> Cover Story> Full Story
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
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
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
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.
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.
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
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