Post on 27-Oct-2019
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