Understanding IEEE 802.11.

The IEEE 802.11 standard for wireless LANs (WLANs) -- due to be finalized late this year -- results from seven years of effort by leading scientists in the wireless LAN field. What can we expect from it?

In addition to providing high-performance and robust systems, the standard also promises multivendor interoperability. This means customers are freer to mix and match vendors to meet their requirements for each given application. The new standard will deliver lower-cost components, which will translate into lower prices for users.

The standard specifies a choice of three different physical (PHY) layers, any of which can underlie a single media access control (MAC) layer. Members of the 802.11 working group felt that a choice of PHY implementations was necessary so that systems designers and integrators can choose a technology that matches the price, performance, and operations profile of a specific application.

Specifically, the standard provides for an optical-based layer that uses infrared light to transmit data, and two radio frequency (RF)-based layers, direct sequence spread spectrum (DSSS) and frequency hopping spread spectrum (FHSS).

In 802.11, the DSSS PHY defines both 1- and 2-Mbps peak data rates. The former uses differential binary phase shift keying (DBPSK) and the latter uses differential quadrature phase shift keying (DQPSK).

The standard defines the FHSS PHY to operate at 1 Mbps and allows for optional 2-Mbps operation. The PHY uses 2-or 4-level Gaussian frequency shift keying (GFSK) modulation.

IEEE 802.11 standard specifies that the WLANs operate in the 2.4-GHz band that regulatory agencies around the world have set aside for spread spectrum usage.

KEY MAC-LAYER FEATURES

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

The WLAN standard uses a carrier sense multiple access with collision avoidance (CSMA/CA) MAC scheme, whereas standard Ethernet uses a carrier sense multiple access with collision detection (CSMA/CD) scheme.

The 802.11 standard specifies that the MAC layer handle acknowledgment and that the MAC layer resend lost frames, resulting in more efficient usage of the available bandwidth and quicker acknowledgment.

The Mac-layer implementation eliminates the access latency and allows the acknowledgment to use some of the interframe spacing time period (in which no other activity would occur in any case).

HIDDEN STATION PROVISIONS

A WLAN can suffer from collisions caused by a hidden station. In such a case, station A can communicate with the AP with no problem and station B can communicate with the AP with no problem, but station A and B are physically separated by sufficient distance to prevent direct communications. Since A and B don't directly communicate, the hidden station problem affects only the competition for access, not the actual communication session itself.

The collision avoidance scheme adopted in 802.11 requires a station to avoid transmitting while another station is actively transmitting and can be received well by the first station. Yet node A would not be able to detect that node B was transmitting to the AP in the hidden-station example.

The 802.11 .includes an optional request to send (RTS)/clear to send (CTS) provision to protect against hidden-station interference. All 802.11 receivers must support RTS/CTS, but support is optional in transmitters.

To use the facility, the transmitting node (B in this case) sends an RTS to the AP, requesting to reserve the fixed amount of time necessary to transmit a frame of given length. When the medium is available, the AP broadcasts a CTS message that all stations can hear assuring node B of access to the air for the requested amount of time.

ROAMING PROVISIONS

The roaming provisions built into 802.11 allow a client to roam among multiple APs that can be operating on the same or separate channels. Each AP transmits a beacon signal every 100 msecs. The beacon includes a time stamp for client synchronization, a traffic indication map, an indication of supported data rates, and other parameters.

Roaming clients use the beacon to gauge the strength of their existing connection to an AP. If the connection is judged weak, the roaming station can attempt to associate itself with a new AP.

POWER MANAGEMENT

802.11 adds features to the MAC that can maximize battery life in portable clients via power-management schemes. Power management causes problems with WLAN systems because typical power-management schemes place a system in sleep mode (low or no power) when no activity occurs for some specific or user-definable time period. Unfortunately, a sleeping system can miss critical data transmissions.

To support clients that periodically enter sleep mode, the 802.11 specified that APs include buffers to queue messages. Sleeping clients are required to awaken periodically and retrieve any messages. The APs are permitted to dump unread messages after a specified time.

WIRED EQUIVALENT PRIVACY 

One final area of differentiation between 802.11 and either wired LANs or existing WLAN implementations centers on data security. The standard defines a mechanism through which the WLANs 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 transmission.

While vendors are rushing to bring 802.11-compliant products to market, the committee members and companies behind the standard can also look forward to follow-on efforts that provide continuous improvements. as has been seen within the Ethernet 802.3 world.

For example, 802.11 addresses roaming provided that all APs in an installation were manufactured by the same vendor. The standard does not ensure that clients can roam among APs from different vendors.

But customers may want to mix and match access points. Some customers might need standard commercial-grade bridges in the office and ruggedized bridges for the factory floor.

To address multivendor roaming, Aironet Corp, Digital Ocean, and Lucent Technologies have collaborated to develop the Inter Access Point Protocol (IAPP) specification.

The IAPP will extend the 802.11 multivendor interoperability benefits with comprehensive roaming protocols. Several other companies, including IBM, have voiced support for the IAPP as a necessary step towards true multivendor interoperability.

FASTER DATA RATES

The next step in the evolution of 802.11 is most likely a standard for higher data rates, expected in the 10 Mbps-and-above range. Besides handling bandwidth-hungry applications better, faster peak rates will allow more nodes to connect to a WLAN via a single channel.