BCS-041 Solved Free Assignment 2024-25 Sem 4
Q1. (a) Differentiate between parallel and serial communication. Give an example of each.
    (b) Compare POP and IMAP.
Ans:-Â (a) **Difference Between Parallel and Serial Communication:**
| **Aspect**Â Â Â Â Â Â Â | **Parallel Communication**Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â | **Serial Communication**Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â |
|------------------------|-----------------------------------------------------------------|----------------------------------------------------------|
| **Data Transmission**Â Â | Multiple bits (typically 8, 16, or 32) are transmitted simultaneously over multiple wires. | Data is transmitted one bit at a time, sequentially, over a single wire or channel. |
| **Speed**Â Â Â Â Â Â Â | Typically faster for short distances due to simultaneous transmission of bits. | Generally slower, but can be optimized for high-speed transmission over long distances. |
| **Wiring**       | Requires multiple wires (one for each bit transmitted simultaneously). | Requires fewer wires (typically one or two for data).   |
| **Distance**Â Â Â Â Â Â | Inefficient and unreliable over long distances due to signal degradation and crosstalk. | More reliable for long-distance communication as it reduces signal interference. |
| **Complexity**     | More complex wiring and design, leading to higher costs.     | Simpler wiring and cheaper to implement.         |
| **Example**Â Â Â Â Â Â | Printers connecting to computers using old Centronics ports (parallel ports). | USB (Universal Serial Bus), Ethernet, RS-232 communication. |
 **Example of Parallel Communication:**
- **Parallel Port (Printer Connection):** In older computers, printers connected through a parallel port where data was transferred 8 bits at a time.
 **Example of Serial Communication:**
- **USB (Universal Serial Bus):** A common example of serial communication used for connecting devices like keyboards, mice, and external drives.
---
(b) **Comparison Between POP and IMAP:**
| **Aspect**Â Â Â Â Â Â Â Â Â Â Â | **POP (Post Office Protocol)**Â Â Â Â Â Â Â Â Â Â | **IMAP (Internet Message Access Protocol)**Â Â Â Â Â Â Â Â |
|----------------------------------|---------------------------------------------------|-----------------------------------------------------------|
| **Server Interaction**Â Â Â Â Â Â | Downloads emails from the server to the local device and typically deletes them from the server. | Emails remain on the server; the client downloads copies and synchronizes changes. |
| **Device Synchronization**Â Â Â Â | Primarily designed for use on a single device; emails are downloaded and stored locally. | Designed for multiple devices; changes made on one device (read, delete, move) are synchronized across all devices. |
| **Offline Access**Â Â Â Â Â Â Â Â | Once emails are downloaded, they are available offline. | Access to email is possible offline, but depends on settings for syncing and local storage. |
| **Storage**Â Â Â Â Â Â Â Â Â Â Â | Emails are typically stored locally on the client machine, which may free up server space. | Emails are stored on the server, meaning server storage limits can be reached. |
| **Folder Organization**Â Â Â Â Â | Limited; does not synchronize folder structures across devices. | Supports folder structures and synchronization across devices. |
| **Usage Scenario**Â Â Â Â Â Â Â Â | Suitable for users who access email from one device and prefer to store emails locally. | Ideal for users who access email from multiple devices and want to keep emails on the server. |
| **Example**Â Â Â Â Â Â Â Â Â Â Â | Older email clients like Outlook using POP3 can download emails for offline access. | Modern email services like Gmail or Yahoo! use IMAP to keep emails synced across smartphones, tablets, and computers. |
 Summary:
- **POP** is more suited for users who primarily access email from one device and want to store messages locally, while **IMAP** is ideal for users who need to access their emails from multiple devices with full synchronization across them.
Q2. (a) What is Ad hoc Wireless Communication System? Explain.Â
    (b) What is better for computer communication — analog or digital? Justify your answer.
Ans:-Â Â (a) **Ad hoc Wireless Communication System:**
An **Ad hoc Wireless Communication System** is a decentralized wireless network that operates without relying on a fixed infrastructure such as routers, access points, or centralized administration. In this type of network, each device (or node) communicates directly with other devices in the network, dynamically forming a communication link as needed. This type of system is often used in situations where there is no existing network infrastructure, or it is impractical to set one up.
 **Key Characteristics:**
1. **Decentralization:** No central controller is required. Each device acts as a router and forwards data to other devices.
2. **Dynamic Topology:** Devices can join or leave the network freely, and the network adapts to these changes.
3. **Self-Healing:** If one node becomes unavailable (due to device failure or mobility), the network reconfigures itself by finding alternate communication paths.
4. **Mobility:** Ad hoc networks are often used in mobile scenarios (such as vehicular networks or battlefield communications) where devices are constantly moving and new connections must be established on the fly.
 **Applications:**
- **Military Communication:** Used in military operations where network infrastructure might be unavailable.
- **Disaster Recovery:** During natural disasters, when existing networks are destroyed, ad hoc systems can enable emergency communications.
- **Vehicular Networks:** Vehicles can form ad hoc networks to share information like traffic updates or accident warnings.
---
 (b) **What is Better for Computer Communication: Analog or Digital?**
**Digital communication** is better for computer communication, and here’s why:
 1. **Compatibility:**
  - **Digital Communication:** Computers are inherently digital devices, which process and store data as binary values (0s and 1s). Digital communication is naturally suited for this, as it transmits data in a format that computers can process directly.
  - **Analog Communication:** Analog communication uses continuous signals to transmit data, which would require conversion (A/D and D/A conversion) before computers can process the information, adding extra steps.
 2. **Accuracy and Noise Resistance:**
  - **Digital Communication:** Digital signals are much more resilient to noise and signal degradation. Even if a signal weakens or picks up interference, as long as the binary data can still be interpreted (e.g., 0 or 1), the information can be accurately recovered.
  - **Analog Communication:** Analog signals degrade over distance and can be easily affected by interference. Small distortions in the signal can cause significant errors in the transmitted information.
 3. **Security:**
  - **Digital Communication:** Encryption and other security mechanisms are much easier to implement in digital systems, which makes digital communication more secure.
  - **Analog Communication:** Analog signals are harder to encrypt and secure, making them more vulnerable to interception or unauthorized access.
 4. **Efficiency and Speed:**
  - **Digital Communication:** Modern digital communication technologies (such as fiber optics, Ethernet, Wi-Fi) allow for high-speed, efficient data transfer, which is essential for handling the large volumes of data transmitted in today’s computer networks.
  - **Analog Communication:** Analog systems, by contrast, are slower and less efficient, particularly over long distances.
5. **Compression and Error Detection:**
  - **Digital Communication:** It allows for data compression techniques, which reduce the amount of data transmitted without loss of quality. Additionally, digital systems can detect and correct errors that occur during transmission.
  - **Analog Communication:** Analog communication has no built-in error detection or correction mechanisms, leading to potential data loss or degradation.
Q3. What is Windowing? How is flow control and reliability achieved through windowing at transport layer?
Ans:-Â Â **What is Windowing?**
**Windowing** is a technique used in the transport layer of network communication to manage the flow of data between two devices. It controls how much data can be sent before requiring an acknowledgment from the receiver. The "window" represents the number of data segments (or packets) that the sender can transmit without waiting for an acknowledgment. Windowing is a crucial part of protocols like **TCP (Transmission Control Protocol)** to ensure efficient and reliable communication, preventing congestion and data loss in the network.
 **Types of Windowing:**
1. **Fixed Windowing:** The sender can send a fixed number of packets (the window size) before needing an acknowledgment. This is simpler but less efficient as it may not fully utilize the available bandwidth in varying network conditions.
2. **Sliding Windowing:** The window size dynamically adjusts based on network conditions and the receiver's ability to process data. It allows more efficient use of the available bandwidth.
**Flow Control through Windowing:**
**Flow control** ensures that the sender does not overwhelm the receiver with more data than it can handle. In windowing, flow control is achieved using a dynamic window size.
1. **Sliding Window Protocol:** The window "slides" as acknowledgments are received from the receiver. This means that once the sender has sent the maximum number of packets (equal to the window size), it must wait for an acknowledgment before sending more data. The receiver can control the flow by specifying the size of the window (i.e., how much data it can process at a time) in each acknowledgment. If the receiver’s buffer is full or processing speed is slow, it can reduce the window size to slow down the sender.
2. **Window Size Adjustments:** The receiver informs the sender of the available buffer size through the acknowledgment packet. This dynamically adjusts the sender’s window size. If the receiver’s buffer is nearly full, it sends a smaller window size value, signaling the sender to slow down.
3. **Congestion Avoidance:** If the network is congested, the sender may reduce the window size to avoid packet loss. As the network stabilizes, the window size can increase to make better use of available bandwidth.
 **Reliability through Windowing:**
Windowing ensures reliable transmission of data by using acknowledgments (ACKs) and retransmissions.
1. **Acknowledgments (ACKs):** For every segment of data that the receiver correctly receives, it sends an acknowledgment back to the sender. If the sender doesn’t receive an acknowledgment within a certain time (timeout), it assumes that the packet was lost or corrupted and retransmits it.
2. **Selective Acknowledgment (SACK):** Some advanced versions of windowing allow selective acknowledgment, where the receiver can acknowledge out-of-order packets. This helps to avoid retransmitting already received data.
3. **Retransmissions:** If a packet is lost during transmission, the sender can retransmit the specific missing packet (as identified by the lack of acknowledgment). This helps ensure that all data segments are correctly received by the receiver.
4. **Sliding Window Error Detection:** The sliding window mechanism helps in error detection by using sequence numbers. Each packet is numbered, so the receiver can detect missing or out-of-order packets. If packets are received out of order, the receiver can signal which packet was missing, prompting the sender to resend only that packet.
 **Summary:**
- **Flow Control:** Achieved through the dynamic adjustment of the window size, ensuring that the sender does not overwhelm the receiver.
- **Reliability:** Ensured through acknowledgment of received data, retransmissions of lost or corrupt data, and the use of sequence numbers to track data packets.
Windowing in the transport layer (particularly in **TCP**) provides an efficient way to manage the flow of data, avoid congestion, and ensure reliable delivery of data across networks.
Q4. (a) Compare between CSMA/CD and Ethernet protocol. How does CSMA/CD resolve the problem of line connection? Explain.Â
    (b) Differentiate between circuit switching and virtual circuit. Also explain the effect of router failure in virtual circuits
Ans:-Â Â Â (a) **Comparison Between CSMA/CD and Ethernet Protocol:**
 **CSMA/CD (Carrier Sense Multiple Access with Collision Detection):**
1. **Purpose:** CSMA/CD is a media access control (MAC) protocol used to control how devices share access to a shared communication medium (such as a bus or cable) in Ethernet networks.
2. **Collision Detection:** In CSMA/CD, devices first "listen" to the network to detect if it is free (carrier sense). If the network is idle, the device starts transmitting data. If two devices transmit simultaneously, a collision occurs. The devices detect this collision, stop transmission, and then wait for a random period before trying again (collision detection and backoff mechanism).
3. **Used In:** This protocol was mainly used in **legacy Ethernet** networks, specifically **half-duplex** Ethernet networks where only one device can send data at a time.
4. **Medium:** Typically used over shared communication channels like coaxial cables or hubs.
 **Ethernet Protocol:**
1. **Purpose:** Ethernet is a family of networking technologies that define how data is transmitted over a network. Ethernet encompasses multiple standards (such as Fast Ethernet, Gigabit Ethernet, etc.) and includes both CSMA/CD and modern mechanisms.
2. **Full-Duplex Ethernet:** Modern Ethernet networks use **full-duplex communication**, where devices can send and receive data simultaneously. Full-duplex Ethernet networks eliminate the need for collision detection mechanisms.
3. **Switching:** Modern Ethernet uses **switches**, not hubs, which provide dedicated communication paths between devices, eliminating collisions and the need for CSMA/CD.
4. **Medium:** Can work over various media types (coaxial cables, twisted pair, fiber optics) and at different speeds (10 Mbps, 100 Mbps, 1 Gbps, and beyond).
**Key Differences:**
- **CSMA/CD** is a MAC protocol that controls how data is transmitted on shared Ethernet networks, while **Ethernet** is a broader networking standard that includes both legacy and modern implementations (with or without CSMA/CD).
- **CSMA/CD** is essential for **half-duplex** Ethernet but is not used in modern **full-duplex** Ethernet, where each device has a dedicated communication channel.
 **How CSMA/CD Resolves Line Connection Problems:**
1. **Carrier Sense:** Devices "sense" the network for existing traffic before transmitting. If the medium is busy, they wait until it is free.
2. **Multiple Access:** Multiple devices can attempt to access the network at the same time.
3. **Collision Detection:** If two devices transmit at the same time and a collision occurs, the devices detect it using a change in voltage or signal strength.
4. **Backoff Algorithm:** After a collision, devices wait for a random time (using the **binary exponential backoff** algorithm) before attempting to retransmit, thus reducing the chance of repeated collisions.
 Â
This process resolves the problem of multiple devices sharing the same network medium by minimizing collisions and ensuring fair access to the network.
---
 (b) **Difference Between Circuit Switching and Virtual Circuit:**
**Circuit Switching:**
1. **Dedicated Path:** In circuit switching, a dedicated communication path is established between two endpoints for the duration of the communication session. This path remains reserved, even if no data is being transmitted.
2. **Continuous Data Flow:** Once the circuit is established, data flows continuously along the path without interruptions.
3. **Example:** Traditional telephone networks where a fixed path is set up for the duration of the call.
4. **Resource Allocation:** Resources (such as bandwidth) are reserved for the entire duration of the connection, even if the connection is idle.
5. **Delay:** There is a setup delay at the beginning while establishing the path, but once established, data transmission is fast and continuous.
 Â
 **Virtual Circuit:**
1. **Logical Path:** In virtual circuit switching, a logical path is established across a network for data to travel between two devices, but the actual physical route may change based on the network's conditions.
2. **Packet Switching:** Data is divided into packets, each containing a virtual circuit identifier. These packets travel through different physical routes but follow the same logical path.
3. **Example:** **Frame Relay**, **ATM**, and **MPLS** networks, where data is transmitted in packets but follows a pre-determined logical path.
4. **Resource Allocation:** Unlike circuit switching, resources are allocated dynamically. Multiple virtual circuits can share the same physical infrastructure.
5. **Delay:** Data may experience variable delay due to packetization, queuing, and reordering, but there is flexibility in route selection.
---
 **Effect of Router Failure in Virtual Circuits:**
In virtual circuits, routers play a critical role in forwarding packets along the established logical path. If a router in the path fails, several things may happen:
1. **Disruption of Communication:** If the failed router is part of an active virtual circuit, the communication may be temporarily disrupted until the issue is resolved or an alternate route is established.
2. **Rerouting Mechanism:** Many virtual circuit systems, such as **MPLS**, have built-in mechanisms to reroute packets through an alternate path in case of a router failure. The rerouting may introduce temporary delays, but communication can resume without restarting the entire session.
3. **Need for Reestablishment:** In some systems, a new virtual circuit may need to be established, requiring the session to restart, which can lead to longer disruptions.
4. **Network Overhead:** Rerouting in case of failure can increase network overhead as routers need to recompute paths, update routing tables, and reallocate resources.
 Summary:
- **Circuit switching** is a more traditional method that establishes a dedicated path for communication, while **virtual circuits** use logical paths within a packet-switched network, allowing more efficient use of resources.
- In the event of a router failure in a virtual circuit, communication may be rerouted dynamically, but it could result in delays or disruption until an alternate path is established.
Q5. Given data frame is 1101011011 and generator polynomial G(x) = x4+ x + 1. Derive the transmitted frame using CRC method. Write all the steps involved in the processÂ
Ans:-Â Â Â **Cyclic Redundancy Check (CRC) Method:**
To derive the transmitted frame using CRC (Cyclic Redundancy Check), follow these steps:
 **Given:**
- **Data Frame (D):** 1101011011
- **Generator Polynomial (G(x)):** \( G(x) = x^4 + x + 1 \)
The generator polynomial \( G(x) = x^4 + x + 1 \) can be represented as a binary value:Â Â
\( G = 10011 \)
The goal of the CRC method is to append a **remainder** (CRC bits) to the original data frame to form a **transmitted frame**. The receiver can then use the same generator polynomial to check if the received frame contains errors by checking the remainder.
**Steps:**
 **Step 1: Append Zeros to the Data Frame**
First, append **(n-1)** zeros to the data frame, where **n** is the degree of the generator polynomial. The degree of \( G(x) = x^4 + x + 1 \) is 4, so we append 4 zeros to the original data frame.
- **Data Frame (D):** 1101011011
- **Data Frame with 4 zeros appended (D'):** 1101011011 0000
 **Step 2: Perform Binary Division (Modulo-2 Division)**
Now, we perform a **modulo-2 division** (bitwise XOR operation) between the data frame with zeros appended and the generator polynomial.
- **Dividend (D'):** 1101011011 0000 (15 bits)
- **Divisor (G):** 10011 (5 bits)
We perform bitwise XOR division as follows:
 **Division Process (Binary Long Division):**
1. **Step 1:**
  - Take the first 5 bits of the dividend: **11010**
  - Perform XOR with the divisor (10011):
  ```
  11010
  10011
  -----
  01001 (Remainder after XOR)
  ```
2. **Step 2:**
  - Bring down the next bit from the dividend: **1**
  - New partial dividend: **10011**
  - Perform XOR with the divisor:
  ```
  10011
  10011
  -----
  00000 (Remainder after XOR)
  ```
3. **Step 3:**
  - Bring down the next bit from the dividend: **1**
  - New partial dividend: **00001**
  - Since the leading bit is 0, XOR with divisor won’t work, so continue to the next step.
4. **Step 4:**
  - Bring down the next bit from the dividend: **0110**
  - New partial dividend: **00110**
  - Perform XOR with the divisor:
  ```
  00110
  10011
  -----
  00101
  ```
 **Final Remainder:**
After finishing the division, the final remainder is the **CRC checksum**.
**Remainder:** \( 1001 \)
 **Step 3: Append the CRC to the Original Data Frame**
The remainder (1001) is appended to the original data frame to form the transmitted frame.
- **Original Data Frame (D):** 1101011011
- **Remainder (CRC):** 1001
 **Transmitted Frame:**
- **Transmitted Frame:** \( 1101011011 1001 \)
 **Summary of the Process:**
1. Append four zeros to the original data frame.
2. Perform modulo-2 division (XOR) of the extended data frame by the generator polynomial.
3. The remainder from the division is the CRC, which is appended to the original data frame.
4. The final transmitted frame is \( 1101011011 1001 \).
Q6. Differentiate between public key cryptography and private-key cryptography. Assume two prime numbers p and q are 13 and 17 respectively. Calculate private key and public key using RSA algorithm.
Ans:-Â Â Â **Difference Between Public Key Cryptography and Private-Key Cryptography:**
| **Aspect**Â Â Â Â Â Â Â Â | **Public Key Cryptography (Asymmetric Encryption)**Â Â | **Private-Key Cryptography (Symmetric Encryption)**Â Â Â |
|--------------------------|-------------------------------------------------------|---------------------------------------------------------|
| **Keys Used**      | Uses two different keys: a public key for encryption and a private key for decryption. | Uses a single key for both encryption and decryption.  |
| **Key Distribution**Â Â Â | Public key is openly shared, while the private key is kept secret. | Both parties must have the same secret key, which needs to be securely shared beforehand. |
| **Speed**        | Generally slower due to complex mathematical operations. | Faster because it uses simpler algorithms.        |
| **Security**Â Â Â Â Â Â Â | More secure for key exchange since only the private key is needed to decrypt. | Less secure for key exchange since the key must be shared and can be intercepted. |
| **Usage**        | Used for secure key exchange, digital signatures, and when two parties don’t know each other beforehand. | Used for encrypting large amounts of data in situations where both parties share a key. |
| **Examples**Â Â Â Â Â Â Â | RSA, ECC, DSAÂ Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â | AES, DES, 3DESÂ Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â |
| **Scalability**Â Â Â Â Â | Scalable for systems like email encryption, SSL/TLS, etc., where key distribution is a concern. | Less scalable, as it requires securely sharing keys with every communicating party. |
---
 **RSA Algorithm (Public Key Cryptography):**
**Given:**
- Two prime numbers: \( p = 13 \) and \( q = 17 \)
 **Steps to Calculate Public and Private Key Using RSA:**
1. **Step 1: Calculate \( n \) (Modulus)**Â Â
  The modulus \( n \) is the product of the two prime numbers \( p \) and \( q \).
  \[
  n = p \times q = 13 \times 17 = 221
  \]
2. **Step 2: Calculate \( \phi(n) \) (Euler’s Totient Function)** Â
  \( \phi(n) \) is calculated as:
  \[
  \phi(n) = (p - 1) \times (q - 1) = (13 - 1) \times (17 - 1) = 12 \times 16 = 192
  \]
3. **Step 3: Choose Public Key Exponent \( e \)**Â Â
  \( e \) must be a number that is:
  - Greater than 1
  - Less than \( \phi(n) \)
  - Coprime to \( \phi(n) \) (i.e., gcd(\( e \), \( \phi(n) \)) = 1)
  Let's choose \( e = 5 \), which satisfies these conditions.
4. **Step 4: Calculate Private Key Exponent \( d \)**Â Â
  The private key \( d \) is the modular inverse of \( e \) modulo \( \phi(n) \). In other words, \( d \) is the number such that:
  \[
  e \times d \equiv 1 \ (\text{mod} \ \phi(n))
  \]
  We need to solve for \( d \) such that:
  \[
  5 \times d \equiv 1 \ (\text{mod} \ 192)
  \]
  Using the Extended Euclidean Algorithm to find \( d \):
  \[
  5 \times 77 = 385
  \]
  \( 385 \mod 192 = 1 \), so \( d = 77 \).
5. **Step 5: Public and Private Key**
  - **Public Key:** \( (e, n) = (5, 221) \)
  - **Private Key:** \( (d, n) = (77, 221) \)
---
**Summary of RSA Key Generation:**
- **Public Key (e, n):** \( (5, 221) \)
- **Private Key (d, n):** \( (77, 221) \)
The public key is used to encrypt messages, and the private key is used to decrypt them.
Q7. (a) Differentiate between pure ALOHA and slotted ALOHA. Give formulas for their throughput.     Â
    (b) Explain the importance of Sliding Window Protocol. Also, list the types of sliding window     techniques.Â
Ans:-Â Â Â **(a) Difference Between Pure ALOHA and Slotted ALOHA:**
**ALOHA** is a simple communication protocol used in shared network channels. There are two main types: **Pure ALOHA** and **Slotted ALOHA**.
| **Aspect**Â Â Â Â Â Â Â Â | **Pure ALOHA**Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â | **Slotted ALOHA**Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â |
|---------------------------|--------------------------------------------------|------------------------------------------------|
| **Time Synchronization**Â Â | No time synchronization. Devices can transmit data at any time. | Time is divided into discrete time slots, and devices can only transmit data at the beginning of a time slot. |
| **Vulnerability Period**Â Â | Vulnerable to collisions over a period of **2 times the frame time** (T). | Vulnerable to collisions only during **1 time slot** (T), reducing collision probability. |
| **Efficiency**Â Â Â Â Â Â Â | Lower efficiency because collisions can occur anytime during the frame transmission. | Higher efficiency as collisions are restricted to time slots, reducing the chance of overlap. |
| **Throughput Formula**Â Â Â | Throughput (\(S\)) = \( G \times e^{-2G} \), where \(G\) is the average number of transmission attempts per time unit. | Throughput (\(S\)) = \( G \times e^{-G} \), where \(G\) is the average number of transmission attempts per time unit. |
| **Maximum Throughput**Â Â Â | Maximum throughput is **18.4%** or **0.184 frames/time unit**. | Maximum throughput is **36.8%** or **0.368 frames/time unit**. |
 **Formulas for Throughput:**
- **Pure ALOHA:**
 \[
 S = G \times e^{-2G}
 \]
 Where \( S \) is the throughput, and \( G \) is the average number of packet transmissions per time unit.
- **Slotted ALOHA:**
 \[
 S = G \times e^{-G}
 \]
 Where \( S \) is the throughput, and \( G \) is the average number of packet transmissions per time unit.
**Key Difference:**
- **Pure ALOHA** allows transmissions at any time, while **Slotted ALOHA** requires devices to transmit only at the beginning of predefined time slots. This reduces the chances of collisions, making **Slotted ALOHA** more efficient.
---
 **(b) Importance of Sliding Window Protocol:**
The **Sliding Window Protocol** is crucial in the **Transport Layer** of networking (particularly in **TCP**) to ensure reliable, orderly, and efficient transmission of data between two devices. It manages how much data can be sent before needing an acknowledgment and provides mechanisms for error recovery and flow control.
 **Key Features and Importance:**
1. **Flow Control:** It prevents the sender from overwhelming the receiver with too much data by limiting the amount of unacknowledged data that can be sent. This ensures that the receiver has enough buffer space to process incoming data.
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2. **Reliability:** It ensures that data packets are delivered in order and without loss. If packets are lost or corrupted, the sliding window protocol can detect this through missing acknowledgments and trigger retransmissions.
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3. **Efficiency:** By allowing multiple packets to be sent before requiring acknowledgment (instead of waiting for each packet’s acknowledgment), the protocol improves the overall transmission efficiency, especially in high-latency networks.
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4. **Error Recovery:** The sliding window mechanism uses sequence numbers to track the packets, so in case of errors, it can retransmit only the lost or corrupted packets rather than all the packets.
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5. **Congestion Control:** It helps in controlling congestion in the network by adjusting the window size based on network conditions.
**Types of Sliding Window Techniques:**
1. **Stop-and-Wait ARQ (Automatic Repeat Request):**
  - The sender transmits one frame and waits for its acknowledgment before sending the next frame.
  - Simple but inefficient for long-distance or high-latency networks as it waits for each acknowledgment.
2. **Go-Back-N ARQ:**
  - The sender can transmit multiple frames before receiving an acknowledgment, but if a frame is found missing or corrupted, all subsequent frames are retransmitted starting from the missing frame.
  - More efficient than Stop-and-Wait, but still requires retransmitting several frames.
3. **Selective Repeat ARQ:**
  - The sender transmits multiple frames, and only the lost or corrupted frames are retransmitted. The receiver can accept out-of-order frames and buffer them until the missing frames are received.
  - Most efficient sliding window technique as it minimizes retransmissions, but requires more complex buffering mechanisms.
---
**Summary:**
- **Sliding Window Protocol** is essential for controlling data flow, ensuring reliability, and maximizing efficiency in data transmission.
- **Types of Sliding Window Techniques:**
 - **Stop-and-Wait ARQ:** Simplest but least efficient.
 - **Go-Back-N ARQ:** Allows for multiple frames but requires retransmitting a sequence of frames if errors occur.
 - **Selective Repeat ARQ:** Only retransmits erroneous frames, providing the highest efficiency.
Q8. (a) Write the step-by-step working of Link State Routing. Also, compare it with Distance Vector Routing.Â
    (b) Explain leaky bucket algorithm for congestion control. Also list its advantages and disadvantages.
Ans:-Â Â **(a) Step-by-Step Working of Link State Routing:**
**Link State Routing** is a dynamic routing algorithm where each router has complete knowledge of the entire network's topology. The key algorithm used in Link State Routing is **Dijkstra's Algorithm** for finding the shortest path to every other node in the network.
 **Step-by-Step Working:**
1. **Step 1: Discover Neighboring Routers**
  - Each router sends **HELLO messages** to its directly connected neighbors to discover their presence and establish a connection.
2. **Step 2: Measure the Cost to Each Neighbor**
  - Each router measures the **cost** (in terms of distance, delay, bandwidth, etc.) to each of its neighbors.
3. **Step 3: Create a Link State Packet (LSP)**
  - The router creates a **Link State Packet (LSP)** containing the following information:
   - The router’s **ID**.
   - A list of all the neighbors.
   - The **cost** to reach each neighbor.
4. **Step 4: Flood the LSP to All Routers**
  - The router sends the LSP to all other routers in the network through **flooding**. Each router sends the LSP to its neighbors, ensuring that all routers eventually receive it. Flooding is controlled to avoid sending duplicate information.
5. **Step 5: Build a Topology Map**
  - Once every router has received the LSPs from all other routers, they construct a **complete topology map** of the network, which includes all the routers and the links between them.
6. **Step 6: Run Dijkstra’s Algorithm**
  - Using the complete topology map, each router runs **Dijkstra’s Shortest Path Algorithm** to calculate the shortest path to every other router in the network. This allows the router to build its **routing table**.
7. **Step 7: Update Routing Tables**
  - After calculating the shortest paths, each router updates its routing table with the best routes to reach all destinations.
 **Comparison: Link State Routing vs Distance Vector Routing**
| **Aspect**Â Â Â Â Â Â Â Â Â | **Link State Routing**Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â | **Distance Vector Routing**Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â |
|-----------------------------|--------------------------------------------------------|----------------------------------------------------------|
| **Routing Knowledge**Â Â Â Â | Each router has a complete view of the network topology. | Routers only know the distance to their neighbors. They do not have global knowledge. |
| **Algorithm Used**      | Uses Dijkstra’s algorithm to compute the shortest path. | Uses Bellman-Ford algorithm to find the best path based on distance vectors. |
| **Convergence Speed**    | Faster convergence due to global network knowledge.   | Slower convergence because updates are sent periodically and propagated hop-by-hop. |
| **Updates**Â Â Â Â Â Â Â Â Â | Routers send updates (LSPs) only when there is a change in the network. | Periodic updates of the entire routing table are sent, even if no changes occur. |
| **Scalability**Â Â Â Â Â Â Â | More scalable for large networks since updates are event-driven. | Less scalable because of periodic updates and limited global knowledge. |
| **Complexity**Â Â Â Â Â Â Â Â | More computationally complex due to the shortest path calculation. | Simpler to implement but may suffer from **count-to-infinity** problems. |
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**(b) Leaky Bucket Algorithm for Congestion Control:**
The **Leaky Bucket Algorithm** is used for **congestion control** and **traffic shaping** in networks to ensure smooth and consistent data flow. It enforces a constant output rate (in terms of packets or data rate), even if bursts of traffic are sent.
**Step-by-Step Explanation of Leaky Bucket Algorithm:**
1. **Step 1: The Bucket Concept**
  - Imagine data packets arriving into a bucket (representing a buffer or queue). The bucket has a **fixed capacity**, and data can be added to it at any rate.
2. **Step 2: Constant Draining**
  - The bucket leaks data at a **constant rate** regardless of how fast data is being added to the bucket. The leaking process represents the transmission of data packets at a steady, controlled rate.
3. **Step 3: Overflow Handling**
  - If data arrives too quickly and fills up the bucket beyond its capacity, the extra packets are **discarded** (this mimics network congestion, where packets are dropped when there is no buffer space).
4. **Step 4: Output Regulation**
  - The constant outflow of data from the bucket ensures a **smooth, controlled rate** of packet transmission, preventing sudden bursts from overwhelming the network.
 **Advantages of Leaky Bucket Algorithm:**
1. **Prevents Traffic Bursts:** It smoothens bursty traffic and ensures that data flows at a constant rate.
2. **Simple to Implement:** The algorithm is straightforward and easy to implement in hardware or software.
3. **Fairness:** It enforces a steady, controlled rate of traffic, preventing any one user from monopolizing network resources.
4. **Congestion Control:** It effectively controls network congestion by limiting the rate at which traffic is sent into the network.
 **Disadvantages of Leaky Bucket Algorithm:**
1. **Packet Loss During Bursts:** If a large burst of data arrives, and the bucket is full, excess packets are dropped, which may result in **packet loss**.
2. **Inefficiency for Low Traffic:** If data is arriving at a lower rate than the bucket’s capacity, the algorithm may unnecessarily restrict the flow, under-utilizing network bandwidth.
3. **Not Suitable for Variable Output Rates:** The leaky bucket algorithm forces a constant output rate, which may not be ideal for networks with varying bandwidth needs.
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 **Summary:**
- **Link State Routing** involves routers flooding link state information throughout the network and using Dijkstra’s algorithm to compute shortest paths. It differs from **Distance Vector Routing** in terms of global knowledge, convergence speed, and complexity.
- The **Leaky Bucket Algorithm** controls congestion by regulating the rate at which data is transmitted. It provides smooth, consistent traffic shaping but can cause packet loss during high traffic bursts and may under-utilize bandwidth in low traffic situations.
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