Transcript Chapter 13: Wireless Networks

Chapter 13:
Wireless Networks
Business Data Communications,
4e
Reasons for Wireless Networks
Mobile communication is needed.
Communication must take place in a
terrain that makes wired communication
difficult or impossible.
A communication system must be
deployed quickly.
Communication facilities must be installed
at low initial cost.
The same information must be broadcast
to many locations.
Problems with Wireless Networks
Operates in a less controlled environment,
so is more susceptible to interference,
signal loss, noise, and eavesdropping.
Generally, wireless facilities have lower
data rates than guided facilities.
Frequencies can be more easily reused
with guided media than with wireless
media.
Mobile Telephony
First Generation
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analog voice communication using frequency
modulation.
Second Generation
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digital techniques and time-division multiple access
(TDMA) or code-division multiple access (CDMA)
Third Generation


evolving from second-generation wireless systems
will integrate services into one set of standards.
AMPS - Advanced Mobile Phone Service
Mobile Switching Center (MSC)
Mobile Unit
Base Transceiver Station
AMPS Components
Mobile Units


contains a modem that can switch between
many frequencies
3 identification numbers: electronic serial
number, system ID number, mobile ID
number
Base Transceiver

full-duplex communication with the mobile
Mobile Switching Center (MSC)
AMPS Spectral Allocation
Two 25-MHz bands

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864-894 MHz: Base Station  Mobile Unit
824-849 MHz: Mobile Unit  Base Station
Each band is split in two (i.e. 12.5 MHz)
416 channels (30kHz apart)
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21: control
395: call
Spatial Allocation

Frequency Reuse Patterns (p.343)
GSM Global System for Mobile Communication
Developed to provide common 2ndgeneration technology for Europe
200 million customers worldwide, almost
5 million in the North America
GSM transmission is encrypted
Spectral allocation: 25 MHz for base
transmission (935–960 MHz), 25 MHz for
mobile transmission (890–915 MHz)
GSM Layout
Subscriber
Base Transceiver
Base Station Controller (BSC)
MSSC
Mobile Services Switching Center
GSM Network Architecture
HLR: Home Location Register
VLR: Visitor Location Register
AuC: Authentication Center
EIR: Equipment Identity Register
GPRS
General Packet Radio Services
GSN: GPRS Support Node
SGSN: Serving-GSN
 To support packet data service
GGSN: Gateway-GSN
Multiple Access
Four ways to divide the spectrum among
active users
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frequency-division multiplexing (FDM)
time-division multiplexing (TDM)
code-division multiplexing (CDM)
space-division multiplexing (SDM)
Choice of Access Methods
FDM, used in 1st generation systems, wastes
spectrum
Debate over TDMA vs CDMA for 2nd generation


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TDMA advocates argue there is more successful
experience with TDMA.
CDMA proponents argue that CDMA offers additional
features as well, such as increased range.
TDMA systems have achieved an early lead in actual
implementations
CDMA seems to be the access method of choice for
third-generation systems
Code Division Multiplexing
Based on direct sequence spread
Spectrum (DSSS)
Break each bit into k chips.

k: spreading factor
Ex. k = 6
Code : <1,-1,-1,1,-1,1>
1:
0:
CDMA Example
1,-1,-1,1,-1,1
1, 1,-1,-1,1,1
k =6
1, 1,-1, 1, 1,-1
CDMA Example (cont.)
Receiver receives a chip pattern
d =<d1,d2,d3,d4,d5,d6>
Code of user u
u =<u1,u2,u3,u4,u5,u6>
Decoding function
Su(d)= d1×u1+d2×u2+d3×u3+d4×u4+d5×u5+d6×u6
Orthogonal
1
SA(cB)=0
SA(cA)=6
0
SA(-cB)=0
SA(-cB)=-6
Third Generation Systems
Intended to provide provide high speed
wireless communications for multimedia,
data, and video
Personal communications services (PCSs)
and personal communication networks
(PCNs) are objectives for third-generation
wireless.
Planned technology is digital using TDMA
or CDMA to provide efficient spectrum use
and high capacity
Wireless Application Protocol (WAP)
Programming model based on the WWW
Programming Model
Wireless Markup Language, adhering to
XML
Specification of a small browser suitable
for a mobile, wireless terminal
A lightweight communications protocol
stack
A framework for wireless telephony
applications (WTAs)
WAP Programming Model
WAP’s Optional Proxy Model
WAP 1.0 Protocol Stack
WAP 1.0 Gateway
WAP 2.0 Proxy
What’s New in WAP 2.0
WAP Push
User Agent Profile
Wireless Telephony Application (WTA)
External Functionality Interface (EFI)
Persistent Storage Interface
Data Synchronization (SyncML)
Multimedia Message Service (MMS)
Provisioning
Pictogram
WTA Logical Architecture
Geostationary Satellites
Circular orbit 35,838 km above the earth’s
surface
rotates in the equatorial plane of the earth
at exactly the same angular speed as the
earth
will remain above the same spot on the
equator as the earth rotates.
Advantages of
Geostationary Orbits
Satellite is stationary relative to the earth, so
no frequency changes due to the relative
motion of the satellite and antennas on earth
(Doppler effect).
Tracking of the satellite by its earth stations is
simplified.
One satellite can communicate with roughly a
fourth of the earth; three satellites separated by
120° cover most of the inhabited portions of the
entire earth excluding only the areas near the
north and south poles
Problems with
Geostationary Orbits
Signal can weaken after traveling > 35,000 km
Polar regions and the far northern and southern
hemispheres are poorly served
Even at speed of light, about 300,000 km/sec,
the delay in sending a signal from a point on the
equator beneath the satellite 35,838 km to the
satellite and 35,838 km back is substantial.
LEO and MEO Orbits
Alternatives to geostationary orbits
LEO: Low earth orbiting
MEO: Medium earth orbiting
Satellite Orbits
Types of LEOs
Little LEOs: Intended to work at
communication frequencies below1 GHz
using no more than 5 MHz of bandwidth
and supporting data rates up to 10 kbps
Big LEOs: Work at frequencies above 1
GHz and supporting data rates up to a
few megabits per second