Lec # 13
Computer Communication and Networks
Today’s Menu
↗ Last Lecture Review ↗ Wireless LANs
↗ Introduction
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
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
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)
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
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
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
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?
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
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
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
