Networking Concept

History of communication over distances greater than human voice, hand signals, fire beacons, smoke signals, flags, messengers on horseback, on foot, mechanical semaphores, telegraph, – can be traced to the basic milestones that were discovered thousands years ago. Charles Morrison (1753), Lomond, Nicholas (German), Morse (Morse code, Italy), Charles Whetstone (Electric Telegraph in 1837), Heinrich Rudolf Hertz (1888, German), Guglielmo Marconi (Italy), Lee de Forest (invented first amplifying vacuum tube called Audion or Triod in 1906)etc are foundation stone of our Information technology.

Lee de Forest is regarded as Father of Radio and Electronics because the invention of vacuum tube is proved milestone in the field of electronics. Electrical engineer Guglielmo Marconi had sent radio signal more than a mile in 1895, the foundation stone of wireless communication was laid. Nobel Prize of Physics was awarded to him in 1909 for his work on wireless telephony. The communication is started along with calculating machine Abacus and now a day, it is proved most popular, most improved, reliable, and efficient technology of the world. The networking is the connection of one computer to other through cable or microwave to transfer data from one computer to other computer.


(a) Communication: Transferring of data from one computer to another through medium is known as data-communication or DATACOMM. It is a process of transferring meaningful message from one point to another. This is an age of advanced information technology; recently scientists have discovered ice on the surface of the Mars by the help of sophisticated information technology. Now a day, information revolves around the computer directly or indirectly. Without the knowledge of computer, glimpse and glamour of world cannot be accessed to us. Networks of Internet have been grasping world, so basic knowledge of DATACOMM become most essential for us. At first we discuss about main basic elements of DATACOMM.



There are three elements of data communication: source, medium and receiver. The exchange of data takes place between source and receiver (sink) through medium (cables, microwaves etc.)


(b) Data Transmission:

There are two method of data transmission:

(a) Serial Data transmission, and

(b) Parallel data Transmission


(i) Serial Data Transmission: In serial data transmission only one bit is transmitted at a time through one wire or a pair of wires. It is less costly and noises free. It is very economic for long distance data transmissions. It has three types: (i) Simplex (ii) Half duplex and (iii) Full duplex

  • Simplex: If the transmission is simplex, communication can takes place in only one direction. The simplex mode can be used to transmit data to a computer from a remote source. Devices connected to such a circuit are either a send only or receive only.

Example: Stereo Speaker, TV

Mode of Transmission

Mode of Transmission

  • Half Duplex: Data can be flow both ways, but only one direction at a time. Both stations may transmit, but only one at a time. This mode is called as two-way alternate. This can be compared to a one-lane, two-way bridge. This form of transmission is often used for terminal-to computer interaction.

Example: Walkie-talkie set

  • Full Duplex: Allowing data to be transmitting in the both directions simultaneously. This mode is known as two-way simultaneous and may be compared to a two-lane, two-way bride. For computer to computer data exchange, this mode of transmission is more efficient than half-duplex transmission.

Example: Telephone


(ii) Parallel data Transmission: In parallel data transmission, several bits (8 or 16) are transmitted through several wires. This type of transmission is not appropriate for long distance transmission.

It is not economic because when distance increases, the wire cost also increases. It increases complexity of interface components and noise level is also increased.

Parallel Data Transmission

Parallel Data Transmission



 # Networking Architecture


The networking structure is the combined form of networking topology and mode of data transmission from sender to receiver or vice versa. Networking is a technology of data transmission and there are several techniques are applied to transmit data. It is essential to carefully choose or select which type of network one needs. The choice, of course, should be made upon the user’s requirements and budget. Each type of network has its own pus and minus points. So a mix and match selection is always wise.

The first thing that one needs to understand is the different type of networks based on their functionality.


(a)Peer-To-Peer Architecture:


The technique of data transmission from node to node fashion in which each node behaves like as server or host temporarily is called peer to peer architecture. There are no abusive clientele or good and powerful servers, just peers or equals. In the peer network the network users simply share disk space and other resources, which is good for small and simple networks. If data packet transmitted from A has to reach to his destination node C, the data packet first goes to node B, if data address is not matched with B, then transferred to node C where data address is matched, and communication is started. Communication is established between two nodes, the remaining nodes are free, it has no problem of traffic.


(b) Client Server Architecture:


The client-server architecture is type of broadcasting structure in which workstations or nodes are connected with main server or host. The packet of data sent by any computer, at first reaches to server, and then passed to each local computer. The local computer is called node or client also. If address of packet is matched with node, it is delivered to that node. The server or host behaves like as distribution station. Servers are of various types depending upon the type of function they perform like File server, Printer server, Application server, Message server, Database Server, Web server etc, each server dedicated to a particular function.

A network can have any number of servers ranging from 1 to several hundreds of servers (server-farms), depending upon the size and requirements of the network. Servers are usually most powerful computers in the network, and are critical to functioning of network. At first server receives data, then distributes. The star topology is the suitable example of client server architecture. In this type of networking, all the clients are busy at the time of data distribution by server.


(c) Hybrid Networks:

They are hybrid or a mixture of client server and peer-based networking. These types of networking architecture have benefits of both the types of architectures.



# Types of Networking


There is a tree types: LAN, WAN and WAN



1. It stands for Local Area Network. The computers and peripherals are in physical contact to each other. It is started from 1970 to cover small area. It stands for Wide Area Networks and peripherals are not in physical contact. The first WAN is named as Arpanet which is started to work from 1968.
2. The data transfer speed is 0.1 to 1000 mega bits per second. It has low error rate. The data transfer speed is 1800 to 9600 bits per second. It has high error rate.
3. Economics Not economics like as LAN


There are two categories of WAN: Public and Private. The public WAN contains PSTN (Public Switched Telephone Network), PSDN (Public Switched Data Network), VANS/VADs (Value Added Services), ISDN (Integrated Service Digital Network) etc.


Wireless LAN

A LAN configuration without cable is called wireless LAN in which infrared, ISM band and microwave are used for data transmission from one node to other node without cable, so this type of LAN configuration is called wireless LAN. Its data transfer rate is low and installation cost is more, so it is not more popular configuration.

Categories of wireless LAN technology:

  • Infrared LAN: It uses infrared light for fast communication, but must have line-of-sight arrangements between LAN components and has limited ranges.
  • Spread spectrum LAN: The communication at ISM (Industrial, Scientific, and Medical) band is called spread spectrum LAN. Spread spectrum radio LANS can penetrate masonry walls and link components from 100 to 200 feet away in enclosed area, but are more subject to receive or generate radio interference.
  • Narrow band microwave LAN: It is operated at microwave frequency.



It stands for Metropolitan Area Network. It is used in Metropolitan cities to solve networking problems.

  • It is used to solve networking problems of Metropolitan cities.
  • Kolkata, Mumbai, New Delhi, New York, Tokyo, Washington etc are Metropolitan City where MAN is used to communicate data from computer to computer.
  • MAN is an extension of LAN. Several LANs connected through repeater, Bridge, router and gateway construct MAN.
  • In case of DQDB (Distributed Queue Dual Bus), nodes are connected between two buses.





# Contemporary Networking technology


The contemporary networking technology is related with high speed data transmission. It is a method of transmission of image, audio, video in high speed. It is a transmission technology which can replace traditional analog transmission technology in near futures.

(a) Ethernet: Ethernet is the most popular LAN technology. Ethernet is popular because it strikes a good balance between speed, cost, and ease of installation. These benefits, combined with wide acceptance in the computer marketplace and the ability to support virtually all popular network protocols, make Ethernet an ideal networking technology for most computer users today.

Token Ring, Fast Ethernet, Fiber Distributed Data Interface (FDDI), Local talk, Ether Talk, and Arc net are also types of LAN.

The standard of Ethernet, is defined by IEEE (Institute of Electrical and Electronic Engineers)


(b) Fast Ethernet: The Fast Ethernet is a type of Ethernet with high speed transmission rate for real time video and audio. Its transmission speed is 10 Mbps to 100 Mbps.

(c) FDDI: It stands for Fiber Distributed Data Interface. It is used as backbone for networking and connects more than LANs or WANs.





  • Speed: 100 Mbps
  • It covers distance of 200 Km.
  • It supports dual ring topology. The dual ring consists of a primary and a secondary ring. During normal operation, primary ring is used for data transmission, and secondary ring remains idle.
  • Optical fiber is used as medium.



(d) ATM: It stands for Asynchronous Transfer Mode. It is a fast packet switched cell relay technology in which small packet of fixed 53-byte size are rapidly routed over network. It is also used as backbone networks and connects more than two LANs.


(a) Speed: 155-620 Mbps

(b) Packet switching

(c) Fixed sized packet: 53 bytes (5 bytes header, 48 bytes payload)


(e) ISDN: It stands for Integrated Service Digital Network. It was first lunched in Singapore in 1990. ISDN involves the digitization of the telephone network, which permits voice, data, text, graphics, music, video, and other source material to be transmitted over existing telephone wires. It is used for high bandwidth communication (e.g.: Video conferencing).



(i) Digital dial-up service

(ii) Speed: 64 Kbps-1.54 Mbps

(iii) Processing of various types of information data, voice, video etc.

(iv) Electronic mail box

(v) Tele conferencing

(vi) Telex, Video fax, Telefax etc.


# Modem, Modulation, Analog and Digital Signal


Modem, modulation and signal are integrated to each other’s. The concept of modem is incomplete without the concept of modulation and signals.

Modem: MODEM is an electronic device used to convert digital signal into analog and vice versa. It was developed during 1950 to 1960 connect mainframe computers.

Modem has two parts:

(a) MOdulator and

(b) DEModulator.

Modulator converts digital signal into analog and demodulator converts analog signal into digital signal. The digital signal (square wave) is not suitable for long distance communication through bounded media (cable) because signal suffer from strong attenuation, delay distortion and noises, so, modem is used to convert digital signal into analog signal.

There are three categories of modem available: Internal, External, and PC Card Modem. The PC card modem is a combined form of internal and external modem used in portable categories of computers. It is plugged directly into an external slot available for it.


The differences between external and internal modems:


External Modem

Internal Modem


The external modem is externally attached CPU and it covers space on desktop.

This modem is plugged into expansion slot of motherboard of CPU.


It can be easily detached from one computer and attached to other computer.

To detach the internal modem, at first we unscrew CPU cabinet and take it out and install in other computer. So, it is not easy like as external modem.


It is easy to install and configure modem.

It is tedious to install and configuring modem.


More reliable

Not more reliable.


Generally, process of signal conversion from digital to analog and vice versa is called modulation. The modulation is a non-linear in which two sinusoidal waves are multiplied. When digital signal is given to modem, the modem generates a carrier wave and modulates it according to the digital signal. So, the process of combining input signal and carrier signal into new signal is called modulation. The input signal is called modulating or base band signal and new one is called modulated signals.

There are three types of modulation:

(i) Amplitude Modulation: When amplitude of the carrier wave is changed to encode the digital information is called amplitude modulation. The frequency (oscillation) of carrier wave does not change in this condition.

(ii) Frequency Modulation: The frequency of carrier wave is changed to adjust digital signals but amplitude is unchanged.

(iii) Phase Modulation: In phase modulation the phase of the carrier wave is modulated to encode the digital information.

It was in 1937 the Alec Reeves, an English engineer came up with a novel idea of coding these variable amplitude pulse and converting them into a long stream of bits, all of them having the same amplitudes. This process offers a new method of modulating the analog signal and is known as pulse code modulation (PCM). Reeve’s revolutionary idea received a tremendous boost with the dawn of the semiconductor era. Today, PCM is the order of the day. Semiconductor or integrated circuits known as analog to digital converters (ADC) are extensively used to digitize speech signals for purpose of signal processing and communication.


Analog and Digital Signal:

The differences between analog and digital signal:


Analog Signal

Digital Signal

1. The analog signal is continuously varying electromagnetic waves called sine wave. The digital signal is a sequence of discrete voltage pulse and called square wave.
2. It is suitable for long distance communication through bounded medial (Telephone wire) It us not suitable for long distance communication through bounded media.
3. Demodulator converts analog signal into digital signal. Digital signal is converted Into analog signal through modulator.



# Sharing technology


The Modern, bridge, Router etc are sharing devices or accessories of computer networking.

(a) Repeaters:  Repeaters are used to connect together two or more Ethernet segments of any media type. Ethernet hubs are necessary in star topologies such as 10BASE-T. A multipart twisted pair hub allows several point-to-point segments to be joined into one network. One end of point-to-point link is attached to the hub and the other is attached to the computer. Repeater is a signal amplifier used to amplify weak signal and increase length of LAN. It is a low-level device of transparent nature unable to differentiate message from noise. If noise is present, it amplifies noise with message. It links similar LANs. When Networking Operating System (NOS) are similar for any two or more LAN, then these LANs are called similar LANs.

There are two types of repeater:

(i) Local Repeater and

(ii) Remote Repeater

Local Repeater is used to connect two LAN (segment) separated by small distance while Remote Repeater is used to connect LAN separated by large distance. Remote repeaters are connected through Link Segment.

Barrel connector is used to connect two pieces of cable to make it long. However, connectors weaken the signal and should be used sparingly. One continuous cable is preferable to connecting several smaller ones with connectors. Using too many connectors can prevent the signal from being correctly received.

Barrel Conector

Barrel Conector


(b)Bridge: It is a store and forward device used to connect different or similar networks (LAN) together. Generally, we use it to connect similar LAN. It receives frames from one LAN and transfer to other LAN without any modification and encapsulation.

It is classified into two categories:

(a) Local Bridge

(b) Remote Bridge


Local bridge is a general type of bridge used to link LANs separated by small distance, but in case of large distance separated LANs, Remote Bridge is used. In this case, two Bridges are connected through WAN. More than two LAN can be linked through Bridge.

Bridges operate at the data link layer (layer 2) of the OSI model. As all devices use the same protocols, the amount of processing required at the bridge is minimal. If the distance between the two LANs is large, the user would require two identical bridges at either end of the communication link.


(c) Router: It is a highly intelligent, protocol sensitive linking device used to link similar or dissimilar LANS. Generally, it is used to connect dissimilar networks. If they don’t use the same protocols and / or you don’t want to send all messages between the two, it can get a bit tricky. A router, which can be hardware, software, or both, is a device that connects two networks.


(a) Ethernet to Ethernet

(b) Ethernet to token ring

(c) MAN to WAN.

Routers operate at the network layer (layer 3) of the OSI model. All routers participate in a routing protocol to access the network topology, and based on this information routers compute the best route from a sender to the receiver.


(d) Gateway: It is used to link dissimilar LANs. It also performs all functions of Bridge and Router. It is slower than Bridge and Router because it converts entire protocol of one network to other. When we connect Ethernet (LAN) to token ring (LAN), Gateway converts Ethernet protocol into token ring protocol. Gateway works like as server for other server. Gateways are often used as server-side portals through network firewalls and as protocol translators for access to resources stored on system.

Gateway operates on the application layer (layer 7) of the OSI model. A gateway is required to convert data packets from one protocol format to another before forwarding it, as it connects two dissimilar networks.


(e) Hubs: The hub is the standard and important components of networking, which is used to connect computers. It is a layer-1 device of OSI model. Hub is used to receive incoming data, amplify and distribute. The following figure illustrate hub as a central component of star topology.

There are three types of hubs are used in networking:

(i) Active Hubs: Most hubs are active; that is, they regenerate and retransmit signals in the same way as a repeater does. Because hubs usually have eight to twelve ports for network computers to connect to, they are sometimes called multi-port repeaters. Active hubs require electrical power to run.

(ii) Passive Hubs: Some types of hubs are passive; examples include wiring panels or punch-down blocks. They act as connection points and do not amplify or regenerate the signal; the signal passes through the hub. Passive hubs do not require electrical power to run.

(iii) Hybrid Hubs:  Advanced hubs that will accommodate several different types of cables are called hybrid hubs. The following figure illustrates main hub (hybrid) and three sub-hubs.

Hybrid Hub

Hybrid Hub


# Switching


The establishment of communication link between two nodes in termed as switching. Public telephones are designed to provide access between any source and destination. It general sense, link between transmitting device and receiving device for communication is called switching.

There are three types of switching:

(i) Circuit Switching

(ii) Message Switching, and

(iii) Packet Switching


Circuit Switching

In circuit switching there is a dedicated communication path between the sending and receiving devices. The dedicated path is a connected sequence of links between switching nodes. A conventional telephone network, where a dedicated path is set between the caller and the called party for the duration of a telephone call is an example of circuit switching. The time required to establish a connection is known as the set up time. With modern electric switching the set up time in short. One drawback either circuit switching is that if the exchange is being used to its full capacity then there is possibility of some users being deprived of immediate access.

This is known as blocking. A circuit switch is known as a transparent switch since all that it does is to provide a physical link between two terminals. Circuit switching is ideally suitable for voice signal. Such networks are inefficient when transmitting data.


Suppose, you are out from Nepal and want to establish telephone communication with the friend living in Ilam district, you dial 00977-27-521325, the first connection is established with Nepal (00977) then to Ilam (27), and then to friend’s telephone (521325), is shown here.


Message Switching

It is a store-and-forward network in which message block is stored at first, inspected errors and forwarded toward destination without physical establishment. In message switching, block has no definite size. The modernized version of the switched message scheme is the e-mail. Each switch within the network has a capacity to store messages. The network transfers the message to the next switch and ultimately it reaches final destination as and when it is convenient to do so. There could be a delay in the message reaching its destination which could be a long one.

Message Switching

Message Switching

It does not operate on a real time basis. However, there is no problem of blocking since once a message has entered the network it will sooner or later reach its destination. The signal might get transmitted over different channels such as optical fibers, microwave links and even satellite links.

Example: The message block is flowing from source to destination.


Packet Switching


Packet switching combines the advantage of message and circuit switching. In this type of switching message is broken into several uniformly sized pieces, which are called packet. The packet length may be 80 to 150 characters. These packets travel independently through route (internet) and may take different route to arrive at the intended destination. These packets are reassembled into meaningful message. It is the best choice for long distance transmission of data. X.25 is the standard interface between an end system and pocket-switching network. Pocket switching is designed to provide a more efficient facility than circuit switching for brusty data traffic.

Example: The diagram illustrates packet switching. The message is broken into packets of equal sizes and follow different route to arrive at destination and reassembled.

Packet Switching

Packet Switching


# Public Networks


A Network facility provided by telecommunication authorities for public or organization on the subscription basis is termed as Public network.

(a) PSTN (Public Switched Telephone Network): It transmits human voice in a more or less recognized form. It is suitable for facsimile (FAX) machine. It is used for wide area data communication through telephone and modem. It is a low speed and analog transmission.

(b) PSDN (Public Switched Data Network): It is a highly reliable method of communication that links computer system and network of one organization to server of other origination. It provides facilities of connection public and private mail system to Internet.

(c) BBS (Bulletin Board System): BBS are a popular telecommunications service provided by the Internet, Public Information Services, and thousands of business firms, organizations, and user groups. An electronic bulletin board system allows you to post public or private message that other end users can read by accessing the BBS with their computers. Establishing a small BBS for a business is not that difficult. Minimum requirements are microcomputers with a hard disk drive, custom or packaged BBS software, modem, and a telephone line.

Bulletin board systems serve as a central location to post and pick up messages or upload and download data files or programs 24 hours a day. A BBS helps end users ask questions, get advices, locate and share information, and get in touch with other end users.


# Mobile technologies:


The development of cellular phone is recent one. This is also known as mobile phone and as its name implies it is designed for mobile users who need to make telephone calls from different locations when they are usually away from home or office. It uses radio frequencies to talk to a nearby cell site. Cell cite acts as an access point for cellular calls and the cellular phone regularly communicates with the nearest cell site to inform the network that is connected.

Mobile Technology

Mobile Technology

The cellular network contains overlapped circular or hexagonal cells to eliminate probability of call dropping. This overlapping structure helps in keeping the cell intact as a user moves location from one cell site to another. In this case, the call is transferred to the nearest cell site responsible for that physical area. Each cell site is connected with master site, which acts as an access point for a particular cellular network. Master site furnishes an interconnection to the regular telephone network. Calls handled by each cell site are relayed back to the master cell site, which then relays it to the telephone network.


Mobiles are based on two distinct connectivity technologies

GSM and CDMA. GSM (Global System for Mobile connectivity) mobile phone companies were the first to set up their networks while CDMA (Code Division Multiple Access) networks are quite recent. For instance, Orange is a GSM service provider while Reliance is a CDMA service provider.

GPRS (General Packet Radio Service) technology allows information to be sent back and forth across a mobile network. It is an addition to today’s circuit switched data (CDS) and SMS (Short Message Service).

Cellular telephones are suitable for larger geographical areas including remote sites. It saves the cost of copper wire and efforts in laying the same in densely populated areas.


Satellite Cellular Telephone

It is used to cover much wider geographical area. It works on the same principle as cellular phones but uses LEO (Low Orbiting Satellite). It is particularly a good technology in mountainous terrain and at sea. Unlike cellular phones, satellite cellular phone requires a large number of cells and their accurate positioning to avoid blind spots. Blind spots are the spaces where no cell overlapping or cell is present therefore, no call can be made at such spots.



Pagers are only one way wireless communication device of very small size. It tells the user that a message is waiting for him by ringing or vibration. In this technology, a ground based radio transmitter is used to send out a constant stream of messages on a particular frequency. On the other hand a pager acts as a wireless receiver. The message sent by transmitter in a pager as a stream of messages, which can be monitored by its user on indication. Each pager has built-in address code and whenever a pager detects a message, the bui1t-in address associated with a pager decodes the address to know whether this should be received or discarded.

There are basically four main types of pagers:

  • Cap Code: These are the pagers, which came first in market. They beep when the pager receives the built-in address.
  • Tone voice: These pagers were developed in 1970′s and allow the sender to record and send a short voice message.
  • Digital display: Introduced in the early 1980’s, these are the most widely used pagers. A call back number is entered by the sender, which then appears on the pagers display unit.
  • Alphanumeric pagers: These were introduced in the late 1080′s and allow a text message to be displayed on the pagers display unit. This often has the advantage of not requiring the receiver to dial back the sender.


In recent years, Pocket PC technology has been vigorously developed. These devices are palm-sized and often equal traditional desktop computers in their functionality. PDAs (Personal Digital Assistants) are handheld computers that originally were designed as personal organizers, and they do this really well. The basic features of any PDA are a date book, address book, task list, and memo pad.

There are three major PDA operating systems that are competing for the PDA market. Palm Computing, owned by 3Com, is the manufacturer of the original Pilot organizer and their Palm OS is the market leader in PDA sales. Palm has recently begun to license its operating system out to third-party manufacturers. Handspring, Qualcomm, and TRG have all now released PDA’s based on Palm OS. Nokia and Sony are due to follow in the near future.



The rules around which networking configuration depend, is called protocol. The protocol is designed for standardization of networking of computers. The OSI (Open System Interconnection) models is a protocol in which seven layers are provided for inter connection of devices. The TCP/IP is also a protocol for internet connection. The IP model differs from the ISO seven layer protocol model often illustrated in networking text.

IP uses a simplified five layer model:

(i) Physical Layer: It is a first layer describing the physical medium(copper, fiber, wireless) and the data encoding used to transmit signals on that medium(pulses of light, electrical waves, for instance).

(ii) Data Link Layer: It is a second layer covers media access by network devices and describes how to put data into packets, transmit the data and check it for errors. Ethernet is at this layer, as is 802.11 wireless.

(iii) Network Layer: It is a third layer. It is frequently uses IP and addresses and routes packets.

(iv) Transport Layer: It is a fourth layer of IP protocols. It provides a means for applications to communicate with each other. Common functions of the transport layer include guaranteed delivery, delivery of packet in the order of transmission, flow control, error detection, and error correction. The transport layer is responsible for dividing data streams into packets.

(v) Application Layer: It is a top layer. Unlike the ISO model, the internet model does not distinguish among application, presentation, and session layers. All the upper-layer characteristics, such as character encoding, encryption, GUI, and so on, are part of the application.


OSI Protocol or Layered Protocol

Computer scientists working on developing networking technologies and protocols developed the Open Systems Interconnected Layer model. Here layering was introduced to facilitate protocol design. Layers distinguish functional differences between adjacent protocols. The layered protocol is standard communication architecture used for establishes connections; offer transmissions and development of communication protocol.

This layered protocol is also called OSI (Open System Interconnection) developed by ISO (International Organization for Standardization) in 1983.

This model has seven layers.

OSI protocol

OSI protocol

The layer 1 and 2 are mandatory in order to transmit and receive in any communication system. The controlling network software provides layer 3, 4 and 5. Layer 4, 5, 6 are often combined into one or two layers in existing communication system.

(a) Physical Layer: It determines mechanical, electrical, functional and procedural activities between transmitting and receiving devices. It is a layer of wires, plugs, signals and connections. It accepts data from the data link layer in bit streams for the subsequent transmission over the physical medium.

(b) Data Link Layer: This layer is responsible for transfer of information, flow and error control across physical link. It establishes an error free communications path over the physical channels. It checks integrity of received channels. It is responsible for reliable transfer of data across the physical link.

(c) Network Layer: It establishes link between two parties that are not connected directly. The message to be transmitted is first fragmented into packets at this layer. Then it performs sequencing and error of these packets. Thus the routing decisions are taken here.

(d) Transport Layer: This layer is responsible for data multiplexing, de-multiplexing and breaking message into packets. It also provides end to end error recovery and flow control.

(e) Session Layer: This layer provides dialogue discipline (full duplex or half-duplex), grouping of data flow, and recovery from failure and maintains order. The session layer provides the mechanism for controlling the dialogue between applications in end systems. The key service provided by this layer is: dialogue discipline, grouping, and recovery.

(f) Presentation Layer: It is responsible for code conversion (ASCII to EBCDIC, BCD to binary), data compression, data encryption, and formatting of receiving and transmitting data. The presentation layer performs functions related to the formatting and displaying of received data by terminals and printers.

(g) Application Layer: This layer is a use layer used for various purposes. It is used for email, database query, resource sharing, file transfer, remote file access and network management. It provides support services for user and application tasks. It determines how the user is using the data network.

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