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Explaining 802.11b

Despite the arrival of 802.11a, its older counterpart, 802.11b is still alive and kicking. Its flexible architecture ensures that it will still be relevant in the near future. by CK Mah

The demise of 802.11b has been greatly exaggerated, even though it now faces competition from a faster wireless protocol. 802.11b's underlying architecture is sound and will remain relevant for even next-generation devices. For wireless implementers, it is probably a good idea to have a second look at 802.11b's feature sets so as to be prepared for the future with an older protocol.

The 802.11b standard is limited in scope to the physical (PHY) and medium-access control (MAC) network layers. The PHY layer corresponds directly to the lowest layer defined by the ISO in its layer 7 open system interconnect (OSI) network model. The MAC layer corresponds to the lower half of the second layer of that same model with Logical Link Control (LLC) functions making up the upper half of the OSI Layer 2.

The standard specifies a choice of three different PHY layers, any of which can underlie a single MAC layer. The idea of allowing a choice of PHY implementations was necessary so that 802.11b systems designers and integrators have a choice in the technology that matches the price, performance, and operations profile of a specific wireless product or application. These choices are analogous to choices such as 10Base-T, 10Base-2, and 100Base-T available in the wired Ethernet arena.

The standard allows for an optical-based PHY that uses infrared (IR) light to transmit data, and two radio frequency (RF)-based PHYs that leverage different types of spread-spectrum radio communications.

The IR PHY will typically be limited in range and most practically implemented within a single room, while the RF-based PHYs can be used effectively to cover significant area particularly when deployed using mobile phone like configuration.

The IR PHY relies on pulse position modulation (PPM). This includes direct sequence spread spectrum (DSSS) and frequency hopping spread spectrum (FHSS). Both DSSS and FHSS artificially spread the transmission band so that the transmitted signal can be accurately received and decoded in the face of noise. However, most commercial implementations today use DSSS.

DSSS approaches the task of spreading spectrum by artificially broadening the bandwidth needed to transmit a signal by modulating the data stream with a spreading code. Therefore, the receiver can detect error free data even if there is noise persisting in portions of the transmission band.

One of the key advantages of the RF PHYs is the ability to have a number of distinct channels. This channelization allows users to co-locate channels in the same or adjacent areas to boost aggregate throughput or to deploy an array of channels that support roaming clients. For DSSS, different channels can use different frequency bands.

WLAN Topologies
Three basic topologies are supported in 802.11b WLANs the independent basic service set (IBSS), the basic service set (BSS), and the extended service set (ESS). The MAC layer implements the support for all three configurations.

IBSS configurations are also referred to as an independent network. Logically, an IBSS configuration is analogous to a peer-to-peer office network in which no single node is required to function as a server. In IBSS, base stations can communicate directly with one another and generally cover a limited area. On the other hand, BSS configurations rely on an access point (AP) that acts as the logical server for a single WLAN channel.

While it may seem that the AP adds an unnecessary layer of complexity, it is actually necessary to perform the wired-to-wireless bridging function and connect multiple WLAN channels. ESS WLAN configurations consist precisely of multiple BSS cells that can be linked by either wired or wireless backbones.

MAC Features
The MAC was developed to work seamlessly with standard Ethernet to ensure that wireless and wired nodes on an enterprise LAN are the same logically. 802.11b defines both a frame format and MAC scheme that differs from standard Ethernet. This frame format enables number of features such as fast acknowledge, handling hidden stations, power management, and data security.

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

Under the 802.11b standard, the MAC layer must also handle acknowledgement and resending of lost frames, which results in faster acknowledgement and more efficient bandwidth usage. This ensures that the receiving station can take immediate control of the airwaves rather than compete with other nodes for medium access.

Further, 802.11b includes an option, Request To Send (RTS)/Clear To Send (CTS) provision to protect against hidden-station interference. All 802.11b receivers must support RTS/CTS, but support is optional in transmitters. To use the facility, a transmitting node sends an RTS request to the AP to reserve a fixed amount of time necessary to transmit a frame of given length.

In addition, the MAC also supports a concept called fragmentation that provides for flexibility in transmitter/receiver design, and can be useful in environments with RF interference, such as from a microwave source. The 802.11b standard requires that all receivers support fragmentation but leaves such support optional on transmitters. Designers and potentially end users can determine when or if fragmentation is used.

CK Mah writes for Network Computing-Asian Edition and can be reached at ck_Mah@cmpasia.com.sg. Send your feedback to editor@networkmagazineindia.com

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