Wireless Local Area Networks (WLAN)

Mobile Communication and Mobile Computing Prof. Dr. Alexander Schillhttp://www.rn.inf.tu-dresden.de

Department of Computer Science Institute for System Architecture, Chair for Computer Networks

Network Types

• classification of networks via dimension:− Private Area Network (PAN)− Local Area Network (LAN)− Metropolitan Area Network

(MAN)− Wide Area Network (WAN)− Global Area Network (GAN)

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• wireless versions:

− WPAN, e.g. Bluetooth− WLAN, e.g. 802.11− WMAN, WWAN,

e.g. 802.16 and 802.20,and UMTS / LTE

Wireless LANs: Motivation / Application areas

Advantages• flexibility• widely available Internet

access at low cost • ad-hoc-networks• no problems with cables

Problems• higher error rate on the

transmission link in comparison to Standard-LANs

• security aspects• shared medium• Some national restrictions

concerning the used frequency bands (Industrial Scientific Medical (ISM)- Band)

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Applications• Internet access• networks in exhibition

halls• hospitals• warehouses• airports• structure of networks

in historic buildings• extension of existing

wired local area networks in offices, universities etc.

Wireless Standards in general

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Common Name:

Wi-Fi(Wireless Fidelity)

WiMAX Bluetooth,ZigBee

Standard:IEEE 802.11 IEEE 802.16 /

802.16aIEEE 802.15

Frequency Band: [GHz]

2,4 and 5 2..66 2,4

Data rate:[MBit/s]

300 and more Up to 134 1-20

Reach: about 100 m coverage up to 50 km (cellular)

about 5 m, sometimes up to 50 m

Specifics cheap and flexible, improved security

wireless stationary internet access

voice channel,peripheral devices,sensor networks

Used for WLAN WMAN Remote Devices

802.11 – Frequency Bands

• 2,4 GHz Band− 2,4 to 2,4835 GHz− ISM-Band− License-free− transmitted power max.

100 mW

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• 5 GHz Band− 5,15 - 5,725 GHz in

Europe− License-free− transmitted power

max. 1000 mW with special power control

802.11 - Network Topologies (1)

• infrastructure mode− like a star-network− Access-Point (AP) is a central point− AP coordinates the network nodes and communicates

with other networks

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AP

AP

AP

Three infrastructure APsin one fixed network

Network

802.11– Network Topologies (2)

• Ad-hoc Mode− Like Peer-to-Peer Network− no central Station or

higher-level infrastructure available

− All network nodes are equivalent

Direct connection− the nodes see each other and can

communicate one with each other

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Beaconing-Mechanism− every node sends a “Beacon”-

Signal in certain intervals. Via this signal every node knows its direct neighbors. ad-hoc-nets appear spontaneously and organize and administrate themselves

Indirect connection− no direct communication possible− special routing methods for

transmission of the data (e.g. OLSR Optimized Link State Routing)

802.11 – System Architecture

• Station (STA)− device with 802.11-interface

• Access Point (AP)− Allow access for registered stations

to the distribution system • Basic Service Set (BSS)

− AP and associated stations• Independent BSS (IBSS)

− in Ad-hoc-Mode• Distribution System

− Connects more than one BSS via the access points to form a larger logical network

• Extended Service Set (ESS)− Multiple connected wireless networks

• Portal− Allows entering of other networks

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802.x LAN

Distribution SystemAP

AP

Portal

BSS2

BSS1

STA3STA2

STA1

ESS

802.11 - WLAN standardsStandard Frequency  

Bandwidth  Max.  

data  rate  DRmax

Normal  Data  rate  DR

Trensmission Range  R  (indoor/outdoor)

Remarks

802.11 2,4  GHz 2  MBit/s 1,2  MBit/s

DSSS  (FHSS,  Infrared)

30/300 outdated

802.11a 5  GHz 54  MBit/s 32  MBit/s OFDM 10/100 high  data  rate,  but  incompatible  

to  other  standards,  low  

range802.11b 2,4  GHz 11  MBit/s 7  MBit/s DSSS 30/300 higher  range,  but  

lower  data  rate

802.11g 2,4  GHz 54  MBit/s 32  MBit/s OFDM 30/300 higher  data  rate  and    range,  but  sensitive  to  noise

802.11n802.11ac

2,4  GHz  and  5  GHz  

300  MBit/s(and  more)

~  100MBit/s

OFDM 10/100 very  high  data  rate,  but  also  

sensitive  to  noise

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DSSS ... Direct Sequence Spread SpectrumFHSS … Frequency Hopping Spread SpectrumOFDM ... Orthogonal Frequency Division Multiplexing

802.11 Sub-Standards

Standard Characteristics  802.11a Enhancement  of  the  physical  layer802.11b Enhancement of  the  physical  layer802.11c Define  Wireless  Bridging  between  Access  Points802.11d Country  specific  regulations  for    802.11b802.11e Enhanced  WLAN  for  QoS (Quality  of  Service)802.11f Roaming  and  inter-­operability  between  base  station  different  vendors802.11g Enhancement  of  the  physical  layer802.11h Optional  for  802.11a  for  Europe802.11i Enhancement  of  security  and  authentication802.11j Model  of  802.11a  in  Japan  802.11n,  802.11ac

Enhancements  for  higher  data  rates

802.11o Prioritizing  of  voice  data  in  WLAN  in  opposite  to  the  data  traffic802.11p Enhancement  to  802.11a  for  use  in  vehicular  networks802.11r Fast  Roaming  at  the  swapping  between  Access  Points802.11u Inter-­operability  with  other  non  802  networks 10

OFDM: Example 802.11a

• 64 sub-bearer signals per bearer, use of 64QAM (Quadrature Amplitude Modulation)

• 48 sub-bearers for data modulation, 4 as phase reference, and 12 as distance to next bearer

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OFDM with 802.11b

• ISM frequency band of 2,4 GHz • Bandwidth per channel: 22 MHz, 14 overlapping channels• Channel allocation slightly different in various countries

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802.11n, 802.11ac (1)

• Focus: Higher end applications in WLANs− Wireless Streaming Media − Videoconferencing

• Technical aspects− Actual data rate should reach in the area of 100 MBit/s− Possible gross data rate up to 300 MBit/s− MIMO-techniques (Multiple Input/Multiple Output)− use of several sender-/receiver channels (max. 4)

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Transmitter Receiver

Transmitter Receiver

SISO

MIMO

802.11n, 802.11ac (2)

• OFDM, adaptive modulation with BPSK, QPSK, 16QAM, 64QAM, 256QAM

• 2,4 GHz-band with downward-compatibility to 802.11b/g, 5 GHz band also a possible option

• optional extension of the radio channel from 20 MHz up to 40 MHz• radio interface

• regular sending

• packet aggregation(more efficient)

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Backoff: Jam signal BPSK: Binary Phase Shift KeyingSIFS: Short Inter Frame Spacing for ACK and response to pollingACK: Acknowledgement signal DIFS: Distributed Coordination Function Inter Frame Spacing for

asynchronous services

-> Overhead due to packet aggregation reduced

Specific Problems and Solutions

• Physical problems− interference: spread spectrum techniques, frequency hopping − hidden Terminal problem: use of CSMA/CA (see later)

• Data security− Wired Equivalent Privacy (WEP) service− Current improvements− WPA (WiFi Protected Access), WPA2− 802.11i

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802.11 – Medium Access (1)

• Carrier Sense Multiple Access with collision avoidance (CSMA/CA)− CSMA/CD in wireless networks not possible− No collision detection

− principle: „listen before talking“ (only CSMA without CD)

• Procedure: − 1. a station intends to send, is listening before sending− 2. if medium is free then send− 3. if medium is busy, wait until the „back off“ interval is over

and begin at 1 (repeat until maximum number of attempts)

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802.11 – Medium Access (2)

• Problem of CSMA in the wireless case: hidden terminal − A and C cannot communicate directly− A sends to B and occupies the medium− C also wants to send to B, but does not recognize the

occupied medium− Collision at B, so B cannot reconstruct data− A also does not detect the collision

Ú A is hidden for C and C is hidden for A

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A B C

802.11 – Medium Access (3)

• Further problem: exposed terminal− B sends to A, and C wants to send to an other station (not A

and not B)− C recognizes the signals from B and is waiting until B will

finish its transmission− unnecessary waiting, because signals from C cause no

collision at A

Ú exposed terminal: C is exposed to other stations

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A B C

802.11 – Medium Access (4)• solution: RTS/CTS-Mechanism (Request To Send/Clear To Send)• solution: hidden terminal

− A sends a RTS-Signal to B and B sends CTS after that (ready for transmission)− All other possible senders (C) also get the CTS-signal and will wait and reschedule

their transmission

• solution: exposed terminal− C sends RTS to the receiver and gets CTS, so it can transmit− B hears RTS, too. But B does not receive any CTS (too far away).− So B assumes the channel to be free (which is ok) and can also send.

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A B CRTSCTS CTS

A B CRTSCTS

802.11: Data Security in WLANs

• WEP (Wired Equivalent Privacy)- symmetrical cryptography,

e.g. using RC4

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+clear textof framebody + ICV

secretkey +

Encryption

encodedtext in framebodykey sequence

However: Small key lengths (40-128 bit), same key for multiple clients, low security!

Generator forPseudo-random number

integrity-algorithm Integrity check

value (ICV)

clear text (in frame body)

802.11-Security – WPA / WPA2

WiFi protected access, subset of 802.11i, resolves the WEP problems

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• authentication– Pre-Shared-Key (PSK), 8-64

characters password, used for generation of the session key

– Extensible Authentication Protocol based on 802.1x (e.g. RADIUS-Server – Remote Access Dial-in User Service)

• encryption – Integrity Check– TKIP (Temporal Key

Integrity Protocol) generates dynamic key per packet (WPA)

– RC4 (WPA) or AES (WPA2) for encryption

• remaining security problems– Simple PSK allows “brute force” or dictionary

attack

802.11 Security – Summary

Features   WEP WPA WPA2/IEEE802.11i

Encryption RC4 RC4 AESKey  length  [Bit] 40,  104 128  or  more 128  or  moreData  integrity CRC-­32 “Michael” CCMHeader  integrity   non “Michael” CCMKey  management non EAP-­based EAP-­based  

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• RC4 – R.Rivest Encryption symmetrical method• AES – Advanced Encryption Standard, a symmetrical

cryptosystem, modern DES, RC4 successor • CCM – Counter Mode with Cipher Block Chaining Message

Authentication Code Protocol • EAP – Extensible Authentication Protocol, used on data link layer,

frequently with PPP and SSL/TLS

WPAN (Wireless Personal Area Networks): Bluetooth• Harald Bluetooth was the King of Denmark in the 10th century • Initiated by Ericsson, Intel, IBM, Nokia, Toshiba;

Open Standard: IEEE 802.15.1• Generally for wireless Ad-hoc-piconets (range < 10m);

single-chip solution• Frequency band in 2,4 GHz area• Integrated security (128 bit encryption)• Data rates:

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Basic setup

2,4-Ghz-HF

Bluetooth-Baseband-Controller

Host-System

− 433,9 kBit/s asynchronous-symmetrical− 723,2 kBit/s / 57,6 kbit/s asynchronous-asymmetrical− 64 kBit/s synchronous, voice service− Extensions up to 20 Mbit/s (IEEE 802.15.3a− UWB (Ultra Wide Band))

Bluetooth - Functionality

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connected

t =2 s

t =0,6 s

Not connected; Standby

connection-setupstatus

activestates

Low-Power-states

Standby

Inquiry withknown

Address

Page forUnknownAddress

Send data

PARKAdressresigned

HOLD(paused)

SNIFF(periodic)

Address available

Possible Configurations

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Master

Slave

Piconet(up to 7 “slaves” per “master”)

Scatternet

WPAN: ZigBee• Low-energy network for ad-hoc connectivity of sensors and

other small devices • Standard: IEEE 802.15.4 Low Rate WPAN• Technical data

− Frequency range: 2,4-GHz-Band (16 channels), − Transmission power: only 1 to 10 mW− Reach: up to 50 m− Data rate: 250 kBit/s per channel

• Components− ZigBee End Device (ZED)− ZigBee Router (ZR)− ZigBee Coordinator (ZC)

• Network topology− Star topology with ZEDs− P2P-Topology with ZRs− Up to 65.000 devices

per cluster (ZC)

ZEDZED

ZED

ZED

ZR

ZR ZC

ZED

ZED

ZEDZED

ZEDZED

ZED

ZED

ZED

WPAN: RFID – Radio FrequencyIdentification

• Tags: - Antenna and RFID-Chip− 96-Bit-Identifier, small memory, passive− very cheap, universal usage

• Reader: - active, powerful, own protocols- Sends bearer signal, reflected by Tag

• Backscatter: Tag creates overlay of bearer signal with own transmission bits à Reader can filter and detect these bits

• Multiple access handled by collision detection protocol

RFID-Reader

Reader signal

Tag

Tag

Tag

TagReflectedsignal

WPAN: NFC – Near Field Communication

• Contactless transmission via very small distance (4 cm)(smartcard is being directly attached to reader)

• Transmission− connectionless: passive RFID-Tags− Connection oriented: active transmitters (e.g. smartphone)− Data rate up to 424 kBit/s

• Applications− Payment by smartphone or smartcard− Smartphone as door key

• Problems− Not really secure (controlled by distance only)

WPAN: Applications

• May replace infrared for peripherals

• Wireless headsets (e.g. hands-free interaction with mobile phones)

• Digital image transmission between cameras and archives

• Control of home appliances by networked sensors(examples: heating control, smart electricity metering, alarm systems)

• Digital payment systems

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Some further readings

• IEEE 802.11: standards.ieee.org/getieee802/802.11.htm

• WiFi Alliance (configuration, security): www.wi-fi.org

• Bluetooth special interest group:www.bluetooth.org

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