computer networks basics and osi

Basics of data communication and OSI model

Navneet SoniAsst. ProfessorGDRCST, Bhilai

Data communication is the exchange of data between two devices by making use of some transmission media.

Data Communication may be of two typesLocalRemote

If the communicating devices are in the same building then it is a type of local communication

If the devices are farther apart then it is a type of remote communication

Navneet Soni (Asst. Professor) 2


Effectiveness of data communication depends on 3 fundamental characteristics

DeliveryAccuracyTimeliness

Components of Data communication Protocol Protocol

Sender ReceiverMessage

Sender:- Device that sends the messageReceiver:- Device that receive the messageMedium:-It is the physical path by which message travels from sender to receiver.Protocol:-It is the exchange of data between two separate entities.Message:- Information which is to be communicated.

Networks:

A network is a collection of autonomous computer. The computer which can forcibly start, stop and control another one are not autonomous. Two computer are said to be interconnected if they are able to exchange information.


The communication can be done by copper wire, fiber optics, microwaves and communication satellite.

Networks uses distributed processing in which task is divided among multiple computers.

Advantages of distributed processing areSecurityDistributed DatabaseFaster problem SolvingSecurity Through RedundancyCollaborative Processing


Performance can be measured in many ways including the Transit Time & Response Time. It depends on the following factors, they are:

1. Number of users2. Types of Transmission Media3. Hardware 4. Software


Criteria

Performance Reliability Security

Reliability:- it is measured by frequency of failure, the time it takes link to recover from a failure & Catastrophe.

Security:- Network must be safe guarded from: Unauthorized Access Viruses


Protocol is a set of rules that govern the exchange of data between two separate entities.

A protocol defines: What is communicated, How it is communicated & When it is communicated.

The Key Element of protocol is: Syntax Semantics Timing


Syntax: It refers to the format of the data, i.e. meaning of the order in which they are presented.

Semantics:- It refers to the meaning of each section of the bits.

Timing:- it refers to when the data should be sent and how fast they can be sent.


A standard provides a model for development that makes it possible for a product to work regardless of the individual manufacturer

Data communication standards fall into 2 categories:Defacto (By Fact)Dejure (By Law)

1) Dejure (By Law) : Standards that have not been approved by an organized body but have been adopted as a standard.

2) Defacto (By Fact) standards are of two types:1. Proprietary2. Non Proprietary


Line configuration refers to the way two or more devices attach to the link.

A link is the physical connection i.e. the pathway that transfer the data from one device to another. There are 2 possible line configuration

Point-to-PointMultipoint



Point-to-Point:It provides a dedicated link between two devices. The entire capacity of the channel is reserved for transmission between those two devices.

Multipoint:It is also known as multi-drop line configuration, it is the one in which more than two specific devices share a single link.

Transmission mode defines the direction of single flow between two devices. There are 3 types of transmission mode, they are:

1. Simplex2. Half-Duplex3. Full-Duplex


It is the geometrical representation of how the nodes in the network are attached to each other. Basically there are 5 types of topologies, they are:MeshStarTreeBusRing


A type of network setup where each of the computers and network devices are interconnected with one another, allowing for most transmissions to be distributed, even if one of the connections go down. This type of topology is not commonly used for most computer networks as it is difficult and expensive to have redundant connection to every computer.

16


There are two types of mesh topology they areFully Connected Mesh TopologyPartial Connected Mesh Topology

Advantages:Eliminating the traffic problems.Mesh topology is robust.It provides security.

Disadvantages:The amount of cabling and the ports required

Also known as a star network, a star topology is one of the most common network setups where each of the devices and computers on a network connect to a central hub. A major disadvantage of this type of network topology is that if the central hub fails, all computers connected to that hub would be disconnected.

18

It is a central controlling device in a star network.


http://en.wikipedia.org/wiki/Image:4_port_netgear_ethernet_hub.jpg


There are 2 types of hub1. Active Hub2. Passive Hub

An Active hub contains a repeater which is a hardware device that regenerates the received bit pattern before sending them out.

A Passive hub provides a simple physical connection between the attached devices.


Advantages: It is less expensive than a mesh topology because each device needs

only one link and one I/O port to connect it to any number of others. It includes robustness means if one link fails than only that link is

affected, all the other link remain active.

Disadvantages: If the hub fails then it affects the whole network.

Bus networks use a common backbone to connect all devices. A single cable, the backbone functions as a shared communication medium that devices attach or tap into with an interface connector. A device wanting to communicate with another device on the network sends a broadcast message onto the wire that all other devices see, but only the intended recipient actually accepts and processes the message.


In this nodes are connected to the bus cable by drop lines and taps. A drop line is a connection running between the device and the main cable.

A tap is a connector that splices into the main cable to create a contact with the metallic core.



Advantages: Easy to connect a computer or peripheral to a linear bus. Requires less cable length than a star topology.

Disadvantages: Entire network shuts down if there is a break in the main cable. Terminators are required at both ends of the backbone cable. Difficult to identify the problem if the entire network shuts down. Not meant to be used as a stand-alone solution in a large building.

In this topology each device has a dedicated link only with the two devices on the either side of it. All messages travel through a ring in the same direction (either "clockwise" or "counterclockwise"). A failure in any cable or device breaks the loop and can take down the entire network.



Advantages: It is relative to install and reconfigure In this fault isolation is simplified.

Disadvantages: Unidirectional traffic may be disadvantage. A break in the ring can disable the entire network.

It is a variation of star, nodes in a tree are linked to the central hub. But in this case not all devices are connected to the central hub they are connected to the secondary hub after that all the secondary hub are connected to the central hub.

The central hub is the type of Active hub

The Secondary hub is the type of Passive hub


Advantages:• It allows more devices to be attached to a single central hub.• It allows the network to isolate and prioritize communication from

different computers


A network topology that uses two or more network topologies.


Networks are categorized into 3 categories:1. LAN (Local Area Network)2. MAN (Metropolitan Area Network)3. WAN (Wide Area Network)


When two or more networks are connected together they become an internetwork or internet.


An ISO standard that cover all aspect of network communication is the OSI model.

The purpose of the OSI Model is to provide open communication between different system.

It is a model not a protocol.

The OSI model is built of seven ordered layer.


Physical layer

The physical layer coordinates the function required to transmit a bit stream over a physical medium.

It deals with the electrical and mechanical specification of the interface and the transmission medium means that it defines the procedure and functions that physical devices and interfaces have to perform for transmission to occur


Physical characteristics of Interface and media. Representation of bits. Data Rate. Synchronization of bits. Line configuration. Physical Topology Transmission Mode


It transforms the physical layer a raw transmission facility to a reliable link and it is also responsible from node to node delivery.

Specific responsibility of the physical layer are:FramingPhysical AddressingFlow ControlError ControlAccess Control


The network Layer is responsible for the source to destination delivery of a packet across a multiple links or networks.

Specific responsibilities of the network layer are:Logical AddressingRouting


This layer is responsible for source to destination delivery i.e. end to end delivery of the entire message.

For adding security the transport layer may create a connection between the two end ports.

Specific responsibility of this layer areService Point AddressingSegmentation & ReassemblyConnection ControlFlow ControlError Control


This layer establishes, maintains and synchronizes the interaction between communicating systems

Specific responsibilities performed by this layer is:

Dialog ControlSynchronization


This layer is concerned with the syntax & semantics of the information exchanged between the two system.

Specific responsibilities of this layer is:TranslationEncryptionCompression


This layer enables the user to access the network. It provides user interface and provide support for services.

Specific responsibility performed by this layer is:

Network Virtual TerminalFile Transfer, Access & ManagementMail ServicesDirectory Services


The ARPANET was a research network sponsored by the DOD. It eventually connects hundred of universities and government installations by using leased telephone line.

When satellite and radio networks are attached, the existing protocols had trouble in internetworking with them. For that a new architecture was made which is known as TCP/IP reference model.


Application Layer Transport Layer Network Layer Physical + Data Link Layer---Host-to-Network Layer


This layer contain some higher level protocol, out of which some of them are:

Telnet (Virtual Terminal) FTP

SMTP


In this layer it make use of two protocolTCPUDP

TCP stands for Transmission Control Protocol. It is a type of connection oriented and reliable protocol.

UDP (User Datagram Protocol) which is a connectionless and unreliable protocol.


The job of the network layer is to permit the host to inject packets into any network and have them travel independently to the destination.

The packets may even arrive in a different order than they were sent, in this case it is the job of the higher layer to rearrange them, if inorder delivery is required.


Data Transmission

Parallel Transmission Serial Transmission

SynchronousAsynchronous


To alert the receiver that a new group of data is arriving, two extra bits are added one at the beginning which is known as start bit and the other one is at the end which is known as stop bit . There may be a gap between each byte.

The start bit is always 0 and the stop bit is always 1.

The start bit, stop bit and the gap alert the receiver to the beginning and end of each byte and allow it to synchronize with the data stream


Data can be represented in form of signal and the signal are of two types:

Analog SignalDigital Signal

Analog refers to something that is continuous means a set of specific points of data and all possible points between them. Ex: Human Voice.

Digital refers to something that is discrete means a set of specific points of data with no other points in between. Ex: Data stored in memory.


Both analog and digital signal can be of two forms:Periodic SignalAperiodic Signal

A periodic signal consist of a continuously repeated pattern. The period of signal is expressed in seconds. The completion of one full pattern is known as cycle.

An aperiodic signals have no repetative pattern.


Simple analog signal is known as sine wave. Sine wave is characterized by 3 characteristics:

AmplitudeFrequencyPhase

Amplitude:It refers to the height of the signal. Amplitude is measured in volts, amperes & watts.

Period or Frequency:Period refers to the amount of time a signal needs to complete one cycle.


Frequency is the number of cycles per second. Period is measured in sec, millisec, microsec, picosec, nanosec. Frequency is measured in Hz, KHZ, MHZ, GHZ, THZ.

Phase represents the position of the wave form relative to time zero (0).

Digital to Analog conversion is the process of changing one of the characteristics of an analog signal based on the information in a digital signal.


Amplitude Shift Keying Frequency Shift Keying Phase Shift Keying

Quadrature Amplitude Modulation.


The Major functions of Data Link Layer are

FramingPhysical AddressingLine DisciplineFlow ControlError Control


Line Discipline determines which device can send and when it can send.

Flow control coordinates the amount of data that can be sent before receiving acknowledgement.

Error control means error detection and correction. It allows the receiver to inform the sender about the damaging & losting.


Multipoint Discipline


Error control method refers to the methods of error detection and retransmission. Error control can be done by making use of two methods:

Stop-and-wait ARQSliding Window ARQ

Go-Back-n Selective-Reject


In this method retransmission of data is done in 3 cases:Damaged FrameLost FrameLost Acknowledgement


Sender sends frame 0 and waits to receive ACK 1

When ACK 1 is received, it sends frame 1 and then waits to receive ACK0

repeat from step 1 and 2 and so on.

The ACK must be received before the time set for each frame expires


If an error is discovered in a data frame , indicating that it has been corrupted then the receiver sends an NAK frame which are not numbered that tells the sender to retransmit the last frame sent.


A lost or damaged frame is handled in the same way by the receiver

When the receiver receives a damaged frame, it discards it

The receiver remains silent about a lost frame and keeps its value of R

After the timer at the sender site expires, another copy of frame 1 is sent


Under this there are 2 methods:Go Back-n ARQSelective-Reject ARQ


In this method if one frame is lost or damaged, all frames sent since the last frame acknowledged are retransmitted.

Sequence NumbersFrame from a sending station are numbered

sequentially. If the header of the frame allows m bits fro the sequence number, the sequence numbers range from 0 to pow(2, m-1).

Sender Sliding WindowSender uses window to hold the outstanding

frames until they are acknowledgedFrames to the left of the window are those that

have already been acknowledged and can be purged

Frames to the right of the window can not be sent until the window slides themNavneet Soni (Asst. Professor) 93

Receiver Sliding WindowSize of the window is always 1The receiver is always looking for a

specific frame to arrive in a specific order


Control VariablesThe sender has three variables

S: holds the sequence number of the recently sent frame

Sf : holds the sequence number of the last frame in the window

Sl : Holds the sequence number of the first frame in the window

Size of the window is : The receiver only has one variable: R,

that holds the sequence number of the frame it expects to receive


Only damaged frame is resent Sender and receiver windows

Size of the sender’s window must be one half of Size of the receiver’s window must be same as the sender’s Negative acknowledgement (NACK) is used to define damaged

frame


DTE stands for Data Terminal Equipment. DCE stands for Data Circuit Terminating

Equipment. Both of these equipment is used on both

the side i.e. sender as well as receiver. The DTE generates the data and passes

them along with the necessary control character to a DCE.

The DCE converts the signal to a format appropriate to the transmission medium and introduces it onto the network link.


A DTE is any device that is the source of or destination for binary digital data.

A DTE can be a terminal , micro computer, computer, printer or fax machine.

DTE do not often communicate with one another, they generate and consume information by making use of an intermediary device that is DCE.


A DCE is any device that transmits or receive data in the form of an analog or digital signal through a network.

In any network, a DTE generates digital data and passes them to a DCE, then the DCE converts the data to a form acceptable to the transmission media and sends the converted signal to another DCE on the network. The second DCE takes the signal off the line, converts it to a form usable by its DTE and delivers it. To make this communication possible, both sending and receiving DCE must use the same modulating method.


computer networks basics and osi

Education