Download MOBILE/WIRELESS NETWORKS

WIRELESS LAN (WLAN)
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Selected topics
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Introduction – WLAN Standards
WLAN definition
WLAN characteristics
WLAN design goals
Infrared vs radio transmission
Infrastructure-based vs ad-hoc networks
IEEE 802.11
WLAN Roaming
WLAN Security
» Other technologies
1
Introduction

Several WLAN standards:
– 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 released a new WLAN standard, 802.11a,
operating at 5 GHz and offering gross data rates of 54
Mbit/s
» uses the same physical layer as HiperLAN2 does

tries to give QoS guarantees
– IEEE 802.11g offering up to 54 Mbit/s at 2.4 GHz.2
WLAN Standards
Wireless
LAN
2.4 GHz
5 GHz
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)
3
WLAN definition
A fast-growing 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 and, additionally, to
introduce a higher flexibility for ad hoc
communication in, e.g., group meetings.
4
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)
Disadvantages:
– typically very low bandwidth compared to wired networks (~11 – 54 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
–
many proprietary solutions offered by companies, especially for higher bit-rates,
standards take their time (e.g., IEEE 802.11) – slow standardization procedures
» standardized functionality plus many enhanced features
» these additional features only work in a homogeneous environment (i.e., when adapters from
the same vendors are used for all wireless nodes)
– products have to follow many national restrictions if working wireless, it takes a very
long time to establish global solutions
5
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 (license-free 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
6
Infrared vs radio transmission
Radio
Infrared light


uses IR diodes, diffuse light reflected
at walls, furniture etc, or directed light
if a LOS exists btn sender and receiver

simple, cheap, available in many mobile
devices (PDAs, laptops, mobile phones)
no licenses needed



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)

IrDA (Infrared Data Association)
interface available everywhere
Advantages
Disadvantages
– very limited license free frequency
bands
– shielding more difficult, interference with
other senders, or electrical devices
Example

typically using the license free frequency
band at 2.4 GHz
– experience from wireless WAN
(microwave links) and mobile phones
can be used
– coverage of larger areas possible (radio
can penetrate (thinner) walls, furniture
etc.)
– higher transmission rates (~11 – 54
Mbit/s)
Disadvantages



Advantages




Example
7
– IEEE 802.11, HIPERLAN, Bluetooth
Infrastructure-based vs ad-hoc
wireless networks (I)
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.
8
Infrastructure-based vs ad-hoc
wireless networks (II)

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.
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.
9
Infrastructure-based vs ad-hoc
wireless networks (III)

Infrastructure-based wireless networks lose some of the
flexibility wireless networks can offer in general:
– They cannot be used for disaster relief in cases where no
infrastructure is left.
10
Infrastructure-based vs ad-hoc
wireless networks (III)
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 higher
» implement medium access mechanisms, forwarding data
11
Infrastructure-based vs ad-hoc
wireless networks (IV)

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 forward the message


IEEE 802.11 and HiperLAN2 are typically infrastructurebased networks, which additionally support ad-hoc
networking
Bluetooth is a typical wireless ad-hoc network
12
IEEE 802.11 (I)



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.
13
WLAN components
Figure 2.11 Photographs of popular 802.11b WLAN equipment.
Access points and a client card are shown on left, and PCMCIA
Client card is shown on right. (Courtesy of Cisco Systems, Inc.)
14
IEEE 802.11 (II)
System Architecture of an infrastructure network
Station
802.11 LAN
STA1
802.x LAN
– terminal with access mechanisms
to the wireless medium and radio
contact to the access point
Basic
BSS1
Portal
Access
Point
Distribution System
Service Set (BSS)
– group of stations using the same
radio frequency
Access
Point
– station integrated into the wireless
LAN and the distribution system
Access
Point
ESS
(STA)
Portal
BSS2
– bridge to other (wired) networks
Distribution
STA2
802.11 LAN
STA3
System
– interconnection network to form
one logical network (EES:
Extended Service Set) based 15
on several BSS
IEEE 802.11 (III)
Stations can select an AP and associate with it.
 The APs support roaming (i.e., changing access
points), the distribution system 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.

16
IEEE 802.11 (IV)

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.
17
IEEE 802.11 (V)
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
18
IEEE 802.11 (VI)

Protocol architecture
– Applications should not notice any difference apart
from the lower bandwidth and perhaps higher access
time from the wireless LAN.
» WLAN behaves like a slow 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
19
IEEE 802.11 (VII)
– 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.
20
IEEE 802.11 (VIII)

Physical layer
– Includes the provision of the Clear Channel
Assessment-CCA signal (energy detection).
– This signal is needed for the MAC mechanisms
controlling medium access and indicates if the medium
is currently idle.
21
IEEE 802.11 (IX)

Medium Access Control
– The basic services provided by the MAC layer are the mandatory
asynchronous data service and an optional time-bounded
service.
– IEEE 802.11 offers only the asynchronous data service in ad-hoc
network mode
– Both service types can be offered using an infrastructure-based
network together with the access point coordinating medium
access.
– The asynchronous service supports broadcast and multicast
packets, and packet exchange is based on a “best-effort” model
» no delay bounds can be given for transmission
» cannot guarantee a maximum access delay or minimum transmission
bandwidth
22

Need for a time-bounded service provision
IEEE 802.11 (X)

Medium Access Control (cnt’d)
– Three basic access mechanisms have been defined for IEEE
802.11
» CSMA/CA (mandatory)
» Optional method avoiding the hidden terminal problem
» A contention-free polling method for time-bounded service

access point polls terminals according to a list
– The first two methods are also summarized as distributed
coordination function (DCF)
– The third method is called point coordination function (PCF)
– DCF only offers asynchronous service, while PCF offers both
asynchronous and time-bounded service, but needs an access
point to control medium access and to avoid contention.
23
IEEE 802.11 (XI)

Medium Access Control (cnt’d)
– The medium can be busy or idle (detected by the CCA)
– If the medium is busy this can be due to data frames or other control frames
– During a contention phase several nodes try to access the medium
» Short inter-frame spacing (SIFS)


the shortest waiting time for medium access
defined for short control messages (e.g., ACK of data packets)
» DCF inter-frame spacing (DIFS)


the longest waiting time used for asynchronous data service within a contention period
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)
SIFS + one slot time
24
IEEE 802.11 (XII)
– Medium Access Control (cnt’d)
» The mandatory access mechanism of IEEE 802.11 is based on
carrier sense multiple access with collision avoidance
(CSMA/CA).

a random access scheme with carrier sense (with the help of the Clear
Channel Assessment-CCA signal of the physical layer) and collision
avoidance through random back-off.
» The standard defines also two control frames:


RTS: Request To Send
CTS: Clear To Send
25
IEEE 802.11 (XIII)
» 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 Inter-Frame
Space (DIFS), the 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)
26
IEEE 802.11 (XIV)
» Unicast data transfer
DIFS
sender
data
SIFS
ACK
receiver
DIFS
other
stations
waiting time
data
t
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
27
IEEE 802.11 (XV)
» Sending unicast packets with RTS/CTS control frames
DIFS
sender
RTS
data
SIFS
receiver
other
stations
CTS SIFS
SIFS
NAV (RTS)
NAV (CTS)
defer access
ACK
DIFS
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)
28
– sender can now send data at once, acknowledgement via ACK
– Other stations store medium reservations distributed via RTS and CTS
Masters thesis
http://eeweb.poly.
edu/dgoodman/fai
nberg.pdf
29
30
Coverage
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.)
31
IEEE 802.11 – enhancements (I)

IEEE 802.11b
– 11Mbit/s
– in 2.4GHz frequency band
– widely used

IEEE 802.11a
– offers up to 54 Mbit/s
– 5 GHz band
» Shading is much more severe compared to 2.4 GHz
» Depending on the SNR, propagation conditions and the distance between
sender and receiver, data rates may drop fast
32
IEEE 802.11 – enhancements (II)

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
IEEE 802.11g
– offers up to 54 Mbit/s
– 2.4 GHz band
– Benefits from the better propagation characteristics at 2.4 GHz
compared to 5 GHz
» Backward compatible to 802.11b
33
WLAN Roaming (I)
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)
Reassociation Response
Laptop A
(with WLAN card)
Reassociation Request
Access
Point
B
Access
Point
A
Laptop C
(with WLAN card)
Registering to Access Point B
34
WLAN Roaming (II)


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
35
WLAN Roaming (III)

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
36
WLAN Security (I)

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 now support 128-bit key level encryption
– Also a VPN could operate on top of the WLAN providing increased security

IEEE developing new standards
– 802.11e (Enhanced Security, QoS)
– 802.11i (Advanced Encryption Standard – AES)
– Requires physical replacement of Access Points and WLAN Cards
37
WLAN Security (II)
Late-2001
Mid-2002
• Rapid re-keying
•802.1X
authentication
•WEP
•IEEE 802.1X
Encryption
ratified
•WEP key
•Mutual
tumbling via
Authentication 802.1X key
•Key
management
Management
•WEP
Encryption
•WEP Attacks go
public
Q1 2003
•Wi-Fi Protected
Access
•802.1X
Authentication
•802.1X key
Management
•TKIP data
protection
Q3 2003
•IEEE 802.11i
•802.1X
authentication
•Enhanced
802.1X key
management
•AES-based
Encryption
•Enhanced
support
infrastructure
38
OTHER WIRELESS
TECHNOLOGIES
EPL657
39
HIPERLAN –
High Performance LAN (I)



The European Telecommunications Standards Institute
(ETSI) standardized HIPERLAN as a WLAN allowing
for node mobility and supporting ad hoc and
infrastructure-based topologies.
It is a wireless LAN supporting priorities and packet life
time for data transfer at 23.5 Mbit/s, including forwarding
mechanisms, topology discovery, user data encryption,
network identification and power conservation
mechanisms.
HIPERLANs operate at 5.1 – 5.3 GHz with a range of
50m in buildings at 1 W transmit power.
40
HIPERLAN –
High Performance LAN (II)
The service offered by a HIPERLAN is compatible
with the standard MAC services known from IEEE
802.x LANs.
 The HIPERLAN Channel Access Control
mechanism was specifically designed to provide
channel access with priorities.
 The CAC contains the access scheme EY-NPMA,
which is unique for HIPERLAN.

41
HIPERLAN –
High Performance LAN (III)

Elimination-yield non-preemptive priority multiple
access (EY-NPMA)
– not only a complex acronym, but also the heart of the channel
access providing priorities and different access schemes.
– divides the medium access of different competing nodes into
three phases:
» Prioritization: Determine the highest priority of a data packet ready to be
sent on competing nodes
» Contention: Eliminate all but one of the contenders, if more than one
sender has the highest current priority.
» Transmission: Finally, transmit the packet of the remaining node.
42
HIPERLAN –
High Performance LAN (IV)
– The contention phase is further subdivided into an
elimination phase and a yield phase.
– The purpose of the elimination phase is to eliminate as
many contending nodes as possible. The result is a
more or less constant number of remaining nodes,
almost independent of the initial number of competing
nodes.
– The yield phase completes the work of the elimination
phase with the goal of only one remaining node.
43
HIPERLAN –
High Performance LAN (IV)
– The contention phase is further subdivided into an
elimination phase and a yield phase.
– The purpose of the elimination phase is to eliminate as
many contending nodes as possible. The result is a
more or less constant number of remaining nodes,
almost independent of the initial number of competing
nodes.
– The yield phase completes the work of the elimination
phase with the goal of only one remaining node.
44
BLUETOOTH (I)
Bluetooth technology aims at so-called ad hoc
piconets, which are local area networks with a very
limited coverage and without the need for an
infrastructure.
 Needed to connect different small devices in close
proximity without expensive wiring or the need for
a wireless infrastructure.
 Represents a single-chip, low-cost, radio-based
wireless network technology.

45
BLUETOOTH (II)
Up to now Bluetooth is not a standard like IEEE
802.11 or HIPERLAN, but it soon become a defacto standard – established by the industry and
promoted by the Bluetooth consortium.
 Bluetooth uses the license-free frequency band at
2.4GHz allowing for worldwide operation.

46
BLUETOOTH (III)

Physical layer:
– A frequency-hopping\time-division duplex scheme is used for
transmission with a fast hopping rate of 1,600 hops per second.
The time between two hops is called a slot, which is an interval
of 625μs, thus each slot uses a different frequency.
– On average, the frequency-hopping sequence ´visits´ each hop
carrier with an equal probability.
– All devices using the same hopping sequence with the same
phase form a Bluetooth piconet.
47
BLUETOOTH (IV)
– With transmitting power of up to 100 mW, Bluetooth
devices have a range of up to 10m (or even up to 100m
with special transceivers).
– Having this power and relying on battery power, a
Bluetooth device cannot be in an active transmit mode
all the time.
– Bluetooth defines several low-power states for the
device.
48
BLUETOOTH (V)
– States of a possible Bluetooth device and possible
transitions:
» Standby mode: Every device which is currently not
participating in a piconet (and not switched off)

In this mode, a device listens for paging messages.
» Connections can be initiated by any device which becomes
the master.

This is done by sending page messages if the device already knows
the address of the receiver, or inquiry messages followed by a page
message if the receiver’s address is unknown.
49
BLUETOOTH (VI)
» To save battery power, a Bluetoth device can go into one of
three low power states if no data is ready to be sent:



PARK state: The device has the lowest duty cycle, and thus the lowest
power consumption. The device releases its MAC address, but
remains synchronized with the piconet. The device occasionally
listens to the traffic of the master device to resynchronize and check
for broadcast messages.
HOLD state: The power consumption of this state is a little higher.
The device does not release its MAC address and can resume sending
at once after transition out of the HOLD state.
SNIFF state: It has the highest power consumption of the low-power
states. The device listens to the piconet at a reduced rate.
50
BLUETOOTH (VII)
unconnected
STANDBY
inquiry
transmit
PARK
page
connected
HOLD
SNIFF
connecting
active
low power
51
BLUETOOTH (VIII)

MAC layer:
– Several mechanisms control medium access in a Bluetooth
system.
– First of all, one device within a piconet acts as a master, all other
devices (up to seven) act as slaves.
– The master determines the hopping sequence as well as the phase
of the sequence.
– All Bluetooth devices have the same networking capabilities, i.e.,
they can be master or slave. The unit establishing the piconet
automatically becomes the master and controls medium access;
all other devices will be slaves.
52
WLAN technologies – summary (I)
The basic goals of all three LAN types (WLAN,
HIPERLAN, BUETOOTH) are the provision of a
much higher flexibility for nodes within a network.
 All WLANs suffer from limitations of the air
interface and higher complexity compared to their
wired counterparts but allow for a new degree of
freedom for their users within rooms, buildings etc.

53
WLAN technologies – summary
(II)



The three technologies differ in some respects.
HIPERLAN comprises many interesting features, is much
more powerful than IEEE 802.11, has a higher data rate
(23.5 Mbit/s) but it is questionable if it will ever be a
commercial success.
The 5 GHz band required for HIPERLAN is not available
worldwide compared to the 2.4 GHz used for IEEE
802.11.
54
WLAN technologies – summary
(III)
No standardization body has set up any
specification regarding Bluetooth.
 The primary goal of Bluetooth is not a complex
standard covering many aspects of wireless
networking, but a quick and very cheap solution
enabling ad hoc personal communication within a
short range in the license-free 2.4 GHz band.

55