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21 Wireless Local Area Networks
above MAC is the LLC, where the framing takes place. The LLC inserts certain
fi elds in the frame such as the source address and destination address at the head
end of the frame and error handling bits at the end of the frame.
The 802.11 MAC is similar in concept to 802.3, in that it is designed to
support multiple users on a shared medium by having the sender sense the medium
before accessing it. For the 802.3 Ethernet LAN, the carrier sense multiple access
with collision detection (CSMA/CD) protocol regulates
how Ethernet stations
establish access to the network and how they detect and handle collisions that
occur when two or more devices try to simultaneously communicate over the
LAN. In an 802.11 WLAN, collision detection is not possible due to the
near/far
problem (see Chapter 11). To detect a collision, a station must be able to transmit
and listen
at the same time, but in radio systems the transmission drowns out the
ability of a station to hear a collision.
21.5.4 IEEE 802.11 Medium Access Control
Wireless local area networks operate using a shared, high bit rate transmission
medium to which all devices are attached and information frames relating to all
calls are transmitted. MAC sublayer defi nes how a user obtains a channel when
he or she needs one.
MAC schemes include random access, order access, deterministic access, and
mixed access. The random access MAC protocols are: ALOHA (asynchronous,
slotted), carrier-sense multiple-access (CSMA) (CSMA/collision-detection (CD),
CSMA/collision-avoidance (CA), non-persistent, and p-persistent). The maxi-
mum throughput of slotted ALOHA protocol is about 36%
of the data rate of
the channel (see Chapter 5). It is simple, but not very effi cient. Most WLANs
implement a random access protocol, CSMA/CA with some modifi cation, to deal
with the
hidden node
problem. The CSMA peaks at about 60%. When the traffi c
becomes heavy, it degrades badly. The way of dealing with that situation is to use
p-persistent. Most mobile data networks also use random access protocol, usually
one
that is simpler than CSMA, namely slotted ALOHA. Table 21.10 provides a
comparison of MAC schemes for wireless networks.
Deterministic MAC schemes improve throughput and response time when
traffi c is heavy. They offer the guaranteed bandwidth for isochronous traffi c.
In mixed cases such as CSMA/TDMA, the frame is divided into a random access
part and a reserved part. When the traffi c is light, it is left to be mostly random.
When the traffi c is heavy and throughput is in danger of declining or if a node
requires isochronous bandwidth, the control point allocates bandwidth determin-
istically. CSMA/TDMA approaches CSMA performance under light traffi c, so it
has fast access time. It approaches TDMA performance when the traffi c becomes
heavy, so its throughput can rise close to 100% of the data rate.
IEEE 802.11 uses a modifi ed
protocol known as
carrier sense multiple access
with collision avoidance
(
CSMA/CA
) or distributed coordination function (DCF).
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CSMA/CA attempts to avoid collisions by using
explicit packet acknowledgment
(ACK), which means an ACK packet is sent by the receiving station to confi rm
that the data packet arrived intact.
The CSMA/CA protocol is very effective when the medium is not heav-
ily loaded since it allows stations to transmit with minimum delay. But there is
always a chance of stations simultaneously sensing the medium as being free and
transmitting at the same time, causing a collision. These collisions must be identi-
fi ed so that the MAC layer can retransmit the packet by itself and not by the upper
layers, which would cause signifi cant delay. In the Ethernet with CSMA/CD the
collision is recognized by the transmitting station, which goes into a retransmis-
sion phase based on an exponential random backoff algorithm. While these colli-
sion detection mechanisms are a good idea on a wired LAN, they cannot be used
on a WLAN environment for two main reasons:
Implementing a collision detection mechanism
would require the implemen-
tation of a full duplex radio capable of transmitting and receiving at the
same time, an approach that would increase the cost signifi cantly.
In a wireless environment we cannot assume that all stations hear each
other (which is the basic assumption of the collision detection scheme), and
the fact that a station wants to transmit and senses the medium as free does
not necessarily mean that the medium is free around the receiver area.
To overcome these problems, the 802.11 uses a CA mechanism together
with a positive ACK. The MAC layer of a station wishing to transmit senses the
medium. If the medium is free for a specifi ed time, called
distributed inter-frame
space
(DIFS), then the station is able to transmit the packet; if the medium is
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