Overview of the IEEE 802.11 WLAN Standard
New standard addresses both the physical and media access control layers in WLAN systems.
By Lynn Chroust, Product Marketing Manager, Proxim, Inc.
Wireless local area network (WLAN) technology has found a strong following in the medical design industry over the past year. This technology is allowing engineers to link portable medical devices, such as EKG monitors, to central computer systems in healthcare environments without the need for costly wires or docking stations. By providing this portability, design engineers can produce medical devices and systems that increase efficiency and speed the flow of information in healthcare environments.
In June 1997, the Institute of Electrical and Electronics Engineers (IEEE) passed the 802.11 standard that addresses WLAN device design and development. The IEEE 802.11 standard provides an over-the-air interface between a wireless client and a base station (also called an access point) as well as between wireless clients. Similar to the IEEE 802.3 Ethernet and 802.5 Token Ring standards, the IEEE 802.11 specification addresses both the physical (PHY) and media access control (MAC) layers in WLAN systems.
At the PHY layer, IEEE 802.11 defines three physical characteristics for WLAN systems: diffused infrared (IR), direct sequence spread spectrum (DSSS), and frequency hopping spread spectrum (FHSS). The IR PHY operates at the baseband while the radio-based DSSS and FHSS PHYs operate in the 2.4 GHz Industrial, Scientific, and Medical (ISM) frequency band. All three PHYs specify support for 1 Mb/s and 2 Mb/s data rates.
MAC Layer
The 802.11 MAC layer, supported by an underlying PHY layer, is concerned primarily with rules for accessing the wireless medium. Two network architectures are defined under 802.11: an infrastructure network and an ad hoc network. An infrastructure network is an architecture for providing communication between wireless clients and wired network resources. The transition of data from the wireless to the wired medium is conducted through an access point (AP). The coverage area is defined by an AP and its associated wireless clients. Together, these devices form a basic service set (BSS).
An ad hoc network is an architecture that is used to support mutual communication among wireless clients. Typically created spontaneously, an ad hoc network does not support access to wired networks and does not need an AP to be part of the network.
The primary services provided by the MAC layer include:
1. Data transfer--Wireless clients use a collision sense multiple access with collision avoidance (CSMA/CA) algorithm as the media access scheme.
2. Association--This service enables the establishment of wireless links between wireless clients and APs in infrastructure networks.
3. Re-association--This takes place when a wireless client moves from one BSS to another. Two adjoining BSSs form an extended service set (ESS) if they are defined by a common ESSID. If a common ESSID is defined, a wireless client can roam from one area to another. Although re-association is specified in 802.11, the mechanism that allows AP-to-AP coordination to handle roaming is not specified.
4. Authentication--Authentication is the process of proving a client identity. In IEEE 802.11, this process takes place prior to a wireless client associating with an AP. By default, IEEE 802.11 devices operate in an open system where essentially any wireless client can associate with an access point without the checking of credentials. True authentication is possible with the use of the 802.11 option known as wired equivalent privacy (WEP), where a shared key is configured into the AP and its wireless clients. Only those devices with a valid shared key will be allowed to be associated to the AP.
5. Privacy--By default, data is transferred in the clear in 802.11-compliant WLAN systems. Any 802.11-compliant device can potentially eavesdrop on 802.11 traffic that is within range. The WEP option encrypts data before it is sent wirelessly, using a 40-b encryption algorithm known as RC4. The same shared key used in authentication is used to encrypt or decrypt the data. As a result, only wireless clients with the exact shared key can correctly decipher the data.
6. Power management--IEEE 802.11 defines two power modes--an active mode, where a wireless client is powered to transmit and receive, and a power save mode, where a client is not able to transmit or receive. Actual power consumption is not defined and is dependent upon the implementation.
Interoperability
The IEEE 802.11 was designed to provide interoperability among devices using the same PHY. However, even among devices with the same PHY, a few key ingredients necessary to achieve multivendor interoperability are absent in the ratified standard.
These ingredients include:
1. AP-to-AP coordination for roaming--The standard does not specify the handoff mechanism to allow clients to roam from one AP to another.
2.Data frame mapping--The standard does not state how an AP addresses data framing between the wired and the wireless media.
3.Conformance test suite--There is no conformance test suite specified to verify that a device is compliant with the IEEE 802.11 specification. Vendor claims for compliance to the 802.11 standard will need to be ratified by a neutral third party.
The lack of specification of these items does not inhibit vendors from producing 802.11 products. Rather, each vendor will devise their own algorithm for AP-to-AP roaming and data framing. IEEE 802.11 did not set up a committee for addressing interoperability and interpretation issues. So, determination of a common method for AP-to-AP roaming and data framing will need to be addressed in a group outside of that organization.
More about IEEE 802.11
With standard ratification complete, the IEEE 802.11 committee has refocused its efforts on updating the PHY layer specifications for 802.11 systems. The committee is evaluating new PHY layer standards that will support data rates higher than 2 Mb/s. Further information about current IEEE 802.11 activities can be obtained through IEEE's web site at http://stdsbbs.ieee.org/groups/802/11/.
Lynn Chroust, Product Marketing Manager, Proxim, Inc., 295 North Bernardo Avenue, Mountain View, CA 94043. Phone: 650-526-3753; Fax: 650-960-1984.