WIRELESS LOCAL AREA NETWORKS
Ian F. Akyildiz
Broadband & Wireless Networking Laboratory
School of Electrical and Computer Engineering
Georgia Institute of Technology
Tel: 404-894-5141; Fax: 404-894-7883
Email: ian@ece.gatech.edu
Web: http://www.ece.gatech.edu/research/labs/bwn
WIRELESS LANs
Infrastructure
Network
AP
AP
AP: Access Point
wired network
AP
Ad-hoc Network
IFA’2004
2
Wireless LAN - IEEE 802.11
Reference Architecture
 The terminology and some of the specific
features are unique to this standard and are
not reflected in all commercial products.
 However, it is useful
to be familiar with
the standard since its
features are
representative of the
Wireless LAN
capabilities required.
IFA’2004
3
Reference Architecture of
Wireless LANs
STA1
BSS1
Portal
Access
Point
Distribution System
Access
Point
BSS2
STA2
STA3
ESS
IFA’2004
Station (STA)
– terminal with access mechanisms
to the wireless medium and
radio contact to the access
point
Basic Service Set (BSS)
– group of stations using the same
radio frequency
Access Point
– station integrated into the
wireless LAN and the
distribution system
Portal
– bridge to other (wired) networks
Distribution System
– interconnection network to form
one logical network (ESS:
Extended Service Set) based
on several BSS
4
Reference Architecture
 The smallest building block of a wireless LAN is a
basic service set (BSS), which consists of some
number of stations executing the same MAC protocol
and competing for access to the same shared medium.
 A basic service set may be isolated or it may connect
to a backbone distribution system through an access
point.
 The access point functions as a bridge.
 The MAC protocol may be fully distributed or
controlled by a central coordination function housed in
the access point.
IFA’2004
5
Reference Architecture
 The basic service set generally corresponds to what
is referred to as a cell in the literature.
 An extended service set (ESS) consists of two or
more basic service sets interconnected by a
distribution system.
 Typically, the distribution system is a wired
backbone LAN.
 The extended service set appears as a single logical
LAN to the logical link control (LLC) level.
IFA’2004
6
Reference Architecture
The standard defines three types of stations
based on mobility:
 No transition: A station of this type is either stationary or
moves only within the direct communication range of the
communicating stations of a single BSS.
 BSS transition: This is defined as a station movement from
one BSS to another BSS within the same ESS. In this
case, delivery of data to the station requires that the
addressing capability be able to recognize the new location
of the station.
 ESS transition: This is defined as a station movement from
a BSS in one ESS to a BSS within another ESS. This case
is supported only in the sense that the station can move.
IFA’2004
7
Protocol Architecture
fixed
terminal
mobile terminal
infrastructure
network
access point
application
application
TCP
TCP
IP
IP
LLC
LLC
LLC
MAC
MAC
802.3 MAC
802.3 MAC
PHY
PHY
802.3 PHY
802.3 PHY
IFA’2004
8
Protocol Layers and Functions
PLCP
Physical Layer Convergence
Protocol
 MAC
clear channel assessment
signal (carrier sense)
– access mechanisms,
fragmentation, encryption
PMD
 MAC Management
modulation, coding
MAC
MAC Management
PLCP
PHY Management
PMD
IFA’2004
PHY Management
Station Management
PHY
DLC
– synchronization, roaming,
MIB, power management
LLC
Physical Medium Dependent
channel selection, MIB
Station Management
coordination of all
management functions
9
Basics of Wireless LANs
 Coverage area, data rate, and battery
consumption.
 Characterized by small coverage areas
(~200m), but relatively high bandwidths
(data rates) (upto 50Mbps currently)
 Major standards
– WLAN: IEEE 802.11 and HIPERLAN.
– WPAN: IEEE 802.16 (Bluetooth) and
HomeRF
IFA’2004
10
Advantages of WLANs
– Very flexible within the reception area
– Users can access high speed multimedia applications
anywhere at anytime, with easy implementation, low
cost, and wide user acceptance
- Generally works in industrial, scientific, and
medical (ISM) band, which is un-licensed and
available for public.
– (Almost) no wiring difficulties (e.g. historic buildings,
firewalls)
IFA’2004
11
WLANs – Advantages
 Buildings with large open areas, such as
manufacturing plants, stock exchange
trading floors, and warehouses
 Historical buildings with insufficient
twisted pair and where drilling holes for
new wiring is prohibited
 Small offices where installation and
maintenance of wired LANs is not
economical
IFA’2004
12
Disadvantages of WLANs
– Typically very low bandwidth compared to wired
networks (1-10 Mbit/s)
– Many proprietary solutions, especially for higher bitrates, standards take their time.
– Products have to follow many national restrictions if
working wireless, it takes a very long time to
establish global solutions.
– Interference Problems
IFA’2004
13
Family of Wireless LAN
(WLAN) Standards 802.11
–
–
–
–
–
–
–
802.11a - 5GHz- Ratified in 1999
802.11b - 11Mb 2.4GHz- ratified in 1999
802.11d - Additional regulatory domains
802.11e - Quality of Service
802.11f - Inter-Access Point Protocol (IAPP)
802.11g - Higher Data rate (>20mBps) 2.4GHz
802.11h - Dynamic Frequency Selection and
Transmit Power Control mechanisms
– 802.11i - Authentication and security
IFA’2004
14
WLANs – Current Use













Home wireless networks.
Enterprise wireless networks.
Public access.
Hospitals.
Warehouses.
Consulting and audit teams
Dynamic environments, ad agencies, etc.
Universities
Historic buildings, older buildings.
Meeting rooms.
Retail stores
Restaurants and car rental agencies
Data backup.
IFA’2004
15
In-Building Deployment –
Service Objectives












Greater coverage
High-speed rates
Scalable and manageable bandwidth
Enable new (high-end) services (and keep running the
good old ones)
Service differentiation
Smooth deployment and low maintenance
Interoperable systems
Plug & Play
Extends the local area network
Freedom to access the corporate network
Comparable to those of wired networks
Secure access to important information (e-mail,
corporate data, Internet)
IFA’2004
16
Some Facts

By 2005, more than 1/3rd of Internet users will have
Internet connectivity through a wireless enabled device
(750 million users)!!! (Source: Intermarket group)

By the end of 2001, more than half of the workforce in
the US uses a wireless net device – primarily cellular
phones! (Source: Cahners Intat Group)

By the year 2004 revenue from wireless data will reach
$34B, and by the year 2010 the number of wireless data
subscribers will hit 1B!!
IFA’2004
17
Design Goals for Wireless LANs
– Global, seamless operation (must sell in all countries)
– Low power for battery use (power saving modes and power
management functions)
– No special permissions or licenses needed to use the LAN
– Robust transmission technology (avoid interference)
– Simplified spontaneous cooperation at meetings
– Easy to use for everyone, simple management
– Protection of investment in wired networks (interoperable
with wired LANs)
– Security (no one should be able to read my data), privacy
(no one should be able to collect user profiles), safety (low
radiation)
IFA’2004
18
Topologies
- Single-Cell Wireless LAN
IFA’2004
19
Topologies
- Single-Cell Wireless LAN
 In Figure there is a backbone wired LAN, such as Ethernet, that
supports servers, workstations, and one or more bridges or routers to
link with other networks.
 In addition, there is a control module (CM) (Access Point (AP) before)
that acts as an interface to a wireless LAN. (CM = AP)
 The control module includes either bridge or router functionality to link
the wireless LAN to the backbone.

In addition, it includes some sort of access control logic, such as a
polling or token-passing scheme, to regulate the access from the end
systems.
 Note that some of the end systems are stand-alone devices such as a
workstation or a server.
 In addition, hubs or other user modules (UM) (PORTAL before) that
control a number of stations off a wired LAN may also be part of the
wireless LAN configuration.
IFA’2004
20
Topologies
- Multiple Cell Wireless LAN
Figure 2
IFA’2004
21
Topologies
- Multiple Cell Wireless LAN
 In this case there are multiple control
modules interconnected by a wired LAN.
 Each control module supports a number of
wireless end systems within its transmission
range.
 For example, with an infrared LAN,
transmission is limited to a single room;
therefore, one cell is needed for each room in
an office building that requires wireless
support.
IFA’2004
22
WLANs – 802.11
Protocol Architecture
Real Time Traffic
Normal Data Traffic
(Asynchronous)
Point Coordination
Function (PCF)
MAC
Distributed Coordination Function (DCF)
Physical Layer (PHY)
IFA’2004
23
IEEE 802.11
- Physical Medium
Specification
Three Physical Media:
 INFRARED
Narrowband Microwave
Spread Spectrum
IFA’2004
24
Infrared

Infrared signals used to transmit data (similar to TV remotes!)

Higher data rates possible (than spread spectrum)


Line of sight point-to-point configuration required (or reflection
surface that reflects signals)
Too sensitive to obstacles, line-of-sight requirement, etc.

850-950 nm, diffuse light (to allow point-to-multipoint
communication)

10 m maximum range with no sunlight or heat interfere
IFA’2004
25
Narrowband Microwave
 Typically used to link two WLANs together (for
example, to link WLANs in two buildings)
 Microwave dishes required at both ends of link
 Unlike spread spectrum which operates in the
unlicensed ISM band, narrowband microwave requires
FCC licensing
 Exclusive license typically effective within a 17.5 mile
radius
IFA’2004
26
Spread Spectrum
 Distributed signals over multiple frequencies (to avoid
eavesdropping or jamming)
 Frequency Hopping Spread Spectrum (FHSS)
– Sender transmits over a seemingly random series of frequencies
– Intended receiver aware of sequence of frequencies and hops
accordingly
– Allows the coexistence of multiple networks in the same area by
using different hopping sequences
 Direct Sequence Spread Spectrum (DSSS)
– Sender transmits redundant information called “chips” between
actual data bits
– Intended receiver aware of spread removes redundant information
accordingly
– Preamble and header of a frame is always transmitted with 1
Mbit/s, rest of transmission 1 or 2 Mbit/s
IFA’2004
27
Wireless LAN Classification

Infrared (IR) LANs
– An individual cell of an IR LAN is limited to a single room,
since infrared light does not penetrate opaque walls.
 Spread Spectrum LANs
– In most cases these LANs operate in the ISM (industrial,
scientific, and medical) bands, so no FCC licensing is
required for their use in the United States.
 Narrowband Microwave LANs
– These LANs operate at microwave frequencies but do no
use spread spectrum. Some of these products operate at
frequencies that require FCC licensing; others use one of
the unlicensed ISM bands.
– Table 1 summarizes some of the key characteristics of
these three technologies; the details are explored in the
next three subsections.
IFA’2004
28
Comparison: Infrared
vs. Radio Transmission
 Infrared
– uses IR diodes, diffuse light,
multiple reflections (walls,
furniture etc.)
 Advantages
– simple, cheap, available in many
mobile devices
– no licenses needed
 Disadvantages
– interference by sunlight, heat
sources etc.
– many things shield or absorb IR
light
– low bandwidth
– Line of Sight Problem
 Example
– IrDA (Infrared Data
Association) interface available
everywhere: PDAs, calculators,
laptops, mobile phones...
IFA’2004
 Radio
– typically using the license
free ISM band at 2.4 GHz
 Advantages
– experience from wireless
WAN and mobile phones can
be used
– coverage of larger areas
possible (radio can penetrate
walls, furniture etc.)
 Disadvantages
– very limited license free
frequency bands
– shielding more difficult,
interference with other
electrical devices
 Example
– WaveLAN, HIPERLAN,
Bluetooth
29
Overview of WLAN Classification
IFA’2004
30
Wireless LAN MAC
 CSMA as Wireless MAC?
 Hidden and Exposed Terminal
Problems make the use of CSMA an
inefficient technique
IFA’2004
31
Hidden Terminal Problem
Collision
A





B
C
A talks to B
C senses the channel
C does not hear A’s transmission (out of range)
C talks to B
Signals from A and B collide
IFA’2004
32
Exposed Terminal Problem
A




B
C
Not
possible
D
B talks to A
C wants to talk to D
C senses channel and finds it to be busy
C stays quiet (when it could have ideally
transmitted)
IFA’2004
33
Hidden and Exposed
Terminal Problems
 Hidden Terminal
– More collisions
– Wastage of resources
 Exposed Terminal
– Underutilization of channel
– Lower effective throughput
IFA’2004
34
MACA - Collision Avoidance
 MACA (Multiple Access with Collision Avoidance) uses short
signaling packets for collision avoidance
– RTS (request to send): a sender request the right to send
from a receiver with a short RTS packet before it sends a
data packet
– CTS (clear to send): the receiver grants the right to send
as soon as it is ready to receive
 Signaling packets contain
– sender address
– receiver address
– packet size
 Variants of this method can be found in IEEE802.11 as
DFWMAC (Distributed Foundation Wireless MAC)
IFA’2004
35
Hidden Terminal
Revisited …
A





RTS
CTS
DATA
B
CTS
C
A sends RTS
B sends CTS
C overheads CTS
C inhibits its own transmitter
A successfully sends DATA to B
IFA’2004
36
Hidden Terminal Revisited
 How does C know how long to wait before it
can attempt a transmission?
 A includes length of DATA that it wants to
send in the RTS packet
 B includes this information in the CTS
packet
 C, when it overhears the CTS packet,
retrieves the length information and uses it
to set the inhibition time
IFA’2004
37
Exposed Terminal Revisited
A
RTS
CTS





B
RTS
C
Cannot hear CTS
D
Tx not
inhibited
B sends RTS to A (overheard by C)
A sends CTS to B
C cannot hear A’s CTS
C assumes A is either down or out of range
C does not inhibit its transmissions to D
IFA’2004
38
Collisions
 Still possible – RTS packets can collide!
 Binary exponential backoff performed by
stations that experience RTS collisions
 RTS collisions not as bad as data collisions
in CSMA (since RTS packets are typically
much smaller than DATA packets)
IFA’2004
39
Drawbacks
 Collisions still possible if CTS packets
cannot be heard but carry enough to
cause significant interference
 If DATA packets are of the same
size as RTS/CTS packets, significant
overheads
IFA’2004
40
WLANs – 802.11
Protocol Architecture
Real Time Traffic
Contention Free Service
Normal Data Traffic
(Asynchronous)
Contention Service
Point Coordination
Function (PCF)
MAC
Distributed Coordination Function (DCF)
Physical Layer (PHY)
IFA’2004
41
IEEE 802.11
- Medium Access Control
– Distributed Mode: Distributed Coordination Function (DCF)
* Based on CSMA/CA protocol
* Uses a contention algorithm to provide access to all traffic.
* Ordinary asynchronous traffic uses DCF directly.
– Coordinated Mode: Point Coordination Function (PCF)
*
*
*
*
Supports real time traffic
Based on polling which is controlled by a centralized point coordinator.
Uses a centralized MAC algorithm and provides contention-free service.
PCF is built on top of DCF and exploits features of DCF to assure
access for its users.
Both the DCF and PCF can operate concurrently within
the same BSS to provide alternative contention and
contention-free periods
NOTE:
IFA’2004
42
802.11 - MAC Layer Overview
– DFWMAC-DCF CSMA/CA (Mandatory)
(Distributed Foundation Wireless Medium Access Control
- Distributed Coordinated Function CSMA/CA)
– DFWMAC-DCF w/ RTS/CTS (Optional)
Distributed Foundation Wireless MAC
Avoids Hidden Terminal problem
– DFWMAC- PCF (Optional)
Access point polls terminals according to a list
IFA’2004
43
802.11 - CSMA/CA
Access Method DFWMAC-DCF CSMA/CA
– A station with a frame to transmit senses the medium.
– If the medium is idle, it waits to see if the medium
remains idle for a time equal to IFS. If so, the station
may transmit immediately.
– If the medium is busy (either because the station initially
finds the medium busy or because the medium becomes
busy during the IFS idle time), the station defers
transmission and continues to monitor the medium until the
current transmission is over.
– Once the current transmission is over, the station delays
another IFS.
– If the medium remains idle for this period, the station
backs off using a binary exponential backoff scheme and
again senses the medium.
– If the medium is still idle, the station may transmit.
IFA’2004
44
802.11 - CSMA/CA
Access Method I
DIFS
DIFS
medium busy
direct access if
medium is free  DIFS
IFA’2004
contention window
(randomized back-off
mechanism)
next frame
t
slot time
45
802.11 - CSMA/CA Access Method II
Interframe Spaces (IFS)
 Priorities
– Defined through different inter frame spaces
– SIFS (Short Inter Frame Spacing)
 highest priority, for ACK, CTS, polling response
– PIFS (PCF IFS) - Point Coordination Function Inter-Frame
spacing
 medium priority, for real time service using PCF
 SIFS + one slot time
– DIFS (DCF, Distributed Coordination Function IFS)
 lowest priority, for asynchronous data service
 SFIS + two slot times
IFA’2004
46
Interframe Spaces (IFS)
DIFS
DIFS
medium busy
PIFS
SIFS
direct access if
medium is free  DIFS
IFA’2004
contention
next frame
t
47
IEEE 802.11
- Medium Access Control
In Figure we illustrate the use of these time values.
IFA’2004
48
802.11 - CSMA/CA
Access Method II
– Station has to wait for DIFS before sending data
– Receivers acknowledge at once (after waiting for SIFS) if
the packet was received correctly (CRC))
– Automatic retransmission of data packets in case of
transmission errors
DIFS
sender
data
SIFS
receiver
ACK
DIFS
other
stations
waiting time
IFA’2004
data
t
contention
49
802.11 – DFWMAC w/
RTS/CTS
– Station can send RTS (request to send) with reservation parameter
after waiting for DIFS (reservation determines amount of time the
data packet needs the medium)
– Every node receiving the RTS has to set its Net Allocation Vector
(NAV) in accordance with the duration of the field (NAV specifies
the earliest point at which the station can try to access the
medium
– If receiver receives RTS, it sends CTS (Clear to Send) after
SIFS. CTS again contains duration field and all stations receiving
this packet need to adjust their NAV
– Sender can now send data after SIFS, acknowledgement via ACK
by receiver after SIFS
IFA’2004
50
IEEE 802.11
- Medium Access Control
 Clear to Send (CTS)
– A station can ensure that its data frame will get through by
first issuing a small request to send (RTS) frame.
– The station to which this frame is addressed should respond
immediately with a CTS frame if it is ready to receive.
– All other stations receive the RTS and defer using the
medium until they see a corresponding CTS or until a
timeout occurs.
– PIFS is used by the centralized controller in issuing polls
and takes precedence over normal contention traffic.
– However, those frames transmitted using SIFS have
precedence over a PCF poll.
– Finally, the DIFS interval is used for all ordinary
asynchronous traffic.
IFA’2004
51
802.11 – DFWMAC w/
RTS/CTS
DIFS
sender
RTS
data
SIFS
receiver
other
stations
IFA’2004
CTS SIFS
SIFS
NAV (RTS)
NAV (CTS)
defer access
ACK
DIFS
data
t
contention
52
IEEE 802.11
- Medium Access Control
 Point Coordination Function
– PCF is an alternative access method implemented
on top of the DCF.
– The operation consists of polling with the
centralized polling master (point coordinator).
– The point coordinator makes use of PIFS when
issuing polls.
– Because PIFS is smaller than DIFS, the point
coordinator can seize the medium and lock out
all asynchronous traffic while it issues polls and
receives responses.
IFA’2004
53
IEEE 802.11
- Medium Access Control
 Point Coordination Function (Cont.)
– A wireless network is configured so that a number of
stations with time-sensitive traffic are controlled by the
point coordinator while remaining traffic contends for
access using CSMA.
– The point coordinator could issue polls in a round-robin
fashion to all stations configured for polling.
– When a poll issued, the polled station may respond using
SIFS.
– If the point coordinator receives a response, it issues
another poll using PIFS.
– If no response is received during the expected turnaround
time, the coordinator issues a poll.
IFA’2004
54
IEEE 802.11
- Medium Access Control
 Point Coordination Function (Cont.)
Figure 18 illustrates the use of the superframe.
IFA’2004
Figure 18
55
IEEE 802.11
- Medium Access Control
 Point Coordination Function (Cont.)
– At the beginning of a superframe, the point
coordinator may optionally seize control
and issues polls for a given period of time.
– This interval varies because of the variable
frame size issued by responding stations.
– The remainder of the superframe is
available for contention-based access.
IFA’2004
56
IEEE 802.11
- Medium Access Control
 Point Coordination Function (Cont.)
– At the end of the superframe interval, the
point coordinator contends for access to
the medium using PIFS.
– If the medium is idle, the point
coordinator gains immediate access and a
full superframe period follows.
– However, the medium may be busy at the
end of a superframe.
– In this case, the point coordinator must
wait until the medium is idle to gain
access; this results in a foreshortened
superframe period for the next cycle.
IFA’2004
57
DFWMAC-PCF I
 The access mechanisms presented so far cannot
guarantee a maximum access delay or minimum
transmission bandwidth.
 To provide a time bounded service, the standards
specify a Point Coordination Function (PCF) on top of
the DCF mechanisms.
 Using PCF requires an access point that can controls
medium access and polls the single nodes. Ad Hoc
networks cannot use this function.
IFA’2004
58
DFWMAC-PCF I
 At time t0 the contention-free period should start,
but another station is transmitting data
 After the medium has been idle, the PCF has to
wait for PIFS before accessing the medium.
 The point coordinator now sends data D1 to the
first station. The station can answer after SIFS.
After waiting for SIFS, the point coordinator can
poll the second station by sending D2.
 The second station replies with U2
IFA’2004
59
DFWMAC-PCF I
t0 t1
medium busy PIFS
point
coordinator
wireless
stations
stations‘
NAV
IFA’2004
SuperFrame
SIFS
D1
SIFS
SIFS
D2
SIFS
U1
U2
NAV
60
DFWMAC-PCF II
 Polling continues with the third node which has nothing to answer.
 After waiting for PIFS, the point coordinator can issue an end
marker (CFend), indicating that the contention period may start
again.
 The cycle starts again with the next superframe
t2
point
coordinator
wireless
stations
stations‘
NAV
IFA’2004
D3
PIFS
SIFS
D4
t3
t4
CFend
SIFS
U4
NAV
contention free period
contention
period
t
61
WLAN: IEEE 802.11b

Data rate
–
–

Transmission range
–
–

Free 2.4 GHz ISM-band


Connection set-up time
–
Connectionless/always on
–
Typ. Best effort, no guarantees
(unless polling is used, limited
support in products)
Quality of Service
Manageability
–

–
Limited, WEP (Wired Equivalent
Privacy) insecure, SSID
Availability
–
Many products, many vendors
IFA’2004
Limited (no automated key
distribution, sym. Encryption)
Special Advantages/Disadvantages
Security
–

300m outdoor, 30m indoor
Max. data rate ~10m indoor
Frequency
–

1, 2, 5.5, 11 Mbit/s, depending
on SNR
User data rate max. approx. 6
Mbit/s

–
Advantage: many installed
systems, lot of experience,
available worldwide, free ISMband, many vendors, integrated
in laptops, simple system
Disadvantage: heavy
interference on ISM-band
(Industrial, Scientific, Medical
band), no service guarantees,
slow relative speed only
62
WLAN: IEEE 802.11a

Data rate
–
–
6, 9, 12, 18, 24, 36, 48, 54
Mbit/s, depending on SNR
User throughput (1500 byte
packets): 5.3 (6), 18 (24), 24
(36), 32 (54)
6, 12, 24 Mbit/s mandatory
–
100m outdoor, 10m indoor
–

Transmission range

E.g., 54 Mbit/s up to 5 m, 48 up to 12
m, 36 up to 25 m, 24 up to 30m, 18 up
to 40 m, 12 up to 60 m

Frequency

Security

Availability
–
Free 5.15-5.25, 5.25-5.35,
5.725-5.825 GHz ISM-band
–
Limited, WEP insecure, SSID
–
Some products, some vendors
IFA’2004
 Connection set-up time
– Connectionless/always on
 Quality of Service
– Typ. best effort, no
guarantees (same as all
802.11 products)
 Manageability
– Limited (no automated key
distribution, sym.
Encryption)
 Special
Advantages/Disadvantages
– Advantage: fits into 802.x
standards, free ISM-band,
available, simple system,
uses less crowded 5 GHz
band
– Disadvantage: stronger
shading due to higher
frequency, no QoS
63
WLAN: IEEE 802.11 – Future
Developments (08/2002)


802.11d: Regulatory Domain Update – completed
802.11e: MAC Enhancements – QoS – ongoing
–

802.11f: Inter-Access Point Protocol – ongoing
–



Establish an Inter-Access Point Protocol for data exchange via the
distribution system.
802.11g: Data Rates > 20 Mbit/s at 2.4 GHz; 54 Mbit/s, OFDM – ongoing
802.11h: Spectrum Managed 802.11a (DCS, TPC) – ongoing
802.11i: Enhanced Security Mechanisms – ongoing
–

Enhance the current 802.11 MAC to expand support for applications with
Quality of Service requirements, and in the capabilities and efficiency of the
protocol.
Enhance the current 802.11 MAC to provide improvements in security.
Study Groups
–
–
–
5 GHz (harmonization ETSI/IEEE) – closed
Radio Resource Measurements – started
High Throughput – started
IFA’2004
64
WLANs – 802.11
Compatibility
 802.11a and 802.11b share the same MAC layer
 Significant differences at the physical layer.
– 802.11b: 2.4 GHz, ISM band,
– 802.11a: 5 GHz, U-NII band
– possible to operate both on the same network
concurrently (using the same access points)
 Interoperability
– WECA (Wireless Ethernet Compatibility Alliance):
organization behind Wi-Fi that certifies products
meeting the 802.11b specification
IFA’2004
65
WLANs – Comparison
of Technologies
Characteristic
802.11
802.11b
802.11a
HiperLAN/2
Spectrum
2.4GHz
2.4GHz
5GHz
5GHz
Max Physical Rate
2Mb/s
11Mb/s
54Mb/s
32Mb/s
Max data rate layer 3
1.2Mb/s
5Mb/s
32Mb/s
32Mb/s
Medium access
CSMA/MA
TDMA/TDD
Connectivity
Conn.-less
Conn.-less
Conn.-less
Conn.-oriented
Multicast
Yes
Yes
Yes
Yes
QoS support
No
No
No
Yes
Frequency selection
FH/DSSS
DSSS
Single carrier
Single carrier with
Dynamic Frequency
Selection
Authentication
No
No
No
NAI/IEEE
address/X.509
Encryption
40bit RC4
40bit RC4
40bit RC4
DES, 3DES
Handover support
No
No
No
No
Fixed network support
Ethernet
Ethernet
Ethernet
Ethernet, IP, ATM
UMTS, FireWire, PPP
Management
802.11MIB
802.11MIB
802.11MIB
HiperLAN/2MIB
Radio link quality control
No
No
No
Link adaptation
IFA’2004
66
WLANs – Interference
 An important issue in all wireless systems
because nearby users occupy same bandwidth
and cause co-channel interference.
 For WLANs, in addition to co-channel
interference, other types of interference exist
mainly due to the use of unlicensed ISM band.
Interference to WLANs comes from the
following major sources:
– Co-channel interference.
– Interference from non-wLAN devices in the
same frequency band
– Interference between different wLANs in
the same frequency band.
IFA’2004
67
WLANs – Interference
Reduction

Regulatory and standards
– Change frequency segment of a channel (proposed by
FCC for Bluetooth)
 Usage and practices
– When one wLAN is working, others are banned. Not
practical. Modal operation of wLANs can be a good
alternative
 Technical approaches.
– Driver layer: software above the MAC layer can be
installed for different types of wLANs. Thus,
software switch from one wLAN to another wLAN is
required.
– MAC layer: more attractive but it is an on-going
research topic.
– Physical layer: signal processing techniques and antiIFA’2004 jamming schemes used
68
WLANs – Environmental Issues
 Has been proved that WLAN is safe for health
– Radiation used by this technology, fall well within the limits
of safety guidelines (both in terms of frequency content
and power level) specified by Radio Frequency Safety
Standards and Recommendations.
– Radiation in this frequency range is non-ionizing (as they do
not have enough energy to break the chemical bonds of
genetic material of body cells).
– Vendors designing their products to operate within the
power limit set by the Safety Standards.
 Others
– No wires to hide and maintain
– Looks much better and cleaner compared to the wired one.
– Positive impact on user psychology due to user mobility,
reduced-cost of ownership, real-time-access to
information,
IFA’2004
69
WLANs – Major Suppliers
Three types of products in WLANs:
– Access Point
– LAN Adapters and
– LAN Bridges.
IFA’2004
70
WLANs –Access Point Products
Major
Suppliers
Agere
Products
Standard
Throughput
WEP
Encryption
AP-1000
Operating
Frequency
2.4GHz
802.11b
11Mbps
-
Avaya
I/700022270
2.4GHz
802.11b
11Mbps
-
Nokia
A032
2.4GHz
802.11b
11Mbps
64- and 128-bit
Nortel
e-mobility DSSS
AP DR4000E02
2.4GHz
802.11b
11Mbps
-
Intel
PRO/Wireless
2011B
PRO/Wireless
5000
I-GATE 11M
2.4GHz
802.11b
11Mbps
64- and 128-bit
5.2 GHz
802. 11b
54 Mbs
64- and 128-bit
2.4GHz
802.11b
11Mbps
11Mbps
2.4GHz
802.11b
11Mbps
64-bit
Fujitsu
Access Point
6000
MBH8MB01
2.4GHz
Bluetooth
732.2kbps
-
Compaq
WL510
2.4GHz
802.11b
11Mbps
128bit
Agere
AP-1000
2.4GHz
802.11b
11Mbps
128bit
Apple
AirPort
2.4GHz
802.11b
11Mbps
-
IBM
AP500
2.4GHz
802.11b
11Mbps
-
Cisco
Aironet® 350
Series
2.4GHz
802.11b
11Mbps
-
Intel
Siemens
3Com
IFA’2004
71
WLANs – LAN Adapter
Products
Major
suppliers
Products
Operating
Frequency
Standard
Throughput
Range
Intel
PRO/Wireless
2011B LAN
PC Card
2.4GHz
802.11b
11Mbps
-
Intel
PRO/Wireless
5000 LAN
PCI Adapter
5.2GHz
802.11a
54Mbps
100 feet
Compaq
WL110
2.4GHz
802.11b
11Mbps
-
Agere
ORiNOCO PC
Card
2.4GHz
802.11b
11Mbps
-
Apple
Airport Client
Card
2.4GHz
802.11b
11Mbps
-
Cisco
Aironet® 350
Series
2.4GHz
802.11b
11Mbps
-
IFA’2004
72
WLANs –Bridge Products
Major
suppliers
Products
Operating
Frequency
Standard
Throughput
Range
Intel
Wireless
Gateway
2.4GHz
802.11b
11Mbps
-
Compaq
WL310
2.4GHz
802.11b
11Mbps
160m(525feet)
Agere
RG-1100
Broadband
Gateway
2.4GHz
802.11b
11Mbps
550m(1750ft)
Agere
ORiNOCO
PC Card
2.4GHz
802.11b
11Mbps
-
Cisco
Aironet®
350 Series
Wireless
Bridge
2.4GHz
802.11b
11Mbps
25 miles (40.2
km)
IFA’2004
73
WLANs – Market Segment
by Wireless LAN Association (wLANA )
IFA’2004
74
WLANs – Market Forecast
 Seen as "The Technology" of the future.
– Trend support: Decrease of product price and increase
of network speed,
– Apart from current sectors, more and more new
markets are opening up for this technology. Some of
these are shipping and receiving area, distribution
center, cafeteria, home, train, bus, airport, sport
complexes, trade shows, coffee shops, etc.
 Huge market potential
– Make WLAN as a viable option to connect the
developing nations to the developed part of the world,
making the concept "Global Village" a reality.
 Estimated a five-fold increase in its market by 2005
IFA’2004
75
WLANs – Technology Forecast
 WLAN will have higher speeds
– Wi-Fi5 for IEEE 802.11a up to 54Mbps
– 5.7-GHz band promises to allow for the next
breakthrough data rate of 100 Mbps.
 Provide multimedia and access to 3G-4G Systems
– HIPERLAN/2 to provide high speed access (up to 54
Mbit/s at PHY layer) to 3G mobile core networks, and
Internet.
 More security guarantees.
– Enhancements to Wired Equivalent Privacy (WEP)
IFA’2004
76
WLANs – Technology Forecast
 Even more decreased size, cost, power
consumption
 New approaches in handling network
parameters dynamically to improve
throughput.
 Improved reliability
 Efficient and concurrent uses of
bandwidth via Wideband Orthogonal
Frequency Division Multiplexing (WOFDM)
IFA’2004
77
WLANs – Service Analysis
 Convergence for voice and data networks
– As voice, audio and video are shared among
WLAN-enabled phones, MP3 players, web
cameras, interactive TVs, etc, wireless
applications will move beyond traditional
computer networking.
 Replacement of proprietary cables.
 Portable access to wireless LAN.
 Multimedia over wireless networks.
 Wireless remote data access.
 Data Backup.
IFA’2004
78
WLANs – Network
Design Plan Analysis
 Key parameters:
–
–
–
–
number of expected users
area of coverage
quality of service
service types
 Network topology for WLAN can broadly
be classified into:
– Ad-Hoc WLAN
– Client-Server based WLAN
IFA’2004
79
WLANs – Performance Metrics
 Overall coverage area
– Can be evaluated in terms of received signal
strength intensity (RSSI)
 Throughput
– Can be evaluated by measuring TCP
connection throughputs since wLANs establish
a client-server communication link via TCP
connection,
 Implementations of handoff and dropping are
the responsibility of manufacturers, since they
vary according to different equipments
IFA’2004
80
WLANs – Network
Security Aspects
 Service set identifier (SSID) (can be configured into 802.11
APs)
 Use VPN technologies built into or on top of WLAN products
 Wired equivalent privacy (WEP) of 802.11 or the common
128-bit extension
– Uses shared keys and a pseudo random number (PRN) as
an initial vector (IV) to encrypt the data portion of network
packets, but does not encrypt 802.11b header
– Each station (clients and APs) has a number of keys to
encrypt data before it is transmitted
– Each station can only receive a packet being encrypted
with its appropriate key. Without proper key, the packet
will be discarded
– Some vendors also provide key servers to implement
centralized key management, such as Cisco.
IFA’2004
81
WLANs – Price Comparison
Vendor Name
Access Points
PC card
PCI Adapter
3Com
$545.00
$169.00
$199.00
Wireless
Bridge
$1095.00
Apple
$301.00
$99.00
--
--
Avaya
$685.00
$144.00
$59.00
--
Cisco
$717.00
$193.00
$269.00
$458.00
Compaq
$729.00
$166.00
$208.00
--
Ericsson
$929.00
$186.00
--
--
IBM
$1015.00
$182.00
--
--
Nokia
$751.00
$212.00
$199.00
--
Nortel
$1065.00
$289.00
--
$638.00
Symbol
$1025.00
$181.00
$271.00
$655.00
Zoom
$410.00
$136.00
$136.00
$1888.00
IFA’2004
82
WLANs – Total Cost
 Much smaller (around one year) payback period
– Even though initial capital cost for the WLAN may be
more, but the running cost for the WLAN is much less
 Cost-effective large scale network deployment
– Example: organizations for around 300 users benefited
annual savings of up to $4.9 million, which corresponds
to per user saving of $15,989.00.
 Low initial installation cost (wiring)
– University of Akron, covered their four-story library
with WLAN with a total cost of $80,000, using Cisco
Airnet 350 series and the estimated cost for wired
network for that was $800,000
IFA’2004
83
Conclusions
 Users demand ubiquity: mergence of the
wireless communication services; indoor and
outdoor
 Greater demand for integration with Bluetooth,
wLAN and cellular system
 Technology to meet these demand available
today
 Some key issues:
– interoperability between different wireless
network systems (MORE WORK NEEDED)
– Security: More work needed. An ideal
solution: combination of VPN and IPSec
IFA’2004
84
Conclusions
 Since the needs from various enterprises and
end users are quite different, the service
providers should prepare various network
deployments
– Many issues to be considered: network
capacity, the connectivity to wired network,
QoS, security, price and performance.
 A competitive solution provides seamless endto-end connectivity from mobile users to wired
ones.
 A web-based, centralized network management
is another consideration most customers want
IFA’2004
85
WLANs - Challenging
Issues
 Relatively low data rate.
– Some can achieve very high data rate, e.g., the rate
of IEEE 802.11 WLANs can be as high as 11Mbps,
while some products such as Bluetooth can only
achieve medium speed data rate.
 Lack of support for real time services.
– IEEE 802.11 products, which are based on the
CSMA/CA protocol, are unable to provide QoS
guarantees for voice, video, and other real time
services. (IEEE 802.11 working group E is still working
QoS enhancement)
 Interference between different types of WLANs
 Lack of Interoperability between WLANs
IFA’2004
86