communication and networking



computer network, often simply referred to as a network, is a collection of computers and devices connected by communications channels that facilitates communications among users and allows users to share resources with other users. Networks may be classified according to a wide variety of characteristics. 

Purpose of networking:-

  • Networking for communication medium (have communication among people;internal or external)

  • Resource sharing (files, printers, hard drives, cd-rom/) 

  • Higher reliability ( to support a computer by another if that computer is down)

  • Higher flexibility( different system can be connected without any problems)

  • Scalable (computers and devices can be added with time without changing original network.

  • provides distributed and centralized management system for enterprise.

  • increase productivity.

  • cost reduction by sharing one application/hardware etc.

Application of networking in our daily life:

-E-mail (to send or get mails electronically worldwide)

- Scheduling programs and having meetings at same time across world.

- Video conferencing with sound without delay.

-teleconferencing for people.

-Automate banking facility

-to surf Internet facility

  -telecommuting to work at home by accessing remote computer

-doing business and marketing


Types of signals:

a) Analog signal:- An analog signal varies constantly with time;these changes can be used to represent data and uses sine wave..Or An Analog or analogue signal is any continuous signal for which the time varying feature (variable) of the signal is a representation of some other time varying quantity, i.e., analogous to another time varying signal. For example, human voice, video and music are analog signal. Analog signal once gets distorted, can not be be re-constructed and there would be permanent loss.

An analog signal uses some property of the medium to convey the signal's information. For example, an aneroid barometer uses rotary position as the signal to convey pressure information. Similarly a typical analog device is a clock in which the hands move continuously around the face. Such a clock is capable of indicating every possible time of day. In contrast, a digital clock is capable of representing only a finite number of times (every tenth of a second).

Graphically, it can be represented as,

b) Digital signal: -digital system is a data technology that uses discrete (discontinuous) values. It describes any system based on discontinuous data or events.Although digital representations are discrete, the information represented can be either discrete, such as numbers, letters or icons, or continuous, such as sounds, images, and other measurements of continuous systems. The signal coming out and entering into it is digital and has bandwidth infinite. when the digital signal travels over a medium it travels with same pace but as the medium gets changed the signal becomes distorted and quality degrades. So we use devices like Repeater which repeats the incoming signal digitally and prevents from damages.

when information is stored digitally (may be video,audio or pictures etc),they can be electronically manipulated,processed,preserved and regenerated without any loss and with high speed. Graphically it can be shown as

Data flow in networking:-

  1. Simplex: - in this data flows in only one direction on data communication line. Examples are television and radios broadcasts; they go from station to our home. We can say one way communication takes place.

  2. Half duplex: - the data flows in both directions but only direction at a time on data communication line. Each station can both transmit and receive. Example a talk on walkie–talkie is half duplex. Each person talks on turning.

  3. Full duplex:-The data flows in both directions at same time. The system designed in such a way that data flow occurs in both the sides. Example, mobile phones, land line sets, two ways trafficking system etc.

Types of network (geographically):-

  1. LAN: - 

  • It is a privately-owned network done in single building or room or office to share resources or data, documents (for 100 m). 

  • It occupies small area and small number of computers.

  • Speed with which data is passed is extremely fast(1000mbps).

  • Fast connecting and sharing.

  • Uses medium like Co-axial or utp cable (mostly).

  • can use topology like bus/star/tree etc.

  • fewer error occurrences during transmission.

  • Less congestion

  • can be handled by single person/administrator. 

  • It is cost effective.

  1. MAN:-

  • A metropolitan area network (MAN) is a network that interconnects users with computer resources in a geographic area or region larger than that covered (10km or less than WAN) by even a large local area network (LAN) but smaller than the area covered by a wide area network (WAN). The term is applied to the interconnection of networks in a city into a single larger network (which may then also offer efficient connection to a wide area network). 

  • Faster than LAN and slower than WAN(110 mbps)

  • can use media like guided or unguided.

  • can use combined topology

  • more chances of having error during transmission.

  • More congestion than LAN

  • can be owned by corporate office IT administrator team

  • Costs more to set-up like router,optical fiber,radio device etc 

  1. WAN 

  • is a telecommunications network, usually used for connecting computers, that spans a wide geographical area. WANs can by used to connect cities, states, or even countries. 

  • It occupies broad area(>=50km).

  • Extremely slow transfer rate(150mbps)

  • Slow connecting and file sharing.

  • Mostly uses medium like ,telephone line,leased line or satellite or optical fiber

  • Uses MESH topology.

  • more transmission error

  • high congestion.

  • due to having of complex system, it is handled by a group.

  • Expensive to set-up.

Internet and intranet:-


1) It contains large number of computers.It is completely open/global.

2) Its access is not limited to particular organization.

3)It has/occupies large area.

4) It is not that much secured as Intranet is.

5) It uses/contains different media and technologies.

6)To access Internet, only password from ISP is needed; no more authentication is needed.

7)It is decentralized system with many hosts..



1)An intranet is a private computer network that uses Internet Protocol technologies to securely share any part of an organization's information or operational systems within that organization.

2)Its access is limited to an organization.

3)It has  small coverage.

4)It is safe to access personal information.

5)Locals of intranet can access Internet; and Internet requirement is not mandatory for communication.

6) Employees of an organization can access their intranet by using authorized password by using Internet; otherwise not possible.

7)We can say it as a “Centralized controlling system”.

Transmission media:-

Coaxial cable, or coax,

  •  It is an electrical cable with an inner conductor surrounded by a flexible, tubular insulating layer, surrounded by a tubular conducting shield.

  •  The term coaxial comes from the inner conductor and the outer shield sharing the same geometric axis. 

  • Coaxial cable is used as a transmission line for radio frequency signals, in applications such as connecting radio transmitters and receivers with their antennas, computer network (Internet) connections, and distributing cable television signals and uses many connectors.

  •  It is available in two types thin (10base2; means 100 MBPS and upto 200 meter) and thick (10base5 coax;means 10 MBPS, and upto 5oo meter). They are different in diameter and bandwidth. Most of television operators use this. They are obsolete and no longer in use for computer networking 


  • Unshielded Twisted pair cabling is a type of wiring in which two conductors (the forward and return conductors of a single circuit) are twisted together for the purposes of canceling out electromagnetic interference (EMI) from external sources; for instance, electromagnetic radiation from unshielded twisted pair (UTP) cables, and crosstalk between neighboring pairs. Different pairs(colour) have different function.It has 4-pairs of copper wires.

  • UTP cable is also the most common cable used in computer networking. Modern Ethernet, the most common data networking standard, utilizes UTP cables.

  •  Twisted pair cabling is often used in data networks for short and medium length connections because of its relatively lower costs compared to optical fiber and coaxial cable.

  •  It has bandwidth up to 100mbps but can be increased up to 10000 mbps.

  • The rate at which signal moves is ⅔ of velocity of light.

  • It is available in different categories like CAT5,CAT6 ,CAT7 etc. 


Shielded Twisted pair:-

  • It is also of same category twisted pair. 

  • It contains twisted cables/copper wires.

  • In STP, we can see metal foil as a shield in each pair. It is used to protect or minimize interference.

  • STP cables are bigger than UTP cables, and are more expensive. 

  • Tthey are more fragile than UTP cables, as the shield must be kept intact in order for them to work properly.

  • They are difficult to install than UTPs.

  • They have speed upto 1000mbps.

  • Mostly used for Token-ring topology.

Optical fiber:

An optical fiber or optical fiber

  • is a glass thread with  a bundle of many thin fibers. 

  • transmits the packets with the speed of light.

  • It is less susceptible.

  • It has high/est bandwidth.

  • Fibers are also used for illumination, and are wrapped in bundles so they can be used to carry images, thus allowing viewing in tight spaces. Specially designed fibers are used for a variety of other applications, including sensors and fiber lasers.

  • Optical fiber typically consists of a transparent core surrounded by a transparent cladding material with a lower index of refraction. Light is kept in the core by total internal reflection. This causes the fiber to act as a waveguide.

  • No interference because of no wiring technology

  • Data can be transmitted digitally or in as it is form.

  • Optical fibers are widely used in fiber-optic communications, which permits transmission over longer distances and at higher bandwidths (data rates) than other forms of communication. 


  • In the context of spaceflight, a satellite is an object which has been placed into orbit by human endeavor. Such objects are sometimes called artificial satellites to distinguish them from natural satellites such as the Moon.

  • Satellites are usually semi-independent computer-controlled systems. Satellite subsystems attend many tasks, such as power generation, thermal control, telemetry, altitude control and orbit control

  • satellite has on board computer to control and monitor system.

  • Additionally, it has radio antennae (receiver and transmitter) with which it can communicate with ground crew.

  • Most of the satellites have attitude control system(ACS).It keeps the satellite in right direction.

  • Common types include military and civilian Earth observation satellites, communications satellites, navigation satellites, weather satellites, and research satellites. Space stations and human spacecraft in orbit are also satellites. 

  • Satellite orbits vary greatly, depending on the purpose of the satellite, and are classified in a number of ways. Well-known (overlapping) classes include low Earth orbit, polar orbit, and geostationary orbit. 

Electromagnetic waves(Radio waves/microwaves):-

  • Radio waves are electromagnetic waves. Electromagnetic waves include waves such as X rays, ultraviolet light, visible light, infrared rays and so on,

  •  they are easy to generate

  • they have same velocity in vacuum

  •  they may traverse long distances

  •  they are omni-directional

  •  they can penetrate building (high frequency) easily so they find extensive use in communication both indoor and outdoor.

  • They are different types: VLF(30khz),LF(300khz),MF(3MHZ),HF(30MHZ),VHF(300mhz),UHF(3ghz),SHF(30GHZ),EHF(300ghz).They are frequency dependent. At low frequency they can pass through obstacles well but the power falls off sharply With distance from the source, as power is inversely proportional to cube of the distance from the source. At HF they travel in straight lines and bounce off obstacles.

  • different application like: satellite,RAdio,TV,FM RADAR etc

Network types:-

on the basis of architecture:

Peer to peer(P2P):

  • In this all the computers are treated equally.

  • No one computer is server or client.

  • All computers can be said working as client or server.

  • Computers have self processing capability and do not rely on others.

  • Computers have or can run with normal operating system like, XP, Me etc and application.

  • Easy sharing of files and allows us to have chatting.

  • failure of one does not mean others are down; networking goes on.

  • If heavy load is given, they may not give same performance. etc

  • low level security.

Client –server:

  • In this, one or two computers work as server and left all work as clients.

  • Clients computers give request to server for a task to be performed.

  • Clients computers may or may not have self processing capability. they rely on server.

  • Mostly servers use a powerful operating system like, Linux or Unix or Win advanced server2008 etc.

  • Through server, the sharing of files is done.

  • Everything is controlled by server so in the case of down, services can not be completed.

  • under heavy load, many servers share the tasks.

  • there is high level security in networking.

  • High traffic towards servers while processing.


When we go for networking, we connect some computers in particular layout or design. This design or pattern of connection is called topology.

There are many types;

Bus topology:

A linear bus topology consists of a main run of cable with a terminator at each end (See fig. 1). All nodes (file server, workstations, and peripherals) are connected to the linear cable.

Fig. 1. Linear Bus topology

Advantages of a Linear Bus Topology

  • Installation is easy and cheap to connect a computer or peripheral to a linear bus.

  • Requires less cable length than a star topology.

Disadvantages of a Linear Bus Topology

  • Entire network shuts down if there is a break in the main 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.

Star topology:-

  • A star topology is designed with each node (file server, workstations, and peripherals) connected directly to a central network hub, switch, or concentrator (See fig. 2).

  • Data on a star network passes through the hub, switch, or concentrator before continuing to its destination. The hub, switch, or concentrator manages and controls all functions of the network.

  •  It also acts as a repeater for the data flow. This configuration is common with twisted pair cable; however, it can also be used with coaxial cable or fiber optic cable.

                Fig. 2. Star topology

Advantages of a Star Topology

  • Easy to install and wire.

  • No disruptions to the network when connecting or removing devices.

  • Easy to detect faults and to remove parts.

Disadvantages of a Star Topology

  • Requires more cable length than a linear topology.

  • If the hub, switch, or concentrator fails, nodes attached are disabled.

Mesh topology

  • The mesh topology incorporates a unique network design in which each computer on the network connects to every other, creating a point-to-point connection between every device on the network. The purpose of the mesh design is to provide a high level of redundancy.

  • It needs/uses n(n-1)/2 channels(paths) and n-1 input/ output ports.

  •  If one network cable fails, the data always has an alternative path to get to its destination. Figure 6 shows the mesh topology.

  • As you can see from Figure 6, the wiring for a mesh network can be very complicated. 

  • It is used mostly in telephone network.This allows computer to balance the load by providing alternative paths. It creates a redundant point-to-point network connection between only specific network devices. 

Figure 6. Mesh topology.



Provides redundant paths between devices

Requires more cable than the other LAN topologies.

The network can be expanded without disruption to current users.

more devices needed.

Networking devices:-


A concentrator is a device that provides a central connection point for cables from workstations, servers, and peripherals. In a star topology, twisted-pair wire is run from each workstation to a central switch/hub. Most switches are active, that is they electrically amplify the signal as it moves from one device to another. Switches no longer broadcast network packets as hubs did in the past, they memorize addressing of computers and send the information to the correct location directly. Switches are:

  • Usually configured with 8, 12, or 24 RJ-45 ports

  • Often used in a star or tree topology

  • Sold with specialized software for port management

  • Also called hubs

  • Usually installed in a standardized metal rack that also may store net modems, bridges, or routers


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

  • If you are adding to an older wiring scheme and want the new network to be up-to-date, a bridge can connect the two.

  • A bridge monitors the information traffic on both sides of the network so that it can pass packets of information to the correct location.

  •  Most bridges can "listen" to the network and automatically figure out the address of each computer on both sides of the bridge. The bridge can inspect each message and, if necessary, broadcast it on the other side of the network.

  • The bridge manages the traffic to maintain optimum performance on both sides of the network. You might say that the bridge is like a traffic cop at a busy intersection during rush hour. It keeps information flowing on both sides of the network, but it does not allow unnecessary traffic through. Bridges can be used to connect different types of cabling, or physical topologies. They must, however, be used between networks with the same protocol.


  • Since a signal loses strength as it passes along a cable, it is often necessary to boost the signal with a device called a repeater. 

  • The repeater electrically amplifies the signal it receives and rebroadcasts it. 

  • Repeaters can be separate devices or they can be incorporated into a concentrator. 

  • They are used when the total length of your network cable exceeds the standards set for the type of cable being used.

  • A good example of the use of repeaters would be in a local area network using a star topology with unshielded twisted-pair cabling. The length limit for unshielded twisted-pair cable is 100 meters. The most common configuration is for each workstation to be connected by twisted-pair cable to a multi-port active concentrator.

  •  The concentrator amplifies all the signals that pass through it allowing for the total length of cable on the network to exceed the 100 meter limit.


  • A router translates information from one network to another; it is similar to a super intelligent bridge. 

  • Routers select the best path to route a message, based on the destination address and origin. 

  • The router can direct traffic to prevent head-on collisions, and is smart enough to know when to direct traffic along back roads and shortcuts.

  • While bridges know the addresses of all computers on each side of the network, routers know the addresses of computers, bridges, and other routers on the network.

  •  Routers can even "listen" to the entire network to determine which sections are busiest -- they can then redirect data around those sections until they clear up.

  • If you have a school LAN that you want to connect to the Internet, you will need to purchase a router. In this case, the router serves as the translator between the information on your LAN and the Internet. It also determines the best route to send the data over the Internet. 

Routers can:

  • Direct signal traffic efficiently

  • Route messages between any two protocols

  • Route messages between linear bus, star, and star-wired ring topologies

  • Route messages across fiber optic, coaxial, and twisted-pair cabling


  • A modem is a device that enables a computer to transmit data over, for example, telephone or cable lines. 

  • Modems are referred to as an asynchronous device, meaning that the device transmits data in an intermittent stream of small packets. Once received, the receiving system then takes the data in the packets and reassembles it into a form the computer can use.

  • Computer information is stored digitally, whereas information transmitted over telephone lines is transmitted in the form of analog waves. 

  • A modem converts between these two forms. The modem modulates and de-modulates the data. 

  • MODEM rates are measured in baud(bps) for slow rates and bps for high rates.

  • MODEMS can be internal or external.

There are there types of modulation technique.

FSK-frequency shift keying (it modulates frequency)

PSK- phase shift keying (it modulates phase)

ASK amplitude shift keying (modulates amplitude) 

OSI model:-

The OSI, or Open System Interconnection, model defines a networking framework for implementing protocols in seven layers. Control is passed from one layer to the next, starting at the application layer in one station, and proceeding to the bottom layer, over the channel to the next station and back up the hierarchy.

---------------------->Application (Layer 7)

  • This layer supports application and end-user processes. 

  • Communication partners are identified, quality of service is identified, user authentication and privacy are considered, and any constraints on data syntax are identified. 

----------------------->Presentation layer (layer 6):-

  • This layer provides independence from differences in data representation (encryption) by translating from application to network format, and vice versa. 

  • The presentation layer works to transform data into the form that the application layer can accept.

------------------------>Session (Layer 5)

  • This layer establishes, manages and terminates connections between applications.

----------------------->Transport (Layer 4)

  • This layer provides transparent transfer of data between end systems, or hosts, and is responsible for end-to-end error recovery and flow control.

  •  It ensures complete data transfer.

------------------------->Network (Layer 3)

  • This layer provides switching and routing technologies, creating logical paths, known as virtual circuits, for transmitting data from node to node

------------------------>Data Link (Layer 2)

  • At this layer, data packets are encoded and decoded into bits.

  •  It furnishes transmission protocol knowledge and management and handles errors in the physical layer, flow control and frame synchronization

------------------------->Physical (Layer 1)

  • This layer conveys the bit stream - electrical impulse, light or radio signal -- through the network at the electrical and mechanical level. 

  • It provides the hardware means of sending and receiving data on a carrier, including defining cables, cards and physical aspects


When data has to be passed from one place to anther in the telecommunication system, the data has to be switched i.e. connection is established and then data is passed. This can be done in many ways like:

Circuit Switching

  • Circuit switching is a technique that directly connects the sender and the receiver in an unbroken path.

  •  Telephone switching equipment, for example, establishes a path that connects the caller's telephone to the receiver's telephone by making a physical connection.

  •     With this type of switching technique, once a connection is established, a dedicated path exists between both ends until the connection is terminated.

  •  A good example of circuit switching involves the Public phone network. A data example would be the classic A/B switch! Example if we want to dial from ktm to USA then first of Nepal telecom switches from Nepal to country code and then country to city and city to subscriber.


  1. the channel is dedicated.


     1. has to wait for connection. 

      2. It is expensive.

Packet switching is a digital networking communications method that groups all transmitted data streams. In this switching:

  • A message to be sent is broken into small parts, called packets.

  •  Each packet is tagged with appropriate source and destination addresses.

  • Since packets have a strictly defined maximum length, they can be stored in main memory instead of disk; therefore access delay and cost are minimized.

  • Also the transmission speeds, between nodes, are optimized.

  • With current technology, packets are generally accepted onto the network on a first-come, first-served basis. If the network becomes overloaded, packets are delayed or discarded (``dropped'').

  • The size of the packet can vary from 180 bits.

  • In packet switching, the analog signal from your phone is converted into a digital data stream.

  • They do not necessarily travel together; they do not travel sequentially. They don't even all travel via the same route.


  1. cost effective

  2. can be re-routed

  3. same channel can be shared at same time.


  1. more complex

  2. Low quality packet received at destination..

Message switching:-

A switching technique in which:

  • With message switching there is no need to establish a dedicated path between two stations.

  • When a station sends a message, the destination address is appended to the message.

  •  The message is then transmitted through the network, in its entirety, from node to node.

  •  Each node receives the entire message, stores it in its entirety on disk, and then transmits the message to the next node.Each message is stored (usually on hard drive due to RAM limitations) before being transmitted to the next switch

  • This type of network is called a store-and-forward network.

Email is a common application for Message Switching.


  1. channel efficienty is greater.

  2. traffic congestion can be reduced


  1. needs to have high storage capacity to store long messages

  2. not comaptible with all applications


communications protocol is a formal description of digital message formats and the rules for exchanging those messages in or between computing systems and in telecommunications. Protocols may include signaling, authentication and error detection and correction capabilities. A protocol describes the syntax, semantics, and synchronization of communication and may be implemented in hardware or software, or both.

We have many examples like http, ftp, stp, tcp/IP etc.

                                     TCP is one of the main protocols in TCP/IP networks. Whereas the IP protocol deals only with packets, TCP enables two hosts to establish a connection and exchange streams of data. TCP guarantees delivery of data and also guarantees that packets will be delivered in the same order in which they were sent.

                                                                  Where as IP specifies the format of packets, also called datagram, and the addressing scheme. Most networks combine IP with a higher-level protocol called Transmission Control Protocol (TCP), which establishes a virtual connection between a destination and a source.

IP by itself is something like the postal system. It allows you to address a package and drop it in the system, but there's no direct link between you and the recipient. TCP/IP, on the other hand, establishes a connection between two hosts so that they can send messages back and forth for a period of time. 


  • IP - is responsible for moving packet of data from node to node. IP forwards each packet based on a four byte destination address (the IP number). The Internet authorities assign ranges of numbers to different organizations. The organizations assign groups of their numbers to departments. IP operates on gateway machines that move data from department to organization to region and then around the world.

  • TCP - It is responsible for verifying the correct delivery of data from client to server. Data can be lost in the intermediate network. TCP adds support to detect errors or lost data and to trigger retransmission until the data is correctly and completely received.

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