likely to operate in close proximity.
Consequently, a device assigned to one LAN
may be able to transmit or receive without
authorization on a nearby LAN. To prevent
this, the wireless LAN scheme must use
addressing and access control techniques.
• License-free operation. Users would prefer to
buy and operate wireless LAN products
without having to secure a license for the
frequency band the LAN uses.
• Handoff/roaming. The MAC protocol
the wireless LAN uses should let mobile
stations move from one cell to another.
• Dynamic configuration. The MAC pro-
tocol’s provision for addressing and net-
work management should let
organizations dynamically and automati-
cally add, delete, or relocate end systems with-
out disrupting other network users.
BALANCING STANDARDIZATION
AND FLEXIBILITY
With so many possible applications and wire-
less configurations and the need to meet the spe-
cific requirements of wireless environments,
some standardized approach to product creation
is imperative. Without it, equipment from vari-
ous vendors would not work together. But flexi-
Every LAN consists of devices that must share its transmis-
sion capacity. Thus, an individual LAN needs some way to con-
trol access to the transmission medium so that devices will use
that capacity in an orderly and efficient fashion. This responsi-
bility falls to the medium access control protocol,
which ensures that all the end systems on a LAN
cooperate. The MAC protocol requires that only
one station transmit at a time, and it specifies that
data be transmitted in blocks, or MAC frames. Each
frame includes user data, a destination and source
address, error-detection code, and MAC control
bits.
Every LAN architecture includes a MAC layer,
which is responsible for detecting errors and discarding any
erroneous frames. Each end system monitors the shared
medium for frames whose destination address is a match with
its address and copies those frames.
The MAC layer for IEEE 802.11 is rather complex (see
Figure 3 in the main text). Unlike MAC in Ethernet, for exam-
ple, it includes a distributed coordination function with a rudi-
mentary priority scheme, in which all stations cooperate for
medium access. It also includes a point coordination function,
implemented in a central controller, to accommodate urgent
What Is the MAC Protocol?
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