TDMA (Time Division Multiple Access) is a channel access method used in shared medium networks where multiple users share the same frequency channel by dividing the signal into different time slots. Each user is assigned a specific time ...

MAC Address and CRC Calculation

MAC Address (2 marks)

A MAC (Media Access Control) address is a unique physical hardware address permanently assigned to a Network Interface Card (NIC) by its manufacturer. It operates at the Data Link Layer (Layer 2) of the OSI model and is used to uniquely identify a device on a local network.

Key features:

• It is a 48-bit (6-byte) address, normally written in hexadecimal.
• Example: 00:1A:2B:3C:4D:5E
• The first 3 bytes (24 bits) form the OUI (Organizationally Unique Identifier) identifying the manufacturer.
• The last 3 bytes identify the specific device.
• Also called the physical / hardware address; usually burned into the NIC's ROM and globally unique.

CRC Calculation (3 marks)

Given data

• Message $M$ = 1011011
• Generator $G(x) = x^3 + x^2 + 1 \Rightarrow$ binary 1101
• Degree $r = 3$

Step 1: Append $r = 3$ zeros

$$ M' = 1011011\underbrace{000}_{3\text{ zeros}} = 1011011000 $$

Step 2: Modulo-2 (XOR) division by 1101

Dividend bits: 1 0 1 1 0 1 1 0 0 0

      1101010   <-- quotient (not required)
    ┌───────────
1101│1011011000
     1101
     ────
     0110 1        bring down bit → 1101
     1101
     ────
     0000 11       bring down bits → 0011
          (0011 < 1101) → q=0, bring down → 0111
          (0111 < 1101) → q=0, bring down → 1110
          1101
          ────
          0011 0    bring down → 0110
          (0110 < 1101) → q=0  (no more bits)
     Remainder = 010

Working the XOR steps cleanly:

Let me track the final bits exactly. After processing all 10 dividend bits:

• Start: 1011 → XOR 1101 = 0110
• +bit(0): 1100 → XOR 1101 = 0001
• +bit(1): 0011 → < divisor, q=0
• +bit(1): 0111 → < divisor, q=0
• +bit(0): 1110 → XOR 1101 = 0011
• +bit(0): 0110 → < divisor, q=0
• +bit(0): 1100 → XOR 1101 = 0001

Final 3-bit remainder (CRC) = 001

Verification

Divide the 3 remaining bits carefully; the last register content after all 10 bits gives remainder = 001.

Check: transmitted frame $T = 1011011,001$. Dividing $1011011001$ by 1101 must give remainder 000:

1011011001
1101
────
0110 → 1100 → 0001 → 0011 → 0111 → 1110→0011 → 0110 → 1101 → 0000

$1101 \oplus 1101 = 0000$ ✓ remainder is 0, so the CRC is correct.

Step 3: Transmitted frame

$$ T = \text{Message} + \text{CRC} = 1011011 , \big| , 001 = \boxed{1011011001} $$

CRC (remainder) = 001 Transmitted encoded frame = 1011011001

--- (a) Difference Between Packet Switched and Circuit Switched Network Feature Circuit Switched Network Packet Switched Network --------- Connection A dedicated physical path is established before communication begins No dedicated path;...

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.

Congestion Control

Definition

Congestion in a network occurs when the number of packets being transmitted through the network approaches the packet handling capacity of the network. Congestion control refers to the mechanisms and techniques used to control or prevent congestion so that network performance does not degrade.

Note: No specific reference notes were found for this topic; the answer below is based on standard networking curriculum (Forouzan / Tanenbaum) as taught in BSc CSIT.

Why Congestion Occurs

When too many packets are present in a part of the network, network performance degrades. This happens because:

• Routers have limited buffer (queue) space
• Links have limited bandwidth
• Processors in routers have limited processing speed

If the load exceeds capacity, packets are dropped, retransmissions increase, and the situation worsens -- leading to congestion collapse.

How Congestion Can Be Handled

Congestion control approaches are broadly classified as:

1. Open Loop Congestion Control (Prevention)

Policies are applied before congestion occurs -- at the design stage.

2. Closed Loop Congestion Control (Reaction)

Policies are applied after congestion is detected.

• Back pressure: A congested node sends a signal to the upstream node to slow down.
• Choke packet: A special packet is sent by the router to the source to reduce its transmission rate.
• Implicit signaling: Source detects congestion from delays or dropped packets (e.g., TCP timeout).
• Explicit signaling: Router explicitly marks packets to inform source/destination of congestion (e.g., ECN -- Explicit Congestion Notification).

Open Loop Congestion Control Policies (Detailed)

A. Acknowledgement Policy

The acknowledgement (ACK) policy at the receiver side can greatly affect congestion.

How it works:

• If the receiver sends an acknowledgement for every packet received, it encourages the sender to keep sending packets rapidly, which can contribute to congestion.
• By delaying acknowledgements, the receiver can slow down the sender and reduce network load.

Techniques:

Effect on Congestion:

• Fewer ACKs mean the sender's window advances more slowly.
• This reduces the rate at which new packets are injected into the network.
• Thus, acknowledgement policy directly helps in preventing congestion at the source.

B. Discarding Policy

The discarding policy is applied at routers/switches. When a router's buffer is full or near full, it must decide which packets to drop.

Approaches:

1. Tail Drop (Default / Simple)

• When the buffer is full, the router simply drops all incoming packets (from the tail of the queue).
• Problem: Can cause global synchronization -- many TCP connections reduce their window simultaneously, then increase together, causing oscillation.

2. Random Early Detection (RED)

• The router monitors its average queue length.
• When the queue length exceeds a minimum threshold (min_th), it starts randomly dropping packets with increasing probability.
• When the queue length exceeds a maximum threshold (max_th), all packets are dropped.

Queue Length:
0 -------- min_th -------- max_th -------- Max Buffer
           |                |
           Start random     Drop all
           dropping         packets

• Advantage: Prevents global synchronization; notifies individual connections early.
• Advantage: Maintains low average queue length and low delay.

3. Priority-Based Discarding

• Packets are assigned priority levels.
• When congestion occurs, lower priority packets are dropped first.
• Higher priority packets (e.g., real-time voice/video) are protected.

4. Weighted RED (WRED)

• Extension of RED where different traffic classes have different drop probabilities.
• Higher priority traffic has a lower drop probability.

Summary Table

Conclusion

Congestion control is essential for maintaining network performance and reliability. Open loop methods like acknowledgement policy and discarding policy prevent congestion proactively, while closed loop methods react to congestion after it is detected. Together, these mechanisms ensure fair and efficient use of network resources.

--- (a) Difference Between Packet Switched and Circuit Switched Network Feature Circuit Switched Network Packet Switched Network --------- Connection A dedicated physical path is established before communication begins No dedicated path;...

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.

TDMA (Time Division Multiple Access) is a channel access method used in shared medium networks where multiple users share the same frequency channel by dividing the signal into different time slots. Each user is assigned a specific time ...

MAC Address and CRC Calculation

MAC Address (2 marks)

A MAC (Media Access Control) address is a unique physical hardware address permanently assigned to a Network Interface Card (NIC) by its manufacturer. It operates at the Data Link Layer (Layer 2) of the OSI model and is used to uniquely identify a device on a local network.

Key features:

• It is a 48-bit (6-byte) address, normally written in hexadecimal.
• Example: 00:1A:2B:3C:4D:5E
• The first 3 bytes (24 bits) form the OUI (Organizationally Unique Identifier) identifying the manufacturer.
• The last 3 bytes identify the specific device.
• Also called the physical / hardware address; usually burned into the NIC's ROM and globally unique.

CRC Calculation (3 marks)

Given data

• Message $M$ = 1011011
• Generator $G(x) = x^3 + x^2 + 1 \Rightarrow$ binary 1101
• Degree $r = 3$

Step 1: Append $r = 3$ zeros

$$ M' = 1011011\underbrace{000}_{3\text{ zeros}} = 1011011000 $$

Step 2: Modulo-2 (XOR) division by 1101

Dividend bits: 1 0 1 1 0 1 1 0 0 0

      1101010   <-- quotient (not required)
    ┌───────────
1101│1011011000
     1101
     ────
     0110 1        bring down bit → 1101
     1101
     ────
     0000 11       bring down bits → 0011
          (0011 < 1101) → q=0, bring down → 0111
          (0111 < 1101) → q=0, bring down → 1110
          1101
          ────
          0011 0    bring down → 0110
          (0110 < 1101) → q=0  (no more bits)
     Remainder = 010

Working the XOR steps cleanly:

Let me track the final bits exactly. After processing all 10 dividend bits:

• Start: 1011 → XOR 1101 = 0110
• +bit(0): 1100 → XOR 1101 = 0001
• +bit(1): 0011 → < divisor, q=0
• +bit(1): 0111 → < divisor, q=0
• +bit(0): 1110 → XOR 1101 = 0011
• +bit(0): 0110 → < divisor, q=0
• +bit(0): 1100 → XOR 1101 = 0001

Final 3-bit remainder (CRC) = 001

Verification

Divide the 3 remaining bits carefully; the last register content after all 10 bits gives remainder = 001.

Check: transmitted frame $T = 1011011,001$. Dividing $1011011001$ by 1101 must give remainder 000:

1011011001
1101
────
0110 → 1100 → 0001 → 0011 → 0111 → 1110→0011 → 0110 → 1101 → 0000

$1101 \oplus 1101 = 0000$ ✓ remainder is 0, so the CRC is correct.

Step 3: Transmitted frame

$$ T = \text{Message} + \text{CRC} = 1011011 , \big| , 001 = \boxed{1011011001} $$

CRC (remainder) = 001 Transmitted encoded frame = 1011011001

Link State Routing is a dynamic routing algorithm where each router has complete knowledge of the entire network topology. Every router broadcasts information about its directly connected links (neighbors and link costs) to all other rou...

Note: Reference notes were not available for this topic; the following answer is based on standard networking curriculum covered in TU BSc CSIT. --- ALOHA is one of the earliest random access protocols developed at the University of Hawa...

• Bit stream to encode: $1\ 1\ 1\ 0\ 0\ 1\ 0\ 1\ 1$ (9 bits) - Schemes required: (i) Manchester, (ii) NRZ-I, (iii) NRZ-L Note: These are standard line-coding schemes. I follow the conventions used in Forouzan (TU curriculum standard). An...

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

Congestion Control

Definition

Congestion in a network occurs when the number of packets being transmitted through the network approaches the packet handling capacity of the network. Congestion control refers to the mechanisms and techniques used to control or prevent congestion so that network performance does not degrade.

Note: No specific reference notes were found for this topic; the answer below is based on standard networking curriculum (Forouzan / Tanenbaum) as taught in BSc CSIT.

Why Congestion Occurs

When too many packets are present in a part of the network, network performance degrades. This happens because:

• Routers have limited buffer (queue) space
• Links have limited bandwidth
• Processors in routers have limited processing speed

If the load exceeds capacity, packets are dropped, retransmissions increase, and the situation worsens -- leading to congestion collapse.

How Congestion Can Be Handled

Congestion control approaches are broadly classified as:

1. Open Loop Congestion Control (Prevention)

Policies are applied before congestion occurs -- at the design stage.

2. Closed Loop Congestion Control (Reaction)

Policies are applied after congestion is detected.

• Back pressure: A congested node sends a signal to the upstream node to slow down.
• Choke packet: A special packet is sent by the router to the source to reduce its transmission rate.
• Implicit signaling: Source detects congestion from delays or dropped packets (e.g., TCP timeout).
• Explicit signaling: Router explicitly marks packets to inform source/destination of congestion (e.g., ECN -- Explicit Congestion Notification).

Open Loop Congestion Control Policies (Detailed)

A. Acknowledgement Policy

The acknowledgement (ACK) policy at the receiver side can greatly affect congestion.

How it works:

• If the receiver sends an acknowledgement for every packet received, it encourages the sender to keep sending packets rapidly, which can contribute to congestion.
• By delaying acknowledgements, the receiver can slow down the sender and reduce network load.

Techniques:

Effect on Congestion:

• Fewer ACKs mean the sender's window advances more slowly.
• This reduces the rate at which new packets are injected into the network.
• Thus, acknowledgement policy directly helps in preventing congestion at the source.

B. Discarding Policy

The discarding policy is applied at routers/switches. When a router's buffer is full or near full, it must decide which packets to drop.

Approaches:

1. Tail Drop (Default / Simple)

• When the buffer is full, the router simply drops all incoming packets (from the tail of the queue).
• Problem: Can cause global synchronization -- many TCP connections reduce their window simultaneously, then increase together, causing oscillation.

2. Random Early Detection (RED)

• The router monitors its average queue length.
• When the queue length exceeds a minimum threshold (min_th), it starts randomly dropping packets with increasing probability.
• When the queue length exceeds a maximum threshold (max_th), all packets are dropped.

Queue Length:
0 -------- min_th -------- max_th -------- Max Buffer
           |                |
           Start random     Drop all
           dropping         packets

• Advantage: Prevents global synchronization; notifies individual connections early.
• Advantage: Maintains low average queue length and low delay.

3. Priority-Based Discarding

• Packets are assigned priority levels.
• When congestion occurs, lower priority packets are dropped first.
• Higher priority packets (e.g., real-time voice/video) are protected.

4. Weighted RED (WRED)

• Extension of RED where different traffic classes have different drop probabilities.
• Higher priority traffic has a lower drop probability.

Summary Table

Conclusion

Congestion control is essential for maintaining network performance and reliability. Open loop methods like acknowledgement policy and discarding policy prevent congestion proactively, while closed loop methods react to congestion after it is detected. Together, these mechanisms ensure fair and efficient use of network resources.

The leaky bucket algorithm is a traffic shaping and congestion control mechanism used in computer networks to control the rate at which data packets are sent into the network. It smooths out bursty traffic and converts it into a steady, ...

A reliable protocol is a communication protocol that guarantees delivery of data from sender to receiver. It ensures that: - Data arrives without errors - Data arrives in order - No data is lost or duplicated - The sender receives an ack...

Recursive resolution is a name resolution strategy used in the Domain Name System (DNS) where a DNS resolver takes full responsibility for resolving a query on behalf of the client. Instead of referring the client to another server, the ...

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.

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.