B.A. (Mahayana Studies)
000-209 Introduction to Computer Science
November 2005 - March 2006
11. Communications Systems
We look at the basic elements
of a communications systems,
and the various services
available.

Overview
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1. What is a Communications System?
2. Bandwidth
3. Network Topology
4. Protocols
5. Packet Switching
6. Circuit Switching
7. Analog <-> Digital Conversion
8. Telephone Services
9. Cable-based Services
10. Wireless
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1. What is a Communications System?

A communications system is a combination of
hardware, software, and connecting links that
transport data between a sender and a receiver.

A sender and receiver
are linked by a
communications channel
 e.g.
telephone lines,
fiber-optic cable
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2. Bandwidth



The amount of data that can be sent over a network
in a certain period of time.
Usually measured in bits per second (bps), kilobits
per second (kbps), or megabits per second (mps).
Two main types:
 broadband

(high capacity)
e.g. by using fibre-optic cable
 narrowband

(less capacity)
e.g when using the telephone system
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3. Network Topology

The shape of the interconnections
in a communications system
is its topology.

Star topology - the communications lines fan out
from a central location
 every
connection is dedicated to one user
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continued 5

A bus topology provides a common or shared
communications link
 used
by cable TV companies
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continued 6

A ring topology connects devices in a continuous
loop
 used
by older local area networks
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4. Protocols

A communications protocol is a set of rules
about how a sender and receiver should
communicate.

The rules specify such things as
 data
representation, signalling, authentication, and
error detection
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5. Packet Switching
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Data is separated into
small packets.
Each packet is sent through the network using the
best route available at that time.
At the receiving end, the packets are reassembled
into the original data.
Packet switching is used by the Internet.
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continued 9
2. Each packet is
addressed to its
destination.
1. A message is
divided into
packets.
4. If a route is
congested or
inoperable, packets
can be rerouted to
other links.
000-209 Intro to CS. 11/Comm. Systems
3. A packet might
travel the shortest
path to its
destination.
5. When the
packets arrive at
their destination,
they are
reassembled.
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Advantages of Packet Switching

Packet switching uses network bandwidth
efficently.

It minimizes transmission latency
 the

time it takes for data to pass across the network
Packet switching can deal with network failure.
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6. Circuit Switching

A dedicated communications path is established
between two devices through one or more
switching nodes.

Unlike packet switching, digital data is sent as a
continuous stream of bits.

The telephone system uses circuit switching.
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Advantages of Circuit Switching

Bandwidth is guaranteed.

Any communication delay is only due to
propagation time.

Primary advantage of the telephone system for
computer communications is that it's cheap for
users.
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Twisted Pair Cable

Telephone systems use miles and miles of
twisted-pair cables.
Twisted-pair cable
terminates with a
plastic RJ-45
connector.
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7. Analog <-> Digital Conversion


Old phone lines use analog sound signals.
Modern systems use digital signals:
 less
susceptible to noise
 require simpler circuitry

It's possible to convert analog signals into
digital signals (and vice versa).
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Analog and Digital Signals
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The Conversion Process
Transmission to
neighborhood
switch box
(within 1800
yards)
Central office
PoP - incoming
digital may be
converted to
analog
Neighborhood switch
If not analog already,
converted at this point
National Backbone,
All digital networks
Modem in computer
Converts analog to digital
Transmission to
neighborhood
Switch box
(within 1800 yards)
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Neighborhood switch
May convert to digital.
May remain analog
Central office PoP
Definitely digital now
Transmission to city
Point of Presence
central switch
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8. Telephone-based Services

POTS (plain old telephone service) is an analog
service.

Voiceband modem - converts digital pulses into
analog tones to send digital computer data over a
POTS line.
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continued 18

ISDN
a
standard for digital transmission of voice and
data
 uses circuit switching with ordinary telephone
wire (and other media, such as fibre-optic cable)
 it requires hardware adapters at both ends of the
transmission
 basic (64 kbps); enhanced (128 kbps)
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continued 19

ADSL (Asymmetric Digital Subscriber Line)
 for
transmitting digital information at a high
bandwidth on existing phone lines

ADSL offers asymmetric data rates
 1.5
to 9 Mbps when receiving (the downstream rate)
 16 to 640 Kbps when sending (the upstream rate)

Speeds depends on distance from the telephone
company office
 at
most ~3 miles away
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continued 20

T1 Line - provides 1.544 Mbps send and
receive capacity over a dedicated line

T3 Line - uses fiber-optic cables to provide
service with a capacity of 44.736 Mbps
 the
equivalent of 28 T1 lines
 enough to show full-screen, full-motion video
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Fiber-Optic Cable

Fiber-optic cable is a bundle of extremely small
tubes of glass called optical fibers.
 thinner
than human hair
Fiber-optic cable is replacing
twisted-pair cable where high
bandwidth is required
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continued 22

How fiber-optic cable works:
 miniature
lasers send pulses of light
 each fiber is a one-way communications
channel
 light signals encounter little resistance
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9. Cable TV-based Services

Cable TV companies in the US have installed
miles of high-bandwidth coaxial cables.
 carrying

capacity far in excess of POTS lines
A cable modem is a device designed to
demodulate a signal from the
cable and translate it back
into Internet data.
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Coaxial Cable

Coaxial cable - high-capacity communications
cable consisting of a copper wire conductor
 common
use is to carry television signals
Coaxial cable
contains shielding
which increases
bandwidth.
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continued 25

Coaxial cable:
 has
excellent bandwidth, but not as good as
fibre-optic cable
 more expensive
 more difficult to work with than twisted-pair
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10. Wireless Communications

Radio-based systems that allow transmission of
information without a physical connection,
 no
need for copper wire or fiber-optic

Cellular, infrared, microwave, and satellite
broadcasting are forms of wireless communication.

Common wireless standards include 802.11b, and
Bluetooth.
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Infrared Wireless

Infrared transmissions occur below the
visible light of spectrum.
 requires
 sender
000-209 Intro to CS. 11/Comm. Systems
line-of-sight communication
must be able to see the receiver
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Microwaves
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High-frequency radio waves used for point-topoint and one-directional communication of
audio and data.
Requires line-of-sight, and ground stations must
be within 30 miles of each other.
Many communications systems transmit
microwave signals between a land-based ground
station and a satellite.
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Satellite

Direct satellite service (DSS) uses a
geosynchronous or low-earth orbit satellite to
send television, voice or computer data directly
to a satellite dish.
in one direction – downstream
 requires a standard modem and phone line for
upstream transmission
 transmits
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801.11

802.11 is a family of specifications developed by
the IEEE for wireless communications.
 IEEE
= Institute of Electrical and Electronics
Engineers

802.11b: provide data rates of up to 11 Mbps per
second at distances up to approximately 300 feet.
 also
called Wi-Fi
 line-of-sight not required
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Bluetooth

An open standard for short-range (30 feet) and
low speed (up to 1Mbps) wireless transmission of
digital voice and data.
 line-of-sight
not required
 low-power (unlike 802.11b), so suitable for portable
devices
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