05 - WLAN · Chapter 3.2: WLAN Page 3 Structure of a WLAN 1. Infrastructure network • Access...

Lehrstuhl für Informatik 4

Kommunikation und verteilte Systeme

Chapter 3.2: WLAN

Wireless Ethernet

• Wireless equivalent to Ethernet: “Wireless LAN” (WLAN)• Exclusively data-oriented, wide-band Internet access solution

• Standardized by the IEEE as IEEE 802.11� 1997: IEEE 802.11 (capacity of maximally 2 MBit/s)

� IEEE 802.11a with 54 MBit/s, use of a (more susceptible for disturbances) frequency band

� 1999: IEEE 802.11b (data rate of 11 MBit/s with a utilizable data rate of of up to 6-7 MBit/s)

� IEEE 802.11g: enhancement of 802.11b with up to 54 MBit/s� …

802.11• 1 or 2 MBit/s• 2.4 GHz• FHSS, DSSS

802.11a

• 54 MBit/s• 5 GHz• FHSS, DSSS

802.11b

• 11 MBit/s• 2.4 GHz• only DSSS

802.11g• 54 MBit/s• 2.4 GHz• only DSSS



Chapter 3.2: WLAN

Wireless LANs: Design Goals

• Global, seamless operation

• Low power for battery use • No special permissions or licenses needed to use the LAN

• Robust transmission technology• Simplified spontaneous cooperation at meetings • Easy to use for everyone, simple management

• Protection of investment in wired networks • Security (no one should be able to read my data), privacy (no one should be able

to collect user profiles), safety (low radiation)

• Transparency concerning applications and higher layer protocols, but also location awareness if necessary



Chapter 3.2: WLAN

Structure of a WLAN

1. Infrastructure network

• Access Points (APs) are attached to an existing fixed network (Ethernet, Satellites, …)

• Each AP manages all communication in its reception range

• APs using the same frequency range must have enough distance to avoid disturbances

• Control functionality (medium access, mobility management, authentication, …) are realized within the infrastructure, wireless devices only need a minimum of functionality

2. Ad-hoc Network• If no AP is available, stations also can

build up an own LAN

• The transmission now takes place directly between the stations

• Higher complexity needed within the stations (control functionality)

Fixed networkL a p to pAP

APAPL a p to pL a p to p L a p to p L a p to p

LaptopLaptop

Laptop



Chapter 3.2: WLAN

Infrastructure Network

Distribution System

Portal

802.x LAN

AccessPoint

802.11 LAN

BSS2

802.11 LAN

BSS1

AccessPoint

STA1

STA2STA3

ESS

• Station (STA)Computer with access mechanism to the wireless medium and by this radio connection to the AP

• Access Point (AP)Station which is integrated both in the radio and the wired network (distribution system)

• Basic Service Set (BSS)Group of stations incl. the AP within an AP transmission range

• PortalGateway to another fixed network

• Distribution systemConnection of different AP areas to one logical network (EES: Extended service set). Simplest principle: switch



Chapter 3.2: WLAN

Ad-hoc Network

802.11 LAN

IBSS2

802.11 LAN

IBSS1

STA1

STA4

STA5

STA2

STA3

Direct communication within limited range

• Station (STA)Computer with access mechanism tothe wireless medium

• Independent Basic Service Set (IBSS)Group of stations which use the same carrier frequency within a transmission range

Different IBSS are possible by spatial separation or by using different carrier frequenciesNo designated stations for the forwarding of data, routing,… …



Chapter 3.2: WLAN

802.11 Protocols

Medium Access Control• Access mechanism, fragmenting, encryption

• MAC management: synchronization, roaming between APs, power management

Physical layer• Channel selection, modulation, coding

Applications should not be aware of the existence of the wireless network (except capacity, longer access times)



Chapter 3.2: WLAN

802.11 Layers

• PLCP– Clear Channel Assessment

Signal (Carrier Sense)

• PMD– Modulation, coding

• PHY Management– Channel selection, MIB

• Station Management– Coordination of management

functions

PMDPhysical Medium Dependent

PLCPPhysical Layer

Convergence Protocol

MACMedium Access Control

LLCLogical Link Control

MAC Management

PHY Management

• MAC

– Access mechanism, fragmentation, encryption

• MAC Management

– Synchronization, Roaming, MIB, power management

PH

YM

AC

Sta

tion

Man

agem

ent



Chapter 3.2: WLAN

IEEE 802.11 Variants

Enhancement for a future, faster WLAN with data rate of 108 – 320 MBit/s802.11n

Summary of earlier enhancements, correction of errors in former specifications (maintenance)

802.11m

Improved measurement/evaluation/management of radio parameters (e.g. signal strength), e.g. for enabling location based services

802.11k

Japanese variant of 802.11a for the frequency range of 4,9 GHz - 5 GHz802.11j

Authentication/encryption for 802.11a/b/g/h802.11i

54 MBit/s WLAN in the 5 GHz band with dynamic adaptation of channel and frequency choice as well as automatic adaptation of transmission power (enhancement of IEEE 802.11a for Europe)

802.11h

54 MBit/s WLAN in the 2,4 GHz band 802.11g

Roaming for 802.11a/g/h (Inter Access Point Protocol IAPP) between Access Points of different vendors

802.11f

QoS und streaming enhancement for 802.11a/g/h 802.11e

"World Mode", Adaptation to regional regulations (e.g. used frequency ranges)802.11d

Wireless Bridging between Access Points802.11c

11 MBit/s WLAN in the 2,4 GHz band 802.11b

54 MBit/s WLAN in the 5 GHz band 802.11a



Chapter 3.2: WLAN

802.11 – Physical Layer

Variants for transmission: 2 using radio (in the 2.4 GHz band), 1 using infrared

• FHSS (Frequency Hopping Spread Spectrum)

– 2 frequencies for 1 MBit/s, 4 frequencies for 2 MBit/s– 79 different channels with 1 MHz bandwidth each

– min. 2.5 hops/sec– GFSK modulation

– Max. transmission power: 1 W (USA)/100 mW (EU), min. 1 mW

• DSSS (Direct Sequence Spread Spectrum)

– DBPSK modulation for 1 MBit/s (Differential Binary Phase Shift Keying), DQPSK for 2 MBit/s (Differential Quadrature PSK)

– Chipping sequence: (+1, -1, +1, +1, -1, +1, +1, +1, -1, -1, -1), a Barker-Code

– Max. transmission power: 1 W (USA)/100 mW (EU), min. 1 mW

• Infrared

– 850-950nm, diffuse light, typically 10 m range



Chapter 3.2: WLAN

IEEE 802.11b

• Data rate– 1, 2, 5.5, 11 MBit/s, depending

on SNR – User throughput max. approx.

6 MBit/s• Transmission range

– 300m outdoor, 30m indoor (directed links: several km)

– Max. data rate ~ 10m (indoor)

• Frequency range– Free 2.4 GHz ISM band

• Security– Limited, WEP insecure, SSID

• Availability– Several products, several

vendors

• Connection setup time

– Connectionless, „always on“• QoS

– Best effort, no guarantees (in principle weak QoS possible, but not implemented)

• Manageability

– Limited (no automatic key distribution, symmetrical encryption)

• Special advantages/disadvantages

– Advantages: many installed systems, lot of experience, available worldwide, free ISM band, many vendors, simple system, integrated into laptops

– Disadvantage: heavy interferences on the ISM band, no QoS, relatively low data rates



Chapter 3.2: WLAN

Channels in IEEE 802.11b

2400 [MHz]2412 2483.52442 2472

Channel 1 Channel 7 Channel 13

22 MHz

• Two APs using the same frequency would have interferences in the overlapping area – thus: divide the whole frequency range in channels

• Each channel in IEEE 802.11b has a bandwidth of 22 MHz• 13 channels in Germany (2412, 2417, 2422, …, 2472 MHz), 11 in USA/Canada

• Channels overlap! Non-overlapping choice of channels:

• Ideal case: only use channels 1, 6 und 11:

116

1

611

1



Chapter 3.2: WLAN

802.11b – Physical Layer

Achieved bits/symbol

Used Symbol Rate

ModulationCode lengthData Rate

81,375 MS/sQPSK8 (CCK)11 Mbit/s

41,375 MS/sQPSK8 (CCK)5,5 Mbit/s

21 MS/sQPSK11 (barker code)2 Mbit/s

11 MS/sPSK11 (barker code)1 Mbit/s

Dynamic Rate ShiftingAdjustment of the data rate to the transmission quality:

• Only DSSS

• CCK: Complementary Code Keying

Use of an 8-Chips spreading sequence: select 64 (11 Mbit/s) resp. 4 (5,5 Mbit/s) of the 48 possible states, which have as good cross correlation characteristics as possible. I.e.: use spreading for the transmission of several bits at the same time Thus the transmission becomes clearly more susceptible for disturbances than for 1 resp. 2 Mbit/s



Chapter 3.2: WLAN

Channels

The whole 2.4GHz ISM band is divided into several channels. To avoid interference, distances must be left between the channels. To avoid collisions, when configuring an access point a channel is assigned to it.

• FHSS: The band is divided into 79 sub-bands, the channel number determines the hop sequence

• DSSS: The band is divided into 11 resp. 13 sub-bands, each of these forms an own channel. Signal spreading is performed in the sub-bands:

→One sub-band has a bandwidth of 22 MHz. The sent data are spread to those bandwidth to avoid environmental disturbances:

Channel n

22 MHz

Purpose: even if the frequency range is disturbed partly, enough of the signal power reaches the receiver. If only on one frequency transmission would take place, the whole data would be lost.

Signal is spread to 11 frequencies (when using a barker code)



Chapter 3.2: WLAN

Range of IEEE 802.11b

10 30 60 100 m0

2

4

6

8

10

Data rate

Mbit/s

Distance

802.11

802.11b

Due to missing spreading, the higher data transmission rates are more susceptible for disturbances. Thus, a smaller range results:



Chapter 3.2: WLAN

Range of 802.11b



Chapter 3.2: WLAN

IEEE 802.11a

• Data rates

– 6, 9, 12, 18, 24, 36, 48, 54 MBit/s, depending on SNR

– User Throughput: max. 32 MBit/s

– 6, 12, 24 MBit/s mandatory• Transmission range

– 100m outdoor, 10m indoor (e.g. 54 Mbit/s up to 5 m, 48 up to 12 m, 36 up to 25 m, 24 up to 30 m, 18 up to 40 m, 12 up to 60 m)

• Frequency range– Free 5.15-5.35, 5.725-5.825 GHz

ISM band

• Security– Limited, WEP insecure, SSID

• Availability– Several products, several vendors

• Connection setup time

– Connectionless, „always on“• QoS

– Best effort, no guarantees (same as for 802.11b)

• Manageability– Limited (same as for 802.11b)

• Special advantages/disadvantages– Advantages: uses less crowded

free ISM band, available worldwide, simple system, many vendors

– Disadvantages: strong shading due to high frequencies, no QoS



Chapter 3.2: WLAN

Channels in IEEE 802.11a

5150 [MHz]5180 53505200

36 44

16,6 MHz

center frequency = 5000 + 5·channel-no. [MHz]

channel-no.40 48 52 56 60 64

149 153 157 161

5220 5240 5260 5280 5300 5320

5725 [MHz]5745 58255765

16,6 MHz

channel-no.

5785 5805



Chapter 3.2: WLAN

subcarriernumber

Modulation in 802.11a: OFDM

• OFDM (Orthogonal Frequency Division Multiplexing) with 52 used subcarriers(64 in total, 6 as guard space on each side)

• Subcarriers overlap, but orthogonality of chosen frequencies allows for clear separation

• 48 data subchannels + 4 subchannels for phase reference (pilot)

• 312,5 kHz spacing

1 7 21 26-26 -21 -7 -1

channel center frequency

312,5 kHzphase reference (pilot)



Chapter 3.2: WLAN

Medium Access Control

„Wireless Ethernet“ – MAC protocol is oriented at CSMA/CD• Hidden Station Problem• Exposed Station Problem

Solution of the problems, especially Hidden Station

CSMA/CA – CSMA with Collision Avoidance

Types of traffic

• Asynchronous data service (standard)– Exchange of data by „best effort“

– Support of broadcast and multicast• Time-bound services (optional)

– Implementation of some degree of QoS– Only for infrastructure networks



Chapter 3.2: WLAN

802.11 – MAC Layer: DFWMAC

Access strategies

• DFWMAC-DCF CSMA/CA (standard)– DFWMAC: Distributed Foundation Wireless MAC

– DCF: Distributed Coordination Function– collision avoidance by random access with backoff mechanism– Minimum time between two frames

– ACKs for acknowledging correct receipt (not for broadcast)

• DFWMAC-DCF with RTS/CTS (optional)

– Avoidance of Hidden Stations– MACA variant (Multiple Access with Collision Avoidance)

• DFWMAC-PCF (optional)– PCF: Point Coordination Function

– Collision-free, centralized Polling strategy where the AP has a list of all connected stations



Chapter 3.2: WLAN

802.11 – MAC Layer

Priorities for medium access

• defined through different timing intervals

• no guaranteed priorities

• SIFS (Short Inter Frame Spacing) – 10µs

– highest priority, used for ACK, CTS, polling response

• PIFS (PCF IFS) – 30µs

– medium priority, for time-bounded services using PCF

• DIFS (DCF IFS) – 50µs

– lowest priority, für asynchronous data service

t

Medium busy SIFSPIFS

DIFSDIFS

next framecontention

direct access, if time the medium is free ≥ DIFS



Chapter 3.2: WLAN

t

Medium busy SIFSPIFS

DIFSDIFS

next frame

contention window(randomized backoffmechanism)

802.11 - CSMA/CA Method

time slot (20 µs)waiting time

• Mandatory for all implementations• Before sending, a station performs carrier sense

• If the medium is free for at least the duration of a DIFS, the station may send • If the medium is occupied, when becoming free the station waits for one DIFS and

then randomly chooses a backoff time (collision avoidance, in multiples of a slot time). The station continues to listen to the medium

• If the medium is occupied by another station during the backoff time, the backofftimer stops. In the next try, no new backoff time is chosen randomly, but the old timer is gone on with.

• Also usable for broadcast



Chapter 3.2: WLAN

Competing Stations

boe

boe

boe

t

busy

Station1

Station2

Station3

Station4

Station5

DIFSboe

boe

boe

busy

bor

bor

DIFS

boe

boe

boe bor

DIFS

busy

busy

DIFSboe busy

boe

boe

bor

bor

boe

Sending request

elapsed backoff time

bor remaining backoff time

busy Medium busy (Frame, ACK, etc.)

The size of the competition window (Contention Window, CW) affects the efficiency. Therefore (similar to Ethernet) it starts with CW = 7 and is doubled with each collision up to CWmax = 255



Chapter 3.2: WLAN

802.11 - CSMA/CA Method

Unicast transmission: the receipt is additionally confirmed, since collisions possibly are not detected by the transmitter

• Data can be sent after waiting for DIFS

• Receivers answer immediately (after SIFS, without additional backoff time), if the frame arrived correctly (CRC)

• In case of an error the frame is repeated automatically. No special treatment of a transmission repetition, same access mechanism as before

t

SIFS

DIFS

Data

ACK

waiting time

otherstations

receiver

senderData

DIFS

contention



Chapter 3.2: WLAN

802.11 – DFWMAC with RTS/CTS

Optional extension for the avoidance of the hidden station problem:

• RTS with holding time as parameter can be sent after waiting for DIFS (plus backoff time)

• Confirmation of the receiver by CTS after SIFS (also containing holding time)

• Immediate sending of the data is possible, confirmation by ACK• Other stations store the holding time, which were sent in the RTS and CTS, in their

NAV (Network Allocation Vector)

• Collisions are only possible with RTS/CTS messages, but substantial overhead through RTS/CTS messages

twaiting time

otherstations

receiver

sender

contention

SIFS

DIFS

data

ACK

data

DIFS

RTS

CTSSIFS SIFS

NAV (RTS)NAV (CTS)



Chapter 3.2: WLAN

802.11 – DFWMAC with RTS/CTS

t

SIFS

DIFS

data

ACK1

frag1

DIFS

contention

RTS

CTSSIFS SIFS

NAV (RTS)NAV (CTS)

NAV (frag1)NAV (ACK1)

SIFSACK2

frag2

SIFS

otherstations

receiver

sender

• Fragmenting data can decrease the damage caused by transfer errors

• Special mechanism: adapt size of the fragments to current error rate of the medium

• First: normal reservation with RTS/CTS

• Fragments and ACKs (except the last for each case) contain reservation durations



Chapter 3.2: WLAN

DFWMAC-PCF

PIFSD1

U1

SIFS

NAV

SIFSD2

U2

SIFS

SIFS

super-framet0 t1

• PCF for guarantees concerning bandwidth and access delay

• AP controls medium access and cyclic queries all stations (Polling)• Super-frames with competition-free period and competition period (like before)• If the medium gets free (t1) after the begin of the super-frame (t0), the coordinator

cyclic asks all stations x (Dx) for sending needs. If necessary, they answer with Ux(the data to be sent)

• If the phase is ended earlier than planned (t2 instead of t3), more time remains for the competition phase (end is announced by a control frame CFend)

t

D3

PIFSD4

U4

SIFS

SIFSCFend

contentioncontention-free period

t2 t3 t4



Chapter 3.2: WLAN

Frame Format

• Types

– control frames, management frames, data frames• Sequence numbers

– Important against duplicated frames due to lost ACKs

• Adresses– Receiver, transmitter (physical), BSS identifier, sender (logical)

• Misc– Sending time, checksum, frame control, data

FrameControl

Duration/ID

Address1

Address2

Address3

SequenceControl

Address4

Data CRC

2 2 6 6 6 62 40-2312bytes

Protocolversion

Type SubtypeToDS

MoreFrag

RetryPowerMgmt

MoreData

WEP

2 2 4 1

FromDS

1

Order

bits 1 1 1 1 1 1



Chapter 3.2: WLAN

Frame Format

Frame Control• Protocol version, frame type (administration, control, data), fragmenting, encryption

information, meaning of the following address fields

Duration ID• Sent along with RTC, CTS for setting the NAV

Addresses• In each case contains 48-Bit MAC addresses. MAC frames can be transferred

between two stations, between station and AP or between two APs by the distribution system. In the field Frame Control, two bits are determining the current meaning of the addresses. Addresses can be: Final destination, source address, BSS Identifier, intermediate sender address, intermediate receiver address

Sequence Control

• Recognition of duplicated frames



Chapter 3.2: WLAN

MAC Address Format

DS: Distribution SystemAP: Access PointDA: Destination AddressSA: Source AddressBSSID: Basic Service Set IdentifierRA: Receiver AddressTA: Transmitter Address

scenario to DS fromDS

address 1 address 2 address 3 address 4

ad-hoc network 0 0 DA SA BSSID -infrastructurenetwork, from AP

0 1 DA BSSID SA -

infrastructurenetwork, to AP

1 0 BSSID SA DA -

infrastructurenetwork, within DS

1 1 RA TA DA SA



Chapter 3.2: WLAN

Special Frames

FrameControl

DurationReceiverAddress

TransmitterAddress

CRC

2 2 6 6 4bytes

FrameControl


CRC

2 2 6 4bytes

FrameControl


CRC

2 2 6 4bytes

Acknowledgement, ACK

Request to Send, RTS

Clear to Send, CTS



Chapter 3.2: WLAN

FHSS Frame Format (PHY)

Synchronization SFD PLW PSF HEC Payload

Preamble Header

80 16 12 4 16 variable Bits

• Synchronization

– Synchronization of receivers by the pattern 010101... • SFD (Start Frame Delimiter)

– 0000110010111101 to announce start of frame• PLW (PLCP_PDU Length Word)

– Length of payload including the 32 Bit CRC (at the end of the payload). Allowed values are between 0 and 4095

• PSF (PLCP Signaling Field)

– Data rate of payload (1 or 2 Mbit/s)• HEC (Header Error Check)

– CRC with x16+x12+x5+1

transmission with 1 Mbit/s

transmission with1 or 2 Mbit/s



Chapter 3.2: WLAN

DSSS Frame Format (PHY)

Synchronization SFD Signal Service HEC Payload

Preamble Header


Length

16

• Synchronization– Snychronization, gain setting, energy detection, frequency offset

compensation• SFD (Start Frame Delimiter)

– 1111001110100000 as start pattern• Signal

– Data rate of payload (0A: 1 Mbit/s DBPSK; 14: 2 Mbit/s DQPSK)

• Service– Reserved for future use, standard: 00 for 802.11 frames

• Length (length of payload) and HEC (CRC) as for FHSS

transmission with 1 Mbit/s

transmission with1 or 2 Mbit/s



Chapter 3.2: WLAN

IEEE 802.11b – Frame Format (PHY)

synchronization SFD signal service HEC payload

Preamble Header


length

16

192 µs at 1 Mbit/s DBPSK 1, 2, 5.5 or 11 Mbit/s

short synch. SFD signal service HEC Payload

Preamble(1 Mbit/s, DBPSK)

Header(2 Mbit/s, DQPSK)


length

16

96 µs 2, 5.5 or 11 Mbit/s

Long frame format:

Short frame format, optional:



Chapter 3.2: WLAN

IEEE 802.11a – Frame Format (PHY)

rate service payload

variable Bits

6 Mbit/s

Preamble, SFD Signal Data

Symbols12 1 variable

reserved length tailparity tail pad

616611214 variable

6, 9, 12, 18, 24, 36, 48, 54 Mbit/s

PLCP-Kopf



Chapter 3.2: WLAN

802.11 - MAC Management

• Synchronization– Find a LAN, try to remain in the LAN

– Synchronization of internal clocks (e.g. FHSS, PCF, power savingmechanisms)

– Timer etc.

• Power management– Sleep mode without missing a message

– Periodic sleeping, frame buffering, traffic monitoring

• Association/Re-association

– Integration into a LAN– Roaming, i.e. moving between networks from one Access Point to another– Scanning, i.e. active search for a network

• MIB - Management Information Base– Managing, read, write



Chapter 3.2: WLAN

tMedium

AP

busy

B

busy busy busy

B B B

value of the timestamp B beacon frame

Synchronization using a Beacon

• Beacon frame contains time stamps and administrative information for power saving mechanisms and roaming

• Varying times between beacon frames, since the medium can be occupied

• In infrastructure networks: AP takes over the sending of the beacons

Interval of the periodic radio

signal (beacon): 20ms - 1s



Chapter 3.2: WLAN

Synchronization using a Beacon (Ad-hoc)

tMedium

Station1

busy

B1

beacon interval

busy busy busy

B1

value of the timestamp B beacon frame

Station2B2 B2

random backoff

• All stations try to send a Beacon frame in fixed intervals

• Standard access procedure with backoff

• One station wins and sends a beacon frame at first. All other stations synchronize to this frame.



Chapter 3.2: WLAN

Power Management

• Idea: Switch off the sending/receiving device when not needed• Timing Synchronization Function

– Regular activation of all stations. Transmissions for sleeping stations are buffered; when waking up, the stations receive the transmission

• Infrastructure:

– AP can store all pending frameworks for sleeping stations– With each beacon frame, a Traffic Indication Map (TIM) is sent along which

indicates, for which stations frames are buffered.

– Additionally: List for broadcast/multicast receivers (Delivery Traffic Indication Map, DTIM)

• Ad-hoc

– Ad-hoc Traffic Indication Map (ATIM)• The storing stations announce the receivers of stored packages

• More complex, since no central AP: all stations have to temporarily store frames

• Collisions of ATIMs possible (scalability?)



Chapter 3.2: WLAN

Power Management with Wake-up Patterns (Infrastructure)

TIM interval

t

Medium

AP

busy

D

busy busy busy

T T D

T TIM D DTIM

DTIM interval

BB

B Broadcast/Multicast

Station

awake

p PS Poll

p

d

d

d Data transmissionfrom/to the station



Chapter 3.2: WLAN

Power Management with Wake-up Patterns (Ad-hoc)

awake

A ATIM transmission D data transmission

t

Station1B1 B1

B beacon frame

Station2B2 B2

random backoff

A

a

D

d

ATIMwindow beacon interval

a ACK for ATIM d ACK for data



Chapter 3.2: WLAN

802.11 - Roaming

Bad or even no connection?

• Scanning– Scanning of environment (listen for beacons of APs or send a probe and

wait for a response)

• Reassociation Request– Station requests joining the network to AP(s)

• Reassociation Response– If an AP responds, the station takes part in the network

– Otherwise, go on scanning

• AP accepts Reassociation Request

– Announce new station to the Distribution System– Distribution System updates its databases (location information)

– The old AP is informed by the Distribution System



Chapter 3.2: WLAN

WLAN in Aachen: MoPS

MoPS: Mobile Professors and Students

Goal: Enhance RWTH core network by a wireless infrastructure

First: installation of APs in lecture halls and central institutions of RWTH

Today: Installation of outdoor antennas for covering places of public interest and areas with many students



Chapter 3.2: WLAN



Chapter 3.2: WLAN

Configuration

Infrastructure or ad-hoc network?

SSID: Identifier for a WLAN.• Here: mops• Only components with the

same SSID are forming a network

• SSID „any“ is accepted by any station

Simple installation of software on a computer

even simpler the Access Point is installed: connecting… works!!!



Chapter 3.2: WLAN

Secure or not Secure…

Within a WLAN „data are flying free through the air“.Within WLAN everybody in transmission range can share your Access Point

Thus: security!

WEP: Wired Equivalent Privacy• Authentication at the Access Point• Connection is only possible if knowing the WEP key

• But: no key management, short keys

Data encryption

• Encryption of data before transmission

... but many users are overtaxed with configuring an Access Point – they carry it back to the shop because it is defective. The vendor is highly pleasured and by default deactivates all security mechanisms.

Registration of allowed MAC addresses• But: MAC addresses can be faked, large effort for large networks



Chapter 3.2: WLAN

Wardriving

New kind of sports: search for open WLANs.Just take:

• A notebook with WLAN card and a connector for a GPS device• A software for detcting Access Points,

e.g. Network Stumbler

• A GPS receiver• Time for driving around



Chapter 3.2: WLAN

Warchalking

What can be found at walls after a wardiver has passed...



Chapter 3.2: WLAN

• Bluetooth may act like a rogue member of a 802.11 network– does not know anything about gaps, IFS etc.

• IEEE 802.15-2 discusses these problems– Proposal: Adaptive Frequency Hopping (only co-existence, no collaboration)

• real effects? Many different opinions, tests, formulae, …– Results from complete breakdown to almost no effect

– Bluetooth (FHSS) seems to be more robust than 802.11b (DSSS)

802.11 vs. 802.15/Bluetooth

t

f [MHz]

2402

2480 802.11b 3 channles(separated by installation)

AC

K

DIF

S

DIF

S

SIF

S

1000 byte

SIF

S

DIF

S

500 byte

AC

K

DIF

S

500 byte

SIF

SA

CK

DIF

S

500 byte

DIF

S 100byte S

IFS

AC

K

DIF

S 100byte S

IFS

AC

K

DIF

S 100byte S

IFS

AC

K

DIF

S 100byte S

IFS

AC

K

DIF

S 100byte S

IFS

AC

K

802.15 79 channels(separated by hopping pattern)



Chapter 3.2: WLAN

„Competitor“ to WLAN: HIPERLAN

HIPERLAN 1 HIPERLAN 2 HIPERLAN 3 HIPERLAN 4Application wireless LAN access to ATM

fixed networkswireless local

looppoint-to-pointwireless ATMconnections

Frequency 5.1-5.3GHz 17.2-17.3GHz

Topology decentralized ad-hoc/infrastructure

cellular,centralized

point-to-multipoint

point-to-point

Antenna omni-directional directionalRange 50 m 50-100 m 5000 m 150 mQoS statistical ATM traffic classes (VBR, CBR, ABR, UBR)Mobility <10m/s stationaryInterface conventional LAN ATM networks

Data rate 23.5 Mbit/s >20 Mbit/s 155 Mbit/sPowerconservation

yes not necessary

HIPERLAN 1 never reached product status, the other standards have been renamed/modified !

05 - WLAN · Chapter 3.2: WLAN Page 3 Structure of a WLAN 1. Infrastructure network • Access...

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