Protocol Data Units at each layer in Computer Networks - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is a Protocol Data Unit (PDU)?

A Protocol Data Unit (PDU) is a unit of data specified at a particular layer of the network model. It includes the data and the control information needed for communication at that layer.

beginner

What is the PDU called at the Application layer?

At the Application layer, the PDU is called Data. It represents the actual information sent by the user or application.

intermediate

Name the PDU at the Transport layer and its role.

The PDU at the Transport layer is called a Segment (TCP) or Datagram (UDP). It breaks data into manageable pieces and adds control information for delivery.

intermediate

What is the PDU at the Network layer and what does it do?

At the Network layer, the PDU is called a Packet. It contains the data plus source and destination addresses to route the data across networks.

intermediate

Identify the PDU at the Data Link layer and its purpose.

The PDU at the Data Link layer is called a Frame. It packages data for transmission over a physical link and includes error checking information.

What is the PDU called at the Physical layer?

A Segments

B Frames

C Packets

D Bits

At the Physical layer, data is transmitted as raw bits (0s and 1s) over the physical medium.

Which PDU contains source and destination IP addresses?

A Segment

B Packet

C Frame

D Data

Packets at the Network layer include source and destination IP addresses for routing.

Which layer's PDU is responsible for error detection on a local link?

A Application layer

B Transport layer

C Data Link layer

D Network layer

The Data Link layer's Frame includes error checking to detect transmission errors on the local link.

What is the main difference between a Segment and a Datagram?

A Segment is used in TCP, Datagram in UDP

B Segment is at Network layer, Datagram at Transport layer

C Segment contains IP addresses, Datagram does not

D Segment is raw bits, Datagram is framed data

Segments are used in TCP (connection-oriented), while Datagrams are used in UDP (connectionless) at the Transport layer.

Which PDU is directly transmitted over the physical medium?

A Bits

B Frame

C Segment

D Packet

Bits are the raw electrical or optical signals sent over the physical medium at the Physical layer.

Explain the different Protocol Data Units (PDUs) at each layer of the OSI or TCP/IP model and their roles.

Think about how data is packaged and handled as it moves down the layers.

Describe how the PDU changes as data moves from the Application layer down to the Physical layer during transmission.

Consider the encapsulation process in networking.

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