Wireless LAN Standard

A wireless LAN standard has been in the making since 1990. In mid-1997, the IEEE 802.11 committee finally made its most significant progress in issuing a standard. The standard defines a transmission rate of up to 2 Mbps over infrared or radio frequency bands. The standard also includes the media access control protocol Carrier Sense Multiple Access with Collision Avoidance (CSMA/CA), which means devices implementing the standard can interoperate with wired Ethernet LANs.

The new standard specifies a choice of three different physical layers. Members of the 802.11 working group decided that a choice of physical-layer implementations was necessary so systems designers and integrators could choose a technology that matches the price, performance, and operations profile of a specific application.

The standard provides for an optical-based physical-layer implementation that uses infrared light to transmit data. It also provides two radio frequency (RF) based physical-layer choices: Direct Sequence Spread Spectrum (DSSS) and Frequency Hopping Spread Spectrum (FHSS). Both operate in the 2.4-GHz ISM band.

In the standard, a 2-Mbps peak data rate is specified for DSSS with optional fallback to 1 Mbps in very noisy environments. The standard defines the FHSS implementation to operate at 1-Mbps and allows for optional 2-Mbps operation in very clean environments.

The 802.11 media access control (MAC) can work seamlessly with standard Ethernet via a bridge to ensure that wireless and wired nodes on an enterprise LAN are logistically indistinguishable and can interoperate. The 802.11 MAC is necessarily different from the wired Ethernet MAC, but any such differences are masked by an access point that connects a wireless LAN channel to a wired LAN backbone.

The wireless LAN standard uses a carrier sense multiple access with collision-avoidance MAC scheme, whereas standard Ethernet uses a collision-detection scheme.

The roaming provisions built into 802.11 also provide several advantages. The 802.11 standard includes mechanisms to allow a client to roam among multiple access points that can operate on the same or separate channels. For example, an access point transmits a beacon signal at regular intervals that includes a time stamp for client synchronization, a traffic indication map, and indication of supported data rates. Roaming clients use the beacon to gauge the strength of their existing connection to an access point. If the connection is weak, the roaming station can attempt to associate itself with a new access point.

Although the 802.11 standard addresses roaming, it is with the understanding that all the access points in an installation are manufactured by the same vendor. The standard does not ensure that clients can roam among access points from different vendors. With the advent of 802.11 products, however, users may want to mix and match access points. For example, some customers might need standard commercial-grade bridges in the office but want ruggedized bridges for the factory floor.

To address multivendor roaming, Aironet Corp., Digital Ocean, Inc., and Lucent have collaborated to develop the Inter Access Point Protocol (IAPP) specification. That will extend the 802.11 multivendor interoperability benefits with comprehensive roaming protocols. Several others, including IBM, have voiced support for IAPP as a necessary step toward true multivendor interoperability.

The 802.11 specification adds features to the MAC that can maximize battery life in portable clients via power-management schemes. Power management causes problems with wireless LAN systems because typical power-management schemes place a system in sleep mode (low or no power) when no activity occurs for a user-definable time period. This can cause a sleeping system to miss critical data transmissions. To support clients that periodically enter sleep mode, the 802.11 standard specifies that access points include buffers to queue messages.

The 802.11 standard also addresses data security. The standard defines a mechanism through which the wireless LANs can achieve Wired Equivalent Privacy (WEP). The optional WEP mechanism is especially important because RF transmissions—even spread-spectrum transmissions—can be intercepted more easily than wired transmissions.

The next step in the evolution of the 802.11 standard will likely be the incorporation of higher data rates, expected in the 10 Mbps and above range in the 5.2-GHz band. The higher speed could encourage development of such applications as streaming video, telephony, and multimedia.