Computer Networks Basics

E n g i s t a n . c o m

E n g i n e e r ’ s C o m m u n i t y

This E-Book contains basics of Computer Networks for the preparation of IBPS CWE (Common Written Examination) for Specialist Officers.

Topics Covered:

1. Introduction2. Types of Computer Networks3. Network Topology4. Protocols5. Network Hardware6. Network Cabling7. Wireless LAN’s.8. Network Operating Software9. The OSI Model10. The TCP/IP Model

Engistan.com [COMPUTER NETWORKS BASICS ] Computer Networks: A network consists of two or more computers that are linked in order to:

share resources (such as printers and CD-ROMs),

exchange files, or

allow electronic communications

How are they linked?

Computers on a network may be linked through:

cables,

telephone lines,

radio waves, satellites or

wireless technologies

There are two aspects in computer networks.

• Hard Ware : It includes physical connection (using adapter, cable, router, bridge etc)

• Soft Ware : It includes set of protocols (nothing but a set of rules)

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Engistan.com [COMPUTER NETWORKS BASICS ] Basic types of Computer Networks:

1. Local Area Networks (LANs) : These are the simple networks. A set of computers (2 or more) connected within a single building or college campus (a few kilometres range).

2. Metropolitan Area Networks (MANs): These are the larger networks than LANs, these are limited to city or group of Nearby Offices. Simply we can say that a group of LANs are called a MAN (but guys, this is not a standard definition, its just for your understanding purpose).

3. Wide Area Networks (WANs): These type of networks provides long distance transmission of data and voice. (A city, state, country and of course the world). It contains HOST and collection of machines.

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Engistan.com [COMPUTER NETWORKS BASICS ] Comparison chart:

Local Area Network (LAN) Wide Area Network (WAN)

Definition

LAN (Local Area Network) is a computer network covering a small geographic area, like a

home, office, school, or group of buildings.

WAN (Wide Area Network) is a computer network that covers a broad area (e.g., any network whose communications links cross

metropolitan, regional, or national boundaries over a long distance).

Speed High speed (1000 mbps) Less speed (150 mbps)

Data transfer rates

LANs have a high data transfer rate.

WANs have a lower data transfer rate as compared to LANs.

Example Network in an organization can

be a LAN Internet is a good example of a WAN

Connection

One LAN can be connected to other LANs over any distance via telephone lines and radio

waves.

Computers connected to a wide-area network are often connected through public

networks, such as the telephone system. They can also be connected through leased

lines or satellites.

Components Layer 2 devices like switches

and bridges. Layer 1 devices like hubs and repeaters.

Layers 3 devices Routers, Multi-layer Switches and Technology specific devices like

ATM or Frame-relay Switches etc.

Fault Tolerance

LANs tend to have fewer problems associated with them, as there are smaller number of

systems to deal with.

WANs tend to be less fault tolerant as they consist of large number of systems.

Data Transmission

Error

Experiences fewer data transmission errors

Experiences more data transmission errors as compared to LAN

Ownership Typically owned, controlled,

and managed by a single person or organization.

WANs (like the Internet) are not owned by any one organization but rather exist under

collective or distributed ownership and management over long distances.

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Engistan.com [COMPUTER NETWORKS BASICS ]

Network Topology: A network Topology is the method or process which tells us about how to connect the Systems.

• The physical topology of a network refers to the configuration of cables, computers and other peripherals.

Types of Topologies:

Line Topology: If we connect several computers in a line then it is called as the LINE TOPOLOGY

Tree Topology: If we connect several systems in a hierarchical order, then it is called as the TREE TOPOLOGY

Bus Topology: If we connect several computers to a back bone (A wire) then it is called as the BUS TOPOLOGY.

Star Topology: If we connect several computers to a central hub (system) then it is called as the STAR TOPOLOGY.

Ring Topology: In line topology if we connected the last system to the starting system, then it is called as the Ring topology.

Mesh Topology: If we connect several systems with each other (There is no necessity that one system should connect with all the remaining systems in the network, see the diagram) then it is called as the Mesh Topology.

Fully Connected: In a Network if every system is connected with all the remaining systems then it is called as the Fully Connected topology.

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Engistan.com [COMPUTER NETWORKS BASICS ]

Protocol:

• A protocol is a set of rules that governs the communications between computers on a network

• These rules include guidelines that regulate the method of access, types of cabling and speed of data transfer

The most common protocols are:

• Ethernet

• Local Talk

• Token Ring

• FDDI

• ATM

1. Ethernet: • Most widely used • Uses an access method called CSMA/CD (Carrier Sense Multiple Access/Collision

Detection.

What does that mean?

• Each computer in the network ‘listens’ to the cable before sending anything through the network. If the network is clear, the computer will transmit.

• If another computer is already transmitting on the cable, the computer will wait and try again when the line is clear

• Sometimes, two computers attempt to transmit at the same instant. When this happens, a collision occurs. Each computer then backs off and waits a random amount of time before attempting to retransmit. It is normal to have collisions using this method, but the delays caused by collisions and transmissions is small, and does not affect speed of transmission on the network

Did you ever wonder what HTTP in web addresses was

about? It stands for Hypertext Transfer Protocol.

CSMA/CD =Carrier Sense Multiple Access /Collision Detection

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Engistan.com [COMPUTER NETWORKS BASICS ] 2. Fast Ethernet: To allow for faster transmission, the Ethernet protocol has developed a

new standard that supports 100 Mbps • Fast Ethernet requires the use of more expensive equipment and network cards.

3. Gigabit Ethernet: The Ethernet protocol has also developed a new standard that allows transmission of 1 Gbps (gigabit per second).

4. Token Ring: Token ring protocol involves ‘token-passing’. • It is not as popular as Ethernet protocol. • A single electronic ‘token’ moves around the ring from one computer to the next.

If a computer wishes to transit and receives an empty token, it attaches data to the token which then proceeds around the ring until it comes to the computer the data is meant for.

5. FDDI: Stands for Fiber Distributed Data Interface. • Is used mainly to connect two or more LANs, often over large distances. • Can operate over fiber optic cable at 100 Mbps.

6. ATM: Stands for Asynchronous Transfer Mode. • Transmit data at a speed of 155 Mbps and higher • Works by transmitting all data in small packets of fixed size (other protocols

transfer variable size packets) • Like FDDI , is most often used to connect two or more LANs

Where does TCP/IP fit into all this???

• TCP/IP is the protocol that is used for the transmission of information over the Internet.

• IP (Internet Protocol) - the main delivery system for information over the Internet.

• TCP (Transport Control Protocol) - used to break apart and rebuild information that travels over the Internet.

Network Hardware: Network hardware includes, Computers, Peripherals, Interface cards, and Other equipment needed to perform data processing and communications within the network.

One gigabit per second = one thousand megabits per

second.

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Engistan.com [COMPUTER NETWORKS BASICS ]

1. Concentrators / Hubs: A concentrator is a device that provides a central connection point for cables from workstations, servers and peripherals.

• Hubs are multi-slot concentrators.

2. Switches: While hubs provide an easy way to scale up and shorten the distance that the packets must travel to get from one node to another, they do not break up the actual network into discrete segments. That is where switches come in.

• A vital difference between a hub and a switch is that all the nodes connected to a hub share the bandwidth among themselves, while a device connected to a switch port has the full bandwidth all to itself.

• Think of a switch as a ‘clever’ hub.

3. Repeaters: A signal loses strength as it passes along a cable, so it is often necessary to boost the signal with a device called a repeater.

• A repeater might be a separate device, or might be part of a concentrator.

4. Bridges: A bridge is a device that allows you to segment a large network into two smaller, more efficient networks.

5. Routers: A router is a device that forwards data packets between computer networks, creating an overlay internetwork. A router is connected to two or more data lines from different networks.

6. Firewalls: A firewall is a networking device that is installed at the entrance to a LAN when connecting a networks together, particularly when connecting a private network to a public network, such as the internet. The firewall uses rules to filter traffic into and out of the private network, to protect the private network users and data from malevolent hackers.

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Engistan.com [COMPUTER NETWORKS BASICS ] Network Cabling: Cable is the medium through which information usually moves from one network device to another.

Types of cables used in networks:

1. Unshielded Twisted Pair (UTP) Cable: UTP cable is also the most common cable used in computer networking. UTP is susceptible to radio and electrical frequency interference (it should not be too

close to electric motors, fluorescent lights, etc.)

2. Shielded Twisted Pair (STP) Cable: STP is similar to UTP but with each pair covered by an additional copper braid jacket or foil wrapping. This shielding helps protect the signals on the cables from external interference.

3. Coaxial Cable: Coaxial cable has a single copper conductor at its centre with a plastic layer between the centre conductor and the braided metal shield

Although coaxial cabling is difficult to install, it is highly resistant to signal interference

4. Fiber Optic Cable: Fiber optic cabling consists of a center glass core surrounded by Several layers of protective materials It transmits light rather than electronic signals It is the standard for connecting networks between buildings, due to its immunity to

the effects of moisture and light Fiber optic cable has the ability to transmit signals over much longer distances than

coaxial or twisted pair It can also carry information at vastly greater speeds Fiber optic cable is more difficult to install than other cabling

Wireless LANS: Wireless networks use high frequency radio signals to communicate between the workstations and the fileserver or hubs.

Disadvantages of wireless networks are:

they are expensive (relatively),

provide poor security,

are susceptible to interference and

are slower than cabled networks

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Engistan.com [COMPUTER NETWORKS BASICS ]

Network Operating Software: Network operating systems co-ordinate the activities of multiple computers across a network.

The two major types of network OS are:

Peer-to-peer

Client/server

1. Peer to peer network OS: In peer to peer network OS, there is no file server or central management source; all computers are considered equal.

Peer to peer networks are design primarily for small to medium LANS. AppleShare and Windows for Workgroups are examples of programs that can

function as peer to peer.

2. Client/Server network OS: Client/server network OS centralise functions and applications in one or more dedicated file servers.

The file server provides access to resources and provides security. Novelle Netware and Windows NT Server are examples of client/server network

operating systems.

The OSI Model Once there was an Organization called International Standards Organization (ISO in short). It standardized the functions of the Communications System with abstraction layer and developed a Model. They named it as Open Systems Interconnection (OSI). ISO - OSI sounds funny, isn't it ? In this model, similar types of communication functions are grouped into Layers. There are 7 layers in this Model. Those are,

1. Physical Layer 2. Data link Layer 3. Network Layer 4. Transport Layer 5. Session Layer 6. Presentation Layer 7. Application Layer

Each layer helps it's above layer to establish an error free communication, in other words, each layer depends on its below layer to establish an error free communication.

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Engistan.com [COMPUTER NETWORKS BASICS ]

How to remember these names???? Here the order is very important. If anybody asks you about these layers, you should start with Physical and end with Application Layer. You should not jumble these names according to your convenience. So you should remember these names with an order. Well, there are several techniques are there to remember these type of names. One of those technique is, making a funny name or sentence with the STARTING letters. Just, write the starting letters of these layers. P D N T S P A Now make a funny sentence, here are some examples • Please Do Not Touch Steve's Pet Alligator

Or • Please Do Not Throw Sausage Pizza Away

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Engistan.com [COMPUTER NETWORKS BASICS ] Functions of these Layers...

I. Physical Layer : This is the First or Lowest layer of the OSI model. It takes care about the Physical Connections.

II. Data Link Layer : This layer Encodes the data packets and provides error free node to

node transmission. It is divided into two sub layers. Those are Media Access Control Layer (MAC Sublayer in short) It helps the computer to get the access of data transmission Logical Link Control Layer (LLC Sublayer in short) It controls the order, flow and errors of the frames (Frame synchronization)

III. Network Layer : This layer helps in Switching and Routing

(connecting and transmitting), and also creates virtual circuits ( Imaginary paths for transmission)

IV. Transport Layer : This layer helps in providing Transparent data transmission. Its also

checks whether the data completely transmitted or not.

V. Session Layer : This layer is responsible for the Transmission Sessions. I mean Starting, Maintaining and Terminating the Communication.

VI. Presentation Layer : This layer provides the data Independence ( simply you can

remember that this layer is responsible for the data Presentation :) I mean, how you are presenting data (without any leakage)

VII. Application Layer : This is the topmost layer, this layer directly interacts with the end

user. This layer is responsible for checking whether the claimed user is genuine or not.

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Engistan.com [COMPUTER NETWORKS BASICS ]

TCP/IP model (Transport Control Protocol / Internet Protocol): This model has 4 Layers. Those are

1. Network Interface Layer 2. Internet Layer 3. Transport Layer 4. Application Layer

ISO Just elaborated this model and divided some of these layers into EXTRA LAYERS and developed OSI model. have a look at the following Diagram, so that you can understand the difference :)

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