1. Lecture # 13 Computer Communication & Networks

2. Today’s Menu ↗Last Lecture Review ↗Wireless LANs ↗Introduction ↗Flavors of Wireless LANs ↗CSMA/CA Wireless LAN2

3. 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 LAN3

4. 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 LAN4

5. 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 LAN5

6. 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 LAN6

7. 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 LAN7

8. 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 LAN8

9. 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 LAN9

10. 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 LAN10

11. Wireless LANs Wireless LAN11

12. 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 LAN12

13. 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 LAN13

14. Wireless LANs Multiple Access with Collision Avoidance (MACA) Wireless LAN14