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COMPUTER NETWORKS – I[10CS55] unit-7
UNIT – VII
14.1 IEEE 802.11
The standard defines two kinds of services: the basic service set (BSS) and the extended service
Basic Service Set
IEEE 802.11 defines the basic service set (BSS) as the building block of a wireless LAN. A basic
service set is made of stationary or mobile wireless stations and an optional central base station,
known as the access point (AP). The figure shows two sets in this standard.
The BSS without an AP is a stand-alone network and cannot send data to other BSSs. It is
called an ad hoc architecture. In this architecture, stations can form a network without the need
of an AP; they can locate one another and agree to be part of a BSS. A BSS with an AP is
sometimes referred to as an infrastructure network.
Extended Service Set
An extended service set (ESS) is made up of two or more BSSs with APs. In this case, the BSSs
are connected through a distribution system, which is usually a wired LAN. The distribution
system connects the APs in the BSSs. IEEE 802.11 does not restrict the distribution system; it
can be any IEEE LAN such as an Ethernet. Note that the extended service set uses two types of
stations: mobile and stationary. The mobile stations are normal stations inside a BSS. The
stationary stations are AP stations that are part of a wired LAN.
Distributed Coordination Function
One of the two protocols defined by IEEE at the MAC sublayer is called the distributed
coordination function (DCF). DCF uses CSMA/CA as the access method. Wireless LANs cannot
implement CSMA/CD for three reasons:
1. For collision detection a station must be able to send data and receive collision signals at
the same time. This can mean costly stations and increased bandwidth requirements.
2. Collision may not be detected because of the hidden station problem. Computer Network-1 10CS55
Dept of CSE,SJBIT 226
3. The distance between stations can be great. Signal fading could prevent a station at one
end from hearing a collision at the other end.
Bluetooth is a wireless LAN technology designed to connect devices of different functions such
as telephones, notebooks, computers (desktop and laptop), cameras, printers, coffee makers, and
so on. A Bluetooth LAN is an ad hoc network, which means that the network is formed
spontaneously; the devices, sometimes called gadgets, find each other and make a network called
a piconet. A Bluetooth LAN can even be connected to the Internet if one of the gadgets has this
capability. A Bluetooth LAN, by nature, cannot be large. If there are many gadgets that try to
connect, there is chaos. Bluetooth technology has several applications. Peripheral devices such as
a wireless mouse or keyboard can communicate with the computer through this technology.
Monitoring devices can communicate with sensor devices in a small health care center. Home
security devices can use this technology to connect different sensors to the main security
controller. Conference attendees can synchronize their laptop computers at a conference. Computer Network-1 10CS55
Dept of CSE,SJBIT 236
Bluetooth was originally started as a project by the Ericsson Company. It is named for Harald
Blaatand, the king of Denmark (940-981) who united Denmark and Norway. Blaatand translates
to Bluetooth in English. Today, Bluetooth technology is the implementation of a protocol
defined by the IEEE 802.15 standard. The standard defines a wireless personal-area network
(PAN) operable in an area the size of a room or a hall.
Bluetooth defines two types of networks: piconet and scatternet.
A Bluetooth network is called a piconet, or a small net. A piconet can have up to eight stations,
one of which is called the primary;t the rest are called secondaries. All the secondary stations
synchronize their clocks and hopping sequence with the primary. Note that a piconet can have
only one primary station. The communication between the primary and the secondary can be
one-to-one or one-to-many. Figure 14.19 shows a piconet.
Figure 14.19 Piconet
Although a piconet can have a maximum of seven secondaries, an additional eight secondaries
can be in the parked state. A secondary in a parked state is synchronized with the primary, but
cannot take part in communication until it is moved from the parked state. Because only eight
stations can be active in a piconet, activating a station from the parked state means that an active
station must go to the parked state.