Protocol Data Units at each layer in Computer Networks - Mini Project: Build & Apply

Understanding Protocol Data Units at Each Layer

📖 Scenario: You are learning how data is prepared and handled as it travels through different layers of a computer network. Each layer adds its own header or information, creating a specific Protocol Data Unit (PDU) name. Understanding these names helps in troubleshooting and designing networks.

🎯 Goal: Build a simple dictionary that maps each network layer to its Protocol Data Unit (PDU) name. This will help you remember what data is called at each layer.

📋 What You'll Learn

Create a dictionary called pdu_layers with exact layer names as keys

Add a configuration variable called layer_order listing layers in correct order

Use a for loop with variables layer and pdu to iterate over pdu_layers.items()

Add a final statement that confirms the dictionary includes the Application layer

💡 Why This Matters

🌍 Real World

Network engineers and IT professionals use PDUs to understand how data is packaged and transmitted across networks.

💼 Career

Knowing PDUs helps in troubleshooting network issues and designing communication protocols.

1

Create the Protocol Data Units dictionary

Create a dictionary called pdu_layers with these exact entries: 'Application': 'Data', 'Transport': 'Segment', 'Network': 'Packet', 'Data Link': 'Frame', 'Physical': 'Bits'.

Computer Networks

# Create the pdu_layers dictionary with exact layer and PDU names
# Your code here

Hint

Remember to use exact layer names as keys and their PDUs as values in the dictionary.

2

Add the layer order list

Create a list called layer_order with these exact strings in order: 'Application', 'Transport', 'Network', 'Data Link', 'Physical'.

Computer Networks

pdu_layers = {
    'Application': 'Data',
    'Transport': 'Segment',
    'Network': 'Packet',
    'Data Link': 'Frame',
    'Physical': 'Bits'
}
# Create the layer_order list with exact layer names in order
# Your code here

Hint

Make sure the list layer_order has the layers in the correct top-to-bottom order.

3

Iterate over the dictionary to access layers and PDUs

Use a for loop with variables layer and pdu to iterate over pdu_layers.items(). Inside the loop, write a comment that says # Processing layer and its PDU.

Computer Networks

pdu_layers = {
    'Application': 'Data',
    'Transport': 'Segment',
    'Network': 'Packet',
    'Data Link': 'Frame',
    'Physical': 'Bits'
}

layer_order = ['Application', 'Transport', 'Network', 'Data Link', 'Physical']

# Use a for loop to iterate over pdu_layers.items()
# Your code here

Hint

Use for layer, pdu in pdu_layers.items(): to loop through the dictionary.

4

Confirm the dictionary includes the Application layer

Add a final line that creates a variable called has_application and sets it to 'Application' in pdu_layers to confirm the Application layer is in the dictionary.

Computer Networks

pdu_layers = {
    'Application': 'Data',
    'Transport': 'Segment',
    'Network': 'Packet',
    'Data Link': 'Frame',
    'Physical': 'Bits'
}

layer_order = ['Application', 'Transport', 'Network', 'Data Link', 'Physical']

for layer, pdu in pdu_layers.items():
    # Processing layer and its PDU
    pass

# Confirm Application layer is in the dictionary
# Your code here

Hint

Use the in keyword to check if 'Application' is a key in pdu_layers.

Practice

1. Which Protocol Data Unit (PDU) name is used at the Network layer in the OSI model?

easy

A. Segment

B. Frame

C. Packet

D. Bit

Solution

1. Step 1: Recall OSI model layers and their PDUs

   The OSI model has seven layers. Each layer uses a specific PDU name. The Network layer is the third layer.

2. Step 2: Identify the PDU for the Network layer

   The Network layer uses the term 'Packet' for its PDU, which contains logical addressing information.

3. Final Answer:

   Packet -> Option C

4. Quick Check:

   Network layer PDU = Packet [OK]

Hint: Network layer always uses 'Packet' as its PDU name [OK]

Common Mistakes:

• Confusing Frame (Data Link layer) with Packet
• Mixing Segment (Transport layer) with Packet
• Thinking Bit is a PDU name at any layer

2. Which of the following correctly matches the PDU name with the Transport layer?

easy

A. Frame

B. Bit

C. Packet

D. Segment

Solution

1. Step 1: Identify the Transport layer in the OSI model

   The Transport layer is the fourth layer and is responsible for end-to-end communication.

2. Step 2: Recall the PDU name for the Transport layer

   The Transport layer uses 'Segment' as its PDU name, which contains data with transport headers.

3. Final Answer:

   Segment -> Option D

4. Quick Check:

   Transport layer PDU = Segment [OK]

Hint: Transport layer PDU is always called Segment [OK]

Common Mistakes:

• Choosing Frame which belongs to Data Link layer
• Selecting Packet which belongs to Network layer
• Confusing Bit with a PDU name

3. Consider this simplified data flow: Application data is passed down to the Transport layer, then to the Network layer, and finally to the Data Link layer. What is the correct sequence of PDUs at each layer?

medium

A. Packet -> Segment -> Frame

B. Segment -> Packet -> Frame

C. Frame -> Packet -> Segment

D. Bit -> Segment -> Packet

Solution

1. Step 1: Identify PDUs for each layer in order

   Transport layer uses Segment, Network layer uses Packet, Data Link layer uses Frame.

2. Step 2: Arrange PDUs in the order data passes down

   Data flows from Transport (Segment) -> Network (Packet) -> Data Link (Frame).

3. Final Answer:

   Segment -> Packet -> Frame -> Option B

4. Quick Check:

   Transport=Segment, Network=Packet, Data Link=Frame [OK]

Hint: Remember data flows: Segment, Packet, then Frame [OK]

Common Mistakes:

• Mixing order of PDUs
• Confusing Packet and Segment
• Including Bit as a PDU in this sequence

4. A student wrote this statement: "The Data Link layer PDU is called a Packet." What is wrong with this statement?

medium

A. Packet is the PDU for the Network layer, not Data Link layer.

B. Packet is the PDU for the Transport layer, not Data Link layer.

C. Packet is the PDU for the Application layer, not Data Link layer.

D. Packet is the PDU for the Physical layer, not Data Link layer.

Solution

1. Step 1: Identify the correct PDU for Data Link layer

   The Data Link layer uses 'Frame' as its PDU, not 'Packet'.

2. Step 2: Identify which layer uses 'Packet'

   The Network layer uses 'Packet' as its PDU.

3. Final Answer:

   Packet is the PDU for the Network layer, not Data Link layer. -> Option A

4. Quick Check:

   Data Link = Frame, Network = Packet [OK]

Hint: Data Link layer PDU is Frame, not Packet [OK]

Common Mistakes:

• Confusing Packet with Frame
• Mixing up Network and Data Link layers
• Assuming Packet is a general PDU name

5. In a network communication, data is encapsulated at each layer with its PDU. If a message is 1000 bytes at the Application layer, and each layer adds a header of 20 bytes (Transport), 30 bytes (Network), and 14 bytes (Data Link), what is the total size of the Frame sent over the Physical layer?

hard

A. 1064 bytes

B. 1066 bytes

C. 1064 bits

D. 1064 kilobytes

Solution

1. Step 1: Calculate total header size added by layers

   Transport header = 20 bytes, Network header = 30 bytes, Data Link header = 14 bytes. Total headers = 20 + 30 + 14 = 64 bytes.

2. Step 2: Add headers to original Application data size

   Application data = 1000 bytes. Total Frame size = 1000 + 64 = 1064 bytes.

3. Final Answer:

   1064 bytes -> Option A

4. Quick Check:

   1000 + 20 + 30 + 14 = 1064 bytes [OK]

Hint: Add all headers in bytes to original data size [OK]

Common Mistakes:

• Confusing bytes with bits
• Adding headers incorrectly
• Misreading units as kilobytes