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

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 AP1 AP2 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)

11-chip Barker sequence Symbols Transmitted chips Received chips +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 -1 Received chips Barker sequence at the receiver Products +11 -11 Demodulation Receiver -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, …

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

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

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

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 A C B Receiver Sender

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 RTS Data CTS ACK A C B

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 Frame Time IFS DIFS PIFS SIFS Busy medium RTS Time Sender DIFS RTS Data SIFS SIFS SIFS Receiver CTS ACK

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 Backoff time =7 time =0 Time slot DIFS SIFS SIFS SIFS Busy medium RTS CTS Data ACK 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 Objective ACK “Collision detection” problem RTS/CTS “Hidden station” problem Binary exponential backoff Collision avoidance and resolution

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

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 Station 1&2 U1 +ACK U2 +ACK Reset NAV Other stations Network Allocation Vector (NAV) CF_Max_duration B: beacon D: downlink frame U: uplink frame

Summary Point coordination function Logic link control (LLC) sublayer Contention-free service Contention 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