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COMPUTER NETWORKS
(UNIT- 1 )
INTRODUCTION
A Computer Network is simply a group of computers (or other devices like phones, printers, etc.) that are connected together so they can share information and resources. Think of it as a system that allows devices to talk to each other. PARTS :
- Devices: Computers, smartphones, tablets, printers, servers, HUB, SWITCH, BRIDGE, GATEWAY, MODEM, ROUTER, REPEATER etc.
- Connection: The way devices are linked (wires, Wi-Fi, Bluetooth).
- Sharing: The ability to exchange data (files, messages, videos) and resources (like printers or internet access). CORE FEATURES OF COMPUTER NETWORKS : o Resource Sharing: Allowing multiple devices to access and use the same resources (like printers, files, internet connections). o Communication: Enabling devices to exchange information with each other. (Social media is a huge example of network based communication). o Connectivity: Linking devices together, regardless of their physical location. (Accessing your bank account online from your home computer). o Reliability: Ensuring data is transmitted accurately and consistently. o Scalability: The ability to easily expand the network as needed by adding more devices. o Security: Protecting data and devices from unauthorized access or damage. (Using a password to access your Wi-Fi network or your online bank account).
COMPUTER NETWORK TYPES : Computer networks can be categorized in several ways.
- Based on Geographical Area: (5 types) i. Personal Area Network (PAN): This is the smallest type of network, typically covering an area of a few meters. It's used to connect personal devices like smartphones, laptops, and wearables. Range < 10 k.m. ( Ex- Bluetooth connections phone and wireless headphones). ii. Local Area Network (LAN): Connects devices within a limited area, such as a home, office, or school. LANs are typically privately owned. Range < 150 m. ( Ex - Commonly uses Ethernet or Wi-Fi). iii. Metropolitan Area Network (MAN): Larger than a LAN but smaller than a WAN. Covers a city or metropolitan area. Often used by government agencies or large corporations. Range < 50 k.m. iv. Wide Area Network (WAN): Spans a large geographical area, such as a country or continent. The internet is the largest WAN. WANs connect LANs and MANs. v. Wireless Local Area Network (WLAN): A type of LAN that uses wireless radio frequency communication to connect devices. Wi-Fi is the most common example. 2. Other Important Network Types:
- Virtual Private Network (VPN): Creates a secure, encrypted connection over a public network like the internet. Used to protect privacy and security. Allows users to access private networks remotely.
- Storage Area Network (SAN): A high-speed network that connects storage devices to servers. Used for efficient data storage and retrieval. DATA COMMUNICATION COMPONENTS ➢ DATA COMMUNICATION : When we communicate, we are sharing information. This sharing can be local or remote. Between individuals, local communication usually occurs face to face, while remote communication takes place over distance. COMPONENTS: A data communications system has five components. i. Message: The message is the information (data) to be communicated. Popular forms of information include text, numbers, pictures, audio, and video. ii. Sender: The sender is the device that sends the data message. It can be a computer, workstation, telephone handset, video camera, and so on.
➢ DATA REPRESENTATION : Information today comes in different forms such as text, numbers, images, audio, and video
- Text: In data communications, text is represented as a bit pattern, a sequence of bits (Os or Is). Different sets of bit patterns have been designed to represent text symbols. Each set is called a code, and the process of representing symbols is called coding. Today, the prevalent coding system is called Unicode, which uses 32 bits to represent a symbol or character used in any language in the world. The American Standard Code for Information Interchange (ASCII), developed some decades ago in the United States, now constitutes the first 127 characters in Unicode and is also referred to as Basic Latin.
- Numbers: Numbers are also represented by bit patterns. However, a code such as ASCII is not used to represent numbers; the number is directly converted to a binary number to simplify mathematical operations. Appendix B discusses several different numbering systems.
- Images: Images are also represented by bit patterns. In its simplest form, an image is composed of a matrix of pixels (picture elements), where each pixel is a small dot
- Audio: Audio refers to the recording or broadcasting of sound or music. Audio is by nature different from text, numbers, or images. It is continuous, not discrete
- Video: Video refers to the recording or broadcasting of a picture or movie. Video can either be produced as a continuous entity (e.g., by a TV camera), or it can be a combination of images, each a discrete entity, arranged to convey the idea of motion. Again we can change video to a digital or an analog signal. ➢ DATA FLOW NETWORKS : In computer networks, data flow essentially describes how information moves from one point to another; it involves breaking down data into small packets, sending those packets across a network through various devices like routers and switches, and then reassembling them at the destination. Communication between two devices can be simplex, half-duplex, or full-duplex i. Simplex: This is one-way communication. Information travels in only one direction. Real-life example: A radio broadcast. The radio station transmits the signal, and you, the listener, receive it. You cannot transmit back to the radio station through your radio. Another example is a keyboard to a computer. The keyboard sends input, but the computer doesn't send data back to the keyboard in the same way. ii. Half-Duplex: This is two-way communication, but only one party can transmit at a time. Think of it as taking turns. Real-life example: Walkie-talkies. When you press the "talk" button, you can speak, and the other person can listen. When you release the button, they can speak, and you can listen. iii. Full-Duplex: This is two-way communication where both parties can transmit and receive simultaneously. Real-life example: A phone call. You can talk and listen at the same time, and so can the person on the other end.
➢ TOPOLOGY : The physical arrangement of the computer system node, which is connected to each other each other via communication medium is called topology. 6 types of topology: Bus, Ring, Star, Mess, Hybrid & Tree
1. Bus Topology: In bus topology one long c able acts as a single communication channel and all the devices are connected to this cable. Advantages Disadvantages Requires only one cable. If cable is fail then the entire network will be failed It is less expensive. The messages are broadcast. So we can't send private messages. It is very easy to maintain. It takes more time to pass the messages from one place to another place. In case of any computer failure there will be no effect on other devices. The length of cable is limited. It broadcast the messages to each devices which are connected through the cable. In this topology data is transmitted only one direction. 2. Ring Topology: It is called ring topology because it forms a ring. In this topology each node is strongly connected with its adjacent node. Advantages Disadvantages It forms a strong network. Very difficult task to add some new computer. Each & every node can share data with another node connected through a ring topology. If we want to send data from a source to destination machine then data will unnecessary passed to all nodes. example WhatsApp Group. Transmission rate of data is very speed. Single point of failure that means if a node goes down entire network goes down. The data send through ring topology will be broadcast (not personal). We can't send private messages (broadcast). 3. Star Topology: In star topology all the nodes are connected with Central device called HUB. And the sharing of data is only possible through HUB.
❖ QUESTION: If any organization wants two set up a network with 6 computers then how many links can be required for this ANSWER: Links for 6 Computers : The number of links required depends on the chosen topology. Mesh Topology (Full Mesh): In a full mesh topology, every device is directly connected to every other device. For 6 computers, you'd need:
- n(n−1)/2 links, where n is the number of computers.
- 6(6−1)/2=6(5)/2=15 links. If the organization uses a star topology , with a central switch or hub, then 6 links are required, one link per computer. ➢ OSI MODEL : The OSI ( Open Systems Interconnection ) model is essentially a conceptual framework that helps us understand how data travels across a network. Think of it like a set of rules that everyone follows so that computers can talk to each other, even if they're made by different companies. What it does: It divides the complex process of network communication into seven distinct layers & Each layer has a specific job, making it easier to understand and troubleshoot network problems. 7. APPLICATION LAYER: The application layer is the topmost layer in the OSI model. The layer establishes communication between the application on the network and the end user using it by defining the protocols for successful user interaction. It provides network services to applications, like web browsers and email clients. An excellent example of this layer is that of web browsers. Real-life example: The application you use to write and send an email, like your email client or web browser. Like the letter itself and the person that writes and reads the letter. Protocols include Hypertext Transfer Protocol (HTTP), Simple Mail Transfer Protocol (SMTP), and File Transfer Protocol (FTP). 6. PRESENTATION LAYER: This layer translates data into a format that applications can understand. It's responsible for things like encryption and compression. This layer is known to compress data received from layer 7 to reduce the overall size of the data transferred. Real-life example: Translating a letter written in a foreign language into your language, or encrypting sensitive information before sending it. Like the translator that reads the letter, or the special envelope that hides the letter contents.
5. SESSION LAYER: This layer manages the connections between applications. It's responsible for starting, maintaining, and ending communication sessions. This OSI layer is also responsible for data synchronization to maintain smooth data flow. This implies that in situations where large volumes of data are sent at once, layer 5 can break down the data into smaller chunks by adding checkpoints. Real-life example: The conversation between two people on a phone call, where the session is established, maintained, and ended. Like the actual phone call itself. 4. TRANSPORT LAYER: The transport layer allows safe message transfer between the sender and the receiver. It divides the data received from the layer above into smaller segments. It also reassembles the data at the receiver side to allow the session layer to read it. Layer 4 performs two critical functions: flow control & error control. Flow control ensures that the communicating device with a good network connection while Error control refers to the error-checking functionality. Real-life example: The assurance that your letter will arrive complete and in the correct order, even if it gets split into multiple shipments. Like the tracking number that allows you to know the letter is delivered correctly. protocols include transmission control protocol (TCP) and user datagram protocol (UDP). 3. NETWORK LAYER: Handles logical addressing ( IP addresses ) and routing of packets across different networks. It's about finding the best path for data to travel. The network layer enables the communication between multiple networks. It receives data segments from the layer above, further broken down into smaller packets at the sender side. On the receiver side, this layer reassembles the data together. Real-life example: The post office system that determines the best route for your letter to travel from your city to another city. Like the system that decides which truck, plane and train the letter will travel on. 2. DATA LINK LAYER: The data link layer transmits data between two nodes that are directly connected or are operating over the same network architecture. Typically, this layer takes data packets from layer 3 and breaks them down into frames before sending them to the destination. Layer 2 is divided into two sub-layers: media access control (MAC) and logical link control (LLC). - Detects damaged or lost frames and retransmits them. Performs framing where data received from layer 3 is further subdivided into smaller units called frames. Updates headers of created frames by adding the MAC address of the sending device and receiving device Real-life example: The post office sorting mail into bags for specific delivery routes within a city.
➢ SPREAD SPECTRUM : Spread Spectrum Communication, a technique used to enhance the reliability and security of wireless transmissions. Spread spectrum communication involves spreading a signal over a wide range of frequencies, which makes it difficult for unauthorized users to intercept or jam the signal. This technique employs different methods to spread the signal, including direct sequence spread spectrum (DSSS) and frequency hopping spread spectrum (FHSS). DSSS multiplies the original signal with a random sequence of bits, known as a spreading code, to spread the signal over a wide range of frequencies. On the other hand, FHSS hops the signal between different frequencies over time, using a predetermined hopping pattern. o Spread spectrum communication offers several advantages: over traditional communication techniques. It is resistant to interference and jamming, provides increased security, and improves signal quality. The signal spread over a wide range of frequencies makes it less susceptible to interference from other signals, and it is difficult to intercept or jam. Therefore, it is often used for military and government applications. o Spread spectrum communication has various applications: including military and government communications, wireless local area networks (WLANs), and satellite communications. It is utilized to secure communications and prevent unauthorized access in military and government applications. In WLANs, spread spectrum communication provides secure and reliable wireless access to the Internet. In satellite communications, it ensures that data is transmitted accurately over long distances. Spread spectrum communication is a wireless communication technique that provides secure and reliable transmissions by spreading the signal over a wide range of frequencies.
(UNIT- 2 )
2. DATA LINK LAYER & MEDIUM ACCESS SUB LAYER:
CONTEXT OF DATA LINK LAYER AND MEDIUM ACCESS SUBLAYER:
These techniques are crucial in these layers because they deal with the physical transmission of data over a medium (like a wired or wireless connection). This medium is prone to noise and interference, which can introduce errors into the transmitted bits.
- Data Link Layer: This layer is responsible for reliable data transfer between two directly connected nodes. Error detection and sometimes correction are essential to ensure that the data received by the next hop is accurate.
- Medium Access Sublayer (part of the Data Link Layer): When multiple devices share the same communication medium (like in a Wi-Fi network), error detection helps identify if a transmission was corrupted due to collisions or other interference. Why are these important? Without error detection and correction, data corruption could lead to:
- Incorrect file transfers
- Garbled messages
- Malfunctioning applications
- Security vulnerabilities By implementing these techniques, computer networks can achieve more reliable and accurate data communication. Think of it as ensuring your digital letters arrive at their destination intact and understandable. Imagine you're sending a handwritten letter (your data) to a friend. ERROR DETECTION & ERROR CORRECTION Imagine you're sending a handwritten letter (your data) to a friend. ➢ ERROR DETECTION : Knowing Something Went Wrong : The goal here is to figure out if the letter arrived exactly as you wrote it or if some words got smudged or changed during delivery.
- Importance for Error Detection/Correction: A larger Hamming Distance between valid codewords (the original messages with added error-checking bits) allows us to detect and correct more errors. If the valid codewords are very different from each other, a small number of errors is less likely to turn one valid codeword into another valid codeword. ➢ CYCLIC REDUNDANCY CHECK (CRC): A More Sophisticated Checksum : CRC is a more powerful error detection technique than simple checksums or parity bits. It involves treating the data as a large binary number and dividing it by a specific "generator polynomial" (another binary number). The remainder of this division is the CRC code, which is appended to the original data. The receiver performs the same division on the received data (including the CRC). If the remainder is zero, it's highly likely that no errors occurred.
- Real-life Example: Imagine you have a long message (a long number). You agree with your friend on a secret "divisor number." Before sending the message, you perform a special kind of division using this secret number and get a remainder. You attach this remainder to the end of your message. o When your friend receives the message and the remainder, they perform the same division using the same secret number on the entire received data (including the remainder). If they get a remainder of zero, it's a strong indication that the message arrived correctly. If they get a non-zero remainder, they know there was an error. FLOW CONTROL AND ERROR CONTROL PROTOCOLS ➢ WHAT IS FLOW CONTROL? Flow control is a mechanism used in data communication to prevent a fast sender from overwhelming a slow receiver. It ensures that the sender transmits data at a rate that the receiver can handle. 1 Without flow control, the receiver might become overloaded, leading to data loss. Ex- Imagine pouring water into a glass. If you pour too quickly (fast sender), the glass (slow receiver) will overflow, and water will be lost. Flow control is like adjusting the rate at which you pour to match the capacity of the glass. Why is Flow Control Necessary?
- Different Processing Speeds: Senders and receivers often have different processing capabilities and buffer sizes.
- Network Congestion: Intermediate network devices might also have limited capacity, and flow control can indirectly help manage congestion at the edges.
➢ WHAT IS ERROR CONTROL?
Error control refers to the methods used to ensure reliable data transmission over a potentially unreliable communication channel. Errors can occur during transmission due to noise, interference, or other factors, leading to corrupted or lost data. Error control protocols detect these errors and implement mechanisms to recover from them. Ex- Imagine sending a letter through the postal service. There's a chance the letter might get damaged or lost during transit. Error control mechanisms are like using sturdy envelopes, adding tracking numbers, or sending a copy to ensure the message reaches its destination correctly. Why is Error Control Necessary?
- Unreliable Media: Many communication media (like wireless networks) are inherently prone to errors.
- Data Integrity: Ensuring the received data is identical to the transmitted data is crucial for many applications. ➢ STOP AND WAIT ARQ (AUTOMATIC REPEAT REQUEST) : Stop and Wait ARQ is a simple flow and error control protocol. The sender transmits one frame ( a unit of data ) and then waits for an acknowledgment ( ACK ) from the receiver indicating that the frame was received correctly.
- Successful Transmission: If the receiver receives the frame without errors, it sends an ACK back to the sender. Upon receiving the ACK, the sender transmits the next frame.
- Error or Loss: If the frame is lost, corrupted, or the ACK is lost, the sender will eventually time out (a pre-set waiting period). After the timeout, the sender retransmits the same frame.
- Duplicate Reception: To handle the case where an ACK is delayed and the sender retransmits, the receiver needs to identify and discard duplicate frames. This is often done using sequence numbers (0 and 1 alternating for each frame). Real-Life Example: Imagine two people communicating using walkie-talkies in a noisy environment.
- Sender (Shree): "Message one, over." (Sends frame 1)
- Receiver (Vishal): (Receives "Message one" clearly) "Roger that, message one received, over." (Sends ACK for frame 1)
- Sender (Shree): (Receives the ACK) "Message two, over." (Sends frame 2)
- Receiver (Vishal): (Receives "Message two" with static, it's garbled) "..." (Doesn't send ACK)
Q. What happened if message is lost, Acknowledgement lost and timer expires for Go Back-N protocol? ANS- Imagine you're sending numbered postcards (data packets) to a friend: I. Message Lost: You send postcard #3, but your friend never receives it. Your friend sends acknowledgments ( ACKs ) for #1 and #2, but not #3. You have a timer running for each postcard. When the timer for #3 expires, you assume it's lost. Action: You re-send postcard #3, and also #4, #5, etc. (all postcards sent after #3), even if your friend might have received them. (Go Back-N means you go back N, which in this case is all the ones after the lost one.) Real life EX- You send a mail, that never arrives. After waiting a while, you resend the mail, and all mails sent after the lost one. II. ACK Lost: You send postcard #3, and your friend receives it. o Your friend sends an ACK for #3, but you never receive it. Your timer for #3 expires. Action: You re-send postcard #3, and all the subsequent ones. Your friend might get duplicates, but they'll know from the numbers to discard them. Real life EX- The confirmation message that your mail was received, gets lost. You resend the mail, and all mails sent after the lost confirmation. III. Timer Expires: This is the trigger for the retransmission in both cases above. The timer is set to a reasonable time, longer than the expected round-trip time ( the time it takes for a postcard to reach your friend and an ACK to return ). If the timer expires, it means either the postcard or the ACK was lost, or there was a significant delay. Action: You re-send the packet, and all subsequent packets. o Real life: The timer is like a deadline. if you don’t hear back within the deadline, you resend. ➢ PIGGYBACKING : Piggybacking is a technique used in data communication where acknowledgments ( ACKs ) for received data packets are sent along with outgoing data packets. Instead of sending a separate ACK packet, the acknowledgment information is " piggybacked " onto a data frame that is already being transmitted in the reverse direction. Real-life Example: Imagine two friends, ‘ R’ and ‘ V’ , talking using walkie-talkies.
- Without Piggybacking: o ‘R’ says, "Did you get the message about the meeting at 3 PM?" (Data) o ‘V’ waits for his turn to talk and says, "Yes, I got it." (Acknowledgment) o ‘R’ then says, "Are we bringing snacks?" (Data)
o ‘V’ waits for her turn and says, "Yes, I'm bringing chips." (Acknowledgment)
- With Piggybacking: o ‘R’ says, "Did you get the message about the meeting at 3 PM?" (Data) o ‘V’ replies, "Yes, I got it. Are we bringing snacks?" (Acknowledgment piggybacked on Data) o ‘R’ replies, "Yes, I'm bringing chips. Don't forget the drinks!" (Acknowledgment piggybacked on Data) Benefit: Piggybacking reduces the overhead of sending separate acknowledgment packets, making the communication more efficient by utilizing the existing data flow. ➢ RANDOM ACCESS PROTOCOLS : Random access protocols (also known as contention-based protocols) are used in shared communication mediums where multiple nodes can try to transmit data at any time. This can lead to collisions if multiple nodes transmit simultaneously. These protocols define how nodes should behave to minimize collisions and recover from them. ➢ MULTIPLE ACCESS PROTOCOLS ( Pure ALOHA, Slotted ALOHA,CSMA/CD,CDMA/CA) Multiple access protocols are a broader category that includes random access protocols. They are sets of rules and procedures that allow multiple devices to share a common communication medium (like a network cable or radio frequency) effectively. 1. Pure ALOHA: Pure ALOHA is a simple random access protocol where any node can transmit data whenever it has data to send, without any coordination. If two or more nodes transmit at the same time, their packets collide and become garbled. The transmitting nodes then wait for a random amount of time before retransmitting. Real-life Example: Imagine a classroom where students can speak whenever they have a question.
- If only one student speaks at a time, everyone can hear clearly.
- If two or more students speak simultaneously, their voices overlap, and no one can understand what they are saying (collision).
- Each student who caused a collision will wait for a random amount of time before trying to ask their question again. Working Principle of Pure ALOHA: Imagine a group of friends ( computers ) in a room ( network ) who want to share stories ( send data ). Whenever someone has a story, they just start talking ( transmit data ) immediately. If two or more people start talking at the same time, their stories get mixed up (collision), and no one understands anything. Each person waits a random amount of time and then tries telling their story again. This process repeats until everyone successfully shares their story.
CSMA/CD is a more sophisticated protocol used in wired networks like Ethernet. It works as follows: ❖ Carrier Sense (CS): Before transmitting, a node listens to the medium to check if it is currently busy (i.e., if another node is transmitting). ❖ Multiple Access (MA): Multiple nodes can access the medium, but they need to follow the carrier sense rule. ❖ Collision Detection (CD): If a node starts transmitting and detects a collision (by sensing an abnormal increase in signal strength), it immediately stops transmitting to reduce the duration of the collision. It then sends a short "jamming signal" to inform all other nodes about the collision. ❖ Backoff: After a collision, each involved node waits for a random amount of time (determined by a backoff algorithm) before attempting to retransmit. Real-life Example: Imagine a group of people in a hallway trying to pass through a narrow door. ❖ Carrier Sense: Before trying to go through, each person checks if someone else is already going through the door. ❖ Multiple Access: Multiple people can try to use the door, but they should check first. ❖ Collision Detection: If two people try to go through at the exact same time and get stuck, they immediately stop pushing. ❖ Jamming Signal (Analogy): One of them might say loudly, "Oops, collision!" to alert others. ❖ Backoff: Each person who collided will step back and wait for a random short period before trying to go through the door again. The randomness helps prevent them from colliding again simultaneously.
4. CARRIER SENSE MULTIPLE ACCESS WITH COLLISION AVOIDANCE
(CSMA/CA) :
CSMA/CA is primarily used in wireless networks (like Wi-Fi) where it's harder for a transmitting node to listen for collisions while it's transmitting (due to the "hidden terminal" problem and signal fading). Instead of detecting collisions, CSMA/CA tries to avoid them before they happen. Common techniques include: ❖ Inter-Frame Space (IFS): Nodes wait for a short, random period called the IFS before transmitting. Different types of IFS prioritize different traffic. ❖ Contention Window: Nodes that want to transmit enter a contention period. They choose a random backoff time within a contention window. The node with the smallest backoff time transmits first. ❖ Request to Send/Clear to Send (RTS/CTS): Some versions of CSMA/CA use RTS and CTS frames. A node wanting to transmit sends an RTS frame to the access point (or receiving node). If the medium is free, the access point responds with a CTS frame. All other nodes hearing either the RTS or CTS refrain from transmitting for the duration of the intended transmission. This helps to mitigate the hidden terminal problem. Real-life Example: Imagine a group of people trying to speak at a conference call where only one person should speak at a time. ❖ Inter-Frame Space (Analogy): Before speaking, each person waits for a brief, slightly random pause after the previous speaker finishes. ❖ Contention Window (Analogy): If multiple people want to speak, they mentally choose a random number. The person with the smallest number "goes first" after the pause. ❖ Request to Speak/Permission to Speak (Analogy): One person might say, "Can I say something?" (RTS). The moderator might reply, "Go ahead, [Name]" (CTS). Everyone else who heard this knows to be quiet until that person is finished. Necessity in Wireless: CSMA/CA is crucial in wireless environments where collision detection is difficult. By focusing on avoidance mechanisms, it improves the reliability and efficiency of wireless communication.
