wireless ethernet wireless lans: design goals · † fhss (frequency hopping spread spectrum) –...

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� IEEE 802.11 (data rate of 2 MBit/s), standardised in 1997

� IEEE 802.11a with 54 MBit/s, use of a 5 GHz frequency band� IEEE 802.11b with 11 MBit/s in a 2.4 GHz frequency range

� IEEE 802.11g: enhancement of 802.11b with up to 54 MBit/s� IEEE 802.11n: data rates up to several hundreds of MBit/s (not finished)

� …

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

802.11a

• 54 MBit/s• 5 GHz• OFDM

802.11b

• 11 MBit/s• 2.4 GHz• DSSS

802.11g• 54 MBit/s• 2.4 GHz• OFDM, DSSS



Page 2Chapter 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




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




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




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,… …




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)




IEEE 802.11 Variants

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, e.g. WPA802.11i

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

Inter Access Point Protocol (IAPP), allows communication between Access Points of different vendors, e.g. for exchanging roaming information

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




IEEE 802.11 Variants

Support of Virtual WLANs802.11q

3650-3700 MHz Operation in the U.S. 802.11y

Protection of Management Frames 802.11w

Wireless network management802.11v

Interworking with non-802 networks (for example, cellular) 802.11u

Wireless Performance Prediction (WPP) - test methods and metrics802.11t

ESS Mesh Networking 802.11s

Fast roaming between APs to avoid gaps in Voice over WLAN audio802.11r

WAVE - Wireless Access for the Vehicular Environment (such as ambulances and passenger cars)

802.11p

Enhancement for a future, faster WLAN with data rate of 100 - 600 MBit/s802.11n

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

802.11m




802.11 – Physical Layer

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

• FHSS (Frequency Hopping Spread Spectrum)

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

– 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




IEEE 802.11b

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

on SNR – User throughput max. approx.

6 MBit/s• Transmission range

– 100m outdoor, 30m indoor (directed links: several km)

– Max. data rate ~ 10m (indoor)• Frequency range

– Unlicensed 2.4 GHz ISM band• Security

– SSID, WPA2

• Connection setup time– Connectionless, „always on“

• QoS– Best effort, no guarantees (some

defined in “bad” way, later on much better standardized in 802.11e)

• Manageability– Limited (no automatic key distribution,

symmetrical encryption)

• Special advantages/disadvantages– Advantages: free ISM band, many

vendors, simple system

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

• Usage– Preferred version in Europe




Channels in IEEE 802.11b

2400[MHz]

2412 2483.52437 2462

Channel 1 Channel 6 Channel 11

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




Channels in IEEE 802.11b

Available in the ISM band (most of Europe): 2400 – 2483,5 MHz

MHz2400 2410 2420 2430 2440 2450 2460 2470 2480

Channel 12401 2412 2423

Channel 12401 2412 2423

Carrier frequencyChannel 6

2426 2437 2448Channel 11

2451 2462 2473

Channel 22406 2417 2428

Channel 72431 2442 2453

Channel 122456 2467 2478

Channel 32411 2422 2433

Channel 82436 2447 2458

Channel 132461 2472 2483

Channel 42416 2427 2438

Channel 92441 2452 2463

Channel 142473 2484 2495

Channel 52421 2432 2443

Channel 102446 2457 2468

Japan ( 1 – 14)

USA/Canada: channel 1 - 11




Dynamic Rate Shifting

Bits/SymbolUsed Symbol RateModulationCode lengthData Rate

811 Mbit/s

41,375 MS/s

Modified DSSS/QPSK

8 (CCK)5,5 Mbit/s

2DSSS/QPSK2 Mbit/s

11 MS/s

DSSS/PSK11 (barker code)

1 Mbit/s

Adjustment of the data rate to the transmission quality:

CCK: Complementary Code Keying

• Use of an 8-chip spreading sequence where each chip is modulated with QPSK• QPSK has 4 states, chipping sequence has length 8 → 48 resulting states

• Select 64 (for 11 Mbit/s) resp. 4 (for 5,5 Mbit/s) of the states which have as good cross correlation characteristics as possible (i.e. are as different as possible)

• That means: make use of 4 resp. 16 code words which can be transferred instead of only 1 as with the barker code (i.e. skip some robustness)




Channels

The whole 2.4GHz ISM band is divided into 11 resp. 13 overlapping channels. On each channel, DSSS is used for signal spreading:

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

→ The chips of the barker code resp. CCK are sent in sequence – this increases the number of symbols per second compared with “pure” sending of the data, thus a larger bandwidth is needed

→Purpose: even if the frequency range is disturbed partly, enough of the signal power reaches the receiver on the rest of the channel; if a non-spread transmission would take place, the whole data would be lost in case of narrowband interference

→ If CCK is used, we use “several codes” instead of the same chipping sequence everytime - the transmission becomes more susceptible for disturbances thanwith use of the barker code, if we have a distortion (maybe caused by an overlapping channel)!

Channel n

22 MHz




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 “abused” spreading in case of CCK, the higher data transmission rates are more susceptible for disturbances. Thus, a smaller range results:




Range of 802.11b




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– SSID, WPA2

• 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

• Usage– Preferred version in USA




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

Channels are also overlapping, as in 802.11b:




subcarriernumber

Modulation in 802.11a: OFDM

• OFDM with 52 subcarriers (64 in total, 6 as guard space on each side)

• Subcarriers overlap with 312,5 kHz spacing, but orthogonality of chosen frequencies allows for clear separation

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

• Pilots are used by the receiver to deal with multipath propagation: phase references for the whole band are sent here, the receiver can interpolate phase shifts for the data carriers

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

channel center frequency

312,5 kHzphase reference (pilot)

And: IEEE 802.11g simply is introducing OFDM on the existing 802.11b system, i.e. replacing of DSSS by OFDM for higher data rates (while keeping the ability to switch to DSSS for interworking with 802.11b)




Medium Access Control

We can assign one channel with an AP – but then we have to coordinate all mobile stations in their communication with the AP. Chosen for IEEE 802.11a/b/g/…:

„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




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




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




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




Example - Backoff

data

wait

B1 = 5

B2 = 15

B1 = 25

B2 = 20

data

wait

B1 and B2 are backoff intervalsat nodes 1 and 2

B2 = 10




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




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




Competing Stations (with ACK)

t

busy

boij

Station1

Station2

Station3

Station4

Station5

Sending request

SIFSbo11

bo21

bo51

busy

jth backoff time of station ibusy Medium occupied (Frame, ACK, etc.)

DIFS

bo41

bo51

bo11

DIFS

busy

busy

DIFSbo11 busy

bo42

bo52

The size of the competition window (Contention Window, CW) affects the efficiency. Therefore (similar to Ethernet) it starts with CW = 8 and is doubled with each collision up to CWmax = 256

ACK

DIFS

ACK Acknowledgement




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)




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




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




What is implemented?

Any vendor has to implement the standard CSMA/CA variant, the other two are optional

• RTS/CTS very often is implemented by AP manufacturers, but: disabled!

• Usual method:� A frame size threshold is defined, and only frames longer than the threshold

are sent with RTS/CTS (to avoid overhead for small frames)

� The threshold value in basic configuration is sent to maximum allowed frame length…

� Changing the threshold value allows you to enable the RTS/CTS

� Only possibility to really avoid collisions• PCF mechanism usually is not implemented

� Not needed in many cases, and not possible in ad-hoc networks� Would allow for real-time data transmission, but is not good in it, thus it

doesn’t became prominent – instead, a QoS enhancement for real-time transmission was defined (IEEE 802.11e)




Frame Format

• Types

� Control frames, administrative frames, data frames• Sequence numbers

� For detecting duplicated frames due to lost ACKs

• Addresses� Receiver, transmitter (physical), sender (logical), BSS identifier

• Misc� Duration of transmission, 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




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

Sequence Control• Recognition of duplicated frames by sequence numbers

CRC• Checksum for detecting transmission errors

Addresses• Each field contains a 48-Bit MAC address. MAC frames can be transferred

between two stations, between station and AP or between two APs within 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




MAC Address Format

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

Scenario to DS from DS address 1 address 2 address 3 address 4 ad-hoc network 0 0 DA SA BSSID - infrastructure network, from AP

0 1 DA BSSID SA -

infrastructure network, to AP

1 0 BSSID SA DA -

infrastructure network, within DS

1 1 RA TA DA SA




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




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




DSSS Frame Format (PHY)

Synchronization SFD Signal Service HEC Payload

Preamble Header


Length

16

• Synchronization– Synchronization, 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




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:




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-Header




802.11 - MAC Management

• Synchronization

� Find a LAN, try to remain in the LAN� Synchronization of internal clocks (e.g. FHSS, PCF, power saving

mechanisms)

� 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 and write of management attributed and state variables inside APs, the distribution system, etc




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




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.




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

� Similar to the infrastructure mod, an aA-hoc Traffic Indication Map (ATIM) is defined

� Stations, which have data to send, announce the receivers of stored packages

� More complex, no central AP: all stations have to temporarily store frames� Collisions of ATIMs possible (scalability?)




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




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




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




Quality of Service – IEEE 802.11e

The PCF variant of CSMA/CA should allow some quality in data transmission:• By polling at certain times, allow for deterministic delay of information

• Also, guarantee a certain data rate to each participant• But…frames in polling can be between 0 and 2304 bytes… and the data rate on

physical layer can change due to channel conditions…

→ no way to calculate transmission time of a frame in advance, thus the above quality cannot be given

Solution: define additional CSMA/CA variants which can give priority to real-time data (defined in IEEE 802.11e)

• Only an add-on the IEEE 802.11a/b/g, not a stand-alone WLAN standard

• Definition of� Extended Distributed Channel Access (EDCA) as better version of DCF using

several classes of access priority by refining the inter-frame gaps and introducing so-called Transmission Opportunities (TXOP)

� Hybrid Coordination Function Controlled Channel Access (HCCA) as better version of PCF also using TXOP




Extended Distributed Channel Access

The scheme from before (all stations use the DIFS time interval) is refined:

• Assign different priorities to different data streams (traffic classes, TC)• As before, priority is given by waiting times: the Arbitration Inter-Frame space (AIFS)

t

busy SIFS

PIFS

DIFS =AIFS[TC7]

RTS

contention window

AIFS[TC6]

AIFS[TC0]

• Classify all data streams in traffic classes regarding their QoS• 8 priority classes, TC 7 has highest priority

• Give longer waiting times to lower priority – thus higher priority streams can start sending earlier

• Fairness is given – even high priority senders can draw a large backoff number

Best EffortBackgroundBackgroundVideo Probe

VideoVideoVoiceVoice

01122233

01234567

PurposeAccess Category (AC)TC




EDCF Implementation

With EDCF, each station has to handle up to 8 queues performing the same access procedure as “plain” DCF with backoff counter (BC) and contention window (CW):

One more enhancement: each class also a TXOP is assigned, which is a maximum sending duration – after getting medium access, for time of TXOP several frames can be sent (Contention Free Burst)




HCCA

As in PCF, HCCA is a combination of a contention-free period and a contention period

• In the contention-free period the AP polls the stations

�Difference to PCF: stations can place reservations for the polling phase�The AP polls stations by granting a TXOP oriented at reservation wishes and

current traffic load

• In the contention period, EDCF is used

Question: why giving QoS? Why not overprovisioning, i.e. only increase the data rate?




Faster!

Not an end with 802.11a/g – go on with 802.11n• up to 600 MBit/s!

• over 70 – 250m!

How to achieve such a data rate while keeping compatibility to 802.11a/b/g?

• Applied to 2.4 as well as 5 GHz ISM band to only have a single variant for the future

• Modify OFDM with increasing symbol rate and slightly enlarge the bandwidth:→ increase data rate from 54 MBit/s to 65 MBit/s

• Optional: Greenfield mode, i.e. skip support for 802.11a/b/g (an increasing number of legacy devices reduces the average throughput in the whole network)

• Optional: increase a channel’s bandwidth to 40 MHz (dynamic adaptation to other WLANs in the environment necessary!)

• Use MIMO – multiple input multiple output




MIMO

MIMO means: use several antennas in parallel to send data to one receiver• Apply Space Division Multiplexing (SDM) – i.e. split the data stream into multiple

parts (called spatial stream) and transmit each part with a separate antenna (for up to 4 antennas)

• Necessary: power control – only use MIMO if necessary, otherwise lots of power is consumed

• Apply beam-forming to focus the sender’s antennas to the receiver’s antennas

• By antenna diversity, a receiver can find out the angle of incidence of certain spatial streams and thus distinguish between several streams

• Optional: apply diversity on improving signal strength, i.e. improve signal by receiving the same stream with several antennas and combine the outputs (for up to 4 antennas, but only if the number of receiver antennas is larger than the number of spatial streams)




802.11n – MAC Layer

Many improvements on PHY layer, only a few on the MAC layer:• Introduce Reduced Inter-Frame Space (RIFS) to shorten the waiting time after

detecting the medium to be idle• Use frame aggregation, i.e. pack together several frames of one station and

remove redundant header information

Availability of 802.11n?• Draft version 2 finished this year• Lot of products of several vendors (compliance to a non-finished standard?)

• Potential problems with a patent?• Planned release date – varies between September 2008 and March 2009…




802.11s – WLAN Mesh Networking

Other WLAN variant: mesh networks• Classical WLAN: wired

infrastructure between APs

• Sometimes called “Wireless Paradox”

Let APs interconnect in wireless manner, also using WLAN (lower costs, simple installation, resilient, …)

Figures from: IEEE 802.11s tutorial




Mesh Topology

Figures from: IEEE 802.11s tutorial

Mesh PointSpecial component, establishes peer links with neighbors

Mesh APAs mesh point, but additionally implements AP functionallity

Mesh PortalAs mesh point, but additionally connects to some other network

Changes in the 802.11 standard regarding:• Addresses

• MAC scheme (oriented at 802.11e)• Synchronization / power modes

• Security

• And: routing (layer 3!)




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, encryption of data before transmission• Connection is only possible if knowing the WEP key

• But: no key management, short keys• Thus: WPA/WPA2 (Wi-Fi Protected Access) today give much better security

... but many users are overtaxed with configuring an Access Point – even if today a good user guide to install security functions is implemented on APs, there is a lot of open networks...

Registration of allowed MAC addresses

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

Hiding of SSID

• Broadcast of SSID in beacons can be switched of, thus only someone knowing the SSID can join the network (but: intuitive names? Default names?)




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




Warchalking

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




• 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)

– Maybe Bluetooth adaptive frequency hopping has better effect

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

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DIF

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500 byte

AC

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DIF

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500 byte

SIF

SA

CK

DIF

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500 byte

DIF

S 100byte S

IFS

AC

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DIF

S 100byte S

IFS

AC

K

DIF

S 100byte S

IFS

AC

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DIF

S 100byte S

IFS

AC

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S 100byte S

IFS

AC

K

802.15 79 channels(separated by hopping pattern)

wireless ethernet wireless lans: design goals · † fhss (frequency hopping spread spectrum) –...

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