CBSE Class 12 Informatics Practices Communication And Computer Networks Notes

Communication and Computer Networks

Network→ It is an interconnected collection of autonomous computers

Network Goals →
1. Resource sharing --- All programs, data and peripherals are available to anyone on the network irrespective to the physical location of the resource and the user.

2. High Reliability --- A file could be replicated on two or three machines, so if one of them is unavailable (hardware crash), the other copies could be available.

3. Cost Factor --- Personal Computers (PC’s) have a much better price/performance ratio than micro ones.

4. Communication Medium --- A file that was updated/modified on a network, can be seen by the other users on the network immediately.

Network Applications →
1. Sharing of data, programs and peripherals
2. Access to remote programs.
3. Access to remote databases.
4. Value-added communication facilities.

àAdvantages→

1. Speed --- Sharing and transferring files within Networks are very rapid. Thus saving time, while maintaining the integrity of the file.

2. Security --- Sensitive files and programs on a network are passwords protected so no illegal copying of programs is there.

3. Centralized Software Management --- Software can be loaded on one computer eliminating that need to spend time installing on independent computers.

4. Resource Sharing --- Resources such as, printers, fax machines and modems can be shared. Therefore is cheaper than to have these on many systems.

5. Improve Communications --- E-mail aids in personal and professional communication. Electronic mail on a LAN can enable staff to communicate within the building having tot to leave their desk.

àDisadvantages →

1. Cabling can be expensive to install and replace.
2. A fault with the server will prevent the whole network from working.
3. Security measures are needed to restrict access to the network.
4. WANs are vulnerable to hackers and viruses.
5. Server faults stop applications being available
6. Network faults can cause loss of data and resources
7. User work dependent upon network
8. Could degrade in performance
9. Resources could be located too far from users
10. Network management can become difficult for very large network

•Interspace ---- It is a client/serve software program that allows multiple users to communicate online with real time audio, video and text chat in dynamic 3D environments

• TCP ---- It is responsible for dividing the file/message into packets on the source server and also for reassembling the received packets at the destination computer.

• IP ---- It is responsible for handling the address of destination computer so that each packet is routed to its proper destination.
• Nodes ---- The computers that are attached to a network are known as nodes, also known as workstation.
• Server ---- Computer that facilitates the sharing of data, software and hardware resources (e.g. printer, modem etc.) on the network is known as server. There are two types of servers, characteristics and difference is given below :-

• NIU ---- Network Interface Unit is an interpreter (device) that helps establish communication between the server and workstations. Some important point related to NIU :-

a) Each NIU has a unique number known as node address.

b) It is also known as Terminal Access Point (TAP)

c) It is also called as Network Interface Card (NIC)

d) NIC manufactures assigns a unique physical address known as MAC Address

Network Switching Techniques  → These are three switching techniques given below :-

Circuit Switching ----
1. The complete physical connection between two computers is established and then data are transmitted from the source computer to the destination computer.

2. The important property of this switching technique is to setup an end-to-end path (connection) between computers before any data can be sent.

Message Switching ----

1. The source computer sends data or the message to the switching office which sends the data to another switching office until the data is delivered to the destination computers.

2. Its working principle is known as store and forward

Packet Switching ----
1. It places a tight upper limit on block size i.e. a fixed size of packet is transmitted across the network.

2. This improves the performance as the access time (time taken to access a data packet) is reduced,

3. Thus, the throughput (measure of performance) of the network is improved.

   Difference between Message and Packet Switching 

A Comparison of Circuit-Switched and Packet Switched Networks 

Transmission Media
The communication media is divided into two categories :-

1. Guided Media ---- includes cables for transmission. The basic types are :-

a. Twisted Pair Cables

b. Coaxial Cables

c. Optical Fiber Cables

2. Unguided Media ---- includes waves through air, water or vacuum for transmission. The basic types are:-

a. Micro Wave

b. Radio Wave

c. Satellite

d. Infrared

e. Laser

Twisted Pair Cables  → Cables without a shield are called Unshielded Twisted Pair (UTP). As shown in Fig below:-

Important point related to twisted pair cables :-

• The wire comes in pairs and the pair are twisted around each other

• VGM (Voice Grade Medium) for telephone wiring

• DGM (Data Grade Medium) a high quality cable used for LAN applications

• It stops signal interference that is crosstalk (Bleeding of signals from one wire to another) which can corrupt signals and cause network errors.

• The degree of reduction in noise interference is determined by the number of turns per foot. Increasing the number of turns per foot reduces the noise interference.

• To further improve noise rejection, a foil or wire braid shield is woven around the twisted pairs. Cables with a shield are called Shielded Twisted Pair (STP). As shown is figures A and B given below :-

Advantages :-

1. It is simple 5. It is very inexpensive

2. It is physically flexible 6. Easy to add additional network devices, therefore supports large

3. It can be easily connected number of network devices

4. It is easy to install and maintain 7. It has a low weight

Disadvantages :-

1. High attenuation (signal loss) limits individual runs to 100 meters

2. Repeaters should be used to carry signals for long distances

3. Its low bandwidth makes it unsuitable for broadband applications

4. Maximum data rate without conditioning 1Mbps and with conditioning 10 Mbps

UTP cable is available in five grades (or categories) :-

Coaxial Cable 

Important point related to Coaxial pair cables :-

• Coaxial Cable consists of two conductors.

• The inner conductor is held inside an insulator with the other conductor woven around it providing a shield. An insulating protective coating called a jacket covers the outer conductor.

• The outer shield protects the inner conductor from outside electrical signals.

• Coaxial cables are used for Cable TV (in CATV form) and Ethernets

• This cable allows higher data rates to be transferred than the Twisted Pair cable.

Advantages :-
1. Can be used for broadband transmissions i.e. many channels transmitted simultaneously

2. Offer higher bandwidths upto 400 MBPS

3. Data transmission is better than Twisted Pair cable

4. Low cost due to less total footage of cable, hubs not needed

5. Lower attenuation (disturbance)than twisted pair 

Disadvantages :-

1. Expensive as compared to Twisted Pair cables

2. They are not compatible with twisted pair cables

3. Limited in network speed

4. Limited in size of network

5. One bad connector can take down entire network

Important point related to Fiber Optical cables :-

• It consists of thin strands of glass or glass like material that carry light from a source at one end to a detector at the other end

• Light sources used are either LEDs (Light Emitting Diodes) or LDs (Laser Diodes)

• Data transmitted is modulated onto light using frequency modulation techniques

• Bandwidth is very high, for LEDs is 20 to 150 Mbps and for LDs even higher

Advantages :-

1. Can be used for broadband transmissions where many channels are handled in parallel

2. Guarantee Secure Transmission: cannot tap into cable.

3. Noise, Radio-Frequency, Electrical and Magnetic Immunity

4. Longer distances than copper wire

5. Smaller and lighter than copper wire

6. Faster transmission rate

Disadvantages :-

1. Difficult to install

2. Connecting two fibers or a fiber to a light source is a difficult process

3. Light can reach the receiver out of phase

4. Difficult to solder and slice

5. Limited physical arc of cable. Bend it too much & it will break!

6. Expensive than all cables

7. Connection losses are common problem

Micro Wave 

How it works :-
• Microwave Signals are used to transmit data instead of cable which are similar to radio and television signals and are used for long distance communications

• Microwave transmission consists of transmitter, receiver and the atmosphere.

• This transmission is also known as Line-of-Sight transmission. The Transmit station must be in visible contact with the receive station.

• Parabolic antennas are mounted on towers to send a beam to other antenna far away.

• The higher the tower, higher is the range. With 100 meter high tower a distance of 100 Km is feasible

Advantages :-

1. Cheaper than the cabling system

2. Ability to communicate over oceans

3. Low cost land purchase: each tower occupies small area.

4. They can carry high quantities of information due to their high operating frequencies.

5. High frequency/short wavelength signals require small antenna

Disadvantages :-

1. Insure communication

2. Susceptible to weather effects (like rain, snow and fog) and other objects like birds etc.

3. Reflected from flat surfaces like water and metal

4. Refracted by atmosphere, thus causing beam to be projected away from receiver.

5. The code of design, implementation and maintenance is high

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