, Title 47, Section 15.407, regulates these frequencies.

  • Maximum output power is 40 mW for products operating at 5.15 to 5.25 GHz, 200 mW for 5.25 to 5.35 GHz, and 800 mW for 5.725 to 5.825 GHz.

  • The physical layer uses orthogonal frequency division multiplexing (OFDM) for modulating data before transmission. A combination of binary and quadrature phase shift keying (BPSK and QPSK) provide the different data rates.

  • Forward error correction (convolutional coding) compensates for an errored packet without the need for retransmission. FEC reduces overhead when noise or interference causes bit errors.

Existing 802.11b products operate in the 2.4 GHz frequency spectrum, resulting in potential RF interference with Bluetooth products. 802.11a products, however, will operate mostly in the relatively empty 5 GHz bands, encouraging a happy coexistence between wireless LANs and Bluetooth devices.

Which wireless LAN standard should you use? If you need to deploy a system today, then the best alternative is 802.11b. If you need higher data rates and can wait until the beginning of 2001, then 802.11a is an option. If you're simply transmitting bar codes as part of a warehouse inventory system, you can probably get by with a wireless LAN supporting 802.11's data rate of 1 or 2 Mbps. For applications having relatively high utilization, such as video transmission, 802.11b's 11 Mbps may be enough, or the higher data rates of 802.11a may be necessary. Keep in mind, though, that the faster 802.11a products are likely to be more expensive.

Next time, we'll help you define your wireless network requirements.

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