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Chapter 15

WirelessLANs

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Figure 15.1 BSSs

IEEE Specification for Wireless LAN: IEEE 802.11, which covers the physical and data link layers.

Basic Service Set (BSS) is made of stationary or mobile wireless stations and a possible central base station, known as the access point (AP).

BSS without AP is a stand-alone network and cannot send data to other BSS LANs. It is what is called as ad-hoc architecture.

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Figure 15.2 ESS

Extended Service Set (ESS) is made up of two or more BSSs with APs.

BSSs are connected through a Distributed System, which is usually a wired LAN.

Nodes can be mobile or stationary. A mobile can belong to more than one BSS at

the same time.

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CSMA/CA Wireless LAN cannot implement CSMA/CD for three reasons

Station must be able to send and receive data at the same time. Collision may not be detected because of the hidden terminal

problem. Distance between stations in wireless LANs can be great. Signal

fading could prevent a station at one end from hearing a collision at other end.

Before sending a frame, source senses the medium by checking the energy level at the carrier frequency.

Backoff until the channel is idle. After the channel is found idle, the station waits for a period of time

called the Distributed interframe space (DIFS); then the station sends a control frame called request to send (RTS).

After receiving RTS, the destination waits for a period called Short interframe space (SIFS), the destination station sends a control frame, called Clear to Send (CTS) to source. This control frame indicates that the destination station is ready to receive data.

Source sends data after waiting for SITS Destination sends ACK after waiting for SITS.

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Figure 15.7 CSMA/CA flowchart

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Figure 15.8 CSMA/CA and NAV

RTS frame indicates the duration of time that the source needs to occupy the channel.

Stations that are affected by this transmission create a timer called a Network Allocation Vector (NAV) that shows how much time must pass before these stations are allowed to check the channel for idleness.

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Collision during handshaking What happens if there is collision

during the time when RTS or CTS control frames are in transition, often called the handshaking period?

Because there is no mechanism for collision detection, the sender assumes there has been a collision if it has not received a CTS frame from the receiver. The backoff strategy is employed, and the sender tries again.

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Bluetooth is a wireless LAN technology designed to connect devices of different functions such as telephones, notebooks, computers, cameras, printers,…

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 piconet.

Ex. Wireless mouse or keyboard, conference among palmtop computers, …

Bluetooth IEEE 802.15

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Figure 15.15 Piconet

A Bluetooth network is called a piconet, or a small net. It can have up to eight stations, one of which is called

the master; the rest are called slaves. Maximum of seven slaves. But an additional eight slaves can stay in parked state,

which means they can be synchronized with the master but cannot take part in communication until it is moved from the parked state. A victim active station has to be selected.

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Figure 15.16 Scatternet

Piconets can be combined to form what is called a scatternet.

A slave station in one piconet can become the master in another piconet.

Bluetooth devices has a built-in short-range radio transmitter, upto 1 Mbps operating at bandwidth of 2.4GHz.

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Epilogue Benefits of Ethernet WLAN

Cheap Faster to deploy Mobility Watch for security

Need for authentication Need for encryption during transmission

A typical diameter is a room for one AP about 40 meters

Frequencies used and data rate IEEE 802.11b operates at 2.4 GHz range and gives

11Mbps IEEE 802.11a operates at 5 GHz range and gives 54Mbps IEEE 802.11g operates at 2.4 GHz range and gives

54Mbps

Bluetooth gives 1Mbps. Operates in a short diameter (10 meters). Operates at 2.4 - 2.4835 GHz May have interference with Ethernet WLAN APs

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Summary

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Future – 802.11n