Wireless Local Area Networks (LANs)

Wireless Local Area Networks (LANs)

Outline • Introduction to wireless LANs • Wireless LAN physical layer • Wireless LAN medium access control (MAC) • Distributed coordination function (DCF) • Point coordination function (PCF)

Review of Related Lectures • Local area networks (LANs) • Share resources and information • Low-cost, high speed, and error-free communications • Ethernet; token ring networks Transmission medium

Review of Related Lectures (Cont’d) • Random access protocols • ALOHA: “just do it” • A station transmits whenever it has data to transmit • Throughput: 18% • Slotted ALOHA • Time is slotted • Only transmit at the beginning of a time slot • Throughput: 36% • Carrier sensing multiple access with collision detection (CSMA-CD) • Ethernet • Sense before transmission; if channel busy, wait • Continue to sense during transmission • If collision  abort

Introduction to Wireless LANs • What is wireless LAN? • An extension of the wired LAN • Compatible • Coverage: ~ 100 feet • Merits • Convenience • Fast installation • User mobility • Challenges • Smaller bandwidth • Interference/noise  not reliable • Broadcast medium  intercepted by snoopers

Wireless LAN Standards • HiperLAN - European Telecommunications Standards Institute (ETSI) @ 5 GHz unlicensed frequency band • IEEE 802.11 - IEEE 802.11 Worldwide Standard Group @ 2.4 GHz or 5 GHz unlicensed frequency band * IEEE: Institute of Electrical and Electronics Engineers

IEEE 802.11 Family for Wireless LANs • Specify air interface between access points (APs) and stations, or between two stations • Difference: radio frequency band, transmission speed, modulation scheme • 802.11 (see slide #34 for more details) • original wireless LAN standard • 1 - 2 Mbps • 802.11a • Orthogonal frequency division multiplexing (OFDM) • 5 GHz radio frequency • High speed: up to 54 Mbps • 802.11b • DS-SS at 2.4 GHz • Up to 11 Mbps • 802.11e • Support quality-of-service • 802.11g • OFDM • High speed standard at 2.4 GHz • Up to 54 Mbps

WLAN Architecture • Two modes: Ad hoc networks & Infrastructure networks • Basic service set (BSS) • a group of stations that can communicate with each other • Ad hoc network • No infrastructure; temporary • Peer-to-peer • Conference meetings, distributed computer games Ad hoc network

AP1 AP2 Infrastructure Network Server • An AP in each BSS • Distribution system: interconnect BSSs to form an extended service set (ESS) • Portal: bridge to other networks Gateway to the Internet Portal Portal Distribution system ESS B2 A1 B1 A2 BSS A BSS B

Road Map • Introduction to wireless LANs • Wireless LAN physical layer • Wireless LAN medium access control (MAC) • Distributed coordination function (DCF) • Point coordination function (PCF)

Wireless LAN Physical Layers • Physical layer: transfer of bits over a communication channel • IEEE 802.11 wireless LAN physical layer (We discuss) • Infrared • Spread spectrum (SS) at 2.4 GHz Application Presentation Session Transport Network Data link Physical OSI model

Infrared Physical Layer • Coverage: 10 – 20 m • Pulse-position modulation (PPM) • Each transmitted symbol has 16 time slots, one contains a pulse • Four bits  integer in [1, 16] (‘0000’1, ‘1111’16) • The integer determines which slot is used for the pulse • An example ‘0000’  1 Slot 1 Slot 16 ‘1111’  16 Slot 1 Slot 16 Symbol

Infrared Physical Layer (Cont’d) • Advantages • Simple & inexpensive • Constrained by walls  Secured against eavesdropping, low interference • Disadvantages • Interference (sunlight, indoor lighting) • Limited range • Not popular

Spread Spectrum Physical Layer • Spread spectrum: spread the signal energy over a wide frequency band (recall: CDMA) • Frequency hopping (FP) & direct sequence (DS)

Receiver +1 +1 +1 +1 +1 +1 +1 +1 +1 +1 +1 Received chips Demodulation -1 -1 -1 -1 -1 -1 -1 -1 -1 -1 -1 Barker sequence at the receiver +1 +1 +1 +1 +1 +1 +1 +1 +1 +1 +1 +1 -1 -1 -1 -1 -1 -1 -1 -1 -1 -1 +1 +1 +1 +1 +1 +1 +1 +1 +1 +1 +1 Products -1 -1 -1 -1 -1 -1 -1 -1 -1 -1 -1 -11 +11 +1 +1 +1 +1 +1 +1 11-chip Barker sequence -1 -1 -1 -1 -1 +1 (for bit ‘1’) Symbols -1 (for bit ‘0’) Transmitted chips +1 +1 +1 +1 +1 +1 +1 +1 +1 +1 +1 Modulation -1 -1 -1 -1 -1 -1 -1 -1 -1 -1 -1 Sender

Spread Spectrum Physical Layer (Cont’d) • Code-division multiple access (CDMA) channelization is also based on spread spectrum • What’s the difference of spread spectrum (CDMA vs. wireless LANs)?

Spread Spectrum in CDMA • Each station has a unique sequence • Stations’ transmissions can be distinguished by their sequences • Support multiple transmissions Symbols Station 1 Sequence -1 -1 -1 -1 Station 2 -1 +1 -1 +1 Station 3 -1 -1 +1 +1

Spread Spectrum in Wireless LANs • All the stations use the same Barker sequence • Multiple transmissions  collision • Spread spectrum: overcome interference from other networks • Unlicensed frequency band share by Bluetooth, cordless phones, …

Where is MAC in OSI Model? IEEE 802 wired/wireless LAN OSI Network layer Network layer Logical Link Control Data link layer LLC 802.3 Ethernet 802.5 Token ring 802.11 Wireless LAN Other LANs MAC Various physical layer (Infrared, spread spectrum, cable) Physical layer Physical layer • Data link layer: logic link control (LLC) + MAC • MAC: coordinating the access to the shared medium. • LLC: operate over all MAC standards (802.3, 802.5, and 802.11), and offer the network layer a standard set of services

A Similar Scenario • The instructor is holding an office hour… • Who asks the first questions? who next? • Listen and wait  contention based • Polling based on an order  contention free • Coordination functions: determine when to transmit/receive • Distributed coordination function (DCF): “listen and wait”  contention service • Point coordination function (PCF): “polling”  contention-free service

Distributed Coordination Function (DCF) • Mandatory in IEEE 802.11 family • Distributed manner • Asynchronous data transfer & best effort • All stations contend • Recall: Ethernet has carrier sensing multiple access with collision detection (CSMA-CD) • Why not use CSMA-CD in wireless LANs? • Sense the channel before transmission • Channel busy  wait for some time, then try again • During transmission, continue to sense (detect collision) • Collision detected  abort • transmit and sense at the same time

Distributed Coordination Function (DCF) (Cont’d) • Drawbacks of CSMA-CD over wireless LANs • “Collision detection” problem: half-duplex  unable to transmit and sense simultaneously • “Hidden-station” problem (also called “hidden-terminal” problem) • A transmits data frame • C senses medium; hears nothing • C transmits data frame • C collides with A at B • A new MAC: CSMA with collision avoidance(CSMA-CA) Data frame Data frame A C B

ACK A C B Solution to “Collision Detection” Problem • Acknowledgement (ACK) • No ACK  collision • Information exchange handshake: Data + ACK • New problem: ACK collision Data A C B Data

A C B Solution to “Collision Detection” (Cont’d) • Two kinds of carrier sensing • Physical carrier sensing • Virtual carrier sensing: tell others how long I need • Sender: set duration field in the MAC header of the transmitted frames • Indicate the amount of time needed to complete the Data-ACK handshake • Other stations wait until the completion of the exchange, and the waiting time is called network allocation vector (NAV). When the NAV value is set, a station will not attempt to initiate any transmission for that interval, and if any station is running a back-off counter (see slide #28), the counter will be frozen for that interval. We need 500 us to complete Data frame Ok, I can wait 500 us until your ACK Duration =500 ACK MAC header Receiver Sender

CTS CTS A C B Solution to “Hidden-Station” Problem • Request-to-send (RTS)/clear-to-send (CTS) handshake • Four-way handshake: RTS-CTS-Data-ACK Data Data A C B ACK RTS Data

IFS IFS IFS Frame Frame Frame Frame Time Basic CSMA-CA Operation • Interframe space (IFS): “idle gap” between two frame transmissions • Short IFS (SIFS): High-priority frames (such as CTS, ACK) • DCF IFS (DIFS) : for distributed coordination function (DCF) to initiate a contention period - RTS • PCF IFS (PIFS):for point coordination function (PCF) to initiate a contention-free period DIFS PIFS SIFS Busy medium RTS Time DIFS Sender Data RTS SIFS SIFS SIFS Receiver CTS ACK

Backoff time =0 Backoff time =7 Time slot Backoff Procedure • If channel busy • Schedule a random backoff time (an integer) for retry • After DIFS channel idle, count down the backoff time by one when the channel continues to be idle for one more time slot • Transmit when the backoff time reaches 0 • Access time: after DIFS + random backoff time • Can collisions be eliminated completely? • If two stations have the same backoff time? Backoff time (waiting time for retry), e.g., 7 Time slot DIFS SIFS SIFS SIFS ACK Data CTS RTS Busy medium Time Count down backoff time

Collisions and Retransmissions • Each sender • Interprets non-arrival of ACK as collision • Schedule a new backoff time in a double range, • e.g., [0, 7]  [0, 15] • The backoff time is a random number of slot times within this interval • Retransmit when the backoff time counts down to 0 • If collided again, double again • Until ACK or frame dropping at the sender • binary exponential backoff

Summary of CSMA-CA Mechanisms

Point Coordination Function (PCF): • Optional • Connection-oriented, contention-free services through polling • Time bounded transfer (e.g., voice over wireless LANs) • Central controller: point coordinator at AP During the contention free periods the AP polls stations with delay sensitive traffic. The portion of time allocated to the contention free period is variable, and the assignment is made by the AP based on the number of stations requesting contention free service, their transmission requirements and data rates. The AP broadcasts a control message after a PIFS interval causing all stations to reset their NAV to initiate contention free period. As in the RTS/CTS operation, that NAV setting will inhibit stations from sending for the specific amount of time. • Polling table • If polled, transmit without contention • At the end of the contention free period, the network automatically returns to the contention mode.

PCF Procedure • Mandatory DCF + optional PCF • Contention-free period (CFP): by PCF; contention period (CP): by DCF • CFP and CP alternate • CFP starts with a beacon SIFS < PIFS < DIFS Contention-free period (CFP) Contention period (CP) SIFS SIFS SIFS SIFS SIFS AP B D1 +Poll D2 +Poll CF End B PIFS U1 +ACK U2 +ACK Station 1&2 Reset NAV Network Allocation Vector (NAV) Other stations CF_Max_duration B: beacon D: downlink frame U: uplink frame

Summary Logic link control (LLC) sublayer Contention service Contention-free service Point coordination function MAC sublayer CSMA-CA-based distribution coordination function 1, 2 Mbps 6-54 Mbps 5.5, 11 Mbps 6-54 Mbps Physical layer Infrared 2.4 GHz DSSS 2.4 GHz FHSS 5 GHz OFDM 2.4 GHz DSSS 2.4 GHz OFDM IEEE 802.11 802.11a 802.11b 802.11g

Wireless Local Area Networks (LANs)