Wireless Local Area Networks (LANs)

Wireless Local Area Networks (LANs)

2

Outline

Introduction to wireless LANs Wireless LAN physical layer Wireless LAN medium access control (MAC)

Distributed coordination function (DCF) Point coordination function (PCF)

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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

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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

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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

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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

7

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

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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

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Infrastructure Network

An AP in each BSS Distribution system:

interconnect BSSs to form an extended service set (ESS)

Portal: bridge to other networks

BSS A

AP1

Distribution system

AP2

BSS B

PortalPortal

Server

Gateway tothe Internet

ESS

A1A2

B1B2

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Road Map



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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

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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

‘1111’ 16

Slot 1 Slot 16

Slot 1 Slot 16

Symbol

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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

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Spread Spectrum Physical Layer

Spread spectrum: spread the signal energy over a wide frequency band (recall: CDMA)

Frequency hopping (FP) & direct sequence (DS)

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+1

-1

+1 +1

-1

+1 +1 +1

-1 -1 -111-chip Barker sequence

+1

-1

+1 +1

-1

+1 +1 +1

-1 -1 -1 -1

+1

-1 -1

+1

-1 -1 -1

+1 +1 +1

+1 (for bit ‘1’)

-1 (for bit ‘0’)Symbols

Transmitted chips

Modulation

Sender

+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

+1

-1

+1 +1

-1

+1 +1 +1

-1 -1 -1

Received chips

Barker sequenceat the receiver

Products+1 +1 +1 +1 +1 +1 +1 +1 +1 +1 +1

-1 -1 -1 -1 -1 -1 -1 -1 -1 -1 -1

+11 -11

Demodulation

Receiver

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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)?

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Each station has a unique sequence Stations’ transmissions can be distinguished by their sequences Support multiple transmissions

Spread Spectrum in CDMA

-1 -1 -1 -1

-1 +1 -1 +1

-1 -1 +1 +1

Station 1

Station 2

Station 3

Symbols

Sequence

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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, …

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Road Map



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Where is MAC in OSI Model?

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

Physicallayer

OSI

Various physical layer(Infrared, spread spectrum, cable)

802.3Ethernet

802.5Token ring

802.11Wireless

LAN

Other LANs

Logical Link Control

Network layer Network layer

Data link layer

Physical layer

MAC

LLC

IEEE 802 wired/wireless LAN

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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

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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

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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)

Distributed Coordination Function (DCF) (Cont’d)

Data frame

A

B

C

Data frame

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Solution to “Collision Detection” Problem Acknowledgement (ACK)

No ACK collision Information exchange handshake: Data + ACK New problem: ACK collision

AC

B

Data

AC

B

ACK Data

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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.

Duration=500

MAC header

AC

B

We need 500 us to

complete

Ok, I can wait 500 us until your ACK

SenderReceiver

Data frame

ACK

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Solution to “Hidden-Station” Problem Request-to-send (RTS)/clear-to-send (CTS) handshake

Four-way handshake: RTS-CTS-Data-ACK

Data

A

B

C

Data

AC

B

RTS CTS CTSDataACK

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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

SIFSBusy medium

Time

DataDIFSSender

Receiver ACKSIFSSIFS

CTS

SIFS

RTS

RTS

Frame Frame Frame Frame TimeIFS IFS IFS

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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?

Busy medium RTS

Time

Backoff time (waiting time for retry), e.g., 7

Time slot

DIFS

Time slot

Backoff time =7

Backoff time =0

Count downbackoff time

CTS Data ACK

SIFS SIFS SIFS

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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

Collisions and Retransmissions

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Summary of CSMA-CA Mechanisms

Mechanism Objective

ACK “Collision detection” problem

RTS/CTS “Hidden station” problem

Binary exponential backoff Collision avoidance and resolution

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Road Map



32

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.

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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

B D1+Poll

U1+ACK

D2+Poll

U2+ACK

CFEnd B

Network Allocation Vector (NAV)

Reset NAV

CF_Max_duration

B: beacon D: downlink frame U: uplink frame

Contention period (CP)

AP

Station 1&2

SIFS SIFS SIFS SIFS SIFS

Contention-free period (CFP)

PIFS

Other stations

SIFS < PIFS < DIFS

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Summary

Infrared

CSMA-CA-based distribution coordination function

Point coordination function MACsublayer

Logic link control (LLC) sublayer

2.4 GHzDSSS

2.4 GHzFHSS

2.4 GHzDSSS

2.4 GHzOFDM

5 GHzOFDM

1, 2 Mbps

5.5, 11 Mbps

6-54 Mbps

6-54 Mbps Physical

layer

IEEE 802.11 802.11a 802.11g802.11b

Contention-free service Contention service

Documents

Wireless Local Area Networks (LANs)