Data Link Layer Services & Shannon Capacity

Part A: Services Provided by Data Link Layer [6 Marks]

The Data Link Layer (Layer 2 of the OSI model) provides reliable transit of data across a physical link. Its main services are:

1. Framing

The bit stream received from the Network Layer is divided into manageable units called frames. A header and trailer are added to mark the start and end of each frame.

2. Physical (MAC) Addressing

A header containing the MAC addresses of the sender and receiver is added, enabling frame delivery between devices on the same network.

3. Flow Control

When the sender transmits faster than the receiver can absorb, the layer applies a flow-control mechanism to prevent receiver buffer overflow.

4. Error Control

Mechanisms to detect and retransmit damaged or lost frames are provided, along with prevention of duplicate frames. Techniques include CRC, parity, and checksums.

5. Access Control

On a shared medium, protocols decide which device controls the link at a given moment, avoiding collisions (e.g., CSMA/CD, Token passing).

6. Reliable (Node-to-Node) Delivery

Uses acknowledgements (ACK) and retransmission schemes such as ARQ to ensure error-free node-to-node delivery.

Part B: Shannon Capacity Calculation [4 Marks]

Given data

Formula

$$C = B \times \log_2(1 + \text{SNR})$$

Step 1: Substitute

$$C = 5 \times 10^6 \times \log_2(1 + 1000) = 5 \times 10^6 \times \log_2(1001)$$

Step 2: Evaluate the logarithm

$$\log_2(1001) = \frac{\log_{10}(1001)}{\log_{10}(2)} = \frac{3.00043}{0.30103} \approx 9.9672$$

Step 3: Compute capacity

$$C = 5 \times 10^6 \times 9.9672 \approx 49.836 \times 10^6\ \text{bps}$$

$$\boxed{C \approx 49.84\ \text{Mbps} \approx 50\ \text{Mbps}}$$

Interpretation

This is the theoretical maximum data rate of the channel. No transmission scheme, regardless of the number of signal levels, can exceed this limit.

TCP as a Reliable Protocol & Subnetting

STEP 1: Given Data

• Network: 192.168.1.0/24
• Original subnet mask: 255.255.255.0
• Required number of subnets: 8

STEP 2: Solution

(a) Why TCP is a Reliable Protocol?

TCP (Transmission Control Protocol) is called reliable because it guarantees accurate, ordered, and complete delivery of data through the following mechanisms:

1. Connection-Oriented: Establishes a connection via a 3-way handshake (SYN, SYN-ACK, ACK) before data transfer.
2. Acknowledgement (ACK): Receiver acknowledges received segments; unacknowledged segments are retransmitted after timeout.
3. Error Detection (Checksum): Corrupted segments are detected and discarded, then retransmitted.
4. Sequencing: Sequence numbers allow reordering of out-of-order segments and detection of lost data.
5. Flow Control: A sliding window prevents overwhelming the receiver.
6. Congestion Control: Adjusts transmission rate to avoid network congestion and packet loss.

Hence, TCP ensures no data is lost, duplicated, or delivered out of order.

(b) Subnetting 192.168.1.0/24 into 8 Subnets

Step 1 - Subnet bits needed: $$2^n \geq 8 \implies 2^3 = 8 \implies n = 3 \text{ bits}$$

Step 2 - New prefix and mask: $$/24 + 3 = /27$$

4th octet: $11100000_2 = 224$

$$\text{Subnet Mask} = 255.255.255.224$$

Step 3 - Block size: $$256 - 224 = 32 \text{ addresses per subnet (30 usable hosts)}$$

Step 4 - Subnet Table:

Summary:

DHCP and DNS Name Resolution

What is DHCP? [1 Mark]

DHCP (Dynamic Host Configuration Protocol) is a network management protocol used to automatically assign IP addresses and other network configuration parameters (such as subnet mask, default gateway, and DNS server address) to devices on a network, so they can communicate on an IP network without manual configuration.

DNS Name Resolution Process [4 Marks]

DNS (Domain Name System) is a hierarchical, distributed naming system that translates human-readable domain names (e.g., www.example.com) into machine-readable IP addresses (e.g., 93.184.216.34).

Components Involved

Step-by-Step Name Resolution Process

Example: A user types www.example.com in a browser.

User's PC --> Local DNS --> Root Server --> TLD Server --> Authoritative Server

Step 1: Browser Cache Check

• The browser first checks its local cache to see if the IP for www.example.com is already stored.
• If not found, the query is passed to the OS resolver.

Step 2: Query to Local DNS Server (Recursive Resolver)

• The client sends a recursive query to the Local DNS Server (configured via DHCP or manually).
• The local DNS server checks its own cache.
• If not cached, it begins the resolution process on behalf of the client.

Step 3: Query to Root Name Server

• The local DNS server sends a query to one of the 13 Root Name Servers.
• The root server does not know the IP of www.example.com, but it knows the address of the .com TLD Name Server.
• Root server replies: "Ask the .com TLD server at IP X.X.X.X"

Step 4: Query to TLD Name Server

• The local DNS server queries the .com TLD Name Server.
• The TLD server does not know the exact IP, but knows the Authoritative Name Server for example.com.
• TLD server replies: "Ask the authoritative server for example.com at IP Y.Y.Y.Y"

Step 5: Query to Authoritative Name Server

• The local DNS server queries the Authoritative Name Server for example.com.
• This server has the actual A record (Address record) for www.example.com.
• It replies: "www.example.com has IP address 93.184.216.34"

Step 6: Response to Client

• The local DNS server caches the result (for future queries) and sends the IP address back to the client.
• The browser can now connect to 93.184.216.34 to load the website.

Diagram

Client
  |
  | (1) Query: www.example.com?
  v
Local DNS Server
  |
  | (2) Query to Root Server
  v
Root Name Server --> "Try .com TLD Server"
  |
  | (3) Query to TLD Server
  v
.com TLD Server --> "Try example.com Authoritative Server"
  |
  | (4) Query to Authoritative Server
  v
Authoritative Server --> "IP = 93.184.216.34"
  |
  | (5) Final Answer returned to Client
  v
Client connects to 93.184.216.34

Types of DNS Queries

Note: The query from client to local DNS is typically recursive; queries from local DNS to other servers are typically iterative.

Summary

The DNS name resolution process converts www.example.com into an IP address through a hierarchical chain: Local DNS Server → Root Server → TLD Server → Authoritative Server, ultimately returning the IP address to the requesting client.

CRC (Cyclic Redundancy Check) - Worked Solution

Step 1 - EXTRACT: Given Data

• Message bit stream (M): 11011001 (8 bits)
• Generator polynomial: $x^3 + x - 1$
• In modulo-2 (GF(2)) arithmetic, $-1 \equiv +1$, so $x^3 + x - 1 = x^3 + x + 1$
• Generator (G): 1011 (4 bits, degree $r = 3$)

Step 2 - SOLVE

Step 2.1: Append $r = 3$ zeros to the message

$$M' = 11011001 ,|, 000 = 11011001000$$

Step 2.2: Sender-side XOR division of 11011001000 by 1011

        10000101   <-- quotient
      ______________
1011 ) 11011001000
       1011
       ----
        1101
        1011
        ----
         01100
          1011
          ----
           01110
            1011
            ----
             01011
              1011
              ----
               00000
                 000  <-- remainder (CRC)

Step trace (leading-bit alignment, XOR when leading bit = 1):

Remainder (CRC) = 000

Step 2.3: Actual transmitted bit string

$$\text{Transmitted} = M ,|, \text{CRC} = 11011001 ,|, 000$$

$$\boxed{11011001000}$$

Step 2.4: Receiver-side error checking

Receiver divides the received frame 11011001000 by the same generator 1011:

1011 ) 11011001000
       ... (identical division) ...
                 000  <-- remainder

Remainder = 000

Step 2.5: Decision

Since the remainder is 000, the receiver concludes the frame was received without error.

Summary

Differences Between Noise, Distortion, and Attenuation

Definitions and Key Differences

Brief Explanations

1. Noise

• Noise is any unwanted energy that gets mixed with the transmitted signal.
• It is random and unpredictable.
• Types include: thermal noise, impulse noise, crosstalk, and intermodulation noise.
• It degrades the signal-to-noise ratio (SNR).

2. Distortion

• Distortion occurs when the shape of the signal changes during transmission.
• It is common in composite signals where each frequency component travels at a different speed.
• The signal arrives with components out of phase, altering the original waveform.
• It is a deterministic problem related to medium characteristics.

3. Attenuation

• Attenuation is the loss of signal energy as the signal propagates through a medium.
• It increases with distance and frequency.
• Measured in decibels (dB).
• It can be compensated by using amplifiers (analog) or repeaters (digital).

Summary

Noise adds unwanted signals, distortion changes the signal shape, and attenuation reduces the signal strength. All three are major impairments in data communication but differ in their cause, nature, and remedy.

Short Notes

1. Sliding Window Protocol

The Sliding Window Protocol is a flow control and error control mechanism used in data link layer and transport layer protocols to allow efficient transmission of multiple frames before requiring acknowledgment.

Key Concepts

• Instead of sending one frame and waiting for an ACK (as in Stop-and-Wait), the sender can transmit multiple frames up to a window size W without waiting for acknowledgment.
• The window represents the number of frames that can be outstanding (sent but not yet acknowledged) at any time.
• Both sender and receiver maintain a window of sequence numbers.

Working Principle

Sender Window (W=4):
[0][1][2][3] --> can send these without ACK
     ^--- as ACKs arrive, window slides forward

• When an ACK is received for a frame, the window slides forward, allowing new frames to be sent.
• Sequence numbers are used cyclically; for a window size W, sequence numbers range from 0 to 2^n - 1 (where n is the number of bits).

Types

Advantages

• Increases link utilization and throughput
• Reduces idle time compared to Stop-and-Wait
• Efficiency = W / (1 + 2a), where a = propagation delay / transmission time

2. HTTP Protocol (HyperText Transfer Protocol)

HTTP is an application layer protocol used for transferring hypermedia documents (HTML, images, etc.) on the World Wide Web. It follows a client-server model.

Key Features

• Stateless Protocol: Each request is independent; the server does not retain session information between requests.
• Default Port: 80 (HTTP), 443 (HTTPS)
• Connection Types:
  • Non-persistent HTTP: A new TCP connection is established for each object/request.
  • Persistent HTTP: Multiple objects can be sent over a single TCP connection.

HTTP Request Methods

HTTP Message Format

Request:

GET /index.html HTTP/1.1
Host: www.example.com
Connection: keep-alive

Response:

HTTP/1.1 200 OK
Content-Type: text/html
Content-Length: 1024

<html>...</html>

HTTP Status Codes

HTTP Versions

• HTTP/1.0: Non-persistent by default
• HTTP/1.1: Persistent connections, pipelining supported
• HTTP/2: Multiplexing, header compression, faster performance

Advantages

• Simple and human-readable
• Platform independent
• Widely supported across all browsers and servers