Wireless LANs

15.1 INTRODUCTION
15.1.1 Architectural Comparison
15.1.2 Characteristics
15.1.3 Access Control
15.2 IEEE 802.11 PROJECT
15.2.1 Architecture
15.2.2 MAC Sub layer
15.2.3 Addressing Mechanism
15.2.4 Physical Layer
15.3 BLUETOOTH
15.3.1 Architecture
15.3.2 Bluetooth Layers
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 Wireless communication is one of the fastest-
growing technologies.
 The demand for connecting devices without
the use of cables is increasing everywhere.
 Wireless LANs can be found on college
campuses, in office buildings, and in many
public areas.
3

 Medium:
 The medium is air
 The signal is generally broadcast
 Communication is done using share medium
 Hosts
 A host is not physically connected to the network
 It can move freely and can use the services provided
by the network.
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 Isolated LANs: It is called an ad hoc network, is a set of hosts that communicate
freely with each other.
 Connection to Other Networks: A wireless LAN may be connected to a wired
infrastructure network to a wireless infrastructure network
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 WLANs can be classified into two types:
 Infrastructure networks contains access points(APs) and mobile
station(STAs).
 Ad hoc LANs do not need any fixed infrastructure.
 Infrastructure networks
 Provide access to other networks
 Include forwarding functions
 Medium access control
 Ad hoc network is a group of computers each with wireless
adapter, connected as an independent wireless LAN.
 Each node can communicate with other nodes
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 Few Characteristics of wireless LANs:
 Attenuation
 The strength of electromagnetic signals decreases rapidly because the signal
disperses in all directions; only a small portion of it reaches the receiver.
 Interference
 Another issue is that a receiver may receive signals not only from the intended sender,
but also from other senders if they are using the same frequency band.
 Multipath Propagation
 A receiver may receive more than one signal from the same sender because
electromagnetic waves can be reflected back from obstacles such as walls, the ground,
or objects.
 This makes the signal less recognizable.
 Error
 Errors and error detection are more serious issues in a wireless network than in a wired
network. 7

 How a wireless host can get access to the shared medium (air).
 We discussed in Chapter 12 that the Standard Ethernet uses the
CSMA/CD algorithm
 In this method, each host contends to access the medium and sends its
frame if it finds the medium idle.
 If a collision occurs, it is detected and the frame is sent again. Collision
detection in CSMA/CD serves two purposes.
 If a collision is detected, it means that the frame has not been received
and needs to be resent.
 If a collision is not detected, it is a kind of acknowledgment that the
frame was received.
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 The CSMA/CD algorithm does not work in wireless LANs for three reasons:
1. To detect a collision, a host needs to send and receive at the same time, which means
the host needs to work in a duplex mode.Wireless hosts do not have enough power to
do so (the power is supplied by batteries).They can only send or receive at one time.
2. Because of the hidden station problem, in which a station may not be aware of another
station’s transmission due to some obstacles or range problems, collision may occur but
not be detected.
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.
 To overcome the above three problems, Carrier Sense MultipleAccess with Collision
Avoidance (CSMA/CA) was invented for wireless LANs 9

 IEEE has defined the specifications for a wireless LAN, called
IEEE 802.11, which covers the physical and data-link layers.
 It is sometimes called wireless Ethernet
 The public uses the term WiFi (short for wireless fidelity) as a
synonym for wireless LAN.
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Wi-Fi Access points
Wi-Fi Router

 The standard defines two kinds of services
 Basic Service Set (BSS)
 Extended Service Set (ESS)
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 Ad hoc architecture
 The BSS without an AP is a stand-alone network and cannot send data to
other BSSs..
 Infrastructure BSS
 A BSS with anAP is sometimes referred to as an infrastructure BSS.
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(Ad hoc mode) (Infrastructure mode)

 An extended service set (ESS) is made up of two or more BSSs with APs.
 The BSSs are connected through a distribution system
 The extended service set uses two types of stations:
 Mobile Station: Normal stations inside a BSS
 Stationary Station : Are AP stations that are part of a wired LAN
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 IEEE 802.11 defines three types of stations based on their
mobility in a wireless LAN:
 No-Transition
 BSS-Transition
 ESS-Transition Mobility
 A station with no-transition mobility is either stationary (not
moving) or moving only inside a BSS.
 A station with BSS-transition mobility can move from one
BSS to another, but the movement is confined inside one ESS.
 A station with ESS-transition mobility can move from one
ESS to another.
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 IEEE 802.11 defines two MAC sub layers:
 Distributed coordination function (DCF)
 Point coordination function (PCF)
 Figure shows the relationship between the two MAC
sublayers, the LLC sublayer, and the physical layer.
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 Distributed coordination function (DCF) is sublayer of MAC
 DCF uses CSMA/CA as the access method
 Problem in CSMA/CA
 Collision During
Handshaking
 Hidden-Station Problem
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 The point coordination function (PCF) is an optional access method that can
be implemented in an infrastructure network.
 It is implemented on top of the DCF and is used mostly for time-sensitive
transmission.
 PCF has a centralized, contention-free polling access method
 The AP performs polling for stations that are capable of being polled.
 The stations are polled one after another, sending any data they have to the AP.
 PIFS is use to give priority to PCF over DCF, another interframe space.
 PIFS (PCF IFS) is shorter than DIFS.
 This means that if, at the same time, a station wants to use only DCF and an AP
wants to use PCF, the AP has priority.
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 Priority of PCF over DCF, stations that only
use DCF may not gain access to the medium.
 To prevent this, a repetition interval has been
designed to cover both contention-free PCF
and contention-based DCF traffic.
 The repetition interval, which is repeated
continuously, starts with a special control
frame, called a beacon frame.
 When the stations hear the beacon frame,
they start their NAV for the duration of the
contention-free period of the repetition
interval.
 During the repetition interval, the PC (point
controller) can send a poll frame, receive data,
send an ACK, receive an ACK.
 At the end of the contention-free period,
the PC sends a CF end (contention-free
end) frame to allow the contention-based
stations to use the medium.
19

 Bluetooth is a wireless LAN technology
 Designed for connecting devices with different functionalities:
 Telephone, mouse, keyboard, camera, etc.
 Standard  IEEE 802.15.1
 The standard defines a wireless personal-area network (PAN)
operable in an area the size of a room or a hall.
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 Bluetooth defines two types of networks:
 Piconet
 Scatternet.
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 A piconet, or a small net, can
have up to eight stations.
 One station will be the primary;
the rest are called secondaries.
 All the secondary stations
synchronize their clocks and
hopping sequence with the
primary.
 The communication between
the primary and secondary
stations can be one-to-one or
one-to-many.
 additional secondaries can be in
the parked state
 For communication should
moved from the parked state
to the active state
 only eight stations can be
active in a piconet
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 Piconets can be combined to form what is called a scatternet.
 A secondary station in one piconet can be the primary in another piconet.
 A station can be a member of two piconets.
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 L2CAP:The Logical Link Control and Adaptation
Protocol, or L2CAP (L2 here means LL), is roughly
equivalent to the LLC sublayer in LANs.
 It is used for data exchange on an ACL link; SCO channels
do not use L2CAP
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 The 16-bit length field defines the size of the
data, in bytes, coming from the upper layers.
 Data can be up to 65,535 bytes.
 The channel ID (CID) defines a unique identifier
for the virtual channel created at this level
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 The L2CAP has specific duties:
 Multiplexing
 Segmentation and
 Reassembly
 Quality of service (QoS)
 Group Management
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Wireless LANs

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Wireless LANs