Image Processing Reference
In-Depth Information
25.4 IEEE 802.11
25.4.1 Technical Background
IEEE . comprises a number of specifications that define the lower layers (mainly the physical
[PHY] and medium access control [MAC] layers) for WLANs [OP,Insa,Insb,Ins,Insa].
Being part of the IEEE group means that an interface can be used (IEEE .) to connect to the
higher layers, which are then not aware of the—with IEEE . wireless—network that is actually
transporting the data. The key intentions of IEEE . are thus to provide a high throughput and
continuous network connection like available in wired LANs.
To encourage the wide employment of the technology, the use of IEEE . does not incur fre-
quency licensing fees. IEEE . foresees either infrared (IR), transmits in the unlicensed ISM-band
at . GHz (like Bluetooth) or in GHz bands that are license exempt in Europe and unlicensed in
the United States (UNII bands). In contrast to Bluetooth, the companies holding key patents within
IEEE . can charge developers of IEEE . products for using the patents “on reasonable terms”
[Ins,Clause b].
In principle it is possible to have an IEEE . WLAN consisting of mobile stations (MS) only. It
is more likely though that IEEE . is used as a wireless access technology to a wired LAN to which
the connection is made by IEEE . APs. Should the AP only employ the distributed coordination
function,
∗
the MAC layer supports collision avoidance by employing carrier sense multiple access
(CSMA).hismeansthatbeforetransmittingapackettherespectiveunithastolistenfortheavail-
ability of the channel.
†
If the channel is sensed free after having been busy, the unit waits a certain
period(calledDIFS)andthenentersarandomback-ofperiod
‡
of
n
PHY
+
n
r
random
))
CW
(
...min
(
−
,
×
t
slot
(.)
where
n
PHY
is a parameter depending on the type of physical layer chosen
n
r
is the index of the retransmission of the packet
t
slot
represents the slot duration
n
PHY
CW stands for contention window (with CW
min
). If the channel is available after this time
period, the unit transmits its packet (consisting of PHY header, MAC header, and payload). Upon a
correct reception, the addressee responds with an ACK-packet a short period (called SIFS) later (see
also Figure .). .).The realized ARQ mechanism ensures reliable data.
Obviously, the IEEE . WLAN MAC concept was designed for best-effort data traffic. This
was fair enough for the use cases the system had originally been designed for. Services for which
strict delay requirements exist, like speech, were not supported well by the basic IEEE . spec-
ifications. QoS is now nevertheless addressed in the IEEE .e specification [Insb] (see also
Table .). High-priority traffic can access the channel with in average shorter back-offs than low-
priority traffic. This basic QoS function is even mandatory in the commercial “WiFi” use of IEEE
=
−
∗
hisislikelyandassumedinthefollowingofthischapter.Intheory,thestandardalsoprovidesadditionallytheuseofa
centralized PCF.
†
The implementor can choose, whether the units react (a) on just other IEEE . traffic, (b) on just other IEEE .
traffic above a certain receive signal strength, or (c) on any signal above a certain receive signal strength [Insb, Section
...], (d) has been found to give the best performance results in interfered environments [KAT].
‡
The random back-off period is entered only when the channel was busy before. Else, the unit will transmit at once after
DIFS.
