Osi Layers and Basics of Networks

8/11/2019 Osi Layers and Basics of Networks

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Unit-1

Definition:A computer networkor data networkis atelecommunications networkthat

allowscomputersto exchangedata.In computer networks, networked computing devices pass

data to each other along data connections. The connections (network links)between nodes are

established using eithercable mediaorwireless media.The best-known computer network istheInternet.

Components of Computer Network Data Communication:

Servers- Servers are computers that hold shared files, programs, and the network operating

system. Servers provide access to network resources to all the users of the network. There are

many different kinds of servers, and one server can provide several functions. For example, there

are file servers, print servers, mail servers, communication servers, database servers, print servers,

fax servers and web servers, to name a few.

Clients- Clients are computers that access and use the network and shared network resources.Client computers are basically the customers (users) of the network, as they request and receive

services from the servers.

Transmission Media- Transmission media are the facilities used to interconnect computers in a

network, such as twisted-pair wire, coaxial cable, and optical fiber cable. Transmission media are

sometimes called channels, links or lines.

Shared data- Shared data are data that file servers provide to clients such as data files, printer

access programs and e-mail.

Shared printers and other peripherals- Shared printers and peripherals are hardware resourcesprovided to the users of the network by servers. Resources provided include data files, printers,

software, or any other items used by clients on the network.

Network Interface Card- Each computer in a network has a special expansion card called a

network interface card (NIC). The NIC prepares (formats) and sends data, receives data, and

controls data flow between the computer and the network. On the transmit side, the NIC passes

frames of data on to the physical layer, which transmits the data to the physical link. On the

receiver's side, the NIC processes bits received from the physical layer and processes the message

based on its contents.

Local Operating System- A local operating system allows personal computers to access files, print

to a local printer, and have and use one or more disk and CD drives that are located on the

computer. Examples are MS-DOS, Unix, Linux, Windows 2000, Windows 98, Windows XP etc.

Network Operating System- The network operating system is a program that runs on computers

and servers, and allows the computers to communicate over the network.
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Hub- Hub is a device that splits a network connection into multiple computers. It is like a distribution

center. When a computer request information from a network or a specific computer, it sends the

request to the hub through a cable. The hub will receive the request and transmit it to the entire

network. Each computer in the network should then figure out whether the broadcast data is for them

or not.

Switch- Switch is a telecommunication device grouped as one of computer network components.

Switch is like a Hub but built in with advanced features. It uses physical device addresses in each

incoming messages so that it can deliver the message to the right destination or port.

Like hubs, switches dont broadcast the received message to entire network, rather before sending it

checks to which system or port should the message be sent. In other words switch connects the

source and destination directly which increases the speed of the network. Both switch and hub have

common features: Multiple RJ-45 ports, power supply and connection lights.

Types Of data:

Text

Numbers

Audio

Video

Image


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Dataflow:

Simplex

Half-duplex

Full-duplex

3. Distributed Processing

Most networks use distributed processing, in which a task is divided among multiplecomputers. Instead of one single large machine being responsible for all aspects of a process,separate computers (usually a personal computer or workstation) handle a subset.

Network CriteriaA network must be able to meet a certain number of criteria. The most important of these are

performance, reliability, and security.

a. Performance:

Performance can be measured in many ways, including transit time and response time. Transittime is the amount of time required for a message to travel from one device to another. Response

time is the elapsed time between an inquiry and a response. The performance of a network

depends on a number of factors, including the number of users, the type of transmission

medium, the capabilities of the connected hardware, and the efficiency of the software.Performance is often evaluated by two networking metrics: throughput and delay


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b. ReliabilityIn addition to accuracy of delivery, network reliability is measured by the frequency of failure,

the time it takes a link to recover from a failure, and the network's robustness in a catastrophe.

c. SecurityNetwork security issues include protecting data from unauthorized access, protecting data fromdamage and development, and implementing policies and procedures for recovery from breaches

and data losses.

Topology:

a. Link connection:

1. Point to point2. Point to multipoint

b. Topology:1. mesh

2. bus

3. star

4. ring

1.Mesh Topology:Mesh In a mesh topology, every device has a dedicated point-to-point link to every other device.

The term dedicated means that the link carries traffic only between the two devices it connects.To find the number of physical links in a fully connected mesh network with n nodes, we firstconsider that each node must be connected to every other node. Node 1 must be connected to n -I nodes, node 2 must be connected to n 1 nodes, and finally node n must be connected to n - 1nodes. We need n(n - 1) physical links. However, if each physical link allows communication inboth directions (duplex mode), we can divide the number of links by 2. In other words, we can

say that in a mesh topology, we need duplex-mode links.

n(n -1) /2


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1.Reduce Network load: It by eliminating the traffic problems that can occur when links must be

shared by multiple devices.

2. Second, a mesh topology is robust. If one link becomes unusable, it does not incapacitate theentire system.

3.Third, there is the advantage of privacy or security.

Disadvantages:

1. the amount of cabling and the number of I/O ports required2. . Second, the sheer bulk of the wiring can be greater than the available space (in walls,

ceilings, or floors) can accommodate. Finally, the hardware required to connect each link(I/O ports and cable) can be prohibitively expensive. For these reasons a mesh topology

is usually implemented in a limited fashion.

2.Star Topology:Star Topology In a star topology, each device has a dedicated point-to-point link only to a centralcontroller, usually called a hub. The devices are not directly linked to one another. Unlike a mesh

topology, a star topology does not allow direct traffic between devices. The controller acts as an

exchange: Ifone device wants to send data to another, it sends the data to the controller, whichthen relays the data to the other connected device. A star topology is less expensive than a mesh

topology. In a star, each device needs only one link and one I/O port to connect it to any number

of others. This factor also makes it easy to install and reconfigure. Far less cabling needs to behoused, and additions, moves, and deletions involve only one connection: between that device

and the hub.

Advantages:

1. Robustness

2. monitoring of link bypass.

Disadvantages:

1. Single point of failure.


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A hybrid topology: a star backbone with three bus networks


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OSI- Layers


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The interaction between layers in the OSI model

The physical layer coordinates the functions required to carry a bit stream over a physi

cal medium. It deals with the mechanical and electrical specifications of the interface and

transmission medium. It also defines the procedures and functions that physical devices

and interfaces have to perform for transmission to Occur .


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The physical layer is also concerned with the following:

1. Physical characteristics of interfaces and medium. The physical layer defines thecharacteristics of the interface between the devices and the transmission medium.

2. It also defines the type of transmission medium.

3. Representation of bits. The physical layer data consists of a stream of bits (sequence ofOs or 1s) with no interpretation. To be transmitted, bits must be encoded into signals--electrical or optical. The physical layer defines the type of encoding (how Os and I s are

changed to signals).

4. Data rate. The transmission rate-the number of bits sent each second-is also defined bythe physical layer. In other words, the physical layer defines the duration of a bit, which

is how long it lasts.

5. Synchronization of bits. The sender and receiver not only must use the same bit rate butalso must be synchronized at the bit level. In other words, the sender and the receiverclocks must be synchronized.

6. Line configuration. The physical layer is concerned with the connection of devices to the

media. In a point-to-point configuration, two devices are connected through a dedicatedlink. In a multipoint configuration, a link is shared among several devices.

7. Physical topology. The physical topology defines how devices are connected to make anetwork. Devices can be connected by using a mesh topology (every device is connected

to every other device), a star topology (devices are connected through a central device), aring topology (each device is connected to the next, forming a ring), a bus topology

(every device is on a common link), or a hybrid topology (this is a combination of two or

more topologies).

8. Transmission mode. The physical layer also defines the direction of transmission betweentwo devices: simplex, half-duplex, or full-duplex. In simplex mode, only one device can

send; the other can only receive. The simplex mode is a one-way communication. In the

half-duplex mode, two devices can send and receive, but not at the same time. In a full-duplex (or simply duplex) mode, two devices can send and receive at the same time.

Osi Layers and Basics of Networks

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