Transcript Information Representation: Summary

Computer Networks
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A computer network is an interconnected collection of computers.
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The goals of a computer network include:
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Resource sharing: programs (O.S., applications), data, equipment
(printers, disks) are available to all users of the network regardless of
location.
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High availability: By replicating files on different machines and having
spare CPUs, users are more immune from hardware/software failure.
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Less cost: Small machines have about 1/10 the power of a
mainframe but 1/1000 the cost. A network of such machines can be
installed at low cost. It is easy to increase the capacity by adding new
machines.
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Communications medium: Users have access to email and the
Internet
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Computer Networks
• A Network may be classified as:
– Local Area Network: LAN
• Operates over small geographical area (1 to 2 kms) such as office
complex,hospital, university campus e.g. UCD network
• High-speed networks: 10 to 100 Mbps are common but up to
1000MBps (Gigabit)
– Wide Area Network: WAN
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Operates over long distances - national or global
e.g. a Bank’s national/international network
Speed depends on communication medium used
From slow 56Kbps to x Mbps
– Also
– Tiny Area Network: TAN - e.g. network of a few PCs in the home
– Metropolitan Area Network: MAN: network in a single city
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Client - Server Model
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Networks provide services on one or more computers for all users
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The machines providing a service are called servers.
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The machines (users) using the service are called clients.
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Several servers may be used to replicate data/services so that if one
crashes, users can still access their data and the network services.
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Such a Client-Server Model is decentralised
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Example: A LAN with file server(s). Users store files and access
software on the file server, but use their own PCs for processing.
Dedicated servers may also be used for email or for printing: mail
servers and print servers.
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One advantage of client-server model is that users can avail of local
processing on their own machines as well as the shared services of the
network.
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Terminal-Mainframe Model
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Centralised Model
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This model uses a central mainframe computer to do ALL processing
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Users use terminals to access mainframe computer
– Terminal is a screen and keyboard (can use a PC)
– Terminals can be local or remote (in a different building/city/country)
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Disadvantage: if mainframe crashes or needs to be serviced NO work
can be done - single point of failure
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Advantages
– Can have enormous computing power to run applications which PCs
cannot cope with
– Easier to maintain and secure - as there is only one computer
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Used for
– large commercial applications: e.g. banking, insurance
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Terminal-Mainframe Model
• Network Topologies.
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Networks can be interconnected in a number of ways
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The method used, determines the shape or topology of the network
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Common topologies include:
– Bus: used in LANs
– Ring: used in LANs
– Star: used in terminal mainframe model
– Single point of failure
– Tree: Land-line phone network
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Network Topology
b) Bus
a) Ring
c) Complet e
e) T ree
d) Loop
f) Int ersect ing Loop
g) St ar
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Network Configuration.
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The user machines in a network are called hosts.
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The hosts are connected by a subnet
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Subnet carries messages between hosts.
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The subnet is made up of
1.
transmission lines (trunks, channels, circuits) and
–
2.
transmission lines carry bits
switching elements (computers).
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switching elements connect the transmission lines
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Broadcast Networks
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In this system a message is broadcast over the network
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All machines have the possibility of receiving the
message.
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Each machine has its own unique address and
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typically machine will only "listen" to messages that are
sent to this address
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LANs are usually broadcast networks
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Ethernet is commonest example and is used in UCD
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Point to Point Networks
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Message is transmitted from one computer (point) to another
(point) and so on, until the destination computer is reached.
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This is analogous to the postal system
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Letter is transferred from post-office to post-office and finally to the
addressee.
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WANs usually use point to point
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Message is usually broken into smaller parts or packets
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Each packet is treated as a separate unit for delivery
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Packets can take different routes through the network
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e.g. if one point fails packets can be re-routed
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Data Transmission
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How data is is physically transmitted
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Medium: carries the data e.g. copper wire, glass fibre
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Shannon’s Law
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There is a maximum limit to the amount of data that can be
transmitted using any transmission medium.
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Data is transmitted as signals
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The number of signals per second is the frequency of the signal.
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This is measured in Hertz (Hz).
– One cycle per second is 1 Hz.
– 1,000 cycles per second is 1 KHz
– One million cycles per second is 1 MHz.
– 1,000 million cycles per second is 1 GHz
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Data Transmission
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Bandwidth
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Bandwidth of a medium is the maximum range of frequencies
that can be transmitted using that medium
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determines the maximum amount of data that can be
transmitted
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A phone cable might have a bandwidth of .5 to 4 MHz
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An optic fibre has a bandwidth of up to 108 MHz.
The higher the bandwidth the more data that can be
transmitted per second
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Transmission Media
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Copper Wires
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Fibre Optic
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
Twisted Pair (TP)
Coaxial Cable (coax) - two types (thick and thin!)
 10-Base-5; 10-Base-2 standards
 Used with Ethernet
 10 to 100 Mbps
Glass fibre
Up to 1000 Mbps
Wireless (x Kbps to x Mbps)
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
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Radio
 wireless LAN (wi-fi) 10-50 Mbps in UCD
Microwave
Infrared
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Twisted Pairs (tp)
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They are used in telephone network for the local loop (connection
between your home phone and the local telephone exchange).
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They carry electrical signals.
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A tp consists of two insulated copper wires (1mm diameter) twisted to
reduce electrical interference.
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Capacity: dependent on the distances involved but can be up to several
Mbps over a few Kms.
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For example ISDN (Integrated Services Digital Network) lines offer
speeds from 64Kbps to over 1 Mbps
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More recently (2003), DSL (Digital Subscriber Line) and in particular
ADSL (Asymmetric DSL) lines are available to home users with speeds
of 1.5 to 6 Mbps.
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ISDN and ADSL both use digital transmission and so must use a digital
line unlike the standard analog telephone line where a modem is used.
You must install an ISDN card or an ADSL card into your PC to use an
ISDN or ADSL line.
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A Network Interface Card (NIC) is used to connect a PC to a LAN
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Twisted Pairs (tp)
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TPs may be shielded (stp) or unshielded (utp) with the
shielded having extra insulation.
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They are also classified into Category-5 (CAT-5) and
Category-6 (CAT-6).
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CAT-5 can carry 10 or 100 Mbps (10/100Mbps) over short
distances e.g. up to 100 metres approx. The communications
standard used in this case is called 10/100-Base-T.
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This is the type of cable that is often used in building to connect
PCs to a LAN.
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Usually, the CAT-5 cable connects to either a hub or switch
which is less than 100 metres from each PC. There may be a
hub for each floor/laboratory in a building.
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CAT-6 cable operates at 100/1000Mbps (Gigabit Ethernet) and
is typically used to interconnect hubs. It is more expensive than
CAT-5 cable.
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Third Floor
P Cs
P Cs t o hub
via CAT-5
Hub
Route r
Switch
CAT-6
Backbone Cable
Route r
Switch
Servers
First Floor
Duct t o carry
cable
UPS
Three Floor Building with CAT-6 Cable Backbone connecting LANs on each floor
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Twisted Pairs (tp)
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Large organisations frequently have a so-called "backbone"
network that interconnects separate LANs in different
buildings/rooms as in the diagram below.
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Over short distances CAT-6 cable may be used for the backbone
but optic fibre is also often used as it can operate at higher
speeds over longer distances.
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A UPS is also shown in the diagram.
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UPS: Uninterruptable Power Supply - provides battery backup
power in event of power failure
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A UPS can cost from as little as €200 to €x,000 depending on
amount of time it can keep system running
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In life critical systems, backup generators are necessary to cope
in the event of power failure
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Types of Transmission
There are three types of transmission.
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1. Simplex: information is transmitted in one direction only
and the roles of transmitter and receiver are fixed. This form is
not used for conventional data transmission.
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2. Half Duplex (HDX): transmission is allowed in both
directions but in only one direction at a time. Data
communication systems that use the telephone network
sometimes transmit in HDX.
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3. Full Duplex (FDX): sender/receiver can transmit and
receive from each other at the same time. In order to transmit
in FDX, the user usually has private direct lines.
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Types of Transmission
SIMPLEX
HALF DUPLEX
FULL DUPLEX
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Data Transmission: Multiplexing
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With high bandwidth channels it is possible to share the channel so that a
number of users can use the channel at the same time- called multiplexing.
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For example 4 users could each operate terminals at 1Mbps connected to
mainframe over a single 4Mbps line as shown in the figure below.
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The 4 lines from the terminals are connected to a multiplexer which is
connected to another multiplexer by the 4Mbps line. The second multiplexer
de-multiplexes the signal onto 4 separate lines for the computer.
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Multiplexing is also used for voice transmission where optic fibres and
microwave cable can handle from 8000 to 16000 simultaneous
conversations.
There are a number of types of multiplexing.
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Two common ones are Time Division Multiplexing (TDM) and Frequency
Division Multiplexing (FDM).
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Data Transmission
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Multiplexing
M
U
X
High
Speed
Line
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M
U
X
C
o
m
p
u
t
e
r
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Data Transmission: Modulation
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Analog Transmission: Dominated the last 100 years and is still
widely used in phone network.
Network designers made use of telephone network which was aimed at voice
transmission. This is very poor for computer networking e.g. 2 computers
connected by a direct cable can achieve a data rate of 100 Mbps upwards
with very low error rate.
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Using phone lines, 56 Kbps is the maximum transmission speed with a
relatively high error rate.
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Modems
Phone lines deal with frequencies of 300 to 3000 Hz. A computer outputs a
digital
serial stream of bits (1’s, 0’s).
A modem is a device that accepts such a bit stream and converts it to
an analog signal, using modulation.
It also performs the inverse conversion.
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Thus two computers can be connected using two modems and phone line.
The serial interface between the modem and computer is governed by the
RS-232 standard (also known as the CCITT V24 standard).
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Data Transmission: Modulation
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A transmitting modem sends a continuous signal (tone) in the range 1000 to
2000 Hz to the receiving modem.
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This carrier signal is modulated to an analog signal as in the figure below.
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Its amplitude, frequency, phase or a combination can be modulated .
DC SIGNALS
FROM TERMINAL
OR COMPUTER
MODEM
MODEM
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DC SIGNALS
TO A TERMINAL
OR COMPUTER
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Frequency Modulation
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One common form of modulation Frequency Modulation (FM).
In FM, the carrier signal is modulated between two different frequencies
(say 1200Hz and 2200Hz) without affecting the amplitude in accordance
with the digital signal that it must send.
0
1
1
0
0
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1
0
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Review
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What is a network and why use one
Client Server and Terminal-Mainframe Models
LANs and WANs
Network Topology: Bus, Ring and Tree
Broadcast and Point-to-Point Networks
Data Transmission: Bandwidth and Shannon’s Law
Transmission Media: utp, coax, fibre, wireless
Hubs and Cabling
ISDN, ADSL, wi-fi
HDX, FDX, Multiplexing
Modulation
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