Computer Networks: Exam Preparation

• This video walkthrough covers the entire subject of Computer Networks, tailored for semester exam preparation.
• It highlights important topics and questions from an exam perspective.
• It offers a comprehensive overview from basic to advanced concepts.
• It suits both beginners and those seeking quick revision.
• The content is based on vast college and university syllabus analysis, ensuring over 95% syllabus alignment.
• Pro-level, detailed notes are available via a link in video description.
• A location-based comment initiative aims to create a map of India in the comment section.

Syllabus Overview

• A standard syllabus was used and refined with specific chapter and topic video markers.
• Top-left feature in video shows video chapter, and chapter topics.

Definition of Computer Network

• A computer network enables autonomous digital devices to exchange data through wired and wireless connection.
• This allows hardware like printers and software like drive sharing resources to be shared.
• Devices in a network may have different hardware/software but share protocols enabling communication.
• Internet of Things is a recent development where various household appliances connect to the internet, expanding the network beyond computers.

Internet Usage Statistics

• Internet access percentages correlate with a nation's development.
• Developed countries connect 80-90% of their population, while developing nations average around 50%.
• Internet access can be an indicator of literacy, GDP, development, and life expectancy

Goals of Computer Networks

• Communication is a primary goal.
• Resource sharing allows for sharing files and databases.
• Centralized databases ease backup and recovery with cost efficiency.
• Enhanced reliability ensures all the systems are working without difficulty.

Applications of Networks

• Used in e-commerce platforms like Amazon.
• Used in colleges for online meetings, seminars, and classes.
• Used in healthcare for teleconsultations and robotic surgeries.
• Used for online government services like reservations and digital documents.
• Used for entertainment, such as streaming services.

Data Communication

• Data communication exchanges data between devices through a transmission medium.
• Five components: message, sender, receiver, transmission medium, and protocols.
• Protocols are rules enabling communication including syntax, semantics, and timing.

Transmission Modes

• Simplex: One direction, one party transmits, the other receives (e.g., radio broadcasting).
• Half Duplex: Communicates both ways, but only one direction at a time (e.g., walkie-talkies).
• Full Duplex: Simulates both directions simultaneously (e.g., phone calls, video conferencing).

Network Criteria

• Delivery and Accuracy: Data must arrive at correct destination without corruption.
• Performance: Considers transmission speed, capacity (bandwidth).
• Reliability: Measured by frequency of failure.
• Security: Protects data through secure connections.

Types of Connections

• Point-to-point: Direct connection between devices without intermediaries.
• Multipoint: Multiple nodes connect through a shared medium.

Topology

• Mesh: Every device connected to every and other, offering redundancy and high robustness.
  • Complex wiring, high cost, difficult reconfiguration.
• Star: All devices connect to a central hub, simplifying management.
  • Relatively easy installation, central point of failure.
• Bus: Devices connect along a central cable backbone, easy cabling, low cost.
  • Failure in Backbone will halt the operation.
• Ring: Devices connected in a circle, data passes through each device until it reaches destination.
  • Simple reconfiguration, failure of link may collapse the whole network.
• Hybrid: Combines different topologies.
  • A mixed bag of each topology depending on arrangement and requirement.

Network Types

• LAN (Local Area Network): Covers a small area like an office.
• MAN (Metropolitan Area Network): Spans a city.
• WAN (Wide Area Network): covers a state or larger area.
• Internet: Is a wide area network, connecting different networks.

Network Models and OSI model

• The Open Systems Interconnection (OSI) model, established by ISO, standardizes network communication dividing it into seven layers.
• The model is not a protocol; it standardizes how to think about setting up protocols.
• Layered architecture divides the complex task into manageable parts, each with specific duties.
• Layers are physical, data link, network, transport, session, presentation, and application.
• The data goes from top to bottom on Source A, then over into the different layers on destination B, which is named encapsulation.

OSI layers and Responsibilities

• Physical Layer: Deals with representation of bits, data rate, line configuration, topology, and transmission mode.
• Data Link Layer: Handles framing, physical addressing (MAC), access control (Aloha), error control (CRC), and flow control (sliding window).
• Network Layer: Responsible for logical addressing (IP) and routing mechanism.
• Transport Layer: Includes service point addressing (ports), segmentation, congestion control, flow control, and error control.
• Session Layer: Manages dialogue control and synchronization.
• Presentation Layer: Translations, encryption/decryption, and compression.
• Application Layer: Provides access to network services (e.g., email) virtualization and file transfer.

Transmission Media

• Guided (Wired) : Uses physical wires or cables that data goes through.
• Unguided (Wireless): Uses radio wavelength that data and signals goes through.
• Multiple types of cables can be used, such as Twisted Pair cables, Coaxial cables, and Optical Fibers.

Cable Types and standards

• Twisted Pair cables are the most common type of cable for LAN networks.
• Cables are wired based on standards known are CAT cables.
• Coaxial Cables were common for cable, or "cable TV" setup.
• Optical Fibers are used more over the years and relies of Fiber Optics technology.

Wireless Media

• Ground Propagation: Waves follow the earth's curve, good for short ranges communications.
• Sky Propagation: Transmits waves towards the sky.
• Line of sight propagation: Must have a clean path from sender and receiver.

Switching Methods

• Circuit Switching: Establishes a dedicated path between two devices using:
  • Time Division / Frequency division.
• Packet Switching: Data is divided into packets and sent across a network using:
  • Data Gram.
  • Virtual Network.

ISDN

• Set of protocols establishing and breaking connections for advanced call features.
• Uses digital signals for data transmission.
• Basic Rate Interface (128 Kbps): The lowest speed tier.
• Primary Rate Interface (PR 1.5-2 Mbps): The highest speed tier.

Data Link Layer Overview

• Subdivided into Media Access Control (MAC), and Logical Link Control (LLC).
  • The lower half deals with medium access control and Ethernet function.
  • While upper half handles flow, and error control.

Access Control Protocols

• Divided into three categories, being Random Access, Controlled Access, and Channelization.
  • Random Access is important for CSIT to know and includes the protocols: Aloha, CSMA/CD, and CSMA/CA.

Transfer Rate Terminologies

• Propagation Delay: The time it takes the data to reach from the receiver to the sender, using the formula:
  • Time=Distance/Speed.
• Transmission Delay: The time it takes to clear the transfer, using the formula:
  • Packet Size/ Bandwidth.

Aloha and it's terminologies

• Aloha, originally was wireless, is now an outdated system, that whoever has something to transfer can at any moment.
• Can cause lots of collisions.
• It is very very random.
• Vulnerable time: 2xTransmission time, is the needed buffer to protect against traffic.
• s=gxe^-2g
  • G was how many packets are prepared during the times of transmission.

Slotted Aloha Algorithm

• Almost identical to Aloha.
• You can only send at the start of each slot.
• The Collision only happens if they both start at the same time, reducing the vulnerability time, doubling the efficiency.

Carrier sense multiple access mechanism. CSMA

• With CSMA you always listen before talking, if the wavelength isn't used, then we give over the package.
• You get collision.
• Vulnerability time: Tp or traffic propagation.
• Persistence method:
  • Non Persistent: Randomizes the time to avoid collision.
  • P persistence: Assigns priority to the devices using random number.

CSMA/ CD Method and Formula

• With the collision detection, you can test when they collide with voltage on Copper cables.
• If A sends to D, is it possible that C starts talking before A to D finishes? Answer is YES because voltage may not yet reach.
• Transmission time= 2x Propagation time = Tt = 2tp.
• Vulnerability= Traffic propagation

CSMA/ CA Method

• Collision avoidance, for wireless since no physical cables to detect with electricity.
• Inter Frame Space measures to constantly sense and also randomize time to avoid collision.
• Contention Window is a random element is introduced to prevent all from talking simultaneously.
• Always has acknowledgment mechanism.

Control flow

• Always make sure that the receiver isn't overflown.

Stop and Wait

• Simplest, only one package can traverse at a time.
• Each sent package waits for a return acknowledgment.
• At the same time, to prevent infinite waiting, you can start timer to resend after period if there's no acknowledgment.
• Also we can use numbering system, so numbers 0 and 1.
  • When requesting Frame 1, even though Frame 0, when the other side responds, it means they now have it since they request for the next.
• Efficiency= Tt/ Tt +Tp +Tp = 1/ 1+2a

Go Back N Technique

• To avoid issues of Tt/ Tt +Tp +Tp, more than 1 packages are sent continuously.
• The receiver must get everything in the right order.
• However if it doesn't it must go again.

Selective Repeat Technique

• If one goes wrong, only that one is resent.
• Requires both receiver and sender to have a buffer.
• Piggy bag technique also saves some steps, by:
  • Allowing two sides that know each other well, to piggyback and save transmission costs.

Type of Errors

• Errors occur during transmission.
• Usually categorized in the form of BITS.
  • When numbers become flipped.

Distances

• There's different variations and complexities of distances calculations.
• Minimum-Maximum, even with the same average there are trade offs.
• The minimum should be always considered, and you may not want averages.

Bit corruption

• Single bit: Only one bit got flipped.
• Burst error: Adjacent bits are flipped.
  • This is dangerous because it allows room for non detection.

Parity check

• Data word: Is the real data you want to send over the channel.
• Code Word: Is the data word + parity bits.
• Checkers and generators are also in the process to double check results.
• Checkers and Generator follow concepts of being odd or even.
  • Even: Is where all "1s" are even numbers.
  • Odd: Is where all "1s" are odd numbers.
• One/ two dimension system with error correction will help check error detection is available.

Hamming Code (For Error Correction)

• Is a calculation code to find source of error and fix it.
• Expressed in the formula: K = 2r -r -1

Checksum.

• Is a unique kind of verification technique that is elegant and widely used.
• All Data is a series of numbers. Performing the check:
  • If I add everything up, does it result in the expected total.
• Modulo 2 helps clean the data for error.
• One's complement makes it all align.

CRC

• CRC or Cyclic redundancy check is another very useful algorithm that presents a clever idea of code transfer.

CRC basics

• Is very similar to other error algorithms and error correction, although:
• The math behind can get a bit hairy.

Ethernet

• Most popular technology and way in this modern point in day, that is constantly evolving.
• Uses twisted pair, 100 m to 1,000m long, and usually has speeds over giga bytes per second.
• The process involves how packet, data, headers work with each other.
• Collision avoidance algorithms also help with error handling.

MANCHESTER encoding

• Used for the communication between computers and data centers, enabling signal conversion in all kinds of media transmission.
• It is the standard to determine what a 0 and what a 1 is
• Follows Ethernet protocol 802. and other variations.

Token Based

• Is an old technique to pass on the access token to devices to talk.
• Not a huge amount of performance with token passing systems overall.
• Mostly obsolete at this point due to other systems now.

How IPv4 Works.

• Its 32 bit, Unique and Universal, with a series of 0s and 1s.
• Still requires finding your number by checking a table that also gives physical access.
• Class full addressing needs further breakdown.

Classfull Addressing

• Consists of a network ID for the first part and a host ID for the second part.

Address Depletion and Problems

• The address allocation is not balanced, with some classes too large and some too small.
• Hard limitations cause wastes.

Classless INter Domain Table

• Uses CIDR for short,.
• Less wasteful.

Supernetting

• Involves combining two or more routes into a single routing table entry.

Routing

• Is how information is sent.
• Designs and extracts data.
• Updates those tables.

Routing Options: Flooding

• Sends data everywhere, all over the internet that has:
  • Pros: Super high delivery change, no configuration or settings to worry about.
  • Cons: Duplicates and traffic issues.

Static routings

• Set in stone system, like a train station timetable and:
  • Is predictable.

Dynamic Routing

• Changes with traffic and is always evolving.
• Has a few options:
  • Intra Domain/ or Intra domain,.
  • Is broken to 2 sides.

Intradomain

• Distance Vector, and Link State make up parts of this setup, including:
  • Cost, and the formula.

Distance Routing

• Uses a table to determine where to, what, where and the next one to talk to. Using the following:
  • If cannot reach, mark infinite.
• It only shares with the immediate neighbor.

Split horizon technique.

• Is used to avoid other issues.
• Each part may do something.

Transport Layer

• Promises a service to service promise, or:
  • To reach a specific process inside a computer,.
• Can be Connection-Less or:
  • Order may need to be enforced

Connection Oriented

• Uses TCP and UDP
  • TCP main protocol.

What the Properties of UDP?

• Low security
• Minimum overhead
• Small short messages
• Real time usage

Port Numbers

• 0-1023: Well Known, and controlled.
  • What service uses what.
• 1024 - 49151: REGISTERED, Not controlled.
• Above that limit:
  • Private.

SOCKET

• IP with all of its other processes connected, known as socket.

TCIP

• Has higher security and takes more time than UDP.
• Uses 6 flags and the window size is a big factor.
• Flags- Urgent, Push, Reset, SYN, and Fin.

UCP

• Very short and easy just remember has has basic stuff, like:
  • Port number.

General Useful Concepts

• RTP for transporting and streaming data
• Cryptography
• Coding
• Distances with hamming.

Cryptography

• Is the process of passing a secure message to another computer so that unauthorized parties can't access it.
• Is a science with many factors, that:
  • Involves encryption and description.
• Works in a very basic way, by flipping, numbers ahead to make it make sense.

Symmetric Keys

• Each communicator has its own personal KEY.
• The key is what encodes and decodes.
• In both receiver and sender to prevent anyone else from knowing from the outside.
  • Main con with that is the Key retrieval.

As Simmetric Keys.

• Modern systems use asymmetric keys:
  • One key for opening (public key) and one for closing (private key).
  • A can give public key out , B does not, B holds private key as safely as possible.

Key Differences

• Authentication, is provided by asymmetric but not symmetric keys.
• A has B keys, the same keys for multiple people, but only their code will work with the one on their respective computer.

How to keep Authentications safe

• A sends message locked with private key that can be unlocked by a public key.
• Unlocking with that key allows trust and knowing the identity the other side.

How to have safe Key exchange/ Key generation:

• "Keep the number short for the math. And understand the logic before it."

Email

• Is very important for modern technology, allowing the transmission of text and multimedia, and:
  • It must connect through proper ports.

Email Components

• MTA and MAA are used.
• SMTP is a simple mail transfer protocol.
• It is a push protocol.

Email Protocols

• POPS - Downloads emails only:
  • Only offers download options, not ideal for modern usage.
• IMAP- Is the latest email transfer protocol:
  • Organizes everything with a folder structure to read from.
  • Includes options to create, rename, and copy emails, plus other organizational features.

MIMI

• Supports structured and organized multimedia and non-ASCII data transfer.

Modern Web Technology

• HTTP with all of its other components
• Very convenient.

FTCP

• Uses TCP, which doesn't loose data.

Short Handed TCP Info

• TFTP, small cheap version:
  • For small file transfer but not recommended for long distance.

The Great Web World. WWW

• Invented by Tim Berners at the European Laboratory and physics in the 1980s, and:
  • Allows to work and make a multi web page on a network.
  • Uses Distributors, Clients, and Servers.

URLs and Cookies

• Make the browsers remember your and your credentials.

HTTP

• Hypertext transfer protocol is
  • The brain for all web transfers that uses tcp, providing reliable data transfer on port 80*
  • Non-persistence can be improved with new technology.

DNS

• Translates domain names to IP addresses.
• Facilitates connecting an English name to an algorithm.
• Begins with local lookup and escalates to broader searches.
• Powers the world by helping to translate English to binary for computers.

Domain Name Extensions

• Several domain extensions exist, like .org for various purposes.
• .net and .museum are other possible extensions.
• .gov and .edu are specific extensions and .com is the most commonly used
• Country-specific extensions exist (e.g., .in for India, .us for the USA, .au for Australia, .sg for Singapore).

Domain Name Depth

• Domain names can have up to 128 levels of depth
• Each level represents a deeper subcategory within a website.
• For example, a university website (.edu) may have departments with further subcategories.

Telnet (Telecommunication Network)

• Telnet facilitates remote server access using a text-based protocol.
• Well-known port number 23 is allocated to Telnet.
• It assists in debugging and interactive sessions.
• Telnet's security measures are weak, making transmitting sensitive information risky.
• SSH has largely replaced Telnet due to security concerns.
• Telnet can still be used in legacy systems where security is less critical.

ARPANET (Advanced Research Projects Agency Network)

• ARPANET was the first wide area network, emerging in the 1960s.
• The U.S. Department of Defense designed ARPANET
• ARPANET used packet switching and laid the groundwork for today's Internet.
• It introduced early networking protocols like NCP and enabled applications like email.
• ARPANET connected hundreds of institutions.
• ARPANET was decommissioned 30–35 years ago but had a strong influence on protocol development.

X. 25

• X.25 is an old protocol used in wide area networks.
• Key features include error checking and virtual circuit setup.
• It had usage-based billing.
• X.25 operates across three layers in the OSI model.
• It was used in credit card processing and ATM support.
• X.25 supported slow data rates.
• X.25's use declined due to faster protocols like the Internet.

SNMP (Simple Network Management Protocol)

• SNMP manages and monitors network devices.
• SNMP operates over UDP and intranet.
• It uses a database known as MIBs to control device properties.
• SNMP manages network routers, servers, and multilayer switches remotely.
• Commands include GET, SET, and TRAP.
• Polling and trapping facilitate real-time monitoring using SNMP.

VoIP (Voice over IP)

• VoIP enables voice communication over the Internet.
• It offers cost savings and efficiency.
• VoIP transmits data in packets over the Internet, making it relatively cheaper.
• Potential issues include delays, lag, and security risks.

RPC (Remote Procedure Call)

• RPC allows a program to execute a function on a remote server.
• Variety of protocols such as TCP and HTTP facilitates the execution
• Encryption and authentication are used for security.
• It is often used in client–server architectures and distributed systems.
• Its disadvantages can include complexity and failure handling.

Firewalls

• Firewalls are security mechanisms for checking network traffic, based on IPs and port numbers.
• Firewalls discard suspicious packets.
• They enhance stateful inspection, proxy servers, and VPN support.
• All professional companies mainly use firewalls for security

Repeaters

• Repeaters amplify and boost data signals to compensate for degradation over distance.
• Repeaters don't analyze data packets at any layer, but instead simply boost the signal.

Hubs

• Hubs are outdated devices with multiple input/output ports
• When data is input on one port, it is replicated and sent to all other ports.
• Hubs have been replaced by switches in modern networks.

Bridges

• Bridges operate at the data link layer
• They connect two local networks, making them behave as a single network
• Bridges handle translation between the networks.
• They do not have network layer (IP) understanding, but understand Ethernet frames.

Switches

• Switches are commonly used in modern networks.
• They send data directly to the correct device using MAC addresses.
• Switches identify the destination device before sending data, ensuring efficiency.
• They are smarter than simple devices like repeaters or hubs.

Routers

• Routers connect home networks to the Internet.
• They manage data transmission between multiple lines or Wi-Fi connections.
• Large, professional-grade routers manage Internet traffic using routing tables.

Gateways

• Gateways operate at the application layer and understand all layers of the OSI model.
• Gateways allow data to flow from one discrete network to another.
• They handle server-to-server connections at every level.
• Gateways are more complex than routers or switches.
• They integrate computer programs to perform gateway tasks.