1. Comp 1001: IT & Architecture - Joe Carthy
Computer Networks
A computer network is an interconnected collection of computers.
The goals of a computer network include:
Resource sharing: programs (O.S., applications), data, equipment (printers, disks) are available to all users of the network regardless of location.
High availability: By replicating files on different machines and having spare CPUs, users are more immune from hardware/software failure.
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.
Communications medium: Users have access to and the Internet
Comp 1001: IT & Architecture - Joe Carthy

2. Comp 1001: IT & Architecture - Joe Carthy
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
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
Comp 1001: IT & Architecture - Joe Carthy

3. Comp 1001: IT & Architecture - Joe Carthy
Client - Server Model
Networks provide services on one or more computers for all users
The machines providing a service are called servers.
The machines (users) using the service are called clients.
Several servers may be used to replicate data/services so that if one crashes, users can still access their data and the network services.
Such a Client-Server Model is decentralised
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 or for printing: mail servers and print servers.
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.
Comp 1001: IT & Architecture - Joe Carthy

4. Comp 1001: IT & Architecture - Joe Carthy
Terminal-Mainframe Model
Centralised Model
This model uses a central mainframe computer to do ALL processing
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)
Disadvantage: if mainframe crashes or needs to be serviced NO work can be done - single point of failure
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
Used for
large commercial applications: e.g. banking, insurance
Comp 1001: IT & Architecture - Joe Carthy

5. Comp 1001: IT & Architecture - Joe Carthy
Terminal-Mainframe Model
Network Topologies.
Networks can be interconnected in a number of ways
The method used, determines the shape or topology of the network
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
Comp 1001: IT & Architecture - Joe Carthy

6. Comp 1001: IT & Architecture - Joe Carthy

7. Comp 1001: IT & Architecture - Joe Carthy
Network Configuration.
The user machines in a network are called hosts.
The hosts are connected by a subnet
Subnet carries messages between hosts.
The subnet is made up of
transmission lines (trunks, channels, circuits) and
transmission lines carry bits
switching elements (computers).
switching elements connect the transmission lines
Comp 1001: IT & Architecture - Joe Carthy

8. Comp 1001: IT & Architecture - Joe Carthy
Broadcast Networks
In this system a message is broadcast over the network
All machines have the possibility of receiving the message.
Each machine has its own unique address and
typically machine will only "listen" to messages that are sent to this address
LANs are usually broadcast networks
Ethernet is commonest example and is used in UCD
Comp 1001: IT & Architecture - Joe Carthy

9. Comp 1001: IT & Architecture - Joe Carthy
Point to Point Networks
Message is transmitted from one computer (point) to another (point) and so on, until the destination computer is reached.
This is analogous to the postal system
Letter is transferred from post-office to post-office and finally to the addressee.
WANs usually use point to point
Message is usually broken into smaller parts or packets
Each packet is treated as a separate unit for delivery
Packets can take different routes through the network
e.g. if one point fails packets can be re-routed
Comp 1001: IT & Architecture - Joe Carthy

10. Comp 1001: IT & Architecture - Joe Carthy
Data Transmission
How data is is physically transmitted
Medium: carries the data e.g. copper wire, glass fibre
Shannon’s Law
There is a maximum limit to the amount of data that can be transmitted using any transmission medium.
Data is transmitted as signals
The number of signals per second is the frequency of the signal.
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
Comp 1001: IT & Architecture - Joe Carthy

11. Comp 1001: IT & Architecture - Joe Carthy
Data Transmission
Bandwidth
Bandwidth of a medium is the maximum range of frequencies that can be transmitted using that medium
determines the maximum amount of data that can be transmitted
A phone cable might have a bandwidth of .5 to 4 MHz
An optic fibre has a bandwidth of up to 108 MHz.
The higher the bandwidth the more data that can be transmitted per second
Comp 1001: IT & Architecture - Joe Carthy

12. Comp 1001: IT & Architecture - Joe Carthy
Transmission Media
Copper Wires
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
Fibre Optic
Glass fibre
Up to 1000 Mbps
Wireless (x Kbps to x Mbps)
Radio
wireless LAN (wi-fi) Mbps in UCD
Microwave
Infrared
Comp 1001: IT & Architecture - Joe Carthy

13. Comp 1001: IT & Architecture - Joe Carthy
Twisted Pairs (tp)
They are used in telephone network for the local loop (connection between your home phone and the local telephone exchange).
They carry electrical signals.
A tp consists of two insulated copper wires (1mm diameter) twisted to reduce electrical interference.
Capacity: dependent on the distances involved but can be up to several Mbps over a few Kms.
For example ISDN (Integrated Services Digital Network) lines offer speeds from 64Kbps to over 1 Mbps
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.
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.
A Network Interface Card (NIC) is used to connect a PC to a LAN
Comp 1001: IT & Architecture - Joe Carthy

14. Comp 1001: IT & Architecture - Joe Carthy
Twisted Pairs (tp)
TPs may be shielded (stp) or unshielded (utp) with the shielded having extra insulation.
They are also classified into Category-5 (CAT-5) and Category-6 (CAT-6).
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.
This is the type of cable that is often used in building to connect PCs to a LAN.
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.
CAT-6 cable operates at 100/1000Mbps (Gigabit Ethernet) and is typically used to interconnect hubs. It is more expensive than CAT-5 cable.
Comp 1001: IT & Architecture - Joe Carthy

15. Comp 1001: IT & Architecture - Joe Carthy
Three Floor Building with CAT-6 Cable Backbone connecting LANs on each floor
Comp 1001: IT & Architecture - Joe Carthy

16. Comp 1001: IT & Architecture - Joe Carthy
Twisted Pairs (tp)
Large organisations frequently have a so-called "backbone" network that interconnects separate LANs in different buildings/rooms as in the diagram below.
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.
A UPS is also shown in the diagram.
UPS: Uninterruptable Power Supply - provides battery backup power in event of power failure
A UPS can cost from as little as €200 to €x,000 depending on amount of time it can keep system running
In life critical systems, backup generators are necessary to cope in the event of power failure
Comp 1001: IT & Architecture - Joe Carthy

17. Comp 1001: IT & Architecture - Joe Carthy
Types of Transmission
There are three types of transmission.
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.
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.
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.
Comp 1001: IT & Architecture - Joe Carthy

18. Comp 1001: IT & Architecture - Joe Carthy
Types of Transmission
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19. Comp 1001: IT & Architecture - Joe Carthy
Data Transmission: Multiplexing
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.
For example 4 users could each operate terminals at 1Mbps connected to mainframe over a single 4Mbps line as shown in the figure below.
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.
Multiplexing is also used for voice transmission where optic fibres and microwave cable can handle from 8000 to simultaneous conversations.
There are a number of types of multiplexing.
Two common ones are Time Division Multiplexing (TDM) and Frequency Division Multiplexing (FDM).
Comp 1001: IT & Architecture - Joe Carthy

20. Comp 1001: IT & Architecture - Joe Carthy
Data Transmission
Multiplexing
Comp 1001: IT & Architecture - Joe Carthy

21. Comp 1001: IT & Architecture - Joe Carthy
Data Transmission: Modulation
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.
Using phone lines, 56 Kbps is the maximum transmission speed with a relatively high error rate.
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.
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).
Comp 1001: IT & Architecture - Joe Carthy

22. Comp 1001: IT & Architecture - Joe Carthy
Data Transmission: Modulation
A transmitting modem sends a continuous signal (tone) in the range 1000 to Hz to the receiving modem.
This carrier signal is modulated to an analog signal as in the figure below.
Its amplitude, frequency, phase or a combination can be modulated .
Comp 1001: IT & Architecture - Joe Carthy

23. Comp 1001: IT & Architecture - Joe Carthy
Frequency Modulation
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.
Comp 1001: IT & Architecture - Joe Carthy

24. Comp 1001: IT & Architecture - Joe Carthy
Review
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
Comp 1001: IT & Architecture - Joe Carthy