Introduction of Computer Network

History of Computer Networks

• In 1960, the ARPANET (Advanced Research Projects Agency Network) was widely recognized as the world’s first operational packet-switching computer network and the foundational technology of the modern internet.
  • Developed By: The U.S. Department of Defense’s Advanced Research Projects Agency (ARPA, now DARPA).
  • First Operational: 1969.
  • Key Milestone: The first successful message transmission occurred on October 29, 1969, between nodes at UCLA and the Stanford Research Institute (SRI).
  • Precursor to the Internet: ARPANET directly evolved into the core network of what became the modern Internet.
• The Internet is the world’s largest network by far, both in terms of geographical reach and the sheer number of connected devices and users.

Introduction to Computer Networks

• The process of interconnection between two/more different networks is called internetworking. The main purpose of interconnecting is to allow any node or any network (e.g., Ethernet) to access/share data with any other node on any other network. (e.g., ATM). 
• The process of interconnection within a typical network is called intranetworking.
• All data communication in a computer network is finally in digital form.
• The Computers on a network may be linked through Cables, telephone lines, radio waves, microwaves, satellites, etc.
• A Computer network includes the network operating system in the client and server machines, the cables/air, which connect different computers, and all supporting hardware in between, such as bridges, routers, and switches. In wireless systems, antennas and towers are also part of the network. 
• There are so many common reasons for having different networks in the world(thus different protocols) –
  • Many personal computers use the TCP/IP protocol suite due to their requirements & platforms/OS environment.
  • Many larger business organizations still use IBM mainframes with the specific SNA Protocol.
  • A large number of telecommunications companies also provide/use the ATM network protocol suite.
  • Some PCs still run on Novell’s NCP/IPX or AppleTalk protocol suite.
  • A Wireless Network will have a totally different protocol suite.
• Due to different network architectures, they have different parameters in their encoding techniques at the physical layer, frame formats at the data link layer, packet length in the network layer, quality of services in the transport layer, error handling mechanisms, flow control mechanisms, and congestion control mechanisms in the data link layer, security issues and addressing mechanisms in the network layer. Hence, these parameters must be taken into consideration before making/interconnecting a typical network.

Link for the difference between Data and Signal

Communication/Transmission Process

• • The exchange of information by speaking, writing, or using some other medium.- Oxford Dictionary.
  • Digital communications is the physical transfer of data or information in a computer network in the form of bits over a communication channel from a source to a destination.

Data Communication Model/Components of a Computer Network/Data Communication System

• • A Data Communication Model describes the basic structure and components involved in the transmission of data from one device to another over a communication medium. It explains how data is generated, transmitted, received, and interpreted in a computer network.
  • The data communication model provides a clear framework for understanding how information is exchanged between devices in a computer network. It forms the foundation for all network architectures and communication protocols.
  • In the present world, a computer does not work as a standalone system but as part of a communication system.
  • The data in a communication system may be transmitted either in analog or digital form over a single path serially or several parallel paths.
  • The data in a communication system can be sent asynchronously (when both the source and receiver are not following the proper timing interval) or synchronously (when both sender and receiver agree on the sequence of arrival of data).
  • The data in a communication system follows any transmission mode from all three methods – Simplex, Half duplex, and Full duplex/duplex.
  • The most important factors affecting the transfer of a signal over a medium are noise and attenuation. Noise is the external disturbance, whereas attenuation is defined as the degradation of the signal.

• • The data communication model consists of six main components, and each plays an important role in successful communication. These are –

• • • Sender/Source/Transmitter Devices :
      
      • A source who/which is trying to originates a message to the receiver.
      • The sender is the device that originates the data. It can be a computer, server, smartphone, or any network device that generates the message to be transmitted.
      • The source produces a message or sequence of messages to be communicated to the receiver.
      • The source output may be in many different forms, such as a waveform, a sequence of binary digits, and a set of outputs from sensors in a space probe, or many other similar forms.
      • Normally transmitter transmits the source’s produced data, but in a few cases source and transmitter are different, and in a few cases source and transmitter are the same.
    • Message/Information(Data)/Signal :
      
      • The actual/main content in a communication system is transferred to the receiver.
      • Here, information/data are mainly in electromagnetic signals, such as an electrical voltage, radio wave, microwave, or infrared signal.
      • The message is the information that is to be communicated. It may be text, numbers, images, audio, or video.
      • The data transmitted can be pure digital messages generated from a digital data source, like a computer or a keyboard. However, it may also be an analog signal, such as a human voice over a phone call, which could be digitized.
    • Networking Connecting Devices :
      
      • Several related/networking devices are also used to connect multiple devices, which are finally used to transmit/receive the data with certain specific modifications as per need.
      • For example – Modem, Encoder, Decoder, Switch, Hub, Repeater, Bridges, Routers, NIC, Gateways, Firewalls, Access Points, etc.
      • The specific devices used will depend on the network’s size, complexity, and intended use.
      • Many modern networking devices combine functionalities. For example, a home router often includes a modem, switch, and wireless access point in a single unit for convenience.
      • The explanation of networking devices is as follows –
        
        • Encoder:
          
          • In most cases, data/message is taken from the source and then encoded (by the encoder) for safety reasons, and then put over the medium for transmission.
          • It makes the original data unintelligible/unreadable.
          • The encoder does related processing of the source messages/signals before transmission. The processing might include, for example, any combination of modulation, data reduction, and insertion of redundancy to combat the channel noise.
        • Decoder:
          
          • Decoding (by the decoder) of the encoded message is done at the receiver end before final handover to the receiver.
          • It makes the unintelligent form of data into an original user-defined/readable form.
          • A decoder does the processing of channel-encoded data to produce an accepted replica of the input at the destination.
        • Routers: Direct data traffic between different networks. They use routing tables to determine the best path for data packets.
        • Switches: Connect devices within a single network and use MAC addresses to forward data to the correct destination.
        • Hubs: (Less common now) Simple devices that broadcast data to all connected devices, leading to potential collisions.
        • Modems: Convert digital data from a network into analog signals for transmission over phone lines or cable and vice versa.
        • Access Points: Create wireless networks by broadcasting a signal that devices can connect to.
        • Firewalls: Security devices that monitor and control incoming and outgoing network traffic based on predetermined security rules.
        • Network Interface Cards (NICs): Hardware components that allow computers to connect to a network.
        • Repeaters: Amplify signals to extend the reach of a network.
        • Bridges: Connect two different network segments and filter traffic between them.
        • Gateways: Act as a “gate” between different networks using different protocols, translating data as needed.
    • Communication Medium/Channel :
      
      • The medium (mostly wire/air/wave/water) through which data is transmitted in the form of a signal between the transmitter and receiver.
      • Typically, it may be a telephone line, a high-frequency radio link, a space communication link, or a storage medium.
      • The channel or medium could be air (for wireless/mobile communication), or cables through copper wires, or optical fibers.
      • The transmission medium is the physical or wireless path through which the data travels. Examples include twisted pair cables, optical fiber, radio waves, and microwaves.
    • Receiver/Destination Devices:
      
      • A destination that is trying to receive a message, finally.
      • It may be the person, devices, or other objects for whom the message is intended.
      • It interprets and processes the received message.
    • Communication Protocol:
      
      • A protocol is a set of rules that govern data communication.
      • It defines how data is formatted, transmitted, received, and acknowledged.
      • Common protocols are – TCP, IP, FTP, DNS, Http, etc.
  • Working Process of the Data Communication Model
    • In the data communication model, the sender encodes the message and transmits it through a communication medium using a specific protocol. The receiver receives the signal, decodes it, and interprets the message according to the same protocol. Proper coordination between sender and receiver ensures accurate and reliable communication.
  • For a data communication model to be effective, it must have:
    • Delivery: Data must reach the correct destination.
    • Accuracy: Data must be received without errors.
    • Timeliness: Data must be delivered within an acceptable time.
    • Reliability: Data transfer should be dependable.

Communication Tasks

Definition of Computer Network

• A computer network can be simply defined as the interconnection of two or more independent computers for a specific purpose.
• A computer network is a collection and connection of more than one autonomous computer, servers, mainframes, network devices, peripherals, or other related devices connected to one another in a pre-defined plan to allow the sharing of data/resources.
• A computer network is a collection of hardware components and computers interconnected by communication channels that allow the sharing of resources and information.

Data Transmission Standard

• The data in a communication system follows these data transmission standards.

(A) Data Transmission Types

(a) Serial Data Transmission

• • In computers and networks, serial communication means sending data one bit at a time (like 0s and 1s), in a sequence, over a single communication line or channel (cable, fiber, or wireless).
  • It is a type of digital data communication in which sequential transmission of bits occurs over a single channel.
  • It is cost-effective because it requires fewer wires (just one for data in each direction, plus a ground wire).
  • This transmission is reliable over long distances, as only one bit travels at a time; therefore, errors and interference are less compared to parallel transmission.
  • This is a standard form in networking, i.e., most computer networks, USB ports, and internet systems rely on serial communication.
  • It uses less processing power and has fewer chances for error.
  • It has a lower data transfer rate comparatively.
  • The start and stop of a communication in this transmission is specified by the LSB (Least Significant Bit) and the MSB (Most Significant Bit).
  • In serial transmission, the byte(data) plus the parity bit(for error detection) is transmitted one bit after another in a continuous line.
  • Advantages
    • This transmission has simple wiring and cheaper hardware.
    • This transmission is more reliable over long distances.
    • This transmission is widely adopted in modern networking and devices.
  • Disadvantages
    • This transmission is slower than parallel for short distances.
    • It requires proper synchronization between the sender and the receiver.
  • Examples
    • In the Old COM ports of computers used RS-232 serial communication.
    • Even modern USB works on serial communication (Universal Serial Bus).
    • Data packets over Ethernet, Wi-Fi, or Fiber optics are transmitted serially.
    • In Embedded Systems, microcontrollers use [have UART, SPI(Serial Peripheral Interface), or I²C(Inter-Integrated Circuit)] serial communication to talk to sensors and chips.
  • Types of Serial Transmission Examples
    • Serial data transmission is of two types –

(i) Synchronous Serial Data Transmission

• • • In synchronous transmission, both receiver and sender have an agreement (or are aware) about timing for the sending and receiving data, so that both sender and receiver can coordinate (synchronize) their data signals.
    • In case of this transmission, there is no use of any start and stop bits, but instead of that clock signal (clock is built into each end of transmission) is being used for synchronizing the data transmission at both the receiving and sending end. For this, a constant stream of bits is sent between the sender and receiver. 
    • It has higher transmission speeds than asynchronous, because the system has a lower possibility of error. But, if an error takes place, there is a chance that the complete set of data is lost instead of a single character.
    • This transmission is mainly used for high-speed communication between computers.
    • It is unsuitable when the characters are transferred at irregular intervals.
    • It gives lower overheads because of no start and stop bits and thus, greater throughput.
    • The processing mechanism is more complex.
    • It is not very cost-effective as the hardware used is more expensive.
    • In this transmission, both sender and receiver share a clock signal to stay in sync.
    • The data transmission in this is faster and efficient.
    • Example: SPI (Serial Peripheral Interface), I²C(Inter-Integrated Circuit), etc., devices use this transmission.

(ii) Asynchronous Serial Data Transmission

• • • In asynchronous transmission, both receiver and sender have no agreement (or awareness) about timing for the sending data, i.e., no coordination between sender and receiver before transmission.
    • The asynchronous transmission uses start bits and stop bits to signify the beginning and end of transmission. For example, if a sender wants to send some data such as  10011101, it will be appended with the start bit(0) and stop bit(1), and finally look like “1 10011101 0”, where we have assumed that ‘0’ is the start bit and ‘1’ is the stop bit.
    • Asynchronous transmission is best suitable in the case where the characters are transferred at irregular intervals, as in data entry from the keyboard.
    • In this transmission, each character is surrounded by start and stop bits, i.e., each character is a complete unit, and hence if there is an error in a character, other sequences of characters are not affected. However, errors in start and stop bits (s) may cause serious problems in data transfer.
    • It doesn’t require synchronization of both communication sides.
    •  It is cost-effective.
    • The speed of this transmission is limited.
    • It has large processing overheads because of the presence of a large no. of start and stop bits with the data, which are uniquely used for control purposes.
    • This transmission has no shared clock.
    • Data is sent with start and stop bits to mark timing.
    • Example: RS-232, UART, USB, etc. devices use this type of transmission.

(b) Parallel Data Transmission

Definition

• • Parallel communication is a type of digital data communication in a computer network where simultaneous transmission of multiple bits of data occurs over multiple separate channels or wires.

Characteristics

• • Instead of sending data bit by bit, parallel communication sends an entire group of bits together, which makes the process very fast over short distances. It is commonly used inside computers where speed is more important than distance.
  • In parallel communication, each wire or channel carries one bit, so several bits travel side by side at the same time.
  • Inside computers, parallel communication uses multiple lines (usually 8, 16, 32, or 64) to transmit several bits simultaneously. The number of wires usually depends on the size of the data being sent, such as 8-bit, 16-bit, or 32-bit communication lines. 
  • It can operate in both synchronous mode (with a clock signal) and asynchronous mode (with a start/stop control bit). In synchronous mode, a common clock ensures all bits move together, while in asynchronous mode, start and stop signals are used. Synchronization between the wires is essential because if one bit arrives earlier or later than the others, the message may be corrupted.
  • Synchronization between multiple lines is very important in parallel communication. If one wire carries a bit slightly slower or faster than the others, the data may arrive out of order. Because of these issues, parallel communication is mostly limited to short distances inside a computer system. That’s why modern networks and external connections prefer serial communication.
  • It uses high processing power and has more chances for error.
  • In parallel transmission, 8 bits (a byte) plus a parity bit are transmitted at the same time over nine separate paths. Thus, parallel transmission is generally faster than serial transmission.
  • Despite its limitations, parallel communication is still very useful where speed and immediate data transfer are required over short ranges.

Advantages

• • Parallel communication allows data to be transmitted at a very high speed over short distances.
  • It is suitable for applications where large volumes of data need to be transferred quickly. 
  • It is generally faster than serial communication for short-range data transfer.
  • Since multiple bits are sent at the same time, the overall throughput can be higher compared to serial communication in short-range setups.

• • Parallel communication often requires more hardware since multiple wires and connectors are needed, which makes it more costly and bulky.
  • Parallel communication is not suitable for long distances. As the distance increases, parallel communication suffers from problems like crosstalk, signal distortion, signal loss, and timing errors/mismatches. This is why it is not suitable for long-distance transmission. Also, over long distances, signals can get distorted due to interference and timing mismatches, making them unreliable.
  • The more bits that are transmitted in parallel, the harder it becomes to maintain synchronization between the wires.
  • Power consumption is usually higher because more lines are active at the same time.
  • Due to these limitations, parallel communication is now mostly replaced by serial communication in modern networking and external connections.

Examples

• • The most famous example of parallel communication is the old printer port (also called a parallel port/Centronics, or IEEE 1284 port). Printers used to receive entire chunks of data in parallel for faster printing. 
  • Parallel communication is usually used inside computers or between devices that are physically very close. Data buses inside a computer that connect the CPU, RAM, or Memory, internal buses of the motherboard. and other components use parallel transmission.
  • Data buses inside microprocessors and memory modules also use parallel transmission.
  • Inside the computer, most data transmission is in parallel (many bits at once). A device called a UART (Universal Asynchronous Receiver/Transmitter) converts parallel into serial form. At the receiving end, another UART or communication device reassembles the bits back into parallel form so the computer can understand.
  • Internal connections in microcontrollers and digital systems, where short-distance high-speed data transfer is needed.
  • Advanced versions of communication buses like PCI (Peripheral Component Interconnect) also used parallel communication in early computer systems.

(B) Transmission Models/Communication Modes

• Transmission models describe the direction and manner in which data flows between communicating devices in a network.
• These models define whether communication occurs in one direction or both directions and how devices coordinate during data transfer.
• Normally, information is transmitted between a source and destination during the communication process using any one of three modes –

(a) Simplex Transmission Mode

• • In the simplex transmission model, data/signal flows or travels in only one direction(one-way/unidirectional), from the sender to the receiver. The receiver cannot send any response back to the sender.
  • This model is used where communication is required in only one direction.
  • Examples include television broadcasting, radio transmission, and keyboard-to-computer communication.
  • This mode uses a simple process and hardware.

(b) Half-Duplex Transmission Mode

• • In the half-duplex transmission model, data can flow in both directions, but not at the same time. At any given moment, only one device can transmit while the other receives.
  • Here, both stations may transmit, but only one at a time. It may be that either the sender or the receiver(one side) can transmit at a time.
  • In this transmission mode, the sender and receiver both transmit on the same frequency.
  • This model is commonly used where bidirectional communication is needed, but simultaneous transmission is not required.
  • For example, walky-talky & citizen’s band.

(c) Full Duplex/Duplex Transmission Mode

• • The transmission of data/signal between sender and receiver can take place in both directions at the same time/simultaneously.
  • It is like a two-lane road with traffic moving in both directions at the same time.
  • In this mode of transmission, signals going in either direction share the capacity, i.e., half of the bandwidth is used for sending data in one direction, while the other half is used for receiving data from the other direction.
  • This model provides the highest efficiency and is widely used in modern networks.
  • For example, a telephone or mobile conversation, video calls, and Ethernet communication are examples of full-duplex communication.

(C) Types of Transmitted Data/Signal

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Examples of World-Class Computer Networks

• There are just a few examples below, and the world of computer networks is constantly evolving with new technologies and applications emerging regularly.
• The impact of these networks extends far beyond their technical specifications, shaping communication, commerce, research, and countless other aspects of our lives.
  • The Internet: It is an excellent example of a computer network is the Internet. It is the global network of interconnected networks, the largest and most impactful computer network in existence. It revolutionized communication, information access, and countless aspects of modern life.
  • ARPANET: The precursor to the Internet, developed by the U.S. Department of Defense in the 1960s. It pioneered packet switching technology and laid the foundation for the modern internet.
  • NSFNET: The National Science Foundation Network, which played a crucial role in expanding internet access in the United States during the 1980s and 1990s.
  • Amazon Web Services (AWS): A massive cloud computing network offering a wide range of services, including computing power, storage, and databases. It powers numerous websites, applications, and services worldwide.
  • Microsoft Azure: Another major cloud computing network, offering similar services to AWS and competing for a significant market share.
  • Google Cloud Platform: Google’s cloud computing network, known for its powerful search and data analytics capabilities.
  • Facebook’s Social Network: While not a traditional computer network in the technical sense, Facebook’s vast infrastructure connects billions of users globally, facilitating communication and information sharing on an unprecedented scale.
  • Bitcoin Network: A decentralized peer-to-peer network that underpins the Bitcoin cryptocurrency. It demonstrates the potential of distributed networks for secure and transparent transactions.
  • Content Delivery Networks (CDNs): Networks like Cloudflare and Akamai optimize the delivery of web content by caching it at strategically located servers around the world, reducing latency and improving user experience.
  • Research and Education Networks: Networks like GEANT in Europe and Internet2 in the United States connect universities, research institutions, and government agencies, facilitating collaboration and data sharing for scientific and educational purposes.

Goals/Objectives of Computer Network

The objectives of a typical computer network are as follows –

• Cost reduction by sharing hardware and software resources.
• Provide high reliability due to multiple sources of supply.
• Provide an efficient transportation structure for large volumes of data among various locations, hence high throughput.
• Provide inter-process communication among users and processors.
• Reduction in the delay in driving data transport.
• Increase productivity by making it easier to share data among users.
• Repairs, upgrades, expansions, and changes to the network should be performed with minimal impact on the majority of network users.
• Standards and protocols should be supported to allow many types of equipment from different vendors to share the network (Interoperability).
• Provide centralised/distributed management and allocation of the network resources like host processors, transmission facilities, etc.

Performance of a Network

• Network performance refers to how efficiently and effectively a computer network operates in terms of speed, reliability, and quality of service while transmitting data between devices.
• Network performance determines the overall effectiveness of data communication. High-performance networks provide fast, reliable, and consistent data transfer.
• Key Factors/Parameters Measuring Network Performance
  • Throughput
    • Throughput is the actual amount of data successfully transmitted over the network per unit time.
  • Measured in bps (bits per second)
    • It is often lower than the bandwidth due to overhead and congestion.
    • It represents potential speed, not actual speed.
  • Bandwidth
    • Bandwidth is the maximum data-carrying capacity of a network link.
  • Latency (Delay)
    • Latency is the time taken for data to travel from source to destination. It includes the propagation delay, transmission delay, processing delay, and queuing delay.
  • Jitter
    • Jitter is the variation in packet delay over time.
  • Packet Loss
    • Packet loss occurs when data packets fail to reach the destination. It is caused by congestion, errors, or faulty hardware.
    • It reduces the reliability and quality of communication.
  • Reliability
    • Reliability indicates how consistently the network performs without failure. It is measured by uptime and error rates.
  • Response Time
    • Response time is the total time between a request and the received response.
• Factors Affecting Network Performance
  • Network topology
  • Type of transmission media
  • Number of users
  • Traffic load
  • Hardware and software quality
  • Protocol efficiency
  • Improving Network Performance
  • Increasing bandwidth
  • Using efficient routing protocols
  • Load balancing
  • Reducing congestion
  • Proper network monitoring

Advantages of Computer Networks

• Increases productivity, i.e., makes it easier to share data among other users.
• Store vast amounts of information and reduce waste.
• Keeps us connected to all the nodes of a network.
• Improves system abilities.
• Save time & cost.

Disadvantages of Computer Networks

• Viruses can spread to other computers throughout a computer network.
• Purchasing the network cabling, equipment, and file servers can be expensive.
• A comparatively complex structure than a standalone system. 
• It lacks independence.
• It poses security difficulties.
• It lacks robustness.
• It requires an efficient/skillful handler.
• It requires an expensive setup.

Applications/Uses of Computer Networks

• Applications of computer networks are found everywhere in every field of life. They are used in our homes, schools, colleges, railway stations, offices, businesses, and many other places.
• They help us to send an email, watch a live sports event on our computer, book rail/air tickets, chat with our friends, and several other.
• The following are common/popular applications of a computer network in human life: –

• • Resource sharing
    • It allows sharing of application programs such as MS Office, equipment, and data available to any component on the network, irrespective of the physical location of the resource and the user.
    • A network is needed because of the desire to share the sharable programs, data, and equipment available to anyone on the network without regard to the physical location of the resource and the user. We can also share processing load on various networked resources.
    • Using computer networks, we can share any resource, CPU processing power, peripherals (like printers, scanners, etc), information (like files and data, and even software), etc, among the components of that network. This sharing is done by communicating with the machine through which we want to share.
    • In hardware sharing, users can share devices present in that network, such as printers, scanners, CD/DVD-ROM drives, hard drives, modems, fax machines, etc.
  • Reliability/Flexibility
    • It provides high reliability in its service, having alternative sources of data if one of them is unavailable/corrupt/fails due to hardware failure or any other reason, the other source is available and can be used.
    • A network may have alternative sources of supply (e.g., replicated files, multiple CPUs, etc.). In case of one resource failure, the others could be used, and the system continues to operate at reduced performance. This feature is very important/useful for military, banking, air traffic control, and many other sensitive applications.
  • Scalability
    • Scalability is the ability to increase system performance gradually by adding more processors (i.e., incremental upgrade).
  • Powerful Electronic Communication Medium
    • Using computer networks, users can share their communication/message through the computer networks as email, chatting, audio/video conferencing, etc., no matter their location.
    • It provides a powerful communication medium among widely separated users easily, where it was previously impossible.
    • In the long run, the use of networks to enhance human-to-human communication may prove more important than technical goals such as improved reliability.
  • Inexpensive
    • Data transmission through a computer network is believed to be comparatively inexpensive.
  • Information Broadcasting and Search
    • This is also a mostly used application these days to create new websites, blogs, social networking websites, search engines, etc.
    • Computer networks provide us with tremendous opportunities for information broadcasting, display, searching, and information retrieval. 
  • Storage capacity
    • A computer network provides a huge capacity to store data reliably.
  • Cost Efficient
    • The output of a computer network is believed to be very cost-efficient with respect to a single/standalone system.
  • In Some Specific Applications
    • To make a Computer network environment for a Campus for computing and resource sharing work mainly.
    • In Collaborative research and development work.
    • In developing an Integrated system for design + manufacturing, + inventory.
    • In making Electronic commerce, publishing, and digital libraries.
    • In Multimedia communication (tele-training, video conferencing, etc.)
    • In Healthcare delivery (remote diagnosis, telemedicine, etc.) systems.
    • To use the Video-on-demand facility.
    • In Online learning.