page 1 tatiana k. madsen hans peter schwefel wcpt spring 2004 wireless communication protocols and...
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Page 1Tatiana K. Madsen
Hans Peter SchwefelWCPT Spring 2004
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)
Page 2Tatiana K. Madsen
Hans Peter SchwefelWCPT Spring 2004
Course material
• Download slides www.kom.auc.dk/~tatiana• Books
– Jochen Schiller, ”Mobile Communications”, Addison-Wesley, 1st edition 2000, 2nd edition
Page 3Tatiana K. Madsen
Hans Peter SchwefelWCPT Spring 2004
Mobile vs Wireless
• Mobile vs Stationary• Wireless vs Wired
MobileWireless
user mobility: users communicate (wireless) “anytime, anywhere, with anyone”
device portability: devices can be connected anytime, anywhere to the network
Page 4Tatiana K. Madsen
Hans Peter SchwefelWCPT Spring 2004
Mobile vs Wireless
Wireless vs. mobile Examples stationary computer notebook in a hotel wireless LANs in historic buildings Personal Digital Assistant (PDA)
Page 5Tatiana K. Madsen
Hans Peter SchwefelWCPT Spring 2004
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
Page 6Tatiana K. Madsen
Hans Peter SchwefelWCPT Spring 2004
Chronological list of events of wireless systemsChronological list of events of wireless systems
Source: B. Furht, Handbook of Internet and Multimedia Systems and Applications. IEEE Press, 1999
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
Page 7Tatiana K. Madsen
Hans Peter SchwefelWCPT Spring 2004
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
Page 8Tatiana K. Madsen
Hans Peter SchwefelWCPT Spring 2004
Source: “Mobile data feels pressure from the need for speed”, Network News, 2 June 1998 (CAP Gemini, September 1999)
Mobile Communications Networks
First Generation
• Analogue• Basic voice telephony• Low capacity• Limited local and
regional coverage• E.g. NMT, AMPS,
TACS, C-net
• 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
• Digital:– Packet and circuit
switched
• Advanced data — i.e. multimedia applications
• Fast data access• Global coverage• E.g. UMTS (WCDMA,
TD/CDMA), IMT-2000
Second Generation Third Generation
Wireless data already be introduced in second generation mobile
Development of mobile communications
Page 9Tatiana K. Madsen
Hans Peter SchwefelWCPT Spring 2004
20 155
Indoor
Pedestrian
High SpeedVehicular
Rural
Mobility & Range
Personal Area
VehicularUrban
IEEE 802.11a/b(WLAN),
Hiperlan2,MMAC
0.5 2
UMTS
GS
M
DECT
Possible UMTS extension for high speed data access
with roaming capability
Fixed urban
Total data rate per cell
10
BRAN
B-PAN PANBluetooth
1000 Mb/s
Page 10Tatiana K. Madsen
Hans Peter SchwefelWCPT Spring 2004
Mobile phones per 100 people 1999
0 10 20 30 40 50 60
Finland
Sweden
Norway
Denmark
Italy
Luxemburg
Portugal
Austria
Ireland
Switzerland
Great Britain
Netherlands
France
Belgium
Spain
Greece
Germany
2002
: 50
-70%
pen
etra
tio
n in
Wes
tern
Eu
rop
e
Page 11Tatiana K. Madsen
Hans Peter SchwefelWCPT Spring 2004
Worldwide wireless subscribers
0
100
200
300
400
500
600
700
1996 1997 1998 1999 2000 2001
Americas
Europe
Japan
others
total
http://www.3g.co.uk
Page 12Tatiana K. Madsen
Hans Peter SchwefelWCPT Spring 2004
Simple reference model
Application
Transport
Network
Data Link
Physical
Medium
Data Link
Physical
Application
Transport
Network
Data Link
Physical
Data Link
Physical
Network Network
Radio
Page 13Tatiana K. Madsen
Hans Peter SchwefelWCPT Spring 2004
Signal propagation ranges
distance
sender
transmission
detection
interference
• 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
Broadcast nature of channel
Page 14Tatiana K. Madsen
Hans Peter SchwefelWCPT Spring 2004
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
reflection scattering diffractionshadowing refraction
Page 15Tatiana K. Madsen
Hans Peter SchwefelWCPT Spring 2004
• Signal can take many different paths between sender and receiver due to reflection, scattering, diffraction
• 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
Multipath propagation
signal at sendersignal at receiver
LOS pulsesmultipathpulses
Page 16Tatiana K. Madsen
Hans Peter SchwefelWCPT Spring 2004
Wireless medium
• Time varying channel– Radio signals propagate according to reflection, diffraction and scattering
– The received signal power attenuates as for free space– Multipath propagation– Fading
• Burst channel errors• Broadcast nature of channel
• Half-duplex operation
1, , 2const
r
Page 17Tatiana K. Madsen
Hans Peter SchwefelWCPT Spring 2004
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
Page 18Tatiana K. Madsen
Hans Peter SchwefelWCPT Spring 2004
• 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
• 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
Hidden and exposed terminals
BA C
Page 19Tatiana K. Madsen
Hans Peter SchwefelWCPT Spring 2004
• 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
• 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!
Near and far terminals
A B C
Page 20Tatiana K. Madsen
Hans Peter SchwefelWCPT Spring 2004
Classification of Wireless MAC protocols
Fixed assignment
Random assignment
Demandassignment
TDMA
FDMA
CDMA
MAC Protocols
ALOHA
CSMA
Token
Polling
GAMAs-ALOHA
FAMA
Page 21Tatiana K. Madsen
Hans Peter SchwefelWCPT Spring 2004
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
Page 22Tatiana K. Madsen
Hans Peter SchwefelWCPT Spring 2004
Random Access - Aloha
• Unslotted Aloha
• Slotted Aloha
packet 1 packet 2packet 2
rescheduled
packet 1packet 1
rescheduledUser 1
User 2
Basestation
successfultransmission
collisionsuccessful
transmissionsuccessful
transmission
timet0 t0+tp1 t1 t1+tp2
exp( 2 )S G G
exp( )S G G
Page 23Tatiana K. Madsen
Hans Peter SchwefelWCPT Spring 2004
Throughput curves of Aloha
Page 24Tatiana K. Madsen
Hans Peter SchwefelWCPT Spring 2004
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
Page 25Tatiana K. Madsen
Hans Peter SchwefelWCPT Spring 2004
Non-persistent CSMA
• If the channel is busy, a terminal refrains from transmitting a packet and behaves exactly as if the packet collided.
• a - vulnerable period
a a
T
a
Page 26Tatiana K. Madsen
Hans Peter SchwefelWCPT Spring 2004
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.
Page 27Tatiana K. Madsen
Hans Peter SchwefelWCPT Spring 2004
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
Page 28Tatiana K. Madsen
Hans Peter SchwefelWCPT Spring 2004
Throughput curves
Page 29Tatiana K. Madsen
Hans Peter SchwefelWCPT Spring 2004
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
Page 30Tatiana K. Madsen
Hans Peter SchwefelWCPT Spring 2004
Comparison of Throughput-Load curves
Page 31Tatiana K. Madsen
Hans Peter SchwefelWCPT Spring 2004
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
Page 32Tatiana K. Madsen
Hans Peter SchwefelWCPT Spring 2004
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
Page 33Tatiana K. Madsen
Hans Peter SchwefelWCPT Spring 2004
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
Page 34Tatiana K. Madsen
Hans Peter SchwefelWCPT Spring 2004
IEEE 802.11 standard• 802.3 Ethernet
• 802.5 Token ring
• 802.11 WLAN
• 802.15 WPAN
• Standards specify PHY and MAC, but offers the same interface to higher layers to maintain interoperability
access point
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
LLCLLC LLC
IEEE=Institute of Electrical and Electronics Engineers
Page 35Tatiana K. Madsen
Hans Peter SchwefelWCPT Spring 2004
802.11 - Layers and functions• 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
PMD
PLCP
MAC
LLC
MAC Management
PHY Management
• MAC–access mechanisms, fragmentation, encryption
• MAC Management–association, re-association of a station to an AP, roaming–authentication, encryption–synchronization, power management
PH
YD
LC
Sta
tion
Man
agem
ent
Page 36Tatiana K. Madsen
Hans Peter SchwefelWCPT Spring 2004
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
Access Point
802.11 LAN
BSS2
802.11 LAN
BSS1
Access Point
STA1
STA2 STA3
ESS
System architecture
Page 37Tatiana K. Madsen
Hans Peter SchwefelWCPT Spring 2004
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
Page 38Tatiana K. Madsen
Hans Peter SchwefelWCPT Spring 2004
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
Page 39Tatiana K. Madsen
Hans Peter SchwefelWCPT Spring 2004
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.
Page 40Tatiana K. Madsen
Hans Peter SchwefelWCPT Spring 2004
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
Page 41Tatiana K. Madsen
Hans Peter SchwefelWCPT Spring 2004
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)
Page 42Tatiana K. Madsen
Hans Peter SchwefelWCPT Spring 2004
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.
Page 43Tatiana K. Madsen
Hans Peter SchwefelWCPT Spring 2004
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
t
medium busySIFS
PIFS
DIFSDIFS
next framecontention
direct access if medium is free DIFS
Page 44Tatiana K. Madsen
Hans Peter SchwefelWCPT Spring 2004
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
Page 45Tatiana K. Madsen
Hans Peter SchwefelWCPT Spring 2004
t
medium busy
DIFSDIFS
next frame
contention window(randomized back-offmechanism)
802.11 - CSMA/CA basic access method
– 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
Page 46Tatiana K. Madsen
Hans Peter SchwefelWCPT Spring 2004
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
t
SIFS
DIFS
ACK
defer access
otherstations
receiver
senderdata
DIFS
Medium busy RTS
CTSSIFS SIFS
NAV (RTS)NAV (CTS)
back
off
Page 47Tatiana K. Madsen
Hans Peter SchwefelWCPT Spring 2004
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
Page 48Tatiana K. Madsen
Hans Peter SchwefelWCPT Spring 2004
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
• 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.
Page 49Tatiana K. Madsen
Hans Peter SchwefelWCPT Spring 2004
DFWMAC-PCF
PIFS
stations‘NAV
wirelessstations
point coordinator
D1
U1
SIFS
NAV
SIFSD2
U2
SIFS
SIFS
SuperFramet0
medium busy
t1
• 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
Page 50Tatiana K. Madsen
Hans Peter SchwefelWCPT Spring 2004
DFWMAC-PCF
tstations‘NAV
wirelessstations
point coordinator
D3
NAV
PIFSD4
U4
SIFS
SIFSCFend
contentionperiod
contention free period
t2 t3 t4
Page 51Tatiana K. Madsen
Hans Peter SchwefelWCPT Spring 2004
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
Page 52Tatiana K. Madsen
Hans Peter SchwefelWCPT Spring 2004
Synchronization using a Beacon (infrastructure)
beacon interval
tmedium
accesspoint
busy
B
busy busy busy
B B B
value of the timestamp B beacon frame
Page 53Tatiana K. Madsen
Hans Peter SchwefelWCPT Spring 2004
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
Page 54Tatiana K. Madsen
Hans Peter SchwefelWCPT Spring 2004
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?)
Page 55Tatiana K. Madsen
Hans Peter SchwefelWCPT Spring 2004
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
Page 56Tatiana K. Madsen
Hans Peter SchwefelWCPT Spring 2004
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
Page 57Tatiana K. Madsen
Hans Peter SchwefelWCPT Spring 2004
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
Page 58Tatiana K. Madsen
Hans Peter SchwefelWCPT Spring 2004
IEEE 802.11 AuthenticationOpen System Authentication
Authentication request (OpenSystem Authentication)
Authentication response
Page 59Tatiana K. Madsen
Hans Peter SchwefelWCPT Spring 2004
IEEE 802.11 AuthenticationShared Key Authentication
Authentication request(Shared Key Authentication)
“challenge” text string
“challenge” text string,encrypted with shared key
WEP encryptionof challenge text
Positive or Negative responsebased on decryption result
WEP decryptionof encrypted text
Page 60Tatiana K. Madsen
Hans Peter SchwefelWCPT Spring 2004
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 Frequency Division Multiplexing
Distance Coverage Up to 100 meters 20 meters - speed goes down with increased distance
Interoperability Current problems expected to be resolved in future
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
Page 61Tatiana K. Madsen
Hans Peter SchwefelWCPT Spring 2004
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