CHAPTER 6:
WIRELESS & MOBILE
NETWORKS
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Code Division Multiple Access
Spread Spectrum
Cellular Telephones
4G Technology
IEEE 802.11
Mobile IP
Wireless Security
Satellite Networks
CODE DIVISION MULTIPLE
ACCESS
TDMA: Everyone gets
to talk using the
entire bandwidth, but
they have to take
turns talking.
FDMA: Everyone
gets to talk at the
same time, but
only across their
narrow channels.
CDMA: Everyone gets to talk simultaneously, using the entire
bandwidth! They do this by coding their transmissions in a unique
fashion (as if every pair were speaking a different language, and
each other language merely sounds like background noise).
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COMPARING CELLULAR
APPROACHES
FDMA…
• subject to impairment due
to selective channel
fading
• limited to fixed number of
concurrent users
• not amenable to privacy
needs
• wasteful guard bands are
needed between channels
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TDMA…
• subject to impairment due
to noise bursts
• limited to fixed number of
concurrent users
• not amenable to privacy
needs
• wasteful guard times are
needed between time slots
to ensure synchronization
Chapter 6
CDMA…
• frequency diversity avoids
transmission impairments
• experiences graceful
degradation with more users
• inherent privacy due to noiselike characteristics of other
messages
• efficiently utilizes the entire
available bandwidth spectrum
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SPREAD-SPECTRUM SIGNALING
To eliminate the noise interference and privacy violations
that have plagued wireless communication, spread-spectrum
signaling was developed in two principal forms:
Frequency Hopping Spread Spectrum (FHSS)
• Each transmitter changes
frequencies at a regular time
interval, following a
pseudorandom pattern known
only to the transmitter and the
receiver.
• The receiver demodulates the
received signal, filtering out any
received signals that are not at
the appropriate frequency.
• FHSS is commonly used in
Bluetooth systems.
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Direct-Sequence Spread Spectrum (DSSS)
• Each data bit is multiplied by a long bit sequence and
modulated for transmission (the sequence used for 0 is the
two’s complement of the signal used for 1).
• Other transmitters must use orthogonal sequences so
they’ll be filtered out at the receiver.
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ORTHOGONAL DIGITAL
SIGNALS
A chip set of orthogonal vectors is
used to “spread” the signals.
Chip Code A: (1,1,1,1)
Chip Code B: (1,1,-1,-1)
Chip Code C: (1,-1,-1,1)
Note that the
chip codes are
mutually
orthogonal (i.e.,
the dot product
of any pair of
them is zero).
Chip Code D: (1,-1, 1,-1)
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CODING OUTGOING MESSAGES
A: (1,1,1,1)
B: (1,1,-1,-1)
C: (1,-1,-1,1)
Assume that there are four senders...
Sender Message
D: (1,-1, 1,-1)
±1
Version
Chip Code
Encoded Message
W
010010
-1 1 -1 -1 1 -1
A
(-1,-1,-1,-1), (1,1,1,1),
(-1,-1,-1,-1), (-1,-1,-1,-1),
(1,1,1,1), (-1,-1,-1,-1)
X
111011
1 1 1 -1 1 1
B
(1,1,-1,-1), (1,1,-1,-1),
(1,1,-1,-1), (-1,-1,1,1),
(1,1,-1,-1), (1,1,-1,-1)
C
(-1,1,1,-1), (-1,1,1,-1),
(-1,1,1,-1), (1,-1,-1,1),
(1,-1,-1,1), (1,-1,-1,1)
D
(1,-1,1,-1), (-1,1,-1,1),
(1,-1,1,-1), (-1,1,-1,1),
(-1,1,-1,1), (1,-1,1,-1)
Y
Z
000111
101001
-1 -1 -1 1 1 1
1 -1 1 -1 -1 1
Summing the four encoded messages yields:
(0,0,0,-4), (0,4,0,0), (0,0,0,-4), (-2,-2,-2,2), (2,2,-2,2), (2,-2,-2,-2)
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DECODING INCOMING
MESSAGES
What happens when the incoming
message arrives?
(0,0,0,-4), (0,4,0,0), (0,0,0,-4), (-2,-2,-2,2), (2,2,-2,2), (2,-2,-2,-2)
Receiver
Original
Message
Chip
Code
Decoded
Message
Decoded Message
After Scaling
W′
010010
A
-4, 4, -4, -4, 4, -4
010010
X′
111011
B
4,4,4,-4,4,4
111011
Y′
000111
C
-4,-4,-4,4,4,4
000111
Z′
101001
D
4,-4,4,-4,-4,4
101001
A: (1,1,1,1)
B: (1,1,-1,-1)
C: (1,-1,-1,1)
The decoded message is “scaled” by converting
all positive values to 1, and all non-positive
values to 0.
D: (1,-1, 1,-1)
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CELLULAR TELEPHONES
Cellular phones transmit
and receive on two kinds of
channels (FDM for analog,
TDM for digital):
•Control Channels – Used
for overhead messages
(e.g., network system ID),
pages (incoming call
signals), access info
(connection requests),
and channel assignments
(when connection is
established).
•Communication Channels
– Used for voice/data,
handoff control, and
maintenance monitoring.
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CELLULAR BASE STATIONS
Cellular providers typically have 832
(analog) channels to use.
• 42 are reserved for control info.
• The remaining 790 are split into 395
duplex pairs for voice/data.
• The service region is split into a
hexagonal grid, with approximately
56 channel pairs allocated to each
base station.
1) The caller dials and the phone
sends the phone number,
programmed system ID, and
phone serial number to the
nearest base station.
2) After verifying everything, the
base station relays the info to
a mobile switching center,
which then uses optical fiber
or wireless to forward the
caller’s signals.
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CELLULAR HANDOFF
As a user moves toward the
edge of a cell, the local base
station notes that the user’s
signal strength is diminishing.
Meanwhile, the base station
in the cell towards which
the user is moving (which is
listening and measuring
signal strength on all
frequencies, not just its
own one-seventh) detects
the user’s signal strength
increasing.
The two base stations coordinate with each
other through the Mobile Telephone
Switching Office, and at some point, the
user’s phone gets a signal on a control
channel telling it to change frequencies.
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MULTIPATH PROPAGATION
Cellular signals may be altered via four principal
types of interference.
Shadowing occurs when the signal power fluctuates
due to objects obstructing the propagation path
between the transmitter and the receiver.
Scattering occurs
Reflection occurs
when the signal
when a radio wave
travels through a
collides with an
medium containing
object which has
objects with
very large
dimensions smaller
dimensions
than the signal’s
compared to the
wavelength, such as
wavelength of the
when the
propagating signal.
transmission
This is often caused
Diffraction occurs when the
encounters a rough
by the surface of
path between the transmitter
surface or small
the earth, buildings,
and the receiver is obstructed
objects.
and walls
by an object with sharp edges,
causing secondary waves to
bend around the object and
provide an artificial line-of-sight
between the communicating pair.
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1G  2G  3G  4G
Firstgeneration
cellular
systems were
analog, using
FDMA and
emphasizing
voice
applications.
Secondgeneration
cellular
systems are
digital, using
TDMA (or
CDMA) and
emphasizing email & Internet
access.
Thirdgeneration
cellular
systems are
also digital,
using CDMA
and stressing
video
telephony and
high-speed
Internet
access.
Fourthgeneration
cellular
systems will
extend the
bandwidth and
throughput
capabilities of
3G.
CDMA’s big advantages…
• Frequency diversity limits transmission impairments.
• Noise-like signal system improves privacy.
• Graceful service degradation with increased usage.
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4G’S MULTITECHNOLOGY
APPROACH
Numerous technologies are
competing for a share of the
4G market, including:
Orthogonal Frequency-Division
Multiple Access (OFDMA) supports
multi-user transmissions by separating
them in both the time domain and in the
frequency domain.
Ultra-Wideband (UWB)
uses the radio spectrum
at low energy levels to
produce short-range,
high-bandwidth wireless
communications.
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Multiple-Input/MultipleOutput (MIMO) uses
multiple antennas at both
the transmitter and the
receiver to improve
communication
performance.
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WIRELESS LANS
Medium access control on wireless LANs has
certain similarities to CSMA/CD.
Three “interframe space” time values are defined:
• Short IFS: time interval for high-priority
traffic (e.g., ACKs and Clear-To-Sends)
• Point Coordination Function IFS: time
interval for polling messages
• Distributed Coordination Function IFS:
time interval for regular traffic
A transmitting wireless station waits the appropriate IFS time,
listening for traffic.
If it hears a message, it waits until the message has passed, and then
waits another IFS, plus an added exponential backoff time.
Note that there is no collision detection (it’s too hard to tell the
difference between external noise and one’s own transmission on
such a wide-open medium).
Corrupted signals must be handled at a higher protocol layer.
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IEEE 802.11 TOPOLOGIES
The 802.11 architecture hierarchically defines its topologies.
An Independent Basic
Service Set has wireless
stations communicating
directly with each other in
a peer-to-peer fashion.
An Infrastructure Basic Service Set
has a component called an Access
Point that serves as a relay
through which the stations
communicate with each other and
which provides a connection to an
external Distribution System.
An Extended Service Set is a
set of infrastructure BSS’s,
where the access points
communicate with each other
to forward traffic from one
BSS to another in order to
facilitate movement of
stations between BSS’s.
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IEEE 802.11 FRAME FORMAT
Preamble
PLCP Header
MAC Data
CRC
Preamble: 10 bytes of alternating 0’s and 1’s (for synch) & a 2-byte start delimiter
PLCP (Physical Layer Convergence Protocol) Header: Length & rate fields, and a HEC.
MAC Data: The MAC frame, described below.
CRC: 4 bytes to error-check the entire frame.
IEEE 802.11 MAC FRAME FORMAT
FC DI
Address1
Address2
Address3
SC
Address4
Data
Frame Control: 2-bit protocol version; 6-bit message type; 1-bit flag indicating
transmission to the Distribution System; 1-bit flag indicating transmission from the DS;
1-bit fragmentation flag; 1-bit retransmission flag; 1-bit power management flag
(Power Save vs. Active); 1-bit Access Point polling flag; 1-bit encryption flag; 1-bit
Strictly-Ordered flag
Duration/ID: Either the time that the channel is being allocated, or, in control
frames, the station ID number for Power-Save polling responses.
Address1-4: The 6-byte addresses for the source, destination, source Access Point,
and destination Access Point.
Sequence Control: 4-bit fragment number and 12-bit sequence number.
Data: 0-2312 bytes of LLC data or control info.
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MOBILE IP
Mobile IP was developed to allow users to seamlessly roam among
wireless networks, with no interruptions to IP applications like media
streaming or VoIP as network boundaries are crossed.
Mobile IP service requires every mobile device to contain Mobile Node
software, Home Agent software on a router in the user’s home
network, and (if used) Foreign Agent software on a router in the
remote network.
Mobile IP datagrams may flow in
a network without a Foreign
Agent, as long as the Mobile
Node has a public IP address in
the visited network.
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If the visited network has a Foreign Agent,
the Mobile Node doesn’t require any IP
address, and the Foreign Agent only
requires one public IP address for all
Mobile Nodes.
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WIRELESS SECURITY
Extra security mechanisms are required in wireless
environments, due to the lack of physical connections:
• Authentication - Establishing identity (e.g., passwords)
• Deauthentication - Terminating authentication (e.g., if
reauthentication is needed)
• Encryption - Ensuring privacy (e.g., encoding to inhibit
unauthorized reading)
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SATELLITE NETWORKS
Each method of medium access has problems
when applied to satellites:
FDMA
•Wide guard bands are needed to separate
channels.
•Power must be carefully controlled to preserve
the signal’s integrity.
•Digital communication isn’t supported; only
analog is.
TDMA
•Ground stations have varying propagation times,
so synchronization is tough.
•Ground stations must be capable of high burst
speeds.
CDMA
•Channels have low capacity due to noise and lack
of synchronization.
•Transmitters must have extremely fast rates to
accommodate spread.
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