Wireless/Mobile /

Wireless LAN speeds get a boost

By John Drewry
Network World, 02/14/00

With portable computers and wireless LANs, users can enjoy greater productivity while away from their desks, whether they are in conference rooms, public areas or remote offices.

Until recently, however, wireless LANs were too slow for most enterprise applications. Based on the IEEE 802.11 standard, they ran at 1M to 2M bit/sec.

Now a new high-rate extension to the standard, 802.11b, lets wireless networks support data rates up to 11M bit/sec.

Ratified in 1997, the original 802.11 standard united the wireless industry by defining a low-level protocol architecture that worked with conventional upper-layer enterprise protocol stacks. Also, 802.11 maintained compatibility with the three most popular radio transmission types: direct sequence spread spectrum, frequency-hopping spread spectrum, and infrared.

Essentially, this new architecture added intelligence at the medium access control (MAC) Layer 2 and at the physical (PHY) Layer 1, fostering cooperation between the two layers in performing the critical tasks involved with initiating and maintaining wireless communications.

For instance, to ensure reliability of the wireless link, the MAC and PHY work together to determine if a clear path exists before they start a transmission.

During transmission, they employ special collision-avoidance and arrival-acknowledgement techniques that are not required in wired Ethernet LANs.

These are necessary because, unlike wired Ethernet, which uses the CSMA/CD collision-detection protocol to elicit a retry in the event of a packet collision, wireless 802.11 clients use collision avoidance - known as CSMA/CA, or Carrier Sense Multiple Access/Collision Avoidance. The reason: Collisions are more difficult to detect in wireless transmissions.

Finally, for direct sequence spread spectrum (DSSS) systems, the 802.11 specification includes provisions for implementing PHY-layer modulation techniques called Differential Binary Phase Shifting (DBPS) and Differential Quaterature Phase Shift Keying (DQPSK).

DSSS functions by substituting special codes for the normal bits of information, the "ones" and "zeros," that are transmitted across a network. DSSS represents each bit with a unique code, then spreads the code pattern out across the radio-wave spectrum as a protection against interference that might occur at one point in the spectrum. At the receiving end, the pattern is decoded back into its original information bits.

DBPS and DQPSK increase signal efficiency by modulating, or shifting, the radio waves in smaller increments than normal, and thereby allowing more bit values for each transmitted radio wave.

By using DBPS and DQPSK, DSSS systems are able to attain 1M and 2M bit/sec data rates, respectively, in the 2.4-GHz radio band allocated by the Federal Communications Commission to wireless LANs.

In September 1999, the IEEE approved a new designation, known as 802.11b, as the high-rate extension to 802.11. Intended to retain the error-correction, security, power-management and other advantages of the original, the new 802.11b standard adds a key ingredient - a technique for increasing bandwidth to 11M bit/sec.

Called Complementary Code Keying (CCK) the technique works only in conjunction with the DSSS technology specified in the original standard. It does not work with frequency-hopping or infrared transmissions.

What CCK does is apply sophisticated mathematical formulas to the DSSS codes, permitting the codes to represent a greater volume of information per clock cycle. The transmitter is now able to send multiple bits of information with each DSSS code, enough to make possible the 11M bit/sec of data rather than the 2M bit/sec in the original standard.

The 802.11b standard benefits users by delivering wireless Ethernet speeds of 11M bit/sec that can reliably support everyday business applications, e-mail, Internet and server network access.

With support from the new Wireless Ethernet Compatibility Alliance, founded by 3Com, Lucent, Nokia and several other companies in the wireless LAN business, the new standard will also promise certified interoperability across multivendor platforms.

Finally, the 802.11b standard serves as a rallying point for vendors and users clamoring for a simplified wireless LAN landscape.

Vendors can now focus on a single, high-speed standard, and users can cut through the clutter of wireless options by focusing on a standard that delivers multivendor interoperability and the performance to meet their application needs.

Wireless LAN speeds get a boost

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