Learning Objectives
Tell how IEEE 802.11a networks function,
and how they differ from 802.11b networks
List the advantages and disadvantages of an
IEEE 802.11g network
Describe the HiperLAN/2 networks
Compare low-speed and high-speed WLANs
Explain basic and enhanced WLAN security
features
High Speed WLANs
Three standards for high-speed WLANs
that transmit at speeds over 15 Mbps
IEEE 802.11a
IEEE 802.11g
HiperLAN/2
All WLANs are concerned with security
How to prevent unauthorized access
IEEE 802.11a
Approved in 1999, 802.11a transmits at speeds of
5.5 Mbps and 11 Mbps
Great demand for 802.11a WLANS, also called
Wi-Fi5, with maximum speed of 54 Mbps
Devices use gallium arsenide (GaAs) or silicon
germanium (SiGe) rather than CMOS
semiconductors
Increased speed achieved by higher frequency,
more transmission channels, multiplexing
techniques, and more efficient error-correction
U-NII Frequency Band
802.11b uses unlicensed Industrial, Scientific,
and Medical (ISM) band and specifies 14
frequencies
802.11a uses Unlicensed Information
Infrastructure (U-NII) band
Table 7-1 compares ISM and U-NII
U-NII is divided into three bands, shown in
Table 7-2
U-NII provides more bandwidth, faster
transmission, and increased power
Efforts underway to unify 5 GHz bands globally
ISM vs. U-NII
U-NII Spectrum
Channel Allocation
802.11a WLANs have have 11 channels in USA but
requires 25 MHz passband
See Figure 7.1
Figure 7-2 shows 8 channels in Low and Medium
Bands with 20 MHz channel supporting 52 carrier
signals, each 200 KHz wide
Supports eight networks per AP, as shown in
Figure 7-3
IEEE 802.11e Task Group is working on standard
that supports quality of service (QOS)
802.11b Channels
802.11a Channels
Orthogonal Frequency
Division Multiplexing
Electromagnetic waves reflect off surfaces
and may be delayed in reaching their
destination
Figure 7-4 illustrates multipath distortion
Receiving device waits until all reflections are
received before it can transmit
Increasing speed of WLAN only causes longer
delays waiting for reflections
802.11a uses Orthogonal Frequency
Division Multiplexing (OFDM) to solve
this problem
Orthogonal Frequency
Division Multiplexing
Dating to 1960s, OFDM’s primary role is to
split high-speed digital signal into several
slower signals running in parallel
Sending device breaks transmission into pieces
and sends it over channels in parallel
Receiving device combines signals to re-create
the transmission
See Figure 7-5
Multiple Channels of OFDM
OFDM Breaks 802.11B
Ceiling Limit
Slowing down transmissions actually
delays reflections, increases total
throughput, and results in faster WLAN
See Figure 7-6
802.11a specifies eight overlapping
channels, each divided into 52 subchannels
that are 300 KHz wide
OFDM uses 48 subchannels for data and the
remaining four for error correction
OFDM vs. Single Channel
Modulation Techniques Vary
Depending on Speed
6 Mbps—phase shift keying (PSK)
Encodes 125 Kbps of data on each of 48
subchannels, resulting in 6Mbps data rate
See Figure 7-7
12 Mbps—quadrature phase shift keying
(QPSK)
Encodes 250Kbps per channel for 12 Mbps
data rate
See Figure 7-8
PSK
QPSK
Modulation Techniques Vary
Depending on Speed
24 Mbps—16-level quadrature amplitude
modulation (16-QAM)
16 different signals can encode 500 Kbps per
subchannel
See Figure 7-9
54 Mbps—64-level quadrature amplitude
modulation (64-QAM)
Transmits 1,125 Mbps over each of 48
subchannels
See Figure 7-10
16-QAM
64-QAM
Higher Speeds
Official top speed of 802.11a is 54 Mbps
Specification allows for higher speeds
known as turbo mode or 2X mode
Each vendor can develop 2X mode by
combining two frequency channels
Produces 96 subchannels and speeds up to
108 Mbps
Other 2X mode techniques include increasing
and reallocating individual carriers and using
different coding rate schemes
Error Correction
802.11a transmissions significantly reduce
errors
Minimizes radio interference from outside
sources
801.11a has enhanced error correction
Forward Error Correction (FEC) transmits
secondary copy of information that may be used
if data is lost
Uses 48 channels for standard transmissions
and 4 for FEC transmissions
802.11a Physical Layer
802.11a changed only physical layer
PHY layer is divided into two parts
Physical Medium Dependent (PMD) sublayer
defines method for transmitting and receiving
data over wireless medium
Physical Layer Convergence Procedure
(PLCP) reformats data received from MAC
layer into frame that PMD sublayer can
transmit
PLCP
Based on OFDM, PLCP frame has three
parts
Preamble—allows receiving device to prepare
for rest of frame
Header—provides information about frame
Data—information to be transmitted
See Figure 7-11
802.11a PLCP Frame
Fields in PLCP Frame
Synchronization
Rate
Length
Parity
Tail
Service
Data
Pad
802.11a Rate Field Values
Advantages and
Disadvantages
Advantages
Good for area that need higher transmission
speeds
Disadvantages
Shorter range of coverage
Approximately 225 feet as compared with
375 feet for 802.11b WLAN
IEEE 802.11g
In 2001, IEEE proposed 802.11g draft standard
to combine stability of 802.11b with faster data
transfer rates of 802.11a
Operates in 2.4 GHz ISM frequency
Has two mandatory modes: Complementary Code
Keying (CCK) mode and Orthogonal Frequency
Division Multiplexing (OFDM)
Offers two optional modes: Packet Binary
Convolutional Coding (PBCC-22) and
CCK-ODFM
802.11g products not expected until 2003
HiperLAN/2
Similar to 802.11a, HiperLAN/2 was
standardized by European Telecommunications
Standards Institute
Figure 7-12 shows protocol stack for
HiperLAN/2
Has three basic layers: Physical, Data Link, and
Convergence
Products based on HiperLAN/2 may appear in
2003
HiperLAN/2 Protocol Stack
Physical Layer
PHY layers of IEEE 802.11a and
HiperLAN/2 are almost identical
Operate in 5 GHz band
Use OFDM
Transmit up to 54 Mbps
Connect seamlessly to wired Ethernet networks
Data Link Layer
HiperLAN/2 centralizes control of RF
medium to access point (AP)
AP informs clients, known as mobile terminals
(MTs), when they may send data
Channel allocation is based on dynamic
time-division multiple access (TDMA) that
divides bandwidth into several time slots
Quality of Service (QOS) refers to dynamically
allocated time slots based on needs of MT and
condition of network
Radio Link Control (RLC)
Sublayer
Three primary functions of RLC sublayer
Connection setup procedure and connection
monitoring—authentication and encryption
Radio resource handling, channel monitoring,
and channel selection—automatic transmission
frequency allocation (known as Dynamic
Frequency Selection (DFS)
Association procedure and reassociation
procedure—standardized handoff to nearest AP
by roaming MTs
Logical Link Control (LLC) sublayer, also part of
Data Link Layer, performs error checking
Convergence Layer
HiperLAN/2 offers seamless high-speed
wireless connectivity up to 54 Mbps
Can connect to cellular telephone systems
Can connect to Asynchronous Transfer Mode
(ATMs) systems using fiber-optic media and
transmitting at 622 Mbps
Can connect to IEEE 1394 (also known as
FireWire) high speed external serial bus
transmitting at 400 Mbps
Summary: High- and
Low-Speed WLANs
May compare different types of WLANs
Do not consider them as competing
technologies
Rather, they are complementary technologies,
each with its strengths and weaknesses and
market niche
HomeRF—combines wireless data, cordless
telephony, and streaming media for home
networks
Supports QoS and transmits from 1/6 Mbps to
10 Mbps
WLAN Summary
IEEE 802.11—provides cable-free access
for mobile or fixed location at rate of 1 or 2
Mbps
802.11b (Wi-Fi)—popular choice for
business wireless networks
Transmits at 11 Mbps on three simultaneous
channels but offers no QoS and uses crowded
ISM band
WLAN Summary
802.11a—current leader in business WLANs
Uses U-NII frequency, allows 8 simultaneous
channels, and transmits at 54 Mbps standard, can
be increased to 108 Mbps
802.11g—offers faster data rates while
remaining compatible with 802.11b networks
Uses only three channels and crowded ISM
frequency
WLAN Summary
HiperLAN/2—uses dynamically allocated
time slots and dynamic frequency selection
for high-speed communications
Popular in Europe
Table 7-4 compares WLANs
WLAN Comparison
802.11 Security
Greatest strength of WLANs is ability to
roam freely
Greatest weakness is risk of unauthorized
user receiving RF signals
Some flawed IEEE WLAN security provisions
Basic Security involves two areas:
Authenticating users
Keeping transmissions private
Authentication
Verifies user has permission to access
network
Each WLAN client can be given Service
Set Identifier (SSID) of network
Only clients that know SSID may connect
SSID may be entered manually into wireless
device, but anyone with device has access to
network
Access points (APs) may freely advertise SSID
to any mobile device within range
Privacy
IEEE standard provides optional Wired
Equivalent Privacy (WEP) specification for
data encryption
Two types of keys used for encryption
Public key cryptography uses matched public
and private keys
IEEE uses shared key cryptography with same
key used for encryption and decryption
The longer the key, the more secure it is
See Figure 7-13
WEP
WEP Privacy Concerns
In late 2000, researchers revealed
“initialization vector” used to encrypt
transmissions with WEP were reused about
once every five hours
Makes it easy for anyone to collect data to
break WEP encryption
Researches recovered 128-bit WEP key in less
than 2 hours
Many think IEEE WLANs should be
treated as insecure
Enhanced Security
Administrators must use enhanced security
measures to prevent WLAN attacks
Four kinds of WLAN attacks
Hardware theft
Access point impersonation
Passive monitoring
Denial of service
Additional Security
Procedures
IEEE task group working on draft known
as IEEE 802.1x to allow centralized
authentication of wireless clients
Uses Extensible Authentication Protocol
(EAP)—client negotiates authentication
protocols with separate authentication server
Uses Remote Authentication Dial-In User
Service (RADIUS)—server on wired network
sends security keys to wireless client
See Figure 7-14
802.1x Security
Other Security Steps
Use an access control list with MAC addresses
of approved clients, as seen in Figure 7-15
Use digital certificates issued by trusted third
party for secure, encrypted online
communication
Use digital wrapper or gatekeeper that secures
data by wrapping around another program or
file
Use a Virtual Private Network (VPN), a
secure, encrypted connection between two
points
Access Control List
Higher Levels of Security
Reduce transmission power used in WLANs
Decreases distance radio waves travel, thus
limiting range where hackers can pick up signals
Change default WLAN security settings
Keep WLAN traffic separate from that of
wired network
Use 128-bit WEP keys rather than default
40-bit keys