Lecture # 13

Computer Communication & Networks

Today’s Menu

↗ Last Lecture Review

↗ Wireless LANs

↗ Introduction

↗ Flavors of Wireless LANs

↗ CSMA/CA

Wireless LAN 2

Wireless LANs

IEEE 802.11

↗A wireless LAN or WLAN is a wireless local area network that uses radio waves as its carrier↗Wireless LANs have become popular due to ease of installation, their mobility and the increasing popularity of laptop computers↗They incorporate IEEE 802.11 standard↗Designed for use in a small area (offices, campuses)↗Bandwidth; 11 or 54 Mbps↗Up to 100 Mbps in newer for 802.11n

Wireless LAN 3

Wireless LANs

How are WLANs Different?

↗They use specialized physical and data link protocols↗They integrate into existing networks through access points which provide a bridging function↗They let you stay connected as you roam from one coverage area to another↗They have unique security considerations ↗They have specific interoperability requirements ↗They require different hardware ↗They offer performance that differs from wired LANs

↗Provides all the features and benefits of traditional LAN technologies such as Ethernet and Token Ring, but without the limitations of wires or cables

Wireless LAN 4

Wireless LANs

WLANs Flavors

↗802.11 (1997) offers speed of 1-2 Mbps in the 2.4 GHz spectrum band (infrared, FHSS, DHSS)↗802.11a (1999) offers speeds of 54Mbps in the 5 GHz band (OFDM)↗802.11b (1999) offers speeds of 11Mbps in the 2.4 GHz spectrum band with different modulation scheme (HR-DSSS)↗802.11g (2003) is a new standard for data rates of up to a 54 Mbps at 2.4 GHz and uses the modulation technique of 802.11a (OFDM)

Wireless LAN 5

Wireless LANs

Wireless & Mobility

↗Wireless:↗ Limited bandwidth↗ Broadcast medium: requires multiple access schemes↗ Variable link quality (noise, interference)↗ High latency, higher jitter↗ Heterogeneous air interfaces↗ Security: easier snooping

↗Mobility:↗ User location may change with time↗ Speed of mobile impacts wireless bandwidth↗ Need mechanism for handoff↗ Security: easier spoofing

↗Portability↗ Limited battery, storage, computing, and UI

Wireless LAN 6

Wireless LANs

↗ The standard work in two modes:↗ In the presence of a base station (Infrastructure Mode)↗ In the absence of a base station (Adhoc Mode)

↗ In Infrastructure Mode, all communication go through the base station, called an access point in 802.11 terminology

↗ In Adhoc Mode, the computers just send to one another directly

Wireless LAN 7

Wireless LANs

802.11 MAC

↗A computer on Ethernet always listens to the ether before transmitting↗Only if the ether is idle does the computer begin transmitting↗With wireless LANs, that idea does not work so well↗Suppose that computer A is transmitting to computer B, but the radio range of A's transmitter is too short to reach computer C↗If C wants to transmit to B it can listen to the ether before starting, but the fact that it does not hear anything does not mean that its transmission will succeed

Wireless LAN 8

Wireless LANs

802.11 MAC: CSMA/CA

↗Similar to Ethernet ↗Sense the medium to transmit↗Defer the transmission until the link becomes idle↗Take back off if collision occurs↗Is it sufficient?

↗All nodes are not always within reach of (to hear) each other

Wireless LAN 9

Wireless LANs

Hidden node problem

↗Since not all stations are within radio range of each other, transmissions going on in one part of a cell may not be received elsewhere in the same cell↗In the example, station C is transmitting to station B. If A senses the channel, it will not hear anything and falsely conclude that it may now start transmitting to B

Exposed node problem

↗Here B wants to send to C so it listens to the channel↗When it hears a transmission, it falsely concludes that it may not send to C, even though A may be transmitting to D

As a result of these problems, 802.11 does not use CSMA/CD, as Ethernet does

Wireless LAN 10

Wireless LANs

Wireless LAN 11

Wireless LANs

Hidden and Exposed Nodes Problems

Hidden nodes↗ Sender thinks its OK to send when its not↗ A-C and B-D are hidden nodes in the figure below

Exposed nodes↗ Sender does not send when its OK to send↗ B and C are exposed nodes in the figure below

Wireless LAN 12

Wireless LANs

Multiple Access with Collision Avoidance (MACA)

↗Sender transmits RequestToSend (RTS) frame↗Contains intended time to hold the medium↗Receiver replies with ClearToSend (CTS) frame↗Neighbors of the receiver hear CTS and keep quiet for the intended duration of transmission or till the ACK is heard↗Neighbors of only the sender hear RTS but not CTS so they can transmit

Wireless LAN 13

Wireless LANs

Multiple Access with Collision Avoidance (MACA)

Wireless LAN 14