Download IEEE 802.11

Wireless LANs
EPL 657
Andreas Pitsillides
Contains slides and ideas from
Teknillinen Korkeakoulou, Finland: Wireless personal, local, metropolitan,
and wide area networks, S-72.3240, and
EntNet@Supercom2004 WLAN tutorial, 23/6/2004
1
Feature Topic on the Future of Wi-Fi
• Wi-Fi is a well researched topic with wide
applicability… why study further?
• E.g. call for Papers IEEE Communications
Magazine
– emerging and future generations of IEEE 802.11 Wi-Fi, such as
Wi-Fi CERTIFIED(tm) ac and WiGig CERTIFIED(tm), will be
capable of achieving multiple gigabits per second speeds and be
used to do everything from simple web browsing and peer-to-peer
sharing, to multimedia streaming, real-time teleconferencing, cable
replacement, and wireless docking, to name a few. Coupled with
the recent introduction of Wi-Fi CERTIFIED(tm) Passpoint by the
Wi-Fi Alliance, users can further enjoy seamless and secure
connectivity when roaming between cellular and Wi-Fi and
between Wi-Fi networks.
2
Future wi-fi technologies
• Topics of interest include, but are not limited to, the
following categories:
– - Technological overview of the recent, emerging, and future Wi-Fi
technologies
– - PHY-, MAC-, and network-layer architectures and protocols
– - Privacy and Security
– - Spectrum and Regulatory
– - Wi-Fi and cellular interworking
– - Implementation and deployment challenges
– - Emerging applications and services
3
WIRELESS LAN (WLAN)
• Selected topics
•
•
•
•
•
•
•
•
•
•
•
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Introduction
WLAN aims
WLAN characteristics
WLAN design goals
Infrared vs radio transmission
Infrastructure-based vs ad-hoc networks
WLAN Standards
IEEE 802.11
WLAN Roaming
WLAN Security
WLAN enhancements
WLAN design issues
• Other technologies (separate slides)
4
Why Wireless LANs (WLANs)
• Mobility (portability) and Flexibility
• Places where there is no cabling infrastructure /
Hard to wire areas
• Reduced cost of wireless systems
• Improved flexibility of wireless systems
• Cost
– Relatively low cost of deployment
– Continual drop in price for WLAN equipment
5
Wireless LAN Applications
•
•
•
•
LAN Extension
Cross building interconnection
Nomadic access
Ad hoc networks
6
Vertical Markets

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Factory floor
Home networking
Hospital
Office workers
Retail stores
Warehouse
Stock market

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Airport
Hotel
Starbuck
College campus
Convention Center
Miscellaneous
7
Example WLAN deployment - Hotel

Competing Technologies







Wired Ethernet (802.3)
Phone Line
xDSL
Power Line
Proposed: Wireless LAN (802.11)
Why: Price/Performance and ease of deployment
Current status: almost all major hotel chains in
major (and not so major) cities
8
Wireless LANs
9
Wireless LAN considerations
•
•
•
•
•
•
•
•
•
•
Throughput
Number of nodes
Connection to backbone
Service area
Battery power consumption
Transmission robustness and security
Collocated network operation
License free operation
Handoff/roaming
Dynamic configuration
10
WLANs goal
• A mature market introducing the flexibility of wireless
access into office, home, or production
environments.
• Typically restricted in their diameter to buildings, a
campus, single rooms etc.
• The global goal of WLANs is to replace office
cabling, increase flexibility of connection
especially for portable devices and, additionally,
to introduce a higher flexibility for ad hoc
communication in, e.g., group meetings.
11
WLAN characteristics
• Advantages:
– very flexible within radio coverage
– ad-hoc networks without previous planning possible
– wireless networks allow for the design of small,
independent devices
– more robust against disasters (e.g., earthquakes, fire)
12
WLAN characteristics
• Disadvantages:
– typically lower bandwidth compared to wired
networks (~11 – 300 Mbit/s) due to limitations in
radio transmission, higher error rates due to
interference, and higher delay/delay variation due
to extensive error correction and error detection
mechanisms
• offer lower QoS
– a number of proprietary solutions, especially for
higher bit-rates, and standards take their time (e.g.,
IEEE 802.11n)
• standardized functionality plus many enhanced features
• additional features only work in a homogeneous
environment (i.e., when adapters from same vendors used)
– products have to follow many national restrictions if
working wireless, it takes a very long time to
establish global solutions
13
WLAN design goals
• global, seamless operation of WLAN products
• low power for battery use (special power saving
modes and power management functions)
• no special permissions or licenses needed (licensefree band)
• robust transmission technology
• easy to use for everyone, simple management
• protection of investment in wired networks (support
the same data types and services)
• security – no one should be able to read other’s
data, privacy – no one should be able to collect user
profiles, safety – low radiation
14
Known problems with WLANs
• Wireless link characteristics: media is error prone and
the bit error rate (BER) is very high compared to the
BER of wired networks.
• Carrier Sensing/collision detection is difficult in
wireless networks because a station is incapable of
listening to its own transmissions in order to detect a
collision (more later).
• The Hidden Terminal problem also decreases the
performance of a WLAN (more later).
• Mobility (variation in link reliability, seamless
connections required, battery limitations) (more later)
15
Wireless Link Characteristics
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); also devices (e.g. motors) interfere as well (noise)
– multipath propagation: radio signal reflects off objects, arriving
at destination at slightly different times (channel quality varies
over time)
– shared with other technologies and spectrum users
– more difficult security (shared medium)
…. make communication across (even a point to point)
wireless link much more “difficult”
16
Wireless LAN Radio Technology
•
•
•
•
Infrared (IR) LANs
Spread spectrum LANs
Narrow band microwave
Laser beam
17
ISM frequency bands
ISM (Industrial, Scientific and Medical) frequency bands:
• 900 MHz band (902 … 928 MHz)
• 2.4 GHz band (2.4 … 2.4835 GHz)
• 5.8 GHz band (5.725 … 5.850 GHz)
Anyone is allowed to use radio equipment for transmitting
in these bands (provided specific transmission power
limits are not exceeded) without obtaining a license.
18
WLAN Standards
• Several WLAN standards, e.g.:
– IEEE 802.11b offering 11 Mbit/s at 2.4 GHz
– The same radio spectrum is used by Bluetooth
• A short-range technology to set-up wireless personal area networks
with gross data rates less than 1 Mbit/s
– IEEE 802.11a, operating at 5 GHz and offering gross data
rates of 54 Mbit/s
– IEEE 802.11g offering up to 54 Mbit/s at 2.4 GHz.
– IEEE 802.11n up and coming standard up to 300 Mbit/s
(two spatial streams; 600 Mbit/s with 4 spatial streams)
–…
19
WLAN Standards
Wireless
LAN
2.4 GHz
5 GHz
802.11n
(300Mb/sec)
802.11
802.11b
802.11g
HiSWANa
802.11a
HiperLAN2
(2 Mbps)
(11 Mbps)
(22-54 Mbps)
(54 Mbps)
(54 Mbps)
(54 Mbps)
HomeRF 2.0
Bluetooth
HomeRF 1.0
(10 Mbps)
(1 Mbps)
(2 Mbps)
802.11e
802.11f
802.11h
802.11i
(QoS)
(IAPP)
(TPC-DFS)
(Security)
802.11 Protocols under development
20
IEEE 802 standardisation framework
802.1
802.2 Logical Link Control (LLC)
Management
802.3
802.5
802.11 Medium Access Control (MAC)
MAC
MAC
CSMA/CA
802.3
802.5
802.11
802.11a
802.11b
802.11g
PHY
PHY
PHY
PHY
PHY
PHY
CSMA/CD
(Ethernet)
Token
Ring
CSMA/CA (Wireless LAN)
802.11n
(300Mb/sec)
Many more protocols recently developed or under development
21
Recent IEEE 802 standardisation efforts
A recent call for papers (CFPs):
A new generation of Wireless Local Area Networks (WLANs) is going to make
its appearance in the upcoming years, with the IEEE 802.11aa (Robust Audio
Video Transport Streaming), IEEE 802.11ac (Very-high throughput < 6GHz),
IEEE 802.11af (TV White Spaces) and IEEE 802.11ad (Very-high throughput
~60 GHz), as examples of the most expected ones. Nevertheless, all nextgeneration standards will consider some of the most significant advances on the
wireless communication and networking area in the last decade, developed by a
highly active community, in both academia and industry.
This special issue requests papers that advance the state-of-the-art of the recent
and on-going IEEE 802.11 standards (i.e., IEEE 802.11p, IEEE 802.11s, IEEE
802.11aa, IEEE 802.11ac, IEEE 802.11ad, IEEE 802.11ae, IEEE 802.11ah,
IEEE 802.11af, IEEE 802.11ai, etc.), as well as present mechanisms and
solutions, from MAC or above layers, that could be readily transferred to the
not-yet finished standards or their future amendments.
22
IEEE 802 wireless network technology
options
Network definition
IEEE standard
Known as
Wireless personal area
network (WPAN)
IEEE 802.15.1
Bluetooth
Low-rate WPAN (LRWPAN)
IEEE 802.15.4
ZigBee
Wireless local area
network (WLAN)
IEEE 802.11
WiFi
Wireless metroplitan
area network (WMAN)
IEEE 802.16
WiMAX
23
IEEE 802.11 standard
• As the standards number indicates, this standard
belongs to the group of 802.x LAN standards.
• This means that the standard specifies the physical
and medium access layer adapted to the special
requirements of wireless LANs, but offers the same
interface as the others to higher layers to
maintain interoperability.
• The primary goal of the standard was the
specification of a simple and robust WLAN which
offers time-bounded and asynchronous services.
24
IEEE 802.11 Wireless LAN
• 802.11g
• 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
• Shading is much more severe
compared to 2.4 GHz
• Depending on the SNR,
propagation conditions and
distance between sender and
receiver, data rates may drop
fast


– 2.4-5 GHz range
– up to 54 Mbps
– Benefits from the better
propagation characteristics at
2.4 GHz compared to 5 GHz
• Backward compatible to
802.11b
• 802.11n: multiple antennae
– 2.4-5 GHz range
– typically 200++ Mbps
• IEEE 802.11e
– MAC enhancements for
providing some QoS
• Some QoS guarantees can be
given only via polling using PCF
all use CSMA/CA for multiple access
all have base-station and ad-hoc network versions
25
Characteristics of selected wireless link
standards
Data rate (Mbps)
200
54
5-11
802.11n
802.11a,g
802.11b
4
1
802.11a,g point-to-point
data
802.16 (WiMAX)
UMTS/WCDMA-HSPDA, CDMA2000-1xEVDO
3G cellular
enhanced
802.15
.384
UMTS/WCDMA, CDMA2000
.056
3G
2G
IS-95, CDMA, GSM
Indoor
Outdoor
10-30m
50-200m
Mid-range
outdoor
Long-range
outdoor
200m – 4 Km
5Km – 20 Km
26
Infrastructure-based vs ad-hoc wireless
networks
Infrastructurebased wireless
networks
AP
AP
wired network
AP: Access Point
AP
• Infrastructure networks provide access to other networks.
• Communication typically takes place only between the
wireless nodes and the access point, but not directly between
the wireless nodes.
• The access point does not just control medium access, but
also acts as a bridge to other wireless or wired networks.
27
Infrastructure-based vs ad-hoc wireless
networks
• Several wireless networks may form one logical wireless
network:
– The access points together with the fixed network in between can
connect several wireless networks to form a larger network beyond
actual radio coverage.
• Network functionality lies within the access point (controls
network flow), whereas the wireless clients can remain quite
simple.
• Can use different access schemes with or without collision.
– Collisions may occur if medium access of the wireless nodes and the
access point is not coordinated.
• If only the access point controls medium access, no collisions are possible.
– Useful for quality of service guarantees (e.g., minimum bandwidth for certain
nodes)
– The access point may poll the single wireless nodes to ensure the data rate.
28
Infrastructure-based vs ad-hoc wireless
networks
• Infrastructure-based wireless networks loose some
of the flexibility wireless networks can offer in
general:
– They cannot be used for disaster relief in cases where no
infrastructure is left.
29
Infrastructure-based vs ad-hoc wireless
networks
Ad-hoc
wireless
networks
• No need of any infrastructure to work
– greatest possible flexibility
• Each node communicate with other nodes, so no access point
controlling medium access is necessary.
– The complexity of each node is much higher
• implement medium access mechanisms and forwarding data
30
Infrastructure-based vs ad-hoc wireless
networks
• Nodes within an ad-hoc network can only
communicate if they can reach each other physically
– if they are within each other’s radio range
– if other nodes can/want to forward the message
• IEEE 802.11 WLANs are typically infrastructurebased networks, which additionally support ad-hoc
networking
• Bluetooth is a typical wireless ad-hoc network
31
Elements of a wireless network
network
infrastructure
wireless hosts
 laptop, PDA, IP phone
 run applications
 may be stationary (nonmobile) or mobile
– wireless does not always
mean mobility
32
Elements of a wireless network
network
infrastructure
base station
 typically connected to
wired network
 relay - responsible for
sending packets between
wired network and
wireless host(s) in its
“area”
– e.g., cell towers,
802.11 access points
33
Elements of a wireless network
network
infrastructure
wireless link
 typically used to connect
mobile(s) to base station
 also can be used as
backbone links
 multiple access protocol
coordinates link access
 various data rates,
transmission distance
34
Elements of a wireless network
network
infrastructure
infrastructure mode
 base station connects
mobiles into wired
network
 handoff: mobile changes
base station
35
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
Node disconnected from the
rest of the ad-hoc network
36
WLAN components
Figure 2.11 Photographs of popular 802.11b WLAN equipment.
Access points and a client card are shown on top, and PCMCIA
Client card is shown on left and WLAN router on right. (Courtesy of
Cisco Systems, Inc.)
37
IEEE 802.11 terminology
System Architecture of an
infrastructure network
802.11 LAN
802.x LAN
Basic
Service Set (BSS)
– group of stations using same radio
frequency
Access
STA1
Point (AP)
– station integrated into the wireless LAN
and the distribution system
BSS1
Portal
Access
Point
Distribution System
Access
Point
ESS
Station
(STA)
– terminal with access mechanisms to
wireless medium and radio contact to
access point
Portal
– bridge to other (wired) networks
BSS2
Distribution
STA2
STA3
802.11 LAN
System (DS)
– interconnection network to form one
logical network
Extended
–
Service Set (EES)
based on several BSS
38
IEEE 802.11 BSS
• IEEE 802.11 allows the building of ad hoc networks
between stations, thus forming one or more BSSs.
– In this case, a BSS comprises a group of stations using the
same radio frequency.
– Several BSSs can either be formed via the distance
between the BSSs or by using different carrier frequencies.
39
Distribution System (DS)
• Used to interconnect wireless cells (multiple BSS
to form an ESS)
• Allows multiple mobile stations to access fixed
resources
• Interconnects 802.11 technology
40
Access Points (AP)
• Allows stations to associate with it
• Supports Distributed Coordination Function (DCF)
and Point Coordination Function (PCF)
• Provides management features
– Join/Associate with BSS
– Time synchronisation (beaconing)
– Power management
• all traffic flows through APs
• Supports roaming
41
IEEE standard 802.11
fixed
terminal
mobile terminal
infrastructure
network
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
42
IEEE 802.11 protocol
• Protocol architecture aims
– Applications should not notice any difference apart from
the lower bandwidth and perhaps higher access time
from the wireless LAN.
• WLAN behaves like, perhaps a ‘slower’, wired LAN.
– Consequently, the higher layers (application, TCP, IP)
look the same for the wireless node as for the wired
node.
– The differences are in physical and link layer
• different media and access control
43
IEEE 802.11 protocol
– The physical layer provides a carrier sense signal, handles
modulation and encoding/decoding of signals.
– The basic tasks of the MAC-medium access control
protocol comprise medium access, fragmentation of user
data, and encryption.
• The standard also specifies management layers.
– The MAC management supports the association and reassociation of a station to an access point and roaming
between different APs.
– Furthermore, it controls authentication mechanisms,
encryption, synchronization of a station with regard to an
AP, and power management to save battery power.
44
IEEE 802.11
• Physical layer
– Includes the provision of the Clear Channel AssessmentCCA signal (energy detection).
– This signal is needed for the MAC mechanisms controlling
medium access and indicates if the medium is currently
idle.
– A number of physical channels
Logical Link Control (LLC)
Media Access Control (MAC)
802.11 802.11 802.11 802.11a 802.11b 802.11g
infrared FHSS DSSS OFDM HR-DSSS OFDM
45
Physical layer
Wireless Transmission
Infrared
(IR)
Radio Frequency
(RF)
Spread
Spectrum
Frequency
Hopping
Direct
Sequence
Orthogonal Frequency
Division Multiplexing
46
Infrared vs radio transmission
Infrared light


 typically using the license free frequency
uses IR diodes, diffuse light reflected
band at 2.4 GHz
at walls, furniture etc, or directed light
if a LOS exists btn sender and receiver  Advantages
Advantages



simple, cheap, available in many mobile
devices (PDAs, laptops, mobile phones)
no licenses needed
Disadvantages





Radio
interference by sunlight, heat sources
etc.

many things shield or absorb IR light
cannot penetrate obstacles (e.g., walls)
low bandwidth (~115kbit/s, 4Mbit/s)
Example

IrDA (Infrared Data Association)
interface available everywhere

– experience from wireless WAN
(microwave links) and mobile phones
can be used
– coverage of larger areas possible (radio
can penetrate (thinner) walls, furniture)
– higher transmission rates (~11 – 54
Mbit/s)
Disadvantages
– very limited license free frequency
bands
– shielding more difficult, interference with
other senders, or electrical devices
Example
– IEEE 802.11, HIPERLAN, Bluetooth
47
Example WLAN physical layer
802.11g is the most popular physical layer, operating in
the same band as 802.11b
The signal format is
OFDM (Orthogonal
Frequency Division
Multiplexing)
Data rates supported:
various bit rates from
6 to 54 Mbit/s (same
as 802.11a)
802.11 Medium Access Control (MAC)
CSMA/CA
802.11
802.11a
802.11b
802.11g
PHY
PHY
PHY
PHY
ISM band: 2.4 … 2.4835 GHz
48
ISM frequency band at 2.4 GHz
The ISM band at 2.4 GHz can be used by anyone as long
as (in Europe...)
Transmitters using FH (Frequency Hopping) technology:
• Total transmission power < 100 mW
• Power density < 100 mW / 100 kHz
Transmitters using DSSS technology:
• Total transmission power < 100 mW
• Power density < 10 mW / 1 MHz
ETSI
EN 300 328-1
requirements
49
802.11 spectrum at 2.4 GHz
Divided into overlapping channels.
For e.g. the 2.4000–2.4835 GHz band is divided into 13 channels each of width 22 MHz
but spaced only 5 MHz apart, with channel 1 centred on 2.412 GHz and 13 on 2.472 GHz
Availability of channels is regulated by country (e.g. Japan adds a 14th channel 12 MHz
above channel 13). 3 channels are non overlapping
Given the separation between channels 1, 6, and 11, the signal on any channel should
be sufficiently attenuated to minimally interfere with a transmitter on any other
channel.
50
Recall: Free-space loss is dependent on
frequency
The free-space loss L of a radio signal is:
 4 d   4 df 
L
 

    c 
2
2
where d is the distance between transmitter and
receiver,  is the rf wavelength, f is the radio
frequency, and c is the speed of light. The formula
is valid for d >>  , and does not take into account antenna gains
(=> Friis formula) or obstucting elements causing additional
loss.
51
Free-space loss examples
For example, when d is 10 or 100 m, the free-space
loss values (in dB) for the different ISM bands are:
d = 10 m
d = 100 m
f = 900 MHz
L = 51.5 dB
L = 71.5 dB
f = 2.4 GHz
L = 60.0 dB
L = 80.0 dB
f = 5.8 GHz
L = 67.7 dB
L = 87.7 dB
52
Maximum channel data rates
Network
Maximum data rate
IEEE 802.15.1 WPAN
(Bluetooth)
1 Mbit/s (Bluetooth v. 1.2)
3 Mbit/s (Bluetooth v. 2.0)
IEEE 802.15.4 LRWPAN (ZigBee)
250 kbit/s
IEEE 802.11 WLAN
(WiFi)
11 Mbit/s (802.11b)
54 Mbit/s (802.11g)
300+ Mbit/s (802.11n)
IEEE 802.16 WMAN
(WiMAX)
134 Mbit/s
53
Modulation / Signal spreading
Network
Modulation / spreading method
IEEE 802.15.1 WPAN
(Bluetooth)
Gaussian FSK / FHSS
IEEE 802.15.4 LRWPAN (ZigBee)
Offset-QPSK / DSSS
IEEE 802.11 WLAN
(WiFi)
DQPSK / DSSS (802.11b)
64-QAM / OFDM (802.11g)
IEEE 802.16 WMAN
(WiMAX)
128-QAM / single carrier
64-QAM / OFDM
54
802.11: advanced capabilities
10-1
• base station and
mobile dynamically
change transmission
rate (physical layer
modulation technique)
as mobile moves, SNR
varies
10-2
QAM256 (8 Mbps)
QAM16 (4 Mbps)
BPSK (1 Mbps)
10-3
operating point
BER
Rate Adaptation
10-4
10-5
10-6
10-7
10
20
30
SNR(dB)
40
1. As node moves away from
base station SNR
decreases, BER increase
2. When BER becomes too
high, switch to lower
transmission rate but with
lower BER
55
IEEE 802.11: MAC overview
• Two basic access mechanisms have been defined
for IEEE 802.11
– CSMA/CA (mandatory) summarized as distributed
coordination function (DCF)
• Optional method (RTS/CTS) avoiding the hidden terminal problem
– A contention-free polling method for time-bounded service
called point coordination function (PCF)
• access point polls terminals according to a list
– DCF only offers asynchronous service, while PCF offers
both asynchronous and time-bounded service, but needs
the access point to control medium access and to avoid
contention.
– only asynchronous data service in ad-hoc network mode
56
IEEE 802.11: MAC overview
• Within the MAC layer, Distributed
Coordination Function (DCF)
(asynchronous service) is used as a
fundamental access method, while Point
Coordination Function (PCF) (synchronous
service) is optional.
– DCF is also known as Carrier Sense Multiple
Access with Collision Avoidance (CSMA/CA)
protocol. It is an asynchronous access method
based on the contention for the usage of shared
channels. A contention-free access mechanism is
provided through the RTS/CTS (Request to
Send/Clear to Send) exchange.
– PCF is used for time-bounded transfer of data
57
IEEE 802.11: MAC overview
• most important differences between WLAN and LAN
protocol design is the impossibility to detect all
collisions.
– difficult to receive (sense collisions) when transmitting due to weak
received signals (fading)
• with receiving and sending antennas immediately next to each other, a
station is unable to see any signal but its own.
• As a result, the complete packet will be sent before the incorrect
checksum reveals that a collision has happened.
• Furthermore, receiver and transmitter mostly not on at the same time
– can’t sense all collisions in any case: hidden terminal, fading
A
C
A
B
B
C
C’s signal
strength
A’s signal
strength
space
58
Hidden Station Problem
A
B
C
A is talking to B.
C does not know this communication and starts talking to B.
 Collisions.
59
IEEE 802.11: MAC overview
Utmost importance that number of collisions be
limited to the absolute minimum.
DCFs CSMA/CA (CA-Collision Avoidance) is the
MAC method used in a WLAN. (Wireless stations
cannot detect collisions, i.e. the whole packet will be
transmitted anyway).
Basic CSMA/CA operation:
1) If medium is free, then
Wait a specified time (DIFS),
Transmit frame
2) If medium busy, then backoff
CSMA/CA rule:
backoff before
collision
60
IEEE 802.11: MAC overview
• CSMA/CA protocol basics:
– medium can be busy or idle (detected by the CCA Clear
Channel Assessment-CCA signal of the physical layer)
• If medium busy this can be due to data frames or other control
frames
– during a contention phase several nodes try to access
medium
– optionally, the standard allows for collision free operation
through small reservation packets (RTS, CTS)
61
IEEE 802.11: MAC overview
• Define (802.11b):
– slot = 20 ms (9 or 20 ms for 802.11g)
– Short inter-frame spacing (SIFS) = 10 ms (16 ms for 802.11a)
• shortest waiting time for medium access
• defined for short control messages (e.g., ACK of data packets)
– DCF inter-frame spacing (DIFS) = 50 ms (28 ms for 802.11g)
• longest waiting time used for asynchronous data service within a
contention period DIFS=SIFS + two slot times
– PCF inter-frame spacing (PIFS)
• an access point polling other nodes only has to wait PIFS for medium
access (for a time-bounded service) PIFS=SIFS + one slot time (30 ms
for 802.11b)
• The standard defines also two control frames:
– RTS: Request To Send
– CTS: Clear To Send
62
Interframe Spacing (IFS) and priorities
• PIFS (PCF IFS)
– PCF operation mode, including Beacon,
Retransmitted poll messages
• DIFS (DCF IFS)
– DCF operation mode, including back-off, RTS
• EIFS (Extended IFS)
Shorter to longer time
– ACK, CTS, Poll Messages, Poll responses, CF-End
High to low priority
• SIFS (Short IFS)
– After detection of erroneous frame
63
IEEE 802.11: CSMA/CA
• Collision Avoidance
– idea is to prevent collisions at the moment they are
most likely to occur , i.e. when the bus is released
(since many stations may compete then).
– In the event medium is sensed busy, all clients are
forced to wait for a random number of timeslots and
then sense the medium again, before starting a
transmission.
– If the medium is sensed to be busy, the client freezes
its timer until it becomes free again.
Thus, the chance of two clients starting to send
simultaneously is reduced.
64
IEEE 802.11: CSMA/CA
– the overhead introduced by the Collision Avoidance
delays should be as small as possible.
– the protocol should keep the number of collisions to a
minimum, even under the highest possible load.
• To this end, the range of the random delay, or the contention
window, is set to vary with the load.
• In the case of a collision, the congestion window (CW) is
doubled progressively: 15, 31, 63,...1023, until a successful
transmission occurs and the delay is reset to the minimal value.
• From the number CW (= 15 / 31 … 1023 slots) the random
backoff bn (in terms of slots) is chosen in such a way that bn is
uniformly distributed between 15/31 … CW.
• Since it is unlikely that several stations will choose the same
value of bn, collisions are rare.
• The 802.11 standard does not fix the minimum and maximum
values of the contention window. However, it does advise a
minimum of 15 or 31 and a maximum of 1023.
65
IEEE 802.11: CSMA/CA
• Broadcast data transfer (DCF)
DIFS
DIFS
medium busy
direct access if
medium is free  DIFS
contention window
(randomized back-off
mechanism)
next frame
t
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 a Distributed Coordination
Function Inter-Frame Space (DIFS), then station can start sending
– if the medium is busy, the station has to wait for a free DIFS, then the
station must additionally wait a random back-off time (collision
avoidance)
– if another station occupies the medium during the back-off time of the
station, the back-off timer stops (fairness – during the next phase this
node will continue its timer from where it stopped)
66
IEEE 802.11 : CSMA/CA
• E.g. Unicast data transfer
DIFS
sender
data
SIFS
ACK
receiver
DIFS
other
stations
Channel
sensed busy
data
t
waiting time
contention
– station has to wait for DIFS before sending data
– receivers acknowledge after waiting for a duration of a Short
Inter-Frame Space (SIFS), if the packet was received correctly
67
EE802.11: Exponential backoff
mechanism
binary exponential backoff:
After k collisions, a random number
of slot times between 15 and 2k+5-1
is chosen. So, for the first collision,
each sender might wait between 15
or 31 slot times. After the second
collision, the senders might wait
between 15 and 63 slot times, and so
forth.
As the number of retransmission
attempts increases, the number of
possibilities for delay increases .
Note that the suggested minimum
window is normally set at 15 (or 31)
at start, so as to have some initial
non-zero random delay and there is
a max number prior to declaring the
transmission not possible
68
EE802.11: Exponential backoff
mechanism
Contention window (CW) for 802.11b
If transmission of a frame was unsuccessful and the frame
is allowed to be retransmitted, before each retransmission
the Contention Window (CW) from which bn is chosen (at
random, starting from 15 or 31) is increased.
802.11b
CW
Initial attempt
DIFS
1st retransm.
DIFS
…
CW = 25-1 = 31 slots
(slot = 20 ms)
…
CW = 26-1 = 63 slots
:
5th (and further)
retransmissions
DIFS
:
…
CW = 210-1
= 1023 slots
69
EE802.11: Exponential backoff
mechanism
Contention window (CW) for 802.11g
In the case of 802.11g operation, the initial CW length is
15 slots. The slot duration is 9 ms. The backoff operation
of 802.11g is substantially faster than that of 802.11b.
802.11g
CW
Initial attempt
DIFS
1st retransm.
DIFS
…
CW = 24-1 = 15 slots
(slot = 9 ms)
…
CW = 25-1 = 31 slots
:
6th (and further)
retransmissions
DIFS
:
…
CW = 210-1
= 1023 slots
70
EE802.11: Exponential backoff
mechanism
Selection of random backoff
From the number CW (= 15 / 31 … 2k+5-1 slots) the
random backoff bn (in terms of slots) is chosen in such a
way that bn is uniformly distributed between 0 … CW.
Since it is unlikely that several stations will choose the
same value of bn, collisions are rare.
The next slides show wireless medium access in action.
The example involves four stations: A, B, C and D.
”Sending a packet” means ”Data+SIFS+ACK” sequence.
Note how the backoff time may be split into several parts.
71
EE802.11: Exponential backoff
mechanism
Wireless medium access example
Data+SIFS+ACK
Station A
ACK
Defer
Station B
1
Station C
Contention
Window
Backoff
Defer
2
Station D
DIFS
1) While station A is
sending a packet,
stations B and C
also wish to send
packets, but have to
wait (defer +
backoff)
2) Station C is
”winner” (backoff
time expires first)
and starts sending
packet
72
EE802.11: Exponential backoff
mechanism
Wireless medium access example
3) Station D also
wishes to send a packet
Station A
4
Station B
Station C
ACK
3
Station D
Defer
DIFS
DIFS
4) When medium
becomes idle plus DIFS
elapses,
station B continues to
count down and station
D draws a CW number
D(bn)
station B is ”winner”
After its CW counts
down to zero it starts
sending packet
73
EE802.11: Exponential backoff
mechanism
Wireless medium access example
5) Station D
counts down to
0 and then
starts sending
packet. Now
there is no
competition.
Station A
Station B
ACK
Station C
5
Station D
DIFS
DIFS
74
EE802.11: Exponential backoff
mechanism
No shortcuts for any station…
Transmitted
frame
(A=>B)
DIFS
Backoff
ACK
(B=>A)
SIFS
Next
frame
(A=>B)
DIFS
When a station wants to send more than one frame, it has
to use the backoff mechanism like any other station (of
course it can ”capture” the channel by sending a long
frame, for instance using fragmentation).
75
IEEE 802.11: MAC overview
Avoiding collisions (using extra signalling). How?
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
– RTS packets may still collide with each other (but they are very
short)
• BS broadcasts clear-to-send CTS in response to RTS
• CTS heard by all nodes
– sender transmits data frame
– other stations defer transmissions. For how long?
avoid data frame collisions completely
using small reservation packets!
76
Network Allocation Vector (NAV)



Each RTS frame includes the duration of the time it needs to
occupy the channel.
NAV: a timer on other stations which have to wait NAV
before checking if the channel/medium is free.
When a station (WS1) sends RTS (or CTS), other stations on
the system start NAV (WS2 and WS3 in example below)
RTS
WS3
RTS
WS1
WS2
77
Hidden Station Problem (Solution)
B can hear A and C
A and C cannot hear each other
A and C want to send to B
A
B
RTS/NAV
CTS
Data
C
RTS/NAV
B accepts RTS from A and rejects RTS from C.
CTS/NAV
CTS from B
(actually BS) to A
is also received
on C which starts
the NAV timer in
CTS.
78
Busy Medium


Physically busy: a station senses the wireless medium
to determine if it is busy.
Virtually busy: a station receives a control message
(RTS or CTS) which indicates the wireless medium is
busy for the duration of the NAV timer.
 All stations must monitor the headers of all frames
they receive and store the NAV value in a counter.
 The counter decrements in steps of one
microsecond. When the counter reaches zero, the
channel is available again.
79
IEEE 802.11
• Sending unicast packets with RTS/CTS control frames
DIFS
sender
RTS
data
SIFS
receiver
other
stations
CTS SIFS
SIFS
ACK
NAV (RTS)=3SIFS+CTS+data+ACK DIFS
NAV (CTS)=2SIFS+data+ACK
defer access
data
t
contention
– station can send RTS with reservation parameter after waiting for DIFS
(reservation determines amount of time the data packet needs the medium and
the ACK related to it).
– Every node receiving this RTS now has to set its net allocation vector – it specifies
the earliest point at which the node can try to access the medium again
– 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
80
Collision Avoidance: RTS-CTS exchange
A
AP
B
reservation collision
DATA (A)
defer
time
81
802.11 MAC Timing
82
Example
calculation of
throughput
Masters thesis
http://eeweb.poly.
edu/dgoodman/fai
nberg.pdf
Note that DIFS
should had been part
of the idle time
83
84
Point Coordination Function (PCF)

Optional and implemented on top of DCF.






Must be running in conjunction with DCF.
A single Access Point (AP) controls access to the medium,
and a Point Coordinator Agent resides in the AP.
AP sends a beacon message and all stations stop DCF.
AP polls each station for data, and after a given time interval
moves to the next station.
 Guaranteed maximum latency
No station is allowed to transmit unless it is polled.
AP could have a priority scheme for stations, and support
time-sensitive applications.
85
PCF (cont.)
repetition interval
Contention
free period (CFP)
B
PCF
NAV
Contention
period (CP)
DCF
busy
B
PCF
DCF
NAV
B: beacon message
86
Additional WLAN Features



Positive Acknowledgement
Sequence Control
Fragmentation
Large frames vs. small frames
 Error-prone medium

87
IEEE 802.11 framing and
addressing
88
802.11 frame: addressing
R1 router
H1
Internet
AP
R1 MAC addr H1 MAC addr
dest. address
source address
802.3 frame
AP MAC addr H1 MAC addr R1 MAC addr
address 1
address 2
address 3
802.11 frame
89
802.11 frame: addressing
2
2
6
6
6
frame
address address address
duration
control
1
2
3
2
6
seq address
4
control
0 - 2312
4
payload
CRC
Address 4: see later
Address 1: MAC address
of wireless host or AP
to receive this frame
Address 3: MAC address
of router interface to
which AP is attached
Address 2: MAC address
of wireless host or AP
transmitting this frame
90
Recall: Routing in a (W)LAN
Recall: Routing in a (W)LAN is based on MAC addresses. A
router performs mapping between these two address
types (IP-MAC):
(W)LAN
device
IP network
(W)LAN
00:90:4B:00:0C:72
Router
124.2.10.57

00:90:4B:00:0C:72
Server
124.2.10.57
91
Recall: Address allocation
MAC addresses associated with hardware devices.
IP addresses can be allocated to (W)LAN devices
either on a permanent basis or dynamically from an
address pool using the Dynamic Host Configuration
Protocol (DHCP).
The DHCP server may be a separate network
element (or for example integrated into a RADIUS
server that offers a set of additional features), or
may be integrated with the address-mapping router
and/or access point.
RADIUS = Remote Authentication Dial-In User
Service
92
Recall: Network Address Translation
(NAT)
Recall:
On (W)LAN side of network address translator (NAT
device), different (W)LAN users are identified using
private (reusable, globally not unique) IP
addresses.
On Internet side of NAT device, only one (globally
unique) IP address is used. Users are identified by
means of different TCP/UDP port numbers.
In client - server type of communication,
application on the server is usually behind a certain
TCP/UDP port number (e.g. 80 for HTTP) whereas
clients can be allocated port numbers from a large
address range.
93
Recall: NAT example
IP network
(W)LAN
User 1
NAT
device
IP address for all users
in (W)LAN:
Server
124.0.6.12
User 2
User 1 IP address
10.2.1.57
User 1 TCP port number
14781
User 2 IP address
10.2.1.58
User 2 TCP port number
14782
94
Case study: ADSL WLAN router
1) The ADSL connection to the wide area network (WAN)
is allocated a globally unique IP address using DHCP.
2) We assume that the router has NAT functionality.
Behind the router, in the private LAN network, wireless
and cabled LAN devices are allocated private IP
addresses, again using DHCP (this is a kind of "double
DHCP" scenario).
Although routing in the LAN is based on MAC addresses,
the IP applications running on the LAN devices still need
their own "dummy" IP addresses.
95
802.11 frame: more
frame seq #
(for Reliable Data Transfer)
duration of reserved
transmission time (RTS/CTS)
2
2
6
6
6
frame
address address address
duration
control
1
2
3
2
Protocol
version
2
4
1
Type
Subtype
To
AP
6
2
1
seq address
4
control
1
From More
AP
frag
1
Retry
1
0 - 2312
4
payload
CRC
1
Power More
mgt
data
1
1
WEP
Rsvd
frame type
(RTS, CTS, ACK, data)
96
802.11 Frame Format
2
Frame
Control
2
Duration
ID
6
Address
1
6
Address
2
6
Address
3
Frame Body (0 – 2312 bytes)
2
6
Sequence
Control
Address
4
4
FCS
Ref. IEEE 802.11 standards
Q: Why do we need four address fields in 802.11?
97
802.11 Addresses
Address Address Address Address
1
2
3
4
To
DS
From
DS
0
0
DA
SA
BSSID
N/A
0
1
DA
Sending
AP
SA
N/A
1
0
Receiving
AP
SA
DA
N/A
1
1
Receiving
AP
Sending
AP
DA
SA
DS: Distribution System
BSSID: Basic Service Set ID
DA: Destination Address
SA: Source Address
98
Case - 00
11-22-33-01-01-01
11-22-33-02-02-02
A1: 11-22-33-01-01-01
DA
A2: 11-22-33-02-02-02
SA
A3: BSS ID
A4: not used
99
Case – 01 (wired to wireless)
wireless
802.11
11-22-33-01-01-01
wired
802.3
99-88-77-09-09-09
DA
A1: 11-22-33-01-01-01
Sending AP
A2: 99-88-77-09-09-09
SA
A3: 11-22-33-02-02-02
11-22-33-02-02-02
DA: 11-22-33-01-01-01
SA: 11-22-33-02-02-02
A4: not used
100
Case – 10 (wireless to wired)
wired
802.3
wireless
802.11
11-22-33-01-01-01
99-88-77-09-09-09
Receiving AP A1: 99-88-77-09-09-09
SA
A2: 11-22-33-01-01-01
DA
A3: 11-22-33-02-02-02
11-22-33-02-02-02
DA: 11-22-33-02-02-02
SA: 11-22-33-01-01-01
A4: not used
101
Case – 11 (via wireless)
wired
802.3
11-22-33-01-01-01
wireless
802.11
99-88-77-09-09-09
wired
802.3
99-88-77-08-08-08
11-22-33-02-02-02
DA: 11-22-33-02-02-02
A1: 99-88-77-08-08-08
DA: 11-22-33-02-02-02
SA: 11-22-33-01-01-01
A2: 99-88-77-09-09-09
SA: 11-22-33-01-01-01
A3: 11-22-33-02-02-02
A4: 11-22-33-01-01-01
102
Wireless Bridge
Building A
Ethernet Backbone
Wireless
Bridge
Building B
Ethernet Backbone
Case 11
Wireless
Bridge
103
IEEE 802.11 management
issues: synchronisation,
power management, and
roaming
104
MAC management
• Synchronization
finding and staying with a WLAN
synchronization functions
• Power Management
- sleeping without missing any messages
- power management functions
• Roaming
- functions for joining a network
- changing access points
- scanning for access points
• Management information base (MIB)
-
105
IEEE 802.11 association, roaming,
synchronisation
• Stations can select an AP and associate with it.
• The APs support roaming (i.e. changing access
points), the distribution system (DS) then handles
data transfer between the different APs.
• Furthermore, APs provide synchronization within a
BSS, support power management, and can control
medium access to support time-bounded service.
106
Scanning
• Scanning is required for many functions
- finding and joining a network
- finding a new access point during
roaming
• Passive scanning
- find networks simply by listening for
beacons
• Active scanning
- on each channel send a probe and wait
for probe response
802.11: passive/active scanning
BBS 1
AP 1
BBS 2
1
1
2
AP 2
BBS 1
BBS 2
AP 1
AP 2
1
2
3
2
3
4
H1
H1
Passive Scanning:
Active Scanning:
(1) beacon frames sent from APs
(2) association Request frame sent:
H1 to selected AP
(3) association Response frame sent:
AP to H1
(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: AP to H1
108
802.11: Channels, association
• 802.11b: 2.4GHz-2.485GHz spectrum divided into 13
channels (EU, USA 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
– will typically run DHCP to get IP address in AP’s subnet
109
Synchronization
• Timing synchronization function (TSF)
• Used for power management
– beacons sent at well known intervals
– all station timers in BSS are synchronized
Beacon interval
B
Access B
point
busybusy
medium
B
B
busy
B
busy
t
B beacon
Value of time stamp
110
Power Management
• Mobile devices are battery powered
- power management is important for
mobility
• 802.11 power management protocol
- allows transceiver to be off as much as
possible
- is transparent to existing protocols
Power management approach
• Allow idle stations to go to sleep
station’s power save mode stored in AP
• APs buffer packets for sleeping stations
- AP announces which stations have
frames buffered
- traffic indication map (TIM) sent with
every beacon
• Power saving stations wake up periodically
802.11: Power management
approach


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 APto-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
113
802.11: beacon frames
• Each beacon frame carries the following information in
the frame body:
– Beacon interval. amount of time between beacon transmissions. Before a
station enters power save mode, station needs the beacon interval to
know when to wake up to receive the beacon.
– Timestamp. After receiving beacon frame, station uses timestamp value
to update its local clock. Enables synchronization among all stations
associated with the same access point.
– Service Set Identifier (SSID). The SSID identifies a specific WLAN.
Before associating with a particular WLAN, a station must have the same
SSID as the access point. By default, access points include the SSID in
the beacon frame to enable sniffing functions to identify the SSID and
automatically configure the WLAN NIC with the proper SSID.
– Supported rates. For example, a beacon may indicate that only 1, 2, and
5.5Mbps data rates are available. As a result, an 802.11b station would
stay within limits and not use 11 Mbps. With this information, stations can
use performance metrics to decide which access point to associate with.
114
802.11: beacon frames
– Parameter Sets. The beacon includes information about the
specific signalling methods (such as frequency hopping spread
spectrum, direct sequence spread spectrum, etc.). For example, a
beacon would include in the appropriate parameter set the channel
number that an 802.11b access point is using. Likewise, a beacon
belonging to frequency hopping network would indicate hopping
pattern and dwell time.
– Capability Information. This signifies requirements of stations that
wish to belong to the wireless LAN that the beacon represents. For
example, this information may indicate that all stations must use
wired equivalent privacy (WEP) in order to participate on the
network.
– Traffic Indication Map (TIM). An access point periodically sends
the TIM within a beacon to identify which stations using power
saving mode have data frames waiting for them in the access point's
buffer. The TIM identifies a station by the association ID that the
access point assigned during the association process.
115
802.11: beacon frames and probe
response frames
• 802.11 probe response frame
– similar to a beacon frame, except without TIM info and
are only sent in response to a probe request.
– A station may send a probe request frame to trigger a
probe response when the station needs to obtain
information from another station.
– A radio NIC, for instance, will broadcast a probe
request when using active scanning to determine which
access points are within range for possible association.
– Some sniffing software (e.g., NetStumbler) tools send
probe requests so that access points will respond with
desired info.
116
WLAN Roaming

WLAN Roaming
Computer
Computer
Computer
Corporate Network
Laptop B
(with WLAN card)
Laptop C
(with WLAN card)
Moving to Access Point B
Laptop D
(with WLAN card)
Laptop E
(with WLAN card)
Laptop C
(with WLAN card)
Registering to Access Point B
Reassociation Response
Laptop A
(with WLAN card)
Reassociation Request
Access
Point
B
Access
Point
A

Mobile stations
may move
 beyond the
coverage area
of their AP
 but within range
of another AP
Re association
allows station to
continue
operation.
118
WLAN 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
119
Roaming approach
•
•
•
•
Station decides that link to its current AP is poor
Station uses scanning function to find another AP
Station sends Re-association Request to new AP
If AP accepts Re-association Request
- AP indicates Re-association to the
distributed system
- Distributed system information is
updated
• If Re-association Response is successful
- then station has roamed to the new AP
- else station scans for another AP
Joining a network
• Phase 1
– Scanning
• Active (probe)
• Passive (beacon)
• Phase 2
– Authentication (more later)
• Open system
• Some admission scheme / shared key
• Phase 3
– Association or Reassociation (allows mobility/roaming
more later)
121
WLAN Roaming
• L2 handover
– If handover from one AP to another belonging
to the same subnet, then handover is
completed at L2
• L3 handover
– If new AP is in another domain, then the
handover must be completed at L3, due to the
assignment of an IP belonging to the new
domain – hence routing to the new IP.
• Mobile IP deals with these issues – more later
122
802.11: mobility within same
subnet
• H1 remains in same IP
subnet: IP address
can remain same
• switch: which AP is
associated with H1?
– self-learning: switch will
see frame from H1 and
“remember” which
switch port can be used
to reach H1
router
hub or
switch
BBS 1
AP 1
AP 2
H1
BBS 2
123
Distribution system (DS) - IAPP
DS is the mechanism by which APs and other nodes in the
wired IP subnetwork communicate with each other.
Distribution System (DS)
AP
AP
Router
External
network
(LAN or
Internet)
This communication, using the Inter-Access
Point Protocol (IAPP), is essential for link-layer
mobility (=> stations can seamlessly move
between different BSS networks).
124
Distribution system (cont.)
For instance, when a wireless station moves from one BSS
to another, all nodes must update their databases, so that
the DS can distribute packets via the correct AP.
Distribution System (DS)
AP 1
AP 2
WS
WS moves to another BSS
Router
AP 1, AP 2 and router:
update your databases!
Packets for this WS will
now be routed via AP 2.
125
Basic routing example
When WS associates with AP 2, the router in charge of the
IP subnet addressing obtains an IP address from the
DHCP (Dynamic Host Configuration Protocol) server.
Distribution System (DS)
AP 1
1
Association
2
Fetch IP address
Router
AP 2
2
1
WS
External
network
(LAN or
Internet)
DHCP
Server
126
Basic routing example (cont.)
The router must maintain binding between this IP address
and the MAC address of the wireless station.
Distribution System (DS)
AP 1
124.2.10.57

00:90:4B:00:0C:72
AP 2
00:90:4B:00:0C:72
Router
External
network
(LAN or
Internet)
WS
127
Basic routing example (cont.)
The globally unique MAC address of the wireless station is
used for routing the packets within the IP subnetwork (DS
+ attached BSS networks).
Distribution System (DS)
AP 1
124.2.10.57

00:90:4B:00:0C:72
AP 2
00:90:4B:00:0C:72
Router
External
network
(LAN or
Internet)
WS
128
Basic routing example (cont.)
The dynamic and local IP address of the wireless station is
only valid for the duration of attachment to the WLAN and
is used for communicating with the outside world.
Distribution System (DS)
AP 1
124.2.10.57

00:90:4B:00:0C:72
AP 2
00:90:4B:00:0C:72
Router
External
network
(LAN or
Internet)
WS
129
Basic routing example (cont.)
The router must also know (and use) the MAC address of
the access point via which the packets must be routed.
For this purpose, a special protocol (IAPP) is needed!
Distribution System (DS)
00:03:76:BC:0D:12
AP 1
AP 2
00:90:4B:00:0C:72
Router
124.2.10.57

00:90:4B:00:0C:72
00:03:76:BC:0D:12
External
network
(LAN or
Internet)
WS
130
IAPP (Inter-Access Point Protocol)
IAPP (defined in IEEE 802.11f) offers mobility in the
Data link layer (within an ESS = Extended Service Set).
Distribution System (DS)
AP 1
1
AP 2
2
AP 3
Router
External
network
(LAN or
Internet)
IAPP: APs must be able to communicate
with each other when the station moves
around in the WLAN
131
In addition to IAPP …
IAPP alone is not sufficient to enable seamless handovers in
a WLAN. The stations must be able to measure the signal
strengths from surrounding APs and decide when and to
which AP a handover should be performed (no 802.11
standardised solutions are available for this operation).
In 802.11 networks, a handover means reassociating
with the new AP. There may be two kinds of problems:
• will handover work when APs are from different vendors?
• will handover work together with security solutions?
132
Mobility Management (MM)
There are basically two objectives of Mobility Management:
1. MM offers seamless handovers when moving from one
network/subnetwork/BSS to another
Active network connection – handover
2. MM makes sure that users or terminals can be reached
when they move to another network/subnetwork/BSS
Passive user/terminal – reachability
133
MM in cellular wireless networks (1)
1. Handover: In a cellular wireless network (e.g. GSM),
the call is not dropped when a user moves to another
cell. Handovers are based on measurements performed
by the mobile terminal and base stations.
BS 1
BS 2
134
MM in cellular wireless networks (2)
2. Reachability (allows roaming): In a cellular wireless
network, the HLR (Home Location Register) knows in
which VLR (Visitor Location Register) area the mobile
terminal is located. The VLR then uses paging to find the
terminal.
Paging
Mobile
subscriber
number
points to
HLR
points to
VLR
HLR
135
MM in three different OSI layers
Mobility Management (MM) schemes are possible in three
different layers of the OSI protocol layer model:
Application layer
…
…
Transport layer
Network layer
Data link layer
Physical layer
e.g. SIP (Session Initiation Protocol)
Personal mobility
e.g. Mobile IP
Terminal mobility
IAPP (Inter-Access Point Protocol)
Handovers
137
MM in the Data link layer
Mobility Management (MM) schemes are possible in
three different layers of the OSI protocol layer model:
Application layer
…
…
Transport layer
Network layer
Data link layer
Physical layer
IAPP (IEEE 802.11f):
Seamless roaming within an
ESS network (= IP subnet).
Handover is not possible when
moving from one ESS network
to another.
No reachability solutions.
138
MM in the Network layer
Mobility Management (MM) schemes are possible in
three different layers of the OSI protocol layer model:
Application layer
…
…
Transport layer
Network layer
Data link layer
Physical layer
Mobile IP:
Seamless roaming between
ESS networks (= IP
subnetworks).
Handover is possible when
moving from one ESS (or
WLAN) network to another.
139
MM in the Application layer
Mobility Management (MM) schemes are possible in three
different layers of the OSI protocol layer model:
Application layer
…
…
Transport layer
Network layer
Data link layer
Physical layer
SIP (or other application
layer solutions):
No seamless handovers as
such...
However, the terminal can be
reached from the outside
network, like with Mobile IP.
140
Mobility management summary
Within a WLAN, handovers are possible (based
on IAPP + proprietary solutions in
equipment), but there is no IEEE-supported
reachability solution available.
Handovers between different WLANs require
Mobile IP (which offers also reachability).
Unfortunately, Mobile IP includes a nontransparent mechanism (Discovering Care-of
Address) that must be implemented in all APs.
Global reachability of wireless stations can be
achieved using SIP or similar Application layer
concepts. SIP does not require changes to APs.
141
IEEE 802.11f
f
e
i
IAPP
QoS
Security
802.11 basic protocol
h
d
DFS/TCP
Scanning
a
b
OFDM 5GHz
g
DSSS 2.4GHz OFDM 2.4GHz
The objective: to specify the
Inter-Access Point Protocol
(IAPP) that enables seamless
roaming between different
Access Points within an ESS.
Note: 802.11f is not
concerned with roaming
between ESS networks. For
this purpose, non-802.11
solutions must be used.
142
WLAN: Design and
Deployment
(part of design exercise
Supplemented with Slides by Mr
Mylonas in Lab part)
143
Wireless LAN Design

Several design issues
 Business Case – justify your case and cost
 Product Selection
 Wireless Access points








Location
Frequency/Channel
Security
Performance
Reliability
Management
Scalability
Miscellaneous
144
Product Selection

Wireless Stations





Wireless Access Points (WAP)
Wireless Bridge, if needed:




connecting multiple WLAN segments
A wireless bridge does not support end stations
Wireless Repeater: Bridge + AP


Desktop: PCI or USB
Laptop
PDA
A wireless repeater supports end stations
Wireless Switch
Security Server (RADIUS Server)
Ref: http://www.practicallynetworked.com/networking/wireless_bridge.htm
145
Multiple BSS Configuration
(different channels/frequencies)
Server
Channel 11
Access
Point
LAN Backbone
Access
Point
Channel 1
Wireless
Clients
Access
Point
Wireless
Clients
Channel 6
146
Office Design (802.11b)
Location and Channel Selection
Design: One AP or more APs?
100 metres
Channel 1
Channel 6
Channel 11
Channel 1
Channel 1
100 metres
147
Dense populations
 Case
study:
1000 users in 100mx100m facility
 3 or 4 APs will cover the system (in range)
 Need more APs in the area than physically required?
 to provide Bandwidth that is defined by customer
 What side-effects are created, if any?
 Interference from neighbouring units

148
Determining Cell Density
• Cell size and throughput-based data rate will affect the cell
density (maximum number of users per cell. ).
• To determine cell density for a best-effort network,
• determine average throughput per user
• divide throughput rate of AP by throughput per user.
This provides maximum active transmissions per cell.
• In a best-effort WLAN, data latency does not affect the
outcome.
• In general, throughput will be about half the data rate of the
access point.
Dense Population Area
Reduce Cell size
 Reduce Antenna gain or
transmitter power to
create smaller cell size

Pink: Channel 1
Yellow: Channel 6
Green: Channel 11
100m × 100m area
150
WLAN Performance
802.11b
802.11a
802.11g
Link Rate
(max)
UDP
11M bps
54M bps
54M bps
7.1M bps
30.5M bps
30.5M bps
TCP
5.9M bps
24.4M bps
24.4M bps
The test was conducted in a lab environment, and the
distance is expected to be less than 10m.
Ref. “WLAN Testing with IXIA IxChariot,” IXIA White Paper
Appears in EntNet@Supercom2004, 06/23/2004
151
WLAN Performance (line rate)
WLAN Performance
60
Throughput (Mbps)
50
40
802.11g
30
20
802.11a
10
802.11b
0
0
30
60
90
120
150
Distance (m)
Data Source: Cisco Networking Professional On-Line Live Tech Talk
Appears in EntNet@Supercom2004, 06/23/2004
152
Determining Cell Size
Cell size -- area of coverage provided by an access point.
Size of the cell determined by several factors:
• transmit power and receiver sensitivity of the radios in
AP and client
• antennas used by the AP and client
• data rate used
• frequency and modulation technique
• antenna gain
• Environment (e.g. actual coverage characteristics)
Cell size is limited by the device with the weakest RF
characteristics
Coverage – design issues
Figure 2.13 A predicted coverage plot for three access points
in a modern large lecture hall. (Courtesy of Wireless Valley
Communications, Inc., ©2000, all rights reserved.)
154
Coverage – design issues
Figure 2.15 A typical neighborhood where high speed
license free WLAN service from the street might be
contemplated [Dur98b].
155
Coverage – design issues
Figure 2.16 Measured values of path loss using a street-mounted
lamp-post transmitter at 5.8 GHz, for various types of customer
premise antenna [from [Dur98], ©IEEE].
156
Tools for WLAN planning
Many tools are available offering differing
functionalities for network design, planning,
and monitoring
One example is:
http://www.softpedia.com/get/NetworkTools/Network-Monitoring/NetStumbler.shtml
Another tool
http://www.metageek.net/products/inssider/
Another is
http://www.visiwave.com/index.php/ScrInfoProduct
s.html?sid=EyUcNeJxwlyKbI46
157
WLAN security
intro
158
WLAN Security
• Not so efficient compared with Ethernet security due to the nature of the
medium & the requirements of the users
• Security mechanisms
– Service Set Identifiers (SSID)
• Used to name the network and provide initial authentication for each client
– Wired Equivalent Privacy (WEP)
• Data encryption technique using shared keys and a pseudorandom number as
an initialization vector
• 64-bit key level encryption BUT several vendors support 128-bit key level
encryption
– Wi-Fi Protected Access (WPA(2)) –replaced WEP
• WPA2 uses encryption device which encrypts the network with a 256 bit key
– Also a VPN could operate on top of the WLAN providing increased
security
159
WLAN Security
• IEEE newer standards
– 802.11i (Advanced Encryption Standard – AES uses a symmetric block data encryption technique)
– 802.1X for port based Network Access Control
• provides an authentication mechanism to devices wishing
to attach to LAN/WLAN (governs Extensible
Authentication Protocol (EAP) encapsulation process that
occurs between clients, wireless APs, and authentication
servers (RADIUS)
• EAP allows developers to pass authentication data between
RADIUS servers and wireless APs.
• has a number of variants, including: EAP MD5, EAP-TLS,
EAP-TTLS, LEAP, and PEAP
160
WLAN Security - WEP


Wired Equivalent Privacy (WEP) –
 Least secure - A network that is secured with
WEP has been cracked in 3 minutes by the FBI
Shared key encryption




Stations use the same key for encryption.
RC4 encryption algorithm
Key: 40 bits or 128 bits
User Authentication



Not specified in 802.11.
802.1X
VPN
162
WEP Operation
RC4
Algorithm
40-bit WEP Key
24-bit IV
64-bit RC4
Integrity
check
RC4 Key Stream
24-bit IV
randomly generated
Frame
Header
IV
Header
4 bytes
Frame
Body
ICV
Trailer
FCS
4 bytes
IV: initialization vector ICV: integrity check value
163
WEP Key Distribution Issue



Key is manually set in the driver.
The key cannot be protected from local users.
When a user leaves the organization, technically
you must change the key information on all
stations.


What if a station is stolen?
For a large organization, there is a need to publish
the key which is a security problem.
164
WEP Design Issue

Weakness in the Key Scheduling Algorithm:
“http://www.crypto.com/papers/others/rc4_ksaproc.pdf




A weakness of RC4 in generating the keystream.
Hacker attack: using weak IV to attack a particular byte
of the secret portion of the RC4 key.
The time to attack is a linear algorithm to the key
length.
This is a complete break for WEP.
165
WPA(2)- Wi-Fi Protected Access



WPA intermediate measure to replace WEP pending availability
of full IEEE 802.11i standard.
requires firmware upgrades on wireless NICs and APs.
implements much of IEEE 802.11i standard--adopts Temporal
Key Integrity Protocol (TKIP)

TKIP employs per-packet key; dynamically generates new 128-bit key for
each packet - prevents types of attacks that compromised WEP



WEP used a 40-bit or 128-bit encryption key manually entered on wireless
APs and devices and does not change.
includes message integrity check to prevent an attacker from
capturing, altering and/or resending data packets.
WPA2 replaced WPA.


implements mandatory elements of IEEE 802.11i-- CCMP, AES
based encryption mode
requires testing and certification by Wi-Fi Alliance
166
Solutions to Security Issue

Non-standard solutions
Layer 3 – VPN
 Layer 4 – IP Address Control and Firewall
 Layer 7 –Proxy


Standard solutions
802.11i (including 802.1X)
 802.1X (including EAP)
 Extensible Authentication Protocol (EAP)

167
VPN for WLAN (Layer3)
VPN
Gateway
LAN
Layer 2 tunnel over a layer 3 protocol
Ethernet
RADIUS
server
VPN Tunnel
IP
IP
Wireless LAN
LAN
168
Router/Firewall (Layer4)
LAN
temp IP
authentication
Security
Server
Internet
official IP
1.
2.
3.
4.
Standard WLAN and DHCP procedure for a temp IP to
the wireless station.
The temp IP address is used for authentication only. All
other traffic is blocked by the router.
After user authentication, the station is given an official
IP address which can go through the router.
May also register the MAC address to reduce the risk of
hacker attack.
169
Proxy/Gateway (Layer-7)
Security
Server
LAN
Proxy
Gateway
1.
2.
3.
4.
Standard WLAN and DHCP procedure for an IP
address to the wireless station.
User types any URL and the request is routed to the
security server web page.
• All other traffic is blocked.
After entering account info or credit card, the user is
authenticated.
The gateway authorizes the traffic from the
authenticated station.
Internet
170
WLAN New Security
Standards
802.1X and 802.11i
171
Extensible Authentication Protocol (EAP)


EAP is an IETF standard (RFC 2284) and adopted by
IEEE as the basis for 802.1X. It is called the port
based network access control. (also know as postbased authentication protocol)
EAP supports both wired and wireless authentication.
MD5
TLS
TTLS
LEAP
PEAP
EAP
PPP
802.3
802.11
802.5
TLS: Transport Layer Security TTLS: Tunnel TLS
LEAP: Lightweight EAP PEAP: Protected EAP
172
EAP Authentication Methods





MD5 (Message Digest 5) - Username/Password. This
is similar to MS_CHAP.
TLS (Transport Layer Security) - PKI (certificates),
strong authentication
TTLS (Tunnel TLS) - Username/Password
LEAP - Cisco proprietary lightweight EAP. It is to be
phased out in favor of PEAP.
PEAP – Protected EAP.
173
802.1X
802.1X authentication involves three parties: a supplicant (client
device) , an authenticator (Ethernet switch or wireless AP), and an
authentication server typically a host running software supporting
the RADIUS and EAP protocols.
EAP data is first encapsulated in EAPOL frames between the Supplicant and Authenticator, then reencapsulated between Authenticator and Authentication server using RADIUS or Diameter.
174
802.1X Port-Based Network Access
Control
EAP over
RADIUS
EAP over
LAN
Supplicant
Authenticator
Association
Authentication
Server
(RADIUS)
EAP Request/Identify
EAP Response/Identify
Challenge (auth request)
Response to the challenge
success
Authenticator may set restrictions on the access.
175
New Product: Wireless Switch
What is the issue?
It is not cost effective to implement 802.1X on all
Access points. It is also a management issue.
Authenticator
(Wireless Switch)
RADIUS
If a switch supports 802.1X, could it
perform the same function?
Supplicant
176
802.11i Security Management
EAP over
RADIUS
EAP over
LAN
Supplicant
Authenticator
Authentication
Server
(RADIUS)
Security discovery capability
802.1X Authentication
Key Management
Key Distribution
encryption
Data Protection
177
802.11i Data Protection (encryption)


Need to replace or improve WEP
Wi-Fi Protected Access (WPA) and WPA2



Temporal Key Integrity Protocol (TKIP)





A wrapper around WEP
Use MAC address to create unique key for each station.
Change temporal key every 10,000 packets
It is interoperable with WEP-only device
Advanced Encryption Standard (AES)


This is included in 802.11i.
WPA uses TKIP for encryption.
This is to completely replace WEP.
802.11i makes use of the Advanced Encryption Standard (AES) block
cipher, whereas WEP and WPA use the RC4 stream cipher
178
Windows 7 Wireless Adaptor card options
Security Type:
WPA(2)
shared
802.X
open
CCKM
Encryption Type:
Network security key:
TKIP, AES, WEP
….
Network Authentication:
(P)EAP
(L)
EAP -FAST -SIM –TTLS -AKA
179
WLAN enhancements
180
WLAN Performance
802.11b
802.11a
802.11g
Link Rate
(max)
UDP
11M bps
54M bps
54M bps
7.1M bps
30.5M bps
30.5M bps
TCP
5.9M bps
24.4M bps
24.4M bps
The test was conducted in a lab environment, and the distance is expected to be
less than 10m.
Ref. “WLAN Testing with IXIA IxChariot,” IXIA White Paper
181
WLAN enhancements
• See paper WLAN enhancements
Performance enhancement of WLANs
Methods for improving WLANs performance employ:
• Enhanced hardware in the Physical Layer to achieve better
physical (PHY) layer parameters, such as shorter Slot Time
and shorter Short Inter-Frame Space (SIFS).
• Better tuning of WLAN parameters, such as Fragmentation
Threshold and RTS Threshold [2].
• Adaptive (rather than basic) back-off algorithms in the
MAC layer].
• Proxy approaches in the link-layer, such as snoop protocol.
• Split-connection approaches, such as I-TCP or M-TCP
182
IEEE 802.11 – enhancements
• IEEE 802.11e
– MAC enhancements for providing some QoS
• No QoS in the DCF operation mode
• Some QoS guarantees can be given only via polling using PCF
• For applications such as audio, video, or media stream, distribution
service classes have to be provided
– For this reason, MAC layer must be enhanced
183
WLAN new technologies
802.11ac
Based on Xirrus http://wifi.xirrus.com/abcs11ac?elq=502ceecd98ba417d93b3514b0bb15391&elqCampaignId=29
184