1

Ασύρµατα τοπικά δίκτυα

Σαράντης Πασκαλής <paskalis@di.uoa.gr>Εθνικό και Καποδιστριακό Πανεπιστήµιο Αθηνών

2

Elements of a wireless network

network infrastructure

wireless link

� typically used to connect mobile(s) to base station

� also used as backbone link

� multiple access protocol coordinates link access

� various data rates, transmission distance

3

Characteristics of wireless LANs

� Advantages

� very flexible within the reception area

� Ad-hoc networks without previous planning possible

� (almost) no wiring difficulties (e.g. historic buildings, firewalls)

� more robust against disasters like, e.g., earthquakes, fire - or users pulling a plug...

� Disadvantages

� typically very low bandwidth compared to wired networks (1-10 Mbit/s) due to shared medium

� many proprietary solutions, especially for higher bit-rates, standards take their time (e.g. IEEE 802.11n, ac)

� products have to follow many national restrictions if working wireless, it takes a vary long time to establish global solutions like, e.g., IMT-2000

4

Design goals for wireless LANs

� 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

� …

5

Indoor10-30m

Outdoor50-200m

Mid-rangeoutdoor

200m – 4 Km

Long-rangeoutdoor

5Km – 20 Km

.056

.384

1

4

5-11

54

IS-95, CDMA, GSM 2G

UMTS/WCDMA, CDMA2000 3G

802.15

802.11b

802.11a,g

UMTS/WCDMA-HSPDA, CDMA2000-1xEVDO 3G cellularenhanced

802.16 (WiMAX)

802.11a,g point-to-point

200 802.11n

Dat

a ra

te (

Mbp

s)

data

Characteristics of selected wireless link standards

6

Elements of a wireless network

network infrastructure

infrastructure mode

� base station connects mobiles into wired network

� handoff: mobile changes base station providing connection into wired network

7

Elements of a wireless network

ad hoc mode

� no base stations

� nodes can only transmit to other nodes within link coverage

� nodes organize themselves into a network: route among themselves

8

Comparison: infrastructure vs. ad-hoc networks

infrastructurenetwork

ad-hoc network

APAP

AP

wired network

AP: Access Point

9

802.11 - Architecture of an infrastructure network

� Station (STA)� terminal with access mechanisms to

the wireless medium and radio contact to the access point

� Basic Service Set (BSS)� group of stations using the same

radio frequency

� Access Point� station integrated into the wireless

LAN and the distribution system

� Portal� bridge to other (wired) networks

� Distribution System� interconnection network to form one

logical network (EES: Extended Service Set) based on several BSS

Distribution System

Portal

802.x LAN

AccessPoint

802.11 LAN

BSS2

802.11 LAN

BSS1

AccessPoint

STA1

STA2 STA3

ESS

10

802.11 - Architecture of an ad-hoc network

� Direct communication within a limited range

� Station (STA):terminal with access mechanisms to the wireless medium

� Independent Basic Service Set (IBSS):group of stations using the same radio frequency

802.11 LAN

IBSS2

802.11 LAN

IBSS1

STA1

STA4

STA5

STA2

STA3

11

Wireless network taxonomy

single hop multiple hops

infrastructure(e.g., APs)

noinfrastructure

host connects to base station (WiFi,WiMAX, cellular) which connects to

larger Internet

no base station, noconnection to larger Internet (Bluetooth,

ad hoc nets)

host may have torelay through several

wireless nodes to connect to larger

Internet: mesh net

no base station, noconnection to larger

Internet. May have torelay to reach other a given wireless node

MANET, VANET

12

Wireless Link Characteristics (1)

Differences from wired link ….

� decreased signal strength: radio signal attenuates as it propagates through matter (path loss)

� interference from other sources: standardized wireless network frequencies (e.g., 2.4 GHz) shared by other devices (e.g., phone); devices (motors) interfere as well

� multipath propagation: radio signal reflects off objects ground, arriving ad destination at slightly different times

…. make communication across (even a point to point) wireless link much more “difficult”

13

Wireless Link Characteristics (2)

� SNR: signal-to-noise ratio

� larger SNR – easier to extract signal from noise (a “good thing”)

� SNR versus BER tradeoffs

� given physical layer: increase power -> increase SNR->decrease BER

� given SNR: choose physical layer that meets BER requirement, giving highest thruput

� SNR may change with mobility: dynamically adapt physical layer (modulation technique, rate)

10 20 30 40

QAM256 (8 Mbps)

QAM16 (4 Mbps)

BPSK (1 Mbps)

SNR(dB)

BE

R

10-1

10-2

10-3

10-5

10-6

10-7

10-4

14

Wireless network characteristics

Multiple wireless senders and receivers create additional problems (beyond multiple access):

AB

C

Hidden terminal problem� B, A hear each other

� B, C hear each other

� A, C can not hear each other

means A, C unaware of their interference at B

A B C

A’s signalstrength

space

C’s signalstrength

Signal attenuation:� B, A hear each other

� B, C hear each other

� A, C can not hear each other interfering at B

15

Code Division Multiple Access (CDMA)

� used in several wireless broadcast channels (cellular, satellite, etc) standards

� unique “code” assigned to each user; i.e., code set partitioning� all users share same frequency, but each user has own “chipping”

sequence (i.e., code) to encode data� encoded signal = (original data) X (chipping sequence)� decoding: inner-product of encoded signal and chipping sequence� allows multiple users to “coexist” and transmit simultaneously with

minimal interference (if codes are “orthogonal”)

16

CDMA Encode/Decode

slot 1 slot 0

d1 = -1

1 1 1 1

1- 1- 1- 1-

Zi,m= di.cm

d0 = 1

1 1 1 1

1- 1- 1- 1-

1 1 1 1

1- 1- 1- 1-

1 1 11

1-1- 1- 1-

slot 0channeloutput

slot 1channeloutput

channel output Zi,m

sendercode

databits

slot 1 slot 0

d1 = -1d0 = 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 11

1-1- 1- 1-

slot 0channeloutput

slot 1channeloutputreceiver

code

receivedinput

Di = Σ Zi,m.cmm=1

M

M

17

CDMA: two-sender interference

18

IEEE 802.11 Wireless LAN

� 802.11b

� 2.4-5 GHz unlicensed spectrum

� up to 11 Mbps

� direct sequence spread spectrum (DSSS) in physical layer

� all hosts use same chipping code

� 802.11a

� 5-6 GHz range

� up to 54 Mbps

� 802.11g

� 2.4-5 GHz range

� up to 54 Mbps

� 802.11n: multiple antennae

� 2.4-5 GHz range

� up to 200 Mbps

� all use CSMA/CA for multiple access

� all have base-station and ad-hoc network versions

19

Μη αδειοδοτηµένο φάσµα

� Τα ασύρµατα τοπικά δίκτυα λειτουργούν σε περιοχές µηαδειοδοτηµένου φάσµατος

� 2,45 GHz (ISM band 2400-2500 MHz)

� 5 GHz (διαφορές ανά χώρα/ήπειρο)

20

Μη αδειοδοτηµένο φάσµα 5 GHz

5.200 5.300 5.400 5.500 5.600 5.700 5.800 5.9005.100

Europe

USA

Japan5.150 5.250

5.150 5.350 5.725 5.825

Indoor 250 mW | Outdoor 1 W EIRP Outdoor 4 W EIRP

5.3505.150 5.470 5.725

Outdoor 1W EIRPIndoor 200 mW EIRP

DFS & TPC required in Europe

21

802.11 LAN architecture

� wireless host communicates with base station

� base station = access point (AP)

� Basic Service Set (BSS) (aka “cell”) in infrastructure mode contains:

� wireless hosts

� access point (AP): base station

� ad hoc mode: hosts only

BSS 1

BSS 2

Internet

hub, switchor router

AP

AP

22

IEEE standard 802.11

mobile terminal

access point

fixedterminal

application

TCP

802.11 PHY

802.11 MAC

IP

802.3 MAC

802.3 PHY

application

TCP

802.3 PHY

802.3 MAC

IP

802.11 MAC

802.11 PHY

LLC

infrastructurenetwork

LLC LLC

23

802.11 - Layers and functions

� PLCP Physical Layer Convergence Protocol

� clear channel assessment signal (carrier sense)

� PMD Physical Medium Dependent

� modulation, coding

� PHY Management

� channel selection, MIB

� Station Management

� coordination of all management functions

PMD

PLCP

MAC

LLC

MAC Management

PHY Management

� MAC

� access mechanisms, fragmentation, encryption

� MAC Management

� synchronization, roaming, MIB, power management

PH

YD

LC

Sta

tion

Man

agem

ent

24

802.11 - Physical layer (legacy)

� 3 versions: 2 radio (typ. 2.4 GHz), 1 IR

� data rates 1 or 2 Mbit/s

� FHSS (Frequency Hopping Spread Spectrum)

� spreading, despreading, signal strength, typ. 1 Mbit/s

� min. 2.5 frequency hops/s (USA), two-level GFSK modulation

� DSSS (Direct Sequence Spread Spectrum)

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

� preamble and header of a frame is always transmitted with 1 Mbit/s, rest of transmission 1 or 2 Mbit/s

� chipping sequence: +1, -1, +1, +1, -1, +1, +1, +1, -1, -1, -1 (Barker code)

� max. radiated power 1 W (USA), 100 mW (EU), min. 1mW

� Infrared

� 850-950 nm, diffuse light, typ. 10 m range

� carrier detection, energy detection, synchronization

25

FHSS PHY packet format (legacy)

� Synchronization

� synch with 010101... pattern

� SFD (Start Frame Delimiter)

� 0000110010111101 start pattern

� PLW (PLCP_PDU Length Word)

� length of payload incl. 32 bit CRC of payload, PLW < 4096

� PSF (PLCP Signaling Field)

� data of payload (1 or 2 Mbit/s)

� HEC (Header Error Check)

� CRC with x16+x12+x5+1

synchronization SFD PLW PSF HEC payload

PLCP preamble PLCP header

80 16 12 4 16 variable bits

26Prof. Dr.-Ing. Jochen H. Schiller www.jochenschiller.de MC - 2013

DSSS PHY packet format (legacy)

� Synchronization� synch., gain setting, energy detection, frequency offset compensation

� SFD (Start Frame Delimiter)

� 1111001110100000

� Signal

� data rate of the payload (0A: 1 Mbit/s DBPSK; 14: 2 Mbit/s DQPSK)

� Service

� future use, 00: 802.11 compliant

� Length

� length of the payload

� HEC (Header Error Check)� protection of signal, service and length, x16+x12+x5+1

synchronization SFD signal service HEC payload

PLCP preamble PLCP header

128 16 8 8 16 variable bits

length

16

27

802.11: Channels, association

� 802.11b: 2.4GHz-2.485GHz spectrum divided into 11 channels at different frequencies

� AP admin chooses frequency for AP

� interference possible: channel can be same as that chosen by neighboring AP!

� host: must associate with an AP� scans channels, listening for beacon frames containing AP’s name

(SSID) and MAC address

� selects AP to associate with

� may perform authentication [Chapter 8]

� will typically run DHCP to get IP address in AP’s subnet

28

802.11: passive/active scanning

AP 2AP 1

H1

BBS 2BBS 1

122

3 4

Active Scanning: (1) Probe Request frame broadcast

from H1(2) Probes response frame sent from

APs(3) Association Request frame sent:

H1 to selected AP (4) Association Response frame

sent: H1 to selected AP

AP 2AP 1

H1

BBS 2BBS 1

12

3

1

Passive Scanning:(1) beacon frames sent from APs(2) association Request frame sent:

H1 to selected AP (3) association Response frame sent:

H1 to selected AP

29

802.11 - MAC layer I - DFWMAC

� Traffic services� Asynchronous Data Service (mandatory)

� exchange of data packets based on “best-effort”

� support of broadcast and multicast

� Time-Bounded Service (optional)� implemented using PCF (Point Coordination Function)

� Access methods� DFWMAC-DCF CSMA/CA (mandatory)

� collision avoidance via randomized „back-off“ mechanism

� minimum distance between consecutive packets

� ACK packet for acknowledgements (not for broadcasts)

� DFWMAC-DCF w/ RTS/CTS (optional)� Distributed Foundation Wireless MAC

� avoids hidden terminal problem

� DFWMAC- PCF (optional)� access point polls terminals according to a list

30

802.11 - MAC layer II

� Priorities

� defined through different inter frame spaces

� no guaranteed, hard priorities

� SIFS (Short Inter Frame Spacing)

� highest priority, for ACK, CTS, polling response

� PIFS (PCF IFS)

� medium priority, for time-bounded service using PCF

� DIFS (DCF, Distributed Coordination Function IFS)

� lowest priority, for asynchronous data service

t

medium busy SIFSPIFS

DIFSDIFS

next framecontention

direct access if medium is free ≥ DIFS

31

IEEE 802.11: multiple access

� avoid collisions: 2+ nodes transmitting at same time

� 802.11: CSMA - sense before transmitting

� don’t collide with ongoing transmission by other node

� 802.11: no collision detection!

� difficult to receive (sense collisions) when transmitting due to weak received signals (fading)

� can’t sense all collisions in any case: hidden terminal, fading

� goal: avoid collisions: CSMA/C(ollision)A(voidance)

AB

CA B C

A’s signalstrength

space

C’s signalstrength

32

IEEE 802.11 MAC Protocol: CSMA/CA

802.11 sender

1 if sense channel idle for DIFS then

transmit entire frame (no CD)

2 if sense channel busy then

start random backoff time

timer counts down while channel idle

transmit when timer expires

if no ACK, increase random backoff interval, repeat 2

802.11 receiver

- if frame received OK

return ACK after SIFS (ACK needed due to hidden terminal problem)

sender receiver

DIFS

data

SIFS

ACK

33

t

medium busy

DIFSDIFS

next frame

contention window(randomized back-offmechanism)

802.11 - CSMA/CA access method I

� station ready to send starts sensing the medium (Carrier Sense based on CCA, Clear Channel Assessment)

� if the medium is free for the duration of an Inter-Frame Space (IFS), the station can start sending (IFS depends on service type)

� if the medium is busy, the station has to wait for a free IFS, then the station must additionally wait a random back-off time (collision avoidance, multiple of slot-time)

� if another station occupies the medium during the back-off time of the station, the back-off timer stops (fairness)

slot time (20µs)direct access if medium is free ≥ DIFS

34

802.11 - competing stations - simple version

t

busy

boe

station1

station2

station3

station4

station5

packet arrival at MAC

DIFSboe

boe

boe

busy

elapsed backoff time

bor residual backoff time

busy medium not idle (frame, ack etc.)

bor

bor

DIFS

boe

boe

boe bor

DIFS

busy

busy

DIFSboe busy

boe

boe

bor

bor

35

802.11 - CSMA/CA access method II

� Sending unicast packets

� station has to wait for DIFS before sending data

� receivers acknowledge at once (after waiting for SIFS) if the packet was received correctly (CRC)

� automatic retransmission of data packets in case of transmissionerrors

t

SIFS

DIFS

data

ACK

waiting time

otherstations

receiver

senderdata

DIFS

contention

36

Avoiding collisions (more)

idea: allow sender to “reserve” channel rather than random access of data frames: avoid collisions of long data frames

� sender first transmits small request-to-send (RTS) packets to BS using CSMA

� RTSs may still collide with each other (but they’re short)

� BS broadcasts clear-to-send CTS in response to RTS

� CTS heard by all nodes

� sender transmits data frame

� other stations defer transmissions

avoid data frame collisions completely using small reservation packets!

37

Collision Avoidance: RTS-CTS exchange

APA B

time

RTS(A)RTS(B)

RTS(A)

CTS(A) CTS(A)

DATA (A)

ACK(A) ACK(A)

reservation collision

defer

38

802.11 - DFWMAC

� Sending unicast packets

� station can send RTS with reservation parameter after waiting for DIFS (reservation determines amount of time the data packet needs the medium)

� acknowledgement via CTS after SIFS by receiver (if ready to receive)

� sender can now send data at once, acknowledgement via ACK

� other stations store medium reservations distributed via RTS and CTS

t

SIFS

DIFS

data

ACK

defer access

otherstations

receiver

senderdata

DIFS

contention

RTS

CTSSIFS SIFS

NAV (RTS)NAV (CTS)

39

Fragmentation

t

SIFS

DIFS

data

ACK1

otherstations

receiver

senderfrag1

DIFS

contention

RTS

CTSSIFS SIFS

NAV (RTS)NAV (CTS)

NAV (frag1)NAV (ACK1)

SIFSACK2

frag2

SIFS

40

DFWMAC-PCF I (almost never used)

PIFS

stations‘NAV

wirelessstations

point coordinator

D1

U1

SIFS

NAV

SIFSD2

U2

SIFS

SIFS

SuperFramet0

medium busy

t1

41

DFWMAC-PCF II

tstations‘NAV

wirelessstations

point coordinator

D3

NAV

PIFSD4

U4

SIFS

SIFSCFend

contentionperiod

contention free period

t2 t3 t4

42

802.11 - Frame format

� Types

� control frames, management frames, data frames

� Sequence numbers

� important against duplicated frames due to lost ACKs

� Addresses

� receiver, transmitter (physical), BSS identifier, sender (logical)

� Miscellaneous

� 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

43

MAC address format

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

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

44

Special Frames: ACK, RTS, CTS

� Acknowledgement

� Request To Send

� Clear To Send

FrameControl

DurationReceiverAddress

TransmitterAddress

CRC

2 2 6 6 4bytes

FrameControl

DurationReceiverAddress

CRC

2 2 6 4bytes

FrameControl

DurationReceiverAddress

CRC

2 2 6 4bytes

ACK

RTS

CTS

45

802.11 - MAC management

� Synchronization

� try to find a LAN, try to stay within a LAN

� timer etc.

� Power management

� sleep-mode without missing a message

� periodic sleep, frame buffering, traffic measurements

� Association/Reassociation

� integration into a LAN

� roaming, i.e. change networks by changing access points

� scanning, i.e. active search for a network

� MIB - Management Information Base

� managing, read, write

46

Synchronization using a Beacon (infrastructure)

beacon interval(20ms – 1s)

tmedium

accesspoint

busy

B

busy busy busy

B B B

value of the timestamp B beacon frame

47

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 delay

48

hub or switch

AP 2

AP 1

H1 BBS 2

BBS 1

802.11: mobility within same subnet

router

� H1 remains in same IP subnet: IP address can remain same

� switch: which AP is associated with H1?

� self-learning (Ch. 5): switch will see frame from H1 and “remember” which switch port can be used to reach H1

49

802.11 - Roaming

� No or bad connection? Then perform:

� Scanning� scan the environment, i.e., listen into the medium for beacon signals or send

probes into the medium and wait for an answer

� Reassociation Request� station sends a request to one or several AP(s)

� Reassociation Response� success: AP has answered, station can now participate

� failure: continue scanning

� AP accepts Reassociation Request� signal the new station to the distribution system

� the distribution system updates its data base (i.e., location information)

� typically, the distribution system now informs the old AP so it can release resources

� Fast roaming – 802.11r� e.g. for vehicle-to-roadside networks

50

802.11: advanced capabilities

Rate Adaptation

� base station, mobile dynamically change transmission rate (physical layer modulation technique) as mobile moves, SNR varies

QAM256 (8 Mbps)QAM16 (4 Mbps)BPSK (1 Mbps)

10 20 30 40SNR(dB)

BE

R

10-1

10-2

10-3

10-5

10-6

10-7

10-4

operating point

1. SNR decreases, BER increase as node moves away from base station

2. When BER becomes too high, switch to lower transmission rate but with lower BER

51

802.11: advanced capabilities

Power Management

� node-to-AP: “I am going to sleep until next beacon frame”

� AP knows not to transmit frames to this node

� node wakes up before next beacon frame

� beacon frame: contains list of mobiles with AP-to-mobile frames waiting to be sent

� node will stay awake if AP-to-mobile frames to be sent; otherwise sleep again until next beacon frame

52

Power management

� Idea: switch the transceiver off if not needed

� States of a station: sleep and awake

� Timing Synchronization Function (TSF)

� stations wake up at the same time

� Infrastructure

� Traffic Indication Map (TIM)

� list of unicast receivers transmitted by AP

� Delivery Traffic Indication Map (DTIM)

� list of broadcast/multicast receivers transmitted by AP

� Ad-hoc

� Ad-hoc Traffic Indication Map (ATIM)

� announcement of receivers by stations buffering frames

� more complicated - no central AP

� collision of ATIMs possible (scalability?)

� APSD (Automatic Power Save Delivery)

� new method in 802.11e replacing above schemes

53

Power saving with wake-up patterns (infrastructure)

TIM interval

t

medium

accesspoint

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 transmissionto/from the station

54

Power saving with wake-up patterns (ad-hoc)

awake

A transmit ATIM D transmit data

t

station1B1 B1

B beacon frame

station2B2 B2

random delay

A

a

D

d

ATIMwindow beacon interval

a acknowledge ATIM d acknowledge data

55

WLAN: IEEE 802.11b

� Data rate

� 1, 2, 5.5, 11 Mbit/s, depending on SNR

� User data rate max. approx. 6 Mbit/s

� Transmission range

� 300m outdoor, 30m indoor

� Max. data rate ~10m indoor

� Frequency

� DSSS, 2.4 GHz ISM-band

� Security

� Limited, WEP insecure, SSID

� Availability

� Many products, many vendors

� Connection set-up time� Connectionless/always on

� Quality of Service� Typ. Best effort, no guarantees

(unless polling is used, limited support in products)

� Manageability� Limited (no automated key

distribution, sym. Encryption)

� Special Advantages/Disadvantages� Advantage: many installed systems,

lot of experience, available worldwide, free ISM-band, many vendors, integrated in laptops, simple system

� Disadvantage: heavy interference on ISM-band, no service guarantees, slow relative speed only

56

IEEE 802.11b – PHY frame formats

synchronization SFD signal service HEC payload

PLCP preamble PLCP header

128 16 8 8 16 variable bits

length

16

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

short synch. SFD signal service HEC payload

PLCP preamble(1 Mbit/s, DBPSK)

PLCP header(2 Mbit/s, DQPSK)

56 16 8 8 16 variable bits

length

16

96 µs 2, 5.5 or 11 Mbit/s

Long PLCP PPDU format

Short PLCP PPDU format (optional)

57

Channel selection (non-overlapping)

2400[MHz]

2412 2483.52442 2472

channel 1 channel 7 channel 13

Europe (ETSI)

US (FCC)/Canada (IC)

2400[MHz]

2412 2483.52437 2462

channel 1 channel 6 channel 11

22 MHz

22 MHz

58

WLAN: IEEE 802.11a

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

depending on SNR

� User throughput (1500 byte packets): 5.3 (6), 18 (24), 24 (36), 32 (54)

� 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 30m, 18 up to 40 m, 12 up to 60 m

� Frequency� Free 5.15-5.25, 5.25-5.35, 5.725-

5.825 GHz ISM-band

� Security� Limited, WEP insecure, SSID

� Availability� Some products, some vendors

� Connection set-up time� Connectionless/always on

� Quality of Service

� Typ. best effort, no guarantees (same as all 802.11 products)

� Manageability

� Limited (no automated key distribution, sym. Encryption)

� Special Advantages/Disadvantages

� Advantage: fits into 802.x standards, free ISM-band, available, simple system, uses less crowded 5 GHz band

� Disadvantage: stronger shading due to higher frequency, no QoS

59

IEEE 802.11a – PHY frame format

rate service payload

variable bits

6 Mbit/s

PLCP preamble signal data

symbols12 1 variable

reserved length tailparity tail pad

616611214 variable

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

PLCP header

60

Operating channels of 802.11a in Europe

5150 [MHz]5180 53505200

36 44

16.6 MHz

center frequency = 5000 + 5*channel number [MHz]

channel40 48 52 56 60 64

5220 5240 5260 5280 5300 5320

5470

[MHz]

5500 57255520

100 108

16.6 MHz

channel104 112 116 120 124 128

5540 5560 5580 5600 5620 5640

132 136 140

5660 5680 5700

61

Operating channels for 802.11a / US U-NII

5150 [MHz]5180 53505200

36 44

16.6 MHz

center frequency = 5000 + 5*channel number [MHz]

channel40 48 52 56 60 64

149 153 157 161

5220 5240 5260 5280 5300 5320

5725 [MHz]5745 58255765

16.6 MHz

channel

5785 5805

62

OFDM in IEEE 802.11a

� OFDM with 52 used subcarriers (64 in total)

� 48 data + 4 pilot

� (plus 12 virtual subcarriers)

� 312.5 kHz spacing

subcarriernumber

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

channel center frequency

312.5 kHzpilot

63

WLAN: IEEE 802.11 – current developments

� 802.11c: Bridge Support� Definition of MAC procedures to support bridges as extension to 802.1D

� 802.11d: Regulatory Domain Update� Support of additional regulations related to channel selection, hopping sequences

� 802.11e: MAC Enhancements – QoS� Enhance the current 802.11 MAC to expand support for applications with Quality of Service

requirements, and in the capabilities and efficiency of the protocol� Definition of a data flow (“connection”) with parameters like rate, burst, period… supported by HCCA

(HCF (Hybrid Coordinator Function) Controlled Channel Access, optional) � Additional energy saving mechanisms and more efficient retransmission� EDCA (Enhanced Distributed Channel Access): high priority traffic waits less for channel access

� 802.11F: Inter-Access Point Protocol (withdrawn)� Establish an Inter-Access Point Protocol for data exchange via the distribution system

� 802.11g: Data Rates > 20 Mbit/s at 2.4 GHz; 54 Mbit/s, OFDM� Successful successor of 802.11b, performance loss during mixed operation with .11b

� 802.11h: Spectrum Managed 802.11a� Extension for operation of 802.11a in Europe by mechanisms like channel measurement for dynamic

channel selection (DFS, Dynamic Frequency Selection) and power control (TPC, Transmit Power Control)

� 802.11i: Enhanced Security Mechanisms� Enhance the current 802.11 MAC to provide improvements in security. � TKIP enhances the insecure WEP, but remains compatible to older WEP systems� AES provides a secure encryption method and is based on new hardware

64

WLAN: IEEE 802.11– current developments

� 802.11j: Extensions for operations in Japan� Changes of 802.11a for operation at 5GHz in Japan using only half the channel width at larger range

� 802.11-2007: Current “complete” standard� Comprises amendments a, b, d, e, g, h, i, j

� 802.11k: Methods for channel measurements� Devices and access points should be able to estimate channel quality in order to be able to choose a

better access point of channel

� 802.11m: Updates of the 802.11-2007 standard� 802.11n: Higher data rates above 100Mbit/s

� Changes of PHY and MAC with the goal of 100Mbit/s at MAC SAP� MIMO antennas (Multiple Input Multiple Output), up to 600Mbit/s are currently feasible� However, still a large overhead due to protocol headers and inefficient mechanisms

� 802.11p: Inter car communications� Communication between cars/road side and cars/cars� Planned for relative speeds of min. 200km/h and ranges over 1000m� Usage of 5.850-5.925GHz band in North America

� 802.11r: Faster Handover between BSS� Secure, fast handover of a station from one AP to another within an ESS� Current mechanisms (even newer standards like 802.11i) plus incompatible devices from different

vendors are massive problems for the use of, e.g., VoIP in WLANs� Handover should be feasible within 50ms in order to support multimedia applications efficiently

65

WLAN: IEEE 802.11– current developments

� 802.11s: Mesh Networking� Design of a self-configuring Wireless Distribution System (WDS) based on 802.11� Support of point-to-point and broadcast communication across several hops

� 802.11T: Performance evaluation of 802.11 networks� Standardization of performance measurement schemes

� 802.11u: Interworking with additional external networks� 802.11v: Network management

� Extensions of current management functions, channel measurements� Definition of a unified interface

� 802.11w: Securing of network control� Classical standards like 802.11, but also 802.11i protect only data frames, not the control

frames. Thus, this standard should extend 802.11i in a way that, e.g., no control frames can be forged.

� 802.11y: Extensions for the 3650-3700 MHz band in the USA� 802.11z: Extension to direct link setup� 802.11aa: Robust audio/video stream transport� 802.11ac: Very High Throughput <6Ghz� 802.11ad: Very High Throughput in 60 GHz� 802.11af: TV white space, ah: sub 1GHz, ai: fast initial link set-up; … aq: pre-

association discovery

� Note: Not all “standards” will end in products, many ideas get stuck at working group level

� Info: www.ieee802.org/11/, 802wirelessworld.com, standards.ieee.org/getieee802/

66

IEEE 802.11ac – the new high-speed standard for WLANs

� Final approval scheduled for 2014, several devices available (based on draft versions)

� Single link troughput > 500Mbit/s, multi-station > 1 Gbit/s

� Bandwidth up to 160 MHz (80 MHz mandatory), up to 8x MIMO, up to 256 QAM, beamforming, SDMA via MIMO

� Example home configuration:

� 8-antenna access point, 160 MHz bandwidth, 6.77 Gbit/s

� 4-antenna digital TV, 3.39 Gbit/s

� 2-antenna tablet, 1.69 Gbit/s

� Two 1-antenna smartphones, 867 Mbit/s each