This document summarizes wireless local area networks (WLANs). It discusses that WLANs link devices wirelessly using technologies like Wi-Fi. It describes the categories of wireless networks as infrastructure with a base station or ad hoc between nodes. The physical layers of 802.11 standards for wireless networking are also summarized, including specifications for 802.11b, 802.11a, and 802.11g. Issues like the hidden and exposed terminal problems are outlined, and how Carrier Sense Multiple Access with Collision Avoidance (CSMA/CA) addresses these using request-to-send and clear-to-send messages.

1. WLAN (Wireless Local
Area Networks)
By: Muhammad Hanif

2. Wireless LANs
Eighth Edition
by William Stallings
Lecture slides by Lawrie Brown
Chapter 17 – Wireless LANs

3. Quotes of the day
 GREED AND FAITH CAN NEVER CO-
EXIST IN THE HUMAN HEART.
• Hadith of An-Nasa’i on the authority of Abu
Hurayrah

4. Agenda
 WLAN
 Categories of Wireless Networks
 Wi-Fi
 Wireless Physical Layer
 Two Additional Problems
 CSMA / CA

5. Wireless Local Area Networks
 A wireless local area network (WLAN) links
two or more devices using some wireless
distribution method.
 When two or more than two devices connect
through air medium is called Wireless LAN.
 The laptop computers and other mobile devices
(PDAs and cell phones) created high level
demand for wireless local area networking.
 Companies jumped in, quickly developing
wireless products.

6. Categories of Wireless
Networks
 Infrastructure Wireless :: base station network
is connected to the wired Internet.
 Ad hoc Wireless :: wireless nodes
communicate directly with one another.
 MANETs (Mobile Ad Hoc Networks) :: ad hoc
nodes are mobile.

7. Wireless LANs
Figure(a) Wireless networking with a base station. (b) Ad hoc networking.

8. Wi-Fi (wireless Fidelity)
 Wi-Fi is a technology that allows an
electronic device to exchange data or
connect to the internet wirelessly using
2.4 GHz and 5 GHz radio waves.

9. Wireless Physical Layer
 Physical layer of WLANs include
 1997: 802.11 infrared
 1999: 802.11a OFDM and 802.11b HR-DSSS
 2001: 802.11g OFDM
 And more ……..
 802.11 Infrared
 Two capacities 1 Mbps or 2 Mbps.
 Range is 10 to 20 meters and cannot pass through walls.
 802.11a OFDM (Orthogonal Frequency Divisional
Multiplexing)
 Data rate of 54Mbps and using 5.5 GHz band.
 More difficulty penetrating walls.
 Range is 35m (115 ft) indoor and 100m (330 ft) outdoor.

10. Wireless Physical Layer
 802.11b HR-DSSS (High Rate Direct Sequence Spread
Spectrum)
 11b approved and hit the market before 11a.
 Up to 11 Mbps in 2.4 GHz band.
 Range is 35m (115 ft) indoor and 100m (330 ft) outdoor.
 11b and 11a are incompatible!!
 802.11g OFDM(Orthogonal Frequency Division
Multiplexing)
 An attempt to combine the best of both 802.11a and 802.11b.
 Supports bandwidths up to 54 Mbps.
 Using 2.4 GHz frequency.
 Is compatible with 802.11b.
 Range is 38m (125 ft) indoor and 125m (412 ft) outdoor.

11. Two Additional Problems
 Two additional problems:
 Hidden Terminal Problem
 Exposed Station Problem

12. Two Additional Problem
Figure (a)The hidden station problem. (b) The exposed station
problem.

13. Hidden Terminal Problem
 Wireless stations have transmission
ranges and not all stations are within radio
range of each other.
 Simple CSMA will not work!
 C transmits to B.
 If A “senses” the channel, it will not hear
C’s transmission and falsely conclude that
A can begin a transmission to B.

14. The Exposed Station Problem
 This is the inverse problem.
 B wants to send to C and listens to the
channel.
 When B hears A’s transmission, B falsely
assumes that it cannot send to C.

15. CSMA / CA
 Uses CSMA / CA (CSMA with Collision
Avoidance). solved hidden, exposed terminal:
– Send Ready-to-Send (RTS) and Clear-to-Send
(CTS) first
– Source send RTS before transmission start and
receiver when ready respond with CTS.
– RTS, CTS helps determine who else is in range or
busy (Collision avoidance).

16. CSMA / CA
(a) A sending an RTS to B.(b) B responding with a CTS to A.

17. Summary
 WLAN
 Categories of Wireless Networks
 Wi-Fi
 Wireless Physical Layer
 Two Additional Problems
 CSMA / CA