Wireless Communication Protocols and
Technologies
by Tatiana Madsen & Hans Peter Schwefel
•
Mm1
Introduction. Wireless LANs (TKM)
•
Mm2
Wireless Personal Area Networks and Bluetooth (TKM)
•
Mm3
IP Mobility Support (HPS)
•
Mm4
Ad hoc Networks (TKM)
•
Mm5
Overview of GSM, GPRS, UMTS (HPS)
WCPT Spring 2004
Page 1
Tatiana K. Madsen
Hans Peter Schwefel
Course material
• Download slides www.kom.auc.dk/~tatiana
• Books
– Jochen Schiller, ”Mobile Communications”, Addison-Wesley, 1st
edition 2000, 2nd edition
WCPT Spring 2004
Page 2
Tatiana K. Madsen
Hans Peter Schwefel
Mobile vs Wireless
user mobility: users
communicate (wireless)
“anytime, anywhere, with
anyone”
Wireless
Mobile
device portability: devices
can be connected anytime,
anywhere to the network
• Mobile vs Stationary
• Wireless vs Wired
WCPT Spring 2004
Page 3
Tatiana K. Madsen
Hans Peter Schwefel
Mobile vs Wireless
Wireless vs. mobile




WCPT Spring 2004




Examples
stationary computer
notebook in a hotel
wireless LANs in historic buildings
Personal Digital Assistant (PDA)
Page 4
Tatiana K. Madsen
Hans Peter Schwefel
Outline
•
•
•
•
Introduction - Historical perspectives
Properties of wireless medium
Basic MAC protocols for wireless communication
WLAN
– IEEE 802.11 standard
• Additional information
• http://grouper.ieee.org/groups/802/11/
• B. Crow et al, “IEEE 802.11 Wireless Local Area Networks”, IEEE Comm.
Magazine, September 1997
WCPT Spring 2004
Page 5
Tatiana K. Madsen
Hans Peter Schwefel
Chronological list of events of wireless systems
1860s
J.C. Maxwell postulates electromagnetic waves
1880s
H.R. Hertz provides proof of electromagnetic waves
1895
G. Marconi demonstrates wireless communication and applies for patent
1913
Establishment of marine radio telegraphy
1921
Detroit police conducts field trials with mobile radio
1946
Bell Lab. deploys first commercial mobile radio telephone system
1950
Microwave links are developed
1980s
Wide deployment of analog cellular systems
1992
Introduction of 2nd generation digital cellular systems
1993
Introduction of multiservices capabilities in the 2nd generation systems
2000
Third generation cellular systems with multimedia capabilities are introduced
2003
Start of commercial deployment of 3rd generation systems
Source: B. Furht, Handbook of Internet and Multimedia Systems and Applications. IEEE Press, 1999
WCPT Spring 2004
Page 6
Tatiana K. Madsen
Hans Peter Schwefel
Key problems
• Key problem - Path loss
– Advent of the electron tube amplifier (de Forest, 1915)
• Key problem – Thermal noise
– Claude Shannon, ”A mathematical theory of communication”, 1949
– Advent of the Large Scale Integrated (LSI) circuits and Digital Signal
Processing (DSP)
• Key problem – The limited spectrum
– Only one ”ether, unwanted interference between different users”
– International Telecommunication Union (ITU) deals with these
problems
WCPT Spring 2004
Page 7
Tatiana K. Madsen
Hans Peter Schwefel
Development of mobile communications
Mobile Communications Networks
First Generation
•
•
•
•
Analogue
Basic voice telephony
Low capacity
Limited local and
regional coverage
• E.g. NMT, AMPS,
TACS, C-net
Second Generation
• Digital:
Third Generation
• Digital:
– Circuit switched
• Voice plus basic data
applications:
– Fax
– SMS (small message
services)
– Circuit-switched data
• Low data speed
• Regional coverage,
with trans-national
roaming
• E.g. GSM, D-AMPS,
PDC, IS 95 CDMA
– Packet and circuit
switched
• Advanced data — i.e.
multimedia
applications
• Fast data access
• Global coverage
• E.g. UMTS (WCDMA,
TD/CDMA), IMT-2000
Wireless data already be introduced in second generation mobile
Source: “Mobile data feels pressure from the need for speed”, Network News, 2 June 1998 (CAP Gemini, September 1999)
WCPT Spring 2004
Page 8
Tatiana K. Madsen
Hans Peter Schwefel
Mobility & Range
High Speed
Vehicular
Rural
GSM
Vehicular
Urban
Possible UMTS extension
for high speed data access
with roaming capability
UMTS
Fixed urban
Pedestrian
Indoor
Personal Area
IEEE 802.11a/b
(WLAN),
Hiperlan2,MMAC
DECT
BRAN
Bluetooth
0.5
B-PAN
PAN
2
10
20
155
1000 Mb/s
Total data rate per cell
WCPT Spring 2004
Page 9
Tatiana K. Madsen
Hans Peter Schwefel
Mobile phones per 100 people 1999
2002: 50-70% penetration in Western Europe
Germany
Greece
Spain
Belgium
France
Netherlands
Great Britain
Switzerland
Ireland
Austria
Portugal
Luxemburg
Italy
Denmark
Norway
Sweden
Finland
0
WCPT Spring 2004
10
20
30
40
Page 10
50
60
Tatiana K. Madsen
Hans Peter Schwefel
Worldwide wireless subscribers
700
600
http://www.3g.co.uk
500
Americas
Europe
400
Japan
300
others
total
200
100
0
1996
WCPT Spring 2004
1997
1998
1999
2000
2001
Page 11
Tatiana K. Madsen
Hans Peter Schwefel
Simple reference model
Application
Application
Transport
Transport
Network
Network
Network
Network
Data Link
Data Link
Data Link
Data Link
Physical
Physical
Physical
Physical
Medium
Radio
WCPT Spring 2004
Page 12
Tatiana K. Madsen
Hans Peter Schwefel
Signal propagation ranges
•
•
•
Transmission range
– communication possible
– low error rate
Detection range
– detection of the signal
possible
– no communication
possible
Interference range
– signal may not be
detected
– signal adds to the
background noise
WCPT Spring 2004
sender
transmission
distance
detection
interference
Broadcast nature of channel
Page 13
Tatiana K. Madsen
Hans Peter Schwefel
Signal propagation
•
•
•
•
•
•
•
•
•
Propagation in free space always like light (straight line)
Receiving power proportional to 1/d²
(d = distance between sender and receiver)
Receiving power additionally influenced by
fading (frequency dependent)
shadowing
reflection at large obstacles
refraction depending on the density of a medium
scattering at small obstacles
diffraction at edges
shadowing
WCPT Spring 2004
reflection
refraction
Page 14
scattering
diffraction
Tatiana K. Madsen
Hans Peter Schwefel
Multipath propagation
• Signal can take many different paths between sender and receiver due to
reflection, scattering, diffraction
LOS pulses
multipath
pulses
signal at sender
signal at receiver
• Time dispersion: signal is dispersed over time
 interference with “neighbor” symbols, Inter Symbol Interference (ISI)
• The signal reaches a receiver directly and phase shifted
 distorted signal depending on the phases of the different parts
WCPT Spring 2004
Page 15
Tatiana K. Madsen
Hans Peter Schwefel
Wireless medium
• Time varying channel
– Radio signals propagate according to reflection, diffraction and scattering
– The received signal power attenuates as
– Multipath propagation
– Fading
1
,   const ,   2 for free space

r
• Burst channel errors
• Broadcast nature of channel
• Half-duplex operation
WCPT Spring 2004
Page 16
Tatiana K. Madsen
Hans Peter Schwefel
Wireless networks in comparison to fixed
networks
• Higher loss-rates due to interference
– emissions of, e.g., engines, lightning
• Restrictive regulations of frequencies
– frequencies have to be coordinated, useful frequencies are almost all
occupied
• Low transmission rates
• Higher delays, higher jitter
• Lower security, simpler active attacking
– radio interface accessible for everyone, base station can be
simulated, thus attracting calls from mobile phones
• Always shared medium
– secure access mechanisms important
WCPT Spring 2004
Page 17
Tatiana K. Madsen
Hans Peter Schwefel
Hidden and exposed terminals
• Hidden terminals
– A sends to B, C cannot receive A
– C wants to send to B, C senses a “free” medium
– collision at B, A cannot receive the collision
– A is “hidden” for C
A
B
C
• Exposed terminals
– B sends to A, C wants to send to another terminal (not A or B)
– C has to wait, CS signals a medium in use
– but A is outside the radio range of C, therefore waiting is not
necessary
– C is “exposed” to B
WCPT Spring 2004
Page 18
Tatiana K. Madsen
Hans Peter Schwefel
Near and far terminals
• Terminals A and B send, C receives
– signal strength decreases proportional to the square of the distance
– the signal of terminal B therefore drowns out A’s signal
– C cannot receive A
A
B
C
• If C for example was an arbiter for sending rights, terminal B would
drown out terminal A already on the physical layer
• Also severe problem for CDMA-networks - precise power control needed!
WCPT Spring 2004
Page 19
Tatiana K. Madsen
Hans Peter Schwefel
Classification of Wireless MAC protocols
MAC Protocols
Fixed
assignment
TDMA
CDMA
FDMA
WCPT Spring 2004
Demand
assignment
Random
assignment
s-ALOHA
ALOHA
GAMA
Token
CSMA
FAMA
Polling
Page 20
Tatiana K. Madsen
Hans Peter Schwefel
Access methods SDMA/FDMA/TDMA
• SDMA (Space Division Multiple Access)
– segment space into sectors, use directed antennas
– cell structure
• FDMA (Frequency Division Multiple Access)
– assign a certain frequency to a transmission channel between a
sender and a receiver
– permanent (e.g., radio broadcast), slow hopping (e.g., GSM), fast
hopping (FHSS, Frequency Hopping Spread Spectrum)
• TDMA (Time Division Multiple Access)
– assign the fixed sending frequency to a transmission channel
between a sender and a receiver for a certain amount of time
WCPT Spring 2004
Page 21
Tatiana K. Madsen
Hans Peter Schwefel
Random Access - Aloha
User 1
User 2
packet 1
Base
station
packet 2
successful
transmission
t0
t0+tp1
• Unslotted Aloha
• Slotted Aloha
WCPT Spring 2004
packet 1
rescheduled
packet 1
collision
t1
packet 2
rescheduled
successful
transmission
successful
transmission
time
t1+tp2
S  G exp(2G )
S  G exp(G )
Page 22
Tatiana K. Madsen
Hans Peter Schwefel
Throughput curves of Aloha
WCPT Spring 2004
Page 23
Tatiana K. Madsen
Hans Peter Schwefel
Carrier Sense Multiple Access (CSMA)
• Aloha schemes  “impolite” behavior
• CSMA  “listen before talk”
– Process of listening to the channel is not demanding
– Carrier sensing does not relieve us from collisions
– Variations of CSMA are due to behavior of users when the channel is
busy
• Non-persistent
• 1-persistent
• p-persistent
WCPT Spring 2004
Page 24
Tatiana K. Madsen
Hans Peter Schwefel
Non-persistent CSMA
• If the channel is busy, a terminal refrains from transmitting a packet and
behaves exactly as if the packet collided.
a
a
a
T
• a - vulnerable period
WCPT Spring 2004
Page 25
Tatiana K. Madsen
Hans Peter Schwefel
1-persistent CSMA
• Non-persistent CSMA: there are situations when the channel is idle,
although one or more users have packets to transmit.
• 1-persistent: if the channel is idle, the user waits and transmits as soon
as the channel becomes idle.
WCPT Spring 2004
Page 26
Tatiana K. Madsen
Hans Peter Schwefel
Slotted systems
• The wireless channel is said to be slotted if transmission attempts can
take place at discrete instance in time.
• A slotted system requires network-wide time synchronization
– in centralized network BS is used as a reference
– in distributed networks it is more difficult
• slotted non-persistent and 1-persistent CSMA
WCPT Spring 2004
Page 27
Tatiana K. Madsen
Hans Peter Schwefel
Throughput curves
WCPT Spring 2004
Page 28
Tatiana K. Madsen
Hans Peter Schwefel
CSMA with Collision Detection
• Whenever the transmission of two or more packets overlap in time, all
packets are lost and must be retransmitted
• In some local area networks (such as Ethernet) users can detect
interference among several transmission (including their own) while
transmission is in progress
• If a collision is detected during transmission, the transmission is aborted.
• Consensus reenforcement procedure
• Transmission period in the case of collision:
  2   cd   cr
WCPT Spring 2004
Page 29
Tatiana K. Madsen
Hans Peter Schwefel
Comparison of Throughput-Load curves
WCPT Spring 2004
Page 30
Tatiana K. Madsen
Hans Peter Schwefel
Collision detection in radio systems
• Wire: transmitted and received signals are of the same order of
magnitude
• Wireless: the received signal is considerably weak compared with the
transmitted
•  in radio systems CD is usually not implemented
•  ACK is required
WCPT Spring 2004
Page 31
Tatiana K. Madsen
Hans Peter Schwefel
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
Disadvantages - users expect the same services and capabilities
– 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
WCPT Spring 2004
Page 32
Tatiana K. Madsen
Hans Peter Schwefel
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)
– existing applications should work
WCPT Spring 2004
Page 33
Tatiana K. Madsen
Hans Peter Schwefel
IEEE 802.11 standard
•
•
•
•
802.3 Ethernet
802.5 Token ring
802.11 WLAN
802.15 WPAN
IEEE=Institute of Electrical and
Electronics Engineers
• Standards specify PHY and MAC, but offers the same interface to higher
layers to maintain interoperability
access point
application
application
TCP
TCP
IP
IP
LLC
LLC
LLC
802.11 MAC
802.11 MAC
802.3 MAC
802.3 MAC
802.11 PHY
802.11 PHY
802.3 PHY
802.3 PHY
WCPT Spring 2004
Page 34
Tatiana K. Madsen
Hans Peter Schwefel
802.11 - Layers and functions
PHY
DLC
•
•
MAC
–access mechanisms, fragmentation,
encryption
MAC Management
–association, re-association of a station to
an AP, roaming
–authentication, encryption
–synchronization, power management
WCPT Spring 2004
LLC
MAC
MAC Management
PLCP
PHY Management
PMD
•
•
•
Station Management
•
Page 35
PLCP Physical Layer Convergence Protocol
–clear channel assessment
signal (carrier sense)
PMD Physical Medium Dependent sublayer
–modulation, coding of signals
PHY Management
–channel selection
Station Management
–coordination of all
management functions
Tatiana K. Madsen
Hans Peter Schwefel
System architecture
802.11 - Architecture of an infrastructure network
802.11 LAN
802.x LAN
STA1
BSS1
Portal
Access
Point
Distribution System
Access
Point
ESS
BSS2
STA2
WCPT Spring 2004
802.11 LAN
STA3
•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
Page 36
Tatiana K. Madsen
Hans Peter Schwefel
802.11 - Architecture of an ad-hoc network
802.11 LAN
STA1
•
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
STA3
IBSS1
STA2
IBSS2
STA5
STA4
802.11 LAN
WCPT Spring 2004
Page 37
Tatiana K. Madsen
Hans Peter Schwefel
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)
– separate different networks by using different hopping sequences
– 79 hopping channels; 3 different sets with 26 hopping sequences per set
DSSS (Direct Sequence Spread Spectrum)
– method using separation by code
– 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, indoor
– carrier detection, synchronization
WCPT Spring 2004
Page 38
Tatiana K. Madsen
Hans Peter Schwefel
IEEE 802.11 MAC
• 802.11 supports 2 different fundamental MAC schemes:
• The Distributed Coordination Function (DCF): all users have to contend
for accessing the channel. This is an implementation of ad hoc networks.
• The Point Coordination Function (PCF): is based on polling and is
performed by an AP inside the BSS. In the IEEE 802.11 implementation
of PCF is optionally.
• The PCF is required to coexist with the DCF: when the PCF is available
in a network, there still is a portion of the time allocated to the DCF.
WCPT Spring 2004
Page 39
Tatiana K. Madsen
Hans Peter Schwefel
Access methods
– DFWMAC-DCF CSMA/CA (mandatory) – basic access method
• 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) – handshaking access
method
• avoids hidden terminal problem
– DFWMAC- PCF (optional)
• access point polls terminals according to a list
WCPT Spring 2004
Page 40
Tatiana K. Madsen
Hans Peter Schwefel
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)
WCPT Spring 2004
Page 41
Tatiana K. Madsen
Hans Peter Schwefel
Carrier Sensing
• Carrier sensing is performed at both the air interface, reffered to as
physical carrier sensing, and at the MAC sublayer, reffered to as virtual
carrier sensing.
• Physical c.s. detects activity in the channel via relative signal strength
from other sources
• Virtual c.s. - from header information of frames. The duration field
indicates the amount of time (in microseconds) after the end of the
present frame the channel will be utilized. This time is used to adjust
network allocation vector (NAV).
• The channel is marked busy if one of the c.s. indicate the channel is
busy.
WCPT Spring 2004
Page 42
Tatiana K. Madsen
Hans Peter Schwefel
Priorities
•
Priorities
– priority access to the channel is controlled through the use of interframe
space - mandatory periods of idle time.
– 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
DIFS
DIFS
medium busy
PIFS
SIFS
contention
t
direct access if
medium is free  DIFS
WCPT Spring 2004
next frame
Page 43
Tatiana K. Madsen
Hans Peter Schwefel
Random backoff time mechanism
•
•
•
•
After DIFS period, a station computes a random backoff time
time is slotted to Slot_Time - to define IFS and backoff time
the r.b. Is an integer value that corresponds to a number of time slots
initially it is 0-7
– if the timer reached zero and medium is idle --> transmit
– if the medium becomes busy --> freeze the timer
– if collision --> new backoff time 0-15
• the idle period after DIFS is called contention window
• this method promotes fairness
WCPT Spring 2004
Page 44
Tatiana K. Madsen
Hans Peter Schwefel
802.11 - CSMA/CA basic access method
DIFS
contention window
(randomized back-off
mechanism)
DIFS
medium busy
next frame
t
direct access if
medium is free  DIFS
slot time
– 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)
WCPT Spring 2004
Page 45
Tatiana K. Madsen
Hans Peter Schwefel
DFWMAC
• Network Allocation Vector (NAV) is time field that indicates the duration of
the current transmission
• Backoff procedure is used to randomized access to the channel
Medium busy
sender
DIFS
RTS
data
SIFS
receiver
other
stations
CTS SIFS
NAV (RTS)
NAV (CTS)
defer access
WCPT Spring 2004
SIFS
Page 46
ACK
DIFS
t
Tatiana K. Madsen
Hans Peter Schwefel
Trade-offs with RTS/CTS
+ Collisons are avoided
+ Hidden station problem is solved
– Bandwidth reduction
– Not with multicast and broadcast
Usage
With large frames
When collisions are likely
WCPT Spring 2004
Page 47
Tatiana K. Madsen
Hans Peter Schwefel
Fragmentation
•
•
•
Large frames handed down from the LLC to the MAC may require fragmentation
to increase transmission reliability
Fragmentation_threshold
the channel is not released until the whole frame is transmitted successfully or
the source fails to receive ACK for a fragment.
DIFS
sender
RTS
frag1
SIFS
receiver
CTS SIFS
frag2
SIFS
ACK1 SIFS
SIFS
ACK2
NAV (RTS)
NAV (CTS)
NAV (frag1)
NAV (ACK1)
other
stations
DIFS
contention
WCPT Spring 2004
Page 48
data
t
Tatiana K. Madsen
Hans Peter Schwefel
DFWMAC-PCF
• The beginning of a super frame is indicated by a beacon transmitted by
AP. (synchronization)
• the minimum duration of PCF period is time required to send 2 frames +
overhead + PCF-end-frame
• the maximum duration - time must be allotted for at least one frame to be
transmitted during DCF period
t0 t1
medium busy
point
coordinator
wireless
stations
stations‘
NAV
WCPT Spring 2004
PIFS
SuperFrame
SIFS
D1
SIFS
D2
SIFS
SIFS
U1
U2
NAV
Page 49
Tatiana K. Madsen
Hans Peter Schwefel
DFWMAC-PCF
t2
point
coordinator
D3
PIFS
SIFS
D4
WCPT Spring 2004
t4
CFend
SIFS
U4
wireless
stations
stations‘
NAV
t3
NAV
contention free period
Page 50
contention
period
t
Tatiana K. Madsen
Hans Peter Schwefel
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
WCPT Spring 2004
Page 51
Tatiana K. Madsen
Hans Peter Schwefel
Synchronization using a Beacon
(infrastructure)
beacon interval
access
point
medium
B
B
busy
busy
B
busy
B
busy
t
value of the timestamp
WCPT Spring 2004
B
beacon frame
Page 52
Tatiana K. Madsen
Hans Peter Schwefel
Synchronization using a Beacon (adhoc)
beacon interval
station1
B1
B1
B2
station2
medium
busy
busy
B2
busy
busy
t
value of the timestamp
WCPT Spring 2004
B
beacon frame
Page 53
random delay
Tatiana K. Madsen
Hans Peter Schwefel
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?)
WCPT Spring 2004
Page 54
Tatiana K. Madsen
Hans Peter Schwefel
Power saving with wake-up patterns
(infrastructure)
TIM interval
access
point
DTIM interval
D B
T
busy
medium
busy
T
d
D B
busy
busy
p
station
d
t
WCPT Spring 2004
T
TIM
D
B
broadcast/multicast
DTIM
awake
p PS poll
Page 55
d data transmission
to/from the station
Tatiana K. Madsen
Hans Peter Schwefel
Power saving with wake-up patterns
(ad-hoc)
ATIM
window
station1
beacon interval
B1
station2
A
B2
B2
D
a
B1
d
t
B
beacon frame
awake
WCPT Spring 2004
A transmit ATIM
random delay
a acknowledge ATIM
D transmit data
d acknowledge data
Page 56
Tatiana K. Madsen
Hans Peter Schwefel
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
WCPT Spring 2004
Page 57
Tatiana K. Madsen
Hans Peter Schwefel
IEEE 802.11 Authentication
Open System Authentication
Authentication request (Open
System Authentication)
Authentication response
WCPT Spring 2004
Page 58
Tatiana K. Madsen
Hans Peter Schwefel
IEEE 802.11 Authentication
Shared Key Authentication
Authentication request
(Shared Key Authentication)
“challenge” text string
WEP encryption
of challenge text
“challenge” text string,
encrypted with shared key
Positive or Negative response
based on decryption result
WCPT Spring 2004
Page 59
WEP decryption
of encrypted text
Tatiana K. Madsen
Hans Peter Schwefel
IEEE 802.11 a and b
IEEE 802.11b
IEEE 802.11a
Time Table
Standard in 1997
Standard in 2001
Frequency Band and bandwidth
2.4 GHz
5 GHz
Speed
11 Mbps
54 Mbps
Modulation Techniques
Spread Spectrum
OFDM (Orthogonal
Division Multiplexing
Distance Coverage
Up to 100 meters
20 meters - speed goes down with
increased distance
Interoperability
Current problems
resolved in future
expected to be
Problems now but expect resolution
soon
Cost
Cheaper - $300 for access point and
$75 for adapter
More expensive - $500 in 01/2002 -
Interference with other devices
Band is more polluted - significant
interference here
Less interference because of few
devices in this band
WCPT Spring 2004
Page 60
Frequency
Tatiana K. Madsen
Hans Peter Schwefel
WLAN: IEEE 802.11 – future developments
•
•
•
•
•
•
•
802.11d: Regulatory Domain Update – completed
802.11e: MAC Enhancements – QoS – ongoing
– 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.
802.11f: Inter-Access Point Protocol – ongoing
– 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 – ongoing
802.11h: Spectrum Managed 802.11a (DCS, TPC) – ongoing
802.11i: Enhanced Security Mechanisms – ongoing
– Enhance the current 802.11 MAC to provide improvements in security.
Study Groups
– 5 GHz (harmonization ETSI/IEEE) – closed
– Radio Resource Measurements – started
– High Throughput – started
WCPT Spring 2004
Page 61
Tatiana K. Madsen
Hans Peter Schwefel