CSC 37 – COMPUTER NETWORKS

AL. Hanees, BSc (Hons) Special in Computer ScienceLecturer in Computer Science,Faculty of Applied Sciences,South Eastern University of Sri Lanka.

Introduction to Communication Data communication is the process of sending data electronically from one computer or similar device to another. Linking one computer to another permits the power and resources of that computer to be tapped. It also makes possible the updating and sharing of data at different locations.

• Computers that are located in the same room or office building, can communicate data through a direct cable link.

• Computers located far away use a special form of data communication – telecommunication or telegraph.

People communicate with other people in a variety of ways.

When people use computers to communicate they use a computer network.

This course is about computer networks and how they are used to transmit information between computers and people.

It provides a broad overview and foundation for understanding networks and working in the computer and networking industry.

Why Use Networks? Network: group of computers and other devices

that are connected by some type of transmission media

Advantages of using networks over standalone computers: Networks enable multiple users to share the

network’s resources (devices and data) Networks allow you to manage, or administer,

resources on multiple computers from a central location

Defining a Computer Network A computer network allows users

to communicate with other users on the same network by transmitting data on the cables used to connect them.

A computer network is defined as having two or more devices (such as workstations, printers, or servers) that are linked together for the purpose of sharing information, resources, or both.

How Does a Network Work?

IP Address Postal Address

130.184.237.189 South Eastern University of Sri LankaUniversity Park,

Oluvil,Sri Lanka.

Three common reasons that people use networks to send information from a source, such as a personal computer (PC), to a destination, such as a printer, are: Communicate and collaborate

(i.e., e-mail and newsgroups) Share information (i.e., document sharing)

People use computers and networks for a wide variety of reasons. Share resources (i.e., printers and servers)

Why do I need a Network?

Sharing Data File sharing Email FTP Telnet Active Directory

Examples of sharing resources are: Sharing computer files and disk space Sharing high-quality printers Access to common fax machines Access to common modems Multiple access to the Internet

A source or destination can be any device capable of transferring information electronically from one point (source) to another point (destination).

There are many examples of devices that communicate over a network.These include: PCs Macintosh computers Workstations Printers Servers

Generically speaking, these devices are referred to as nodes. Nodes are the various endpoints on a network, connected together to form a network.

The connection between nodes is made using some type of connection medium.

Examples of connection mediums include: Copper cables Fiber optic cables Radio waves

You will see many examples of devices used as sources and destinations in this course.

You will also see examples of cable types and the way cables are used to attach computers together.

You will understand how information is transferred between the source and destination.

This knowledge will provide a solid foundation for understanding more advanced topics, such as internetworking devices and protocol analysis.

Problems experienced in the early days of microcomputers Hard drives were small and expensive Printers (especially Laser) were expensive, large and had a

high-consumable costs Document sharing required “sneaker-net” No applications, such as email or groupware

To address these problems, networks were created

The 5 Elements of a Network In order to have a network, there must be five

elements: Something to share … Resources Pathways through which the resources can be

shared … Infrastructure / Medium Rules to manage the movement of data through

the infrastructure … Protocols Sender Receiver

Protocols OSI model – open

systems interconnection

TCP/IP

Protocols The “rules of the road”, or how do we agree

on controlling the movement of the information.

The Seven-Layer OSI model (in a nutshell) Specific protocols that we will be working with

Ethernet TCP/IP

Protocol

Definition: An agreed upon set of rules governing

communication between two or more entities Protocols leads to architectures and standards

ConversationI like rabbits

J’aime leslapins

Translators

Secretaries

Generic 3 layer Model

Network Access

Transport

ApplicationContains logic needed to support various applications,such as file transfer, a separate module is required that is particular to that module.

Provides mechanisms for reliable transmission of databetween the applications. These common modules are shared by all applications.

This layer is concerned with the exchange of data between a computer and a network to which it is attached.It does not concern itself with the application but handles address and other network related activities.

OSI Model

The 7-layer model

765432

Application

PresentationSessionTransportNetworkData LinkPhysical

Networkresponsibility

Host responsibility

Principles used

OSI ModelDefinitions

Layer 7 (Application): Provides access to the OSI environment for users and also provides distributed information services.

Layer 6 (Presentation): Provides independence to the application processes from differences in data representation (syntax)

Layer 5 (Session): Provides the control structure for communication between applications; establishes, manages, and terminates connections (sessions) between operating applications.

Layer 4 (Transport): Provides reliable, transparent transfer of data between end points; provides end-to-end error recovery and flow control.

OSI ModelDefinitions (contd.)

Layer 3 (Network): Provides upper layers with independence from data transmission and switching technologies used to connect systems; responsible for establishing, maintaining and terminating connections.

Layer 2 (Data Link): Provides for the the reliable transfer of information across the physical link sends blocks (frames) with the necessary synchronization, error control, and flow control.

Layer 1 (Physical): Concerned with transmission of unstructured bit stream over physical medium; deals with the mechanical, electrical, functional and procedural characteristics across the physical medium.

OSI Model

Peer to Peer Protocol

Application

Data goes through only the first 3-layers in an intermediate node, but through all 7-layers in the hosts.

TCP/IP The TCP/IP stack has a 5-layer architecture

Application

TCP (Transport Control Protocol)

IP (Internet Protocol)

Network Access

Physical

Relation between OSI and TCP/IP

TCP/IP OSI

Physical Physical

NetworkAccess Data Link

Network

Transport

Session

Presentation

Application

OSI vs TCP/IP OSI has become the standard model for classifying

communication functions TCP/IP, on the other hand, is the most widely used

interoperable architecture for data communication TCP/IP is the result of experimental communication network

by ARPANET There are other protocols (like Frame Relay and ATM) are

used by TCP/IP as the lower layer protocols.

TCP/IP Protocol Suite Differences between OSI and TCP/IP? 5 layers:

Physical Data link Network (IP) Transport (TCP) Application (http, ftp, telnet, smtp)

Physical Layer (OSI)

Physical

Data Link

Network

Transport

Session

Presentation

Application

Physical Layer As mentioned earlier Physical layer is concerned with transmission of

unstructured bit stream over physical medium. Deals with the mechanical, electrical, functional and procedural

characteristics across the physical medium. Most of the electrical related matters is dealt with COM part of the

course. Some of the issues are:

Type of physical medium. Bandwidth of the medium. Multiplexing. Modulation Techniques Base band signal encodingetc.

Most of these issues will be discussed in details in com part of the course.We will briefly discuss some of them.

Physical Layer

Fiber Optic Cable Small attenuation and no leakage Single Mode Fibers

very narrow to avoid multiple reflections Multiple Mode Fibers

has multiple reflections Step Index Fiber, or Graded Index Fiber

Media Wired

Coaxial Twisted-Pair Fiber

Wireless High-frequency radio Microwave Satellite

Transmission media

Transmission Media The pathway to send data and information between two or

more entities on a network Bandwidth – transmission capacity Attenuation – weakening of a signal over distance EMI – electromagnetic interference

Cable Media Twisted pair cable Coaxial cable (“coax”) Fiber-optic cable

Twisted pairA basic twisted-pair cable consists of two strands of copper wire twisted together. This twisting reduces the sensitivity of the cable to electro magnetic interference.

Shielded twisted pairShielded twisted-pair cabling consists of one or more twisted pairs of cables enclosed in a foil wrap and woven copper shielding. The shield further reduces the tendency of the cable to radiate EMI and thus reduces the cable’s sensitivity to outside interference.

Unshielded twisted pairUnshielded twisted-pair cable doesn’t incorporate a braided shield into its structure.

Coaxial CableCoaxial cables were the first cable types used in LANs. Gets its name because two conductors share a common axis; the cable is most frequently referred to as coax.

Fiber OpticsThe centre conductor of a fiber-optic cable is a fiber that consists of highly refined glass or plastic designed to transmit light signals with little loss. A glass core supports a longer cabling distance, but a plastic core is typically easier to work with. The fiber is coated with a cladding that reflects signals back into the fiber to reduce signal loss. A plastic sheath protects the fiber.

Wireless Media Infrared line of sight High-frequency radio

Pager – one way wireless Cellular phone – two way wireless Wireless LANs

Microwave Terrestrial Satellite

As shown in figure, microwave communication can take two forms: terrestrial (ground) links and satellite links. The frequencies and technologies employed by these two forms are similar, but as you’ll see, distinct differences exist between them.

Microwave

Uplink Down link

Types of Networks Network: A group of two or more computer

systems linked together. LAN WAN Peer to Peer Client- Server (Routers/Switches) Web,Internet,Intranet

LANs, MANs, and WANs Local area network (LAN): confined to a

relatively small space, such as a building or an office

Metropolitan area network (MAN): larger than a LAN; connects clients and servers from multiple buildings

Wide area network (WAN): connects two or more geographically distinct LANs or MANs

LANs, MANs, and WANs

A simple WAN

Wide Area Network (WAN) Network connecting

two or more geographically distinct LANs

The Internet is an intricate and extensive WAN spanning the globe

Networking Fundamentals Network consists of:

Servers – provides services Clients – requests services Peers – both requests and provides services

Types of networks Server-centric networks Peer-to-peer networks

Network Relationships Peer-to-peer

Client/server

Network Features File server Print server Application server E-mail Web server Remote access Management services

Hardware Common to AllServer-Based Networks Client Server Transmission media Network devices

Routers Switches Hubs

Network topologies

Star network

Ring network

Bus network

Bus Topology Bus: each node is daisy-chained (connected one

right after the other) along the same backbone. Information sent from a node travels along the backbone until it reaches its destination node. Each end of a bus network must be terminated with a resistor to keep the data.

Ring Topology Ring: Similar to a bus network, rings

have nodes daisy chained, but the end of the network in a ring topology comes back around to the first node, creating a complete circuit. Each node takes a turn sending and receiving information through the use of a token. The token along with any data is sent from the first node to the second node which extracts the data addressed to it and adds any data it wishes to send. Then second node passes the token and data to the third node, etc. until it comes back around to the first node again. Only the node with the token is allowed to send data . All other nodes must wait for the token to come to them.

Star Topology In a star network, each node is connected to

a central device called a hub. The hub takes a signal that comes from any node and passes it along to all the other nodes in the network.

•A hub does not perform any type of filtering or routing of the data. •A hub is a junction that joins all the different nodes together.

In a star network, each node is connected to a central device called a hub. The hub takes a signal that comes from any node and passes it along to all the other nodes in the network.

Common Hybrid Topologies Star- Bus

Prob. Most common topology used today. Combines elements of the star and bus topologies to create a versatile network environment.

Nodes in particular areas are connected to hubs (and create star topology), and hubs are connected together along the network backbone (like a bus network).

Often you have stars nested within stars.

Common Hybrid Topologies Star- Bus

Types of Networks Peer-to-peer

Small size (less than 10 hosts) Limited resource sharing Little or no security

Server-Based (also called Client-Server) Much larger (up to hundreds of hosts) Centralized sharing of resources High-levels of access security

Types of Networks: Peer-to-peer Networks Every computer can communicate directly

with every other computer By default, no computer has more authority

than another Every computer is capable of sending and

receiving information to and from every other computer

Peer-to-peer Networks (continued)

Resource sharing on a simple peer-to-peer network

Client/Server Networks Server: central computer used to facilitate

communication and resource sharing between other computers on the network (called clients)

Client/server network: uses a server to enable clients to share data, data storage space, and devices

To function as a server, a computer must be running a network operating system (NOS)

Most of the concepts on the Network+ exam pertain to client/server networks

Client/Server Networks (continued)

Resource sharing on a client/server network

Client/Server Networks (continued) Advantages of using client/server networks

over peer-to-peer networks: User logon accounts and passwords can be

assigned in one place Access to multiple shared resources can be

centrally granted to a single user or groups of users

Problems can be tracked, diagnosed, and often fixed from one location

Client/Server Networks (continued) Advantages of using client/server networks

over peer-to-peer networks (continued): Servers are optimized to handle heavy processing

loads and dedicated to handling requests from clients, enabling faster response time

Because of their efficient processing and larger disk storage, servers can connect more than a handful of computers on a network

Elements Common to Client/Server Networks Client Server Workstation Network interface card (NIC) NOS Host Node Connectivity device

Elements Common to Client/Server Networks (continued)

A LAN backbone

Common network topologies

`Examples of network transmission media

How Networks Are Used Network services: functions provided by a

network File services

Capability of a server to share data files, applications, and disk storage space

File server Print services: used to share printers across

a network

How Networks Are Used (continued) Communications services

Allow remote users to connect to the network Remote access server (also known as

communications server or access server) Mail services

Coordinate the storage and transfer of e-mail between users on a network

Mail server

How Networks Are Used (continued) Internet services

Supplying Web pages, file transfer capabilities, Internet addressing schemes, security filters, and a means for directly logging on to other computers on the Internet

Web server Management services: centrally administer

management tasks on the network Traffic monitoring and control Load balancing

How Networks Are Used (continued) Management services (continued)

Hardware diagnosis and failure alert Asset management License tracking Security auditing Software distribution Address management Backup and restoration of data

Becoming a Networking Professional: Mastering the Technical Challenges

Installing, configuring, and troubleshooting network server software and hardware

Installing, configuring, and troubleshooting network client software and hardware

Understanding the characteristics of different transmission media

Understanding network design

Network Personnel Network administrators Systems programmers Network engineers Network architects and

designers Help desk and technical support E-mail managers Web masters, designers, and

programmers Database administrators

Mastering the Technical Challenges (continued) Understanding network protocols Understanding how users interact with the

network Constructing a network with clients, servers,

media, and connectivity devices

Pursuing Certification Certification: process of mastering material pertaining to

a particular hardware system, operating system (OS), programming language, or other software application, and then proving your mastery by passing a series of exams

A+ Microsoft Certified Systems Engineer (MCSE) Certified NetWare Engineer (CNE) Network+ (Net+) CCNA / CCNP (Cisco Certified System Administrator)

Finding a Job in Networking Search the Web Read the newspaper Visit a career center Network Attend career fairs Enlist a recruiter

Joining Professional Associations Benefits of joining a professional organization Professional organizations in the field of

networking Professional associations that cater to

specific demographic groups

Joining Professional Associations (continued)

Networking organizations

Benefits of Networks Improved internal sharing of resources Enabling collaborative work Automation of production control Enhances timely communications Connection with suppliers and customers for supply

chain management Provide value added services for customers Creates new business opportunities

Improvement Concerns WHAT does the BUSINESS need? Do network shortcomings inhibit achieving business

goals? Could additional network capabilities benefit

attaining business goals? Does the network need changes to support future

business plans? What maintenance does the network need to

maintain current levels of service?

Network Operating System (NOS) Two parts:

System software that runs on the server Client software on each workstation

Examples Banyan Vines Novell NetWare Linux Microsoft LAN Manager Microsoft Windows server 2003 LANtastic

Network Software and Hardware Connectivity hardware

1. Transmission Media Connectors2. Network interface card (NIC)3. Modems4. Repeaters5. Hubs6. Bridges7. Multiplexers8. Routers9. Brouters10. Gateway

Connectivity Devices Hubs

Inexpensive No Setup or programming Slower and less efficient

Switches Unmanaged

Inexpensive (more than hubs, but not much more) No setup or Programming Faster than Hubs

Routers Need setup or Programming

SOHO (NetGear, LinkSys, DLink, etc) General Purpose (Cisco)

PC Basics

AGP Expansion Slot

PCI Expansion Slots

Network Interface Cards

NIC and Modem Installation

Connectivity DevicesHubsThe hub's major function is to replicate data it receives from one device attached to it to all others.

RepeatersAny electrical signal reaching the repeater from one segment, will be amplified and retransmitted to the other segment. Using repeaters slows the signal's propagation, and thus the amount of repeaters should be limited.

BridgesBridges are used to increase the number of addressable nodes on a network or to link two geographically distant but similar networks.

RoutersA major feature of a router is that it chooses the best route between networks that may have multiple paths between them. IP addressing and subnet masking are used to route packets

ModemsA modem is a device that converts digital data originating from a computer to analog signals used by voice communication networks, such as the telephone system. A modem may also convert analogue data originating from a (say, a telephone line) to digital data for use by a computer.

GatewaysGateways interface two dissimilar networks. Gateways translate one network protocol into another, thus overcoming both hardware and software incompatibilities.

Applications of Data Communication and Computer Networks• Internet

• Teller and point-of-sale terminals

• Airlines, car-rentals, railway reservation systems, and hotel reservation systems

Advantages of Computer Networks• Sharing of expensive resources (e.g., A Laser printer, DVD drive)

• Sharing of data bases and other software

• Communication among different computer systems

• Transfer of multimedia files

• Faster transfer of data

Disadvantages of Computer Networks• Initial cost

• Security of information (viruses, hackers, ..)

• System failures

Security Viruses and Worms Hacking Passwords Encryption Authentication Certificates Physical Security

Terminology A network is two or more devices or

computers connected together to share resources Allows users to share data easily Enhances communication between users Allows sharing devices (e.g. printers)

Terminology An internet (inter network) is two or more

networks tied together An internet is any two or more networks tied

together The Internet is the world-wide network that we

use for email, web, etc.

Terminology LAN - Local Area Network

Distance < 1 km WAN - Wide Area Network

Distance > 1 km Others: Metropolitan Area Network (MAN),

Campus Area Network (CAN), Personal Area Network (PAN)

Protocols A protocol is a set of rules that govern

communication between computers Similar to human protocols for behavior Specifies the questions that can be asked, the

possible answers, the vocabulary, rules about who gets to ‘talk’ and when

Most of what we will discuss in this class are protocols of one form or another

Protocols TCP (transmission control protocol) and IP

(internet protocol) are examples TCP/IP refers to a suite (collection) of protocols

including TCP, IP, and many others HTTP (hypertext transfer protocol), FTP (file

transfer protocol), SMB (system message block) are other examples

Clients, Peers, and Servers Server

A computer that shares its resources across the network

Responds to client requests for information by providing the requested information

Client A computer that accesses shared resources Requests information

The Client/Server Relationship

Tittel and Johnson

Client-Server A server is an entity that provides a service

Service could be file or printer sharing, web pages, streaming audio, etc.

A client uses the service by issuing requests to the server

The server provides the service by issuing responses to requests

Clients, Peers, and Servers Client/server network

Certain computers take specialized roles and function mostly as servers

Ordinary users’ machines tend to function mostly as clients

Peer-to-peer network Each computer can be a client to other computers

and act as a server as well

Addresses Each device on a network is identified by one

or more addresses Address is usually formed from one or more

numbers or character strings Many protocols have their own concept of an

addresses Multiple protocols implies multiple addresses

Packets A packet is a unit of data transmitted over a

network Breaking a large amount of data up into packets

makes many networking tasks easier - divide and conquer

Often different protocols have their own name for a packet We will use “packet” for any protocol

Errors The world of networking is far from ideal Some types of errors effecting packets:

Get lost along the way Corrupted - the data is changed Arrive out of order Be duplicated

Communication Mode With Simplex communication, only one way

communication – e.g. TV, Radio With full-duplex communication, two systems

can ‘talk’ simultaneously (two way communication) - e.g. normal telephone

With half-duplex communication, only one system can ‘talk’ at a time - e.g. Walkie-talkie, two-way radio (CB Radio)

Communications Unicast - one computer talking directly to

another computer Broadcast - one computer talking to every

computer on the network Multicast - one computer talking to a subset

of the computers on a network

Summary A network is a group of computers and other

devices that are connected by some type of transmission media

In a peer-to-peer network, every computer can communicate directly with every other computer

A client/server network uses a server to enable clients to share data, data storage space, and devices

A LAN is confined to a relatively small space, such as a building or office

Summary (continued) A MAN is larger than a LAN and connects

clients and servers from multiple buildings A WAN connects two or more geographically

distinct LANs or MANs Elements common to client/server networks:

client, server, workstation, NIC, NOS, host, node, connectivity device, segment, backbone, topology, protocol, data packets, addressing, and transmission media

Summary (continued) Network services include file, print,

communications, mail, Internet, and management services

Certification is the process of mastering material pertaining to a particular hardware system, OS, programming language, or other software application and then proving your mastery by passing a series of exams

CSC 37 – COMPUTER NETWORKS

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