Wireless & Mobile Communications Chapter 5: Wireless LANs
Characteristics IEEE 802.11
PHY MAC Roaming
Bluetooth
Winter 2001ICS 243E - Ch 5 Wireless Lans 5.2
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)
many proprietary solutions, especially for higher bit-rates, standards take their time (e.g. IEEE 802.11)
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
Winter 2001ICS 243E - Ch 5 Wireless Lans 5.3
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
Winter 2001ICS 243E - Ch 5 Wireless Lans 5.4
Comparison: infrared vs. radio transmission
Infrared uses IR diodes, diffuse light,
multiple reflections (walls, furniture etc.)
Advantages simple, cheap, available in
many mobile devices no licenses needed simple shielding possible
Disadvantages interference by sunlight, heat
sources etc. many things shield or absorb
IR light low bandwidth
Example IrDA (Infrared Data
Association) interface available everywhere
Radio typically using the license
free ISM band at 2.4 GHz Advantages
experience from wireless WAN and mobile phones can be used
coverage of larger areas possible (radio can penetrate walls, furniture etc.)
Disadvantages very limited license free
frequency bands shielding more difficult,
interference with other electrical devices
Example WaveLAN, HIPERLAN,
Bluetooth
Winter 2001ICS 243E - Ch 5 Wireless Lans 5.5
Comparison: infrastructure vs. ad-hoc networks
infrastructure network
ad-hoc network
APAP
AP
wired network
AP: Access Point
Winter 2001ICS 243E - Ch 5 Wireless Lans 5.6
Distribution System
Portal
802.x LAN
Access Point
802.11 LAN
BSS2
802.11 LAN
BSS1
Access Point
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 frequencyAccess 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
STA1
STA2 STA3
ESS
Winter 2001ICS 243E - Ch 5 Wireless Lans 5.7
802.11 - Architecture of an ad-hoc network
Direct communication within a limited range
Station (STA):terminal with access mechanisms to the wireless medium
Basic Service Set (BSS):group of stations using the same radio frequency
802.11 LAN
BSS2
802.11 LAN
BSS1
STA1
STA4
STA5
STA2
STA3
Winter 2001ICS 243E - Ch 5 Wireless Lans 5.8
IEEE standard 802.11
mobile terminal
access point
server
fixed terminal
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
infrastructure network
LLC LLC
Winter 2001ICS 243E - Ch 5 Wireless Lans 5.9
802.11 - Layers and functions
PLCP Physical Layer Convergence Protocol
clear channel assessment signal (carrier sense)
PMD Physical Medium Dependent
modulation, codingPHY Management
channel selection, MIBStation 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
Winter 2001ICS 243E - Ch 5 Wireless Lans 5.10
802.11 - Physical layer
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, synchonization
Winter 2001ICS 243E - Ch 5 Wireless Lans 5.11
FHSS PHY packet format
synchronization SFD PLW PSF HEC payload
PLCP preamble PLCP header
80 16 12 4 16 variable bits
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
Winter 2001ICS 243E - Ch 5 Wireless Lans 5.12
DSSS PHY packet format
synchronization SFD signal service HEC payload
PLCP preamble PLCP header
128 16 8 8 16 variable bits
length
16
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 Length
future use, 00: 802.11 compliant length of the payload HEC (Header Error Check)
protection of signal, service and length, x16+x12+x5+1
Winter 2001ICS 243E - Ch 5 Wireless Lans 5.13
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
Winter 2001ICS 243E - Ch 5 Wireless Lans 5.14
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 busySIFS
PIFS
DIFSDIFS
next framecontention
direct access if medium is free DIFS
Winter 2001ICS 243E - Ch 5 Wireless Lans 5.15
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 timedirect access if medium is free DIFS
Winter 2001ICS 243E - Ch 5 Wireless Lans 5.16
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
Winter 2001ICS 243E - Ch 5 Wireless Lans 5.17
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
transmission errors
t
SIFS
DIFS
data
ACK
waiting time
otherstations
receiver
senderdata
DIFS
contention
Winter 2001ICS 243E - Ch 5 Wireless Lans 5.18
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)
Winter 2001ICS 243E - Ch 5 Wireless Lans 5.19
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
Winter 2001ICS 243E - Ch 5 Wireless Lans 5.20
DFWMAC-PCF I
PIFS
stations‘NAV
wirelessstations
point coordinator
D1
U1
SIFS
NAV
SIFSD2
U2
SIFS
SIFS
SuperFramet0
medium busy
t1
Winter 2001ICS 243E - Ch 5 Wireless Lans 5.21
DFWMAC-PCF II
tstations‘NAV
wirelessstations
point coordinator
D3
NAV
PIFSD4
U4
SIFS
SIFSCFend
contentionperiod
contention free period
t2 t3 t4
Winter 2001ICS 243E - Ch 5 Wireless Lans 5.22
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
DurationID
Address1
Address2
Address3
SequenceControl
Address4
Data CRC
2 2 6 6 6 62 40-2312bytes
version, type, fragmentation, security, ...
Winter 2001ICS 243E - Ch 5 Wireless Lans 5.23
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
Winter 2001ICS 243E - Ch 5 Wireless Lans 5.24
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
Winter 2001ICS 243E - Ch 5 Wireless Lans 5.25
Synchronization using a Beacon (infrastructure)
beacon interval
tmedium
accesspoint
busy
B
busy busy busy
B B B
value of the timestamp B beacon frame
Winter 2001ICS 243E - Ch 5 Wireless Lans 5.26
Synchronization using a Beacon (ad-hoc)
tmedium
station1
busy
B1
beacon interval
busy busy busy
B1
value of the timestamp B beacon frame
station2
B2 B2
random delay
Winter 2001ICS 243E - Ch 5 Wireless Lans 5.27
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?)
Winter 2001ICS 243E - Ch 5 Wireless Lans 5.28
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
Winter 2001ICS 243E - Ch 5 Wireless Lans 5.29
Power saving with wake-up patterns (ad-hoc)
awake
A transmit ATIM D transmit data
t
station1
B1 B1
B beacon frame
station2
B2 B2
random delay
A
a
D
d
ATIMwindow beacon interval
a acknowledge ATIM d acknowledge data
Winter 2001ICS 243E - Ch 5 Wireless Lans 5.30
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
Winter 2001ICS 243E - Ch 5 Wireless Lans 5.31
Future developments
IEEE 802.11a compatible MAC, but now 5 GHz band transmission rates up to 20 Mbit/s close cooperation with BRAN (ETSI Broadband Radio Access
Network) IEEE 802.11b
higher data rates at 2.4 GHz proprietary solutions already offer 10 Mbit/s
IEEE WPAN (Wireless Personal Area Networks) market potential compatibility low cost/power, small form factor technical/economic feasibility
Bluetooth
Winter 2001ICS 243E - Ch 5 Wireless Lans 5.32
Bluetooth
Winter 2001ICS 243E - Ch 5 Wireless Lans 5.33
History
Winter 2001ICS 243E - Ch 5 Wireless Lans 5.34
Usage Modes
Data/Voice Access Points
Personal Ad-hoc Networks
Cable Replacement
Winter 2001ICS 243E - Ch 5 Wireless Lans 5.35
Bluetooth
Consortium: Ericsson, Intel, IBM, Nokia, Toshiba - many members Scenarios
connection of peripheral devicesloudspeaker, joystick, headset
support of ad-hoc networkingsmall devices, low-cost
bridging of networkse.g., GSM via mobile phone - Bluetooth - laptop
Simple, cheap, replacement of IrDA, low range, lower data rates 2.4 GHz, FHSS (CDMA), TDD
Winter 2001ICS 243E - Ch 5 Wireless Lans 5.36
Technology Characteristics
Low-cost,
Low-power,
Small-sized,
Short-range,
Robust wireless technology
Winter 2001ICS 243E - Ch 5 Wireless Lans 5.37
General Characteristics
Universal wireless interface
Ad-hoc networking architecture
80 Mhz in unlicenced ISM band at 2.45 Ghz
Gross bitrate 1 Mbps
Simultaneous voice and high speed data support
Evolves from cable replacement - > networking solution
Winter 2001ICS 243E - Ch 5 Wireless Lans 5.38
Protocol Architecture
Bluetooth protocol architecture
Core
Adaptation
StandardApplication
TCP/IP
NAL
L2CAP
Link Manager
Baseband
Radio
Application
TCP/IP
NAL
L2CAP
Link Manager
Baseband
Radio
Host
Host Controller
Host Controller if
Winter 2001ICS 243E - Ch 5 Wireless Lans 5.39
Baseband
2.4 GHz, FHSS (CDMA), TDD
time
Forward
Reverse
fn+2
fn+1
fn
Slot 0.625 msec.
TDD frame 1.25 msec.
time
Forward
Reverse
fn+2
fn+1
fn
Slot 0.625 msec.
TDD frame 1.25 msec.
Winter 2001ICS 243E - Ch 5 Wireless Lans 5.40
FHSS Pattern: Synchronization
Master
Slave
Slave
SlaveSlave
Piconet
The slaves synchronize themselves to the Master Clock
Winter 2001ICS 243E - Ch 5 Wireless Lans 5.41
Frequencsy Selection
Native clock (slave)
Offset (master)
+
Phase
Bleutooth address (master)
Hop frequency
Winter 2001ICS 243E - Ch 5 Wireless Lans 5.42
Piconets and Scatternets
M1
S1
S3
S2
S4
M2
S6
S5 S7
Piconet A Piconet B
Scatternet
Winter 2001ICS 243E - Ch 5 Wireless Lans 5.43
FHSS
Time
Frequency
0
78
Winter 2001ICS 243E - Ch 5 Wireless Lans 5.44
Interference
Sources:
Other equipment in ISM band e.g. WLAN, micro-wave oven,
etc.
Frequency hopping
Remedies:
Frequency hopping
Short range
Power control
FEC and ARQN
Short packets and fast acknowledgements
Winter 2001ICS 243E - Ch 5 Wireless Lans 5.45
Error Control
1/3 rate Forward Error Correction (FEC)
2/3 rate Forward Error Correction (FEC)
Automatic Repeat reQuest (ARQ)
Winter 2001ICS 243E - Ch 5 Wireless Lans 5.46
Link Types
Synchronous Connection Oriented (SCO) Circuit switched typically used for voice Symmetric, synchronous service Slot reservation at fixed intervals Point-to-point
Asynchronous Connectionless Link (ACL) Packet switched Symmetric or asymmetric, asynchronous service Polling mechanism between master and slave(s) Point-to-point and point-to-multipoint
Winter 2001ICS 243E - Ch 5 Wireless Lans 5.47
Flexibility: Multislot Packets
Single slot
Three slot
Five slot
fn fn+1 fn+2 fn+3 fn+4 fn+5
fn fn+5
fn fn+3
No frequency hopping during multislots, sequence continues after transmission
Winter 2001ICS 243E - Ch 5 Wireless Lans 5.48
ACL data rates
Asymmetric (kbps)Packet Timeslots CRC FEC Symmetric(kbps)
Forward Reverse
DM1 1 Yes Yes 108 108 108
DH1 1 Yes - 172 172 172
DM3 3 Yes Yes 258 387 54
DH3 3 Yes - 390 585 86
DM5 5 Yes Yes 286 477 36
DH5 5 Yes - 433 723 57
AUX 1 - - 185 185 185
Winter 2001ICS 243E - Ch 5 Wireless Lans 5.49
ACL Polling Scheme:
Master
Winter 2001ICS 243E - Ch 5 Wireless Lans 5.50
SCO data rates
Packet Timeslots CRC FEC Symmetric(kbps)
HV1 1 - 1/3 rate 64
HV2 1 - 2/3 rate 64
HV3 1 - - 64
DV 1 Data only Voice no FEC,Data 2/3 FEC
64
Winter 2001ICS 243E - Ch 5 Wireless Lans 5.51
Voice and Data Transmission:
Master
Slave
Slave
Slave
SCO ACL ACL SCO SCO ACLACLACL
Winter 2001ICS 243E - Ch 5 Wireless Lans 5.52
Capacity of Piconet
One ACL link (432 kbps symmetric or 721/56 kbps asymetric)
or
Three simultaneous SCO links (64 kbps)
or
A combination of voice/data
M1
S1
S2
S4
S3
Winter 2001ICS 243E - Ch 5 Wireless Lans 5.53
States of a Bluetooth device (PHY layer)
STANDBY
inquiry page
connectedtransmit
PARK HOLD SNIFF
unconnected
connecting
active
low power
Winter 2001ICS 243E - Ch 5 Wireless Lans 5.54
Bluetooth MAC layer
access code packet header payload
72 54 0-2745 bits
MAC address type flow ARQN SEQN HEC
3 4 1 1 1 8 bits
Synchronous Connection-Oriented link (SCO) symmetrical, circuit switched, point-to-point
Asynchronous Connectionless Link (ACL) packet switched, point-to-multipoint, master polls
Access code synchronization, derived from master, unique per channel
Packet header 1/3-FEC, MAC address (1 master, 7 slaves), link type,
alternating bit ARQ/SEQ, checksum
Winter 2001ICS 243E - Ch 5 Wireless Lans 5.55
Error Control Details
Packet header: 1/3 rate FEC
SCO payload: 1/3 rate FEC 2/3 FEC no FEC
ACL payload: ARQN using payload CRC (except AUX packet) 2/3 rate FEC optional
Winter 2001ICS 243E - Ch 5 Wireless Lans 5.56
Connection Set up in an existing Piconet
Master
Slave
Page Page
Resp.
FHS
Resp.
data
data.
Winter 2001ICS 243E - Ch 5 Wireless Lans 5.57
Scatternets
piconets
Each piconet has one master and up to 7 slaves Master determines hopping sequence, slaves have to
synchronize Participation in a piconet = synchronization to hopping
sequence Communication between piconets = devices jumping back
and forth between the piconets
Winter 2001ICS 243E - Ch 5 Wireless Lans 5.58
Summary
Bluetooth: Wireless voice and data for mobile devices
IrDA: Wirelss data cable replacement for devices in line of sight
HomeRF: Networking mobile data and voice devices to a PC anywhere in
the home IEEE 802.11 and Hiperlan 2:
Wireless enterprise networking in the office
Winter 2001ICS 243E - Ch 5 Wireless Lans 5.59
Where Bluetooth fits in
Coverage
Ban
dwid
th
WLAN
BluetoothGSM
DECTWLANIrDA
GPRSUMTS
GPRS
GSMDectBlu
etoo
thIR
UMTS
Winter 2001ICS 243E - Ch 5 Wireless Lans 5.60
Bluetooth functionality