WIRELESS COMMUNICATION

Husnain Sherazi sherazi@leads.edu.pk

Lecture 1

Wireless Communication Networks

• It is a type of computer network which does not

require wires between the network devices.

• It uses radio waves to connect different devices.

• Wireless networks are convenient.

• Mobility.

• Expandable.

• Easy connectivity.

• Normally less costly.

Wireless Networks

Elements of Wireless Networks

Link Characteristics

Wired vs. Wireless NetworksWired Networks Wireless Networks

Based on Cables Based on radio frequencies

Communication within meters Communication over kilometers

Secure Less secure

Not Mobile Mobile

Less interference More Interference

Cable expenses No cable expenses

Difficult to setup Easy to setup

Difficult to manage Easy to manage

Nodes are visible Nodes are not always visible

Wireless Hopping

• Wireless hopping means whether the data is

transferred directly to the end device or by jumping

from one device to other and then the end device.

• Wireless Hopping can be

• Single Hop Infrastructure based

• Single Hop Infrastructure less

• Multi Hops Infrastructure based

• Multi Hops Infrastructure less

Single Hop Infrastructure based• These networks have base stations.

• These base stations are connected with wired networks

• All the communication is done between the base

stations and the end devices.

• All this communication is done directly on a single hop.

• 802.11 Networks fall in this category.

• Wi-Fi networks used in classroom and café etc. are all

part of Single Hop Infrastructure based.

Single Hop Infrastructure based

Single Hop Infrastructure less• These networks have no base stations.

• No base stations are connected with wired networks.

• Communication is done by one coordinator (node).

• This coordinator node transmits for others as well.

• A node connects with other node to be a part of that

network.

• All this communication is done directly on a single hop.

• Bluetooth networks fall in this category.

• Ad-hoc networks are also example of Single Hop

Infrastructure less.

Single Hop Infrastructure less

Multi Hop Infrastructure based

• These networks have base stations.

• These base stations are connected with wired

networks.

• Nodes can communicate with the base station

directly and sometimes by using other nodes for

communication.

• This communication of nodes with base station using

other nodes is done through multi hops.

• Some wireless sensor networks fall in this category.

Multi Hop Infrastructure based

Base StationNode

Node

Node

Multi Hop Infrastructure less

• These networks have no base stations.

• No base stations are connected with wired networks.

• Nodes have to transmit data to the destination by

relaying on other nodes.

• Such communication is done using Multi Hop but

with no base station.

• Mobile Ad-hoc Networks and Vehicular Ad-hoc

Networks are part of this network

Multi Hop Infrastructure less

802.11 and Wireless LANs

• The most commonly used Wireless LAN is IEEE

802.11.

• It is also called Wi-Fi (Wireless Fidelity).

• Several 802.11 standards are available

• 802.11a

• 802.11b

• 802.11g

• 802.11n

• Most popular these days are combination of these

standards (802.11a/b/g) or (802.11b/g).

802.11 and Wireless LANs

• These standards share some same attributes

• CSMA/CA

• Support for Infrastructure and Ad-hoc mode

• Ability to reduce transmission rate to reach out more distance

• A new standard 802.11n is used for multiple inputs and outputs.

802.11 Architecture• The most important building block of a 802.11

Architecture is a Basic Service Set (BSS).

• A BSS can have one or more nodes.

• BSS can also have a central device; Access Point

(AP).

• These APs can connect to other devices like Router

and Switches.

• In a Home network, there is only one Router and one

Access Point which are sometimes combined into a

Wireless Router.

802.11 Architecture

802.11 Architecture

• This network can be Ad-Hoc.

• In an Ad-Hoc network, nodes can group themselves

without any need of a central device (AP).

• In Ad-Hoc network, connection is made by the

devices in near proximity.

• Normally this type of network is made when nodes

want to transfer data to each other and there is no

central device available.

802.11 Architecture

Ad-Hoc Network

Channel and Associations• When AP is installed, it is provided with

• Service Set Identifier (SSID)

• Channel Number

• When a node enters the range of an AP, it must

associate with AP to use the internet.

• Node can associate with exactly one AP.

• The associated AP can only send and receive the

frames to and from the node.

• To associate with an AP, the node should accept the

beacon frames sent by an AP.

Channel and Associations

Beacon Frame

• SSID of AP• MAC address of AP

AP sends beacon frames periodically and nodes can listen to these beacon frames.

Active and Passive Scanning• Nodes can scan different channels for beacon frames

from APs. This scanning is called Passive Scanning.

• Nodes can broadcast a probe frame to all the APs in

range. APs respond to this request from Nodes. This

scanning is called Active Scanning.

• Association Request and Response are used in Passive

Scanning.

• Probe Request, Probe Response and Association

Request and Association Response are used in Active

Scanning.

Active and Passive Scanning

802.11 MAC Protocol• In Wireless communication, there are chances that data can

be lost because of several reasons.

• To avoid data loss, we can use acknowledgment called Link

Layer Acknowledgment Scheme.

• In this scheme, when receiver receives the frame, it wait for

some time and then sends the acknowledgment. This wait

time is called Short Inter-Frame Spacing (SIFS).

• If acknowledgment is not received by sender, it will

retransmit.

• If retransmit acknowledgments are received for fixed time,

the frames are discarded.

Distributed Inter-Frame Space

Short Inter-Frame Space

CSMA/CA

• To overcome the problem of collision, Ethernet uses

a technique called CSMA/CD.

• However in Wireless communication, a different

technique is used to overcome collision.

• In Wireless communication, it is called Carrier

Sense Multiple Access/ Collision Avoidance

(CSMA/CA)

CSMA/CANode checks the channel

Channel Idle?

Node waits for some time called DIFS and

then transmits the frames

Node choses a backoff value and counts it

down till 0.

Transmits the frames if value reaches 0.

Waits for acknowledgment

NoYes

CSMA/CA• In Wireless transmission, the target is to avoid collision

instead of detecting collision.

• So a backoff value is used to count down till 0, before

the frame is sent to destination.

• This wait time is called Distributed Inter-Frame

Space.

• The value of backoff will be reduced only when the

channel is idle.

• If the channel is busy, the value of backoff will be

frozen.

Hidden Terminals

• There are scenarios in which nodes are hidden from

each other.

• If two nodes are associating with an AP.

• Nodes might be very far from each other so that

they cannot listen to each other.

• However AP can listen to both the nodes.

• In this case, there are chances of collision, since

both the nodes do not see each other.

Hidden Terminals

• Hidden terminals• A sends to B, C cannot receive A • C wants to send to B, C senses a “free” medium (CS fails)• collision at B, A cannot receive the collision (CD fails)• A is “hidden” for C

• Exposed terminals• B sends to A, C wants to send to another terminal (not A or B)• C senses carrier, finds medium in use and has to wait• A is outside the radio range of C, therefore waiting is not

necessary• C is “exposed” to B

Hidden and Exposed Terminals

BA C

CSMA/CA• To avoid collision, two concepts are used

• RTS (Request to Send)

• CTS (Clear to Send)

• Whenever a node wants to send some data, it will send

a RTS to AP.

• AP after receiving the request will broadcast a CTS to

make sure no other node uses the channel.

• Node can now send the data to AP.

• AP after receiving the data will send an

acknowledgment.