802.11b Wireless LAN

1. 802.11b Wireless LAN Presentation by Srividya Srinivasan

2. What is unique about wireless • Interference and Noise • Full connectivity cannot be assumed • Hidden terminal problem • Battery Usage • Requires power management • Security • No physical boundaries • Overlapping LANs

3. Requirements of a wireless standard • Single MAC to support multiple PHY mediums • Robust to interference • Need to deal with the hidden terminal problem • Need provision for time bounded services • Support for power management to save battery power • Ability to operate world wide • Hence uses the 2.4GHz ISM band

4. System Architecture • Two basic system architectures • Ad hoc • Infrastructure based

5. Ad hoc wireless LANs • A group of stations using the same radio frequency – Basic Service Set

6. Infrastructure based Wireless LANs • Stations and Access Points • Distribution system connects cells via access points to form a single network • Extends wireless area coverage

7. Access Points • Stations select an AP and “associate” with it • Support roaming • Provide other functions • time synchronization (beaconing) • power management support • point coordination function • Traffic typically (but not always) flows through AP • direct communication possible

8. Distribution system • Used to interconnect multiple wireless cells • Multiple BSS are connected to form an ESS Extended Service Set • Allows mobile stations to access fixed resources • Not part of the 802.11 standard

9. MAC layer • Controls medium access • Distributed coordinated function • Mandatory access mechanism – CSMA/CA • Optional RTS/CTS extension to deal with the hidden terminal problem • Point Coordinated function • Contention free polling method to support time bounded services

10. MAC Services • Synchronization • Specifies the Timing Synchronization function (TCF) • Within a BSS (quasi) periodic transmission of beacon signals – contains timestamp – all local nodes adjust their local timer according to the timestamp.

11. MAC Services • Infrastructure based networks – Access point relays the beacon signal • Ad hoc networks – each node maintains own synchronization timer and starts the transmission of a beacon signal. All stations adjust their clocks to the beacon which wins and suppress their beacons for this cycle

12. MAC Services • Power management • Two states for a station –sleep and awake • Transceiver is switched off whenever it is not needed • Transceiver wakes up periodically and checks whether there are any data frames for it buffered

13. MAC Services • Infrastructure based networks • Access point buffers all frames destined for stations operating in power saving mode • With every beacon signal it transmits a TIM – traffic indication map – list of stations for which data frames are buffered in the access point

14. MAC Services • Ad hoc networks • No access point to buffer the data • Each station needs to have the ability to buffer data if it wants to communicate with a power saving station • All stations now announce a list of buffered frames during a period when they are all awake – the ATIM window

15. MAC Services • Roaming • Station starts scanning for a new access point if current link quality is very poor • Passive scanning and Active scanning • Selects best access point based on signal strength and sends Association request to it • Selected AP informs DS which updates its database

16. PHY layer • Supports 3 different physical layers • Infrared • Radio transmission • Frequency hopping spread spectrum • Direct sequence spread spectrum • These two do not interoperate with one another • The data rates achieved are 1 and 2 Mbps using radio waves

17. 802.11b Enhancements • 802.11b supports two new speeds 5.5 and 11 Mbps • DSSS is the sole physical layer technique • 802.11b systems will interoperate with 1 Mbps and 2 Mbps 802.11 DSSS systems but not with FHSS systems

18. 802.11b Enhancements • Uses Complementary Code keying ( CCK ) to encode 4 bits per career to get 5.5 Mbps and 8 bits per career to get 11 Mbps.