Protocol Data Units at each layer in Computer Networks - Full Explanation

Introduction

Imagine sending a letter through the mail. To get it to the right place, the letter is put inside an envelope, then inside a package, and so on. In computer networks, data is also wrapped in layers to travel from one device to another. Each layer adds its own information, and the wrapped data at each step is called a Protocol Data Unit, or PDU.

Explanation

Application Layer PDU

At the top layer, the data is the actual message or information you want to send, like an email or a web page. This data is called 'Data' because it is the raw content before any network packaging happens.

The Application Layer PDU is the original message or content to be sent.

Transport Layer PDU

Here, the data is divided into smaller pieces called segments or datagrams. The transport layer adds information like source and destination ports to help deliver the data to the right application on the receiving device.

The Transport Layer PDU is called a segment or datagram and includes port information.

Network Layer PDU

At this layer, the segment is wrapped into a packet. The network layer adds source and destination IP addresses to guide the packet through different networks to reach the correct device.

The Network Layer PDU is called a packet and contains IP addresses for routing.

Data Link Layer PDU

The packet is then placed inside a frame at the data link layer. This frame includes physical addresses (MAC addresses) to deliver the data within the local network segment.

The Data Link Layer PDU is called a frame and uses MAC addresses for local delivery.

Physical Layer PDU

Finally, the frame is converted into bits—ones and zeros—that travel over cables, wireless signals, or other physical media. This raw bit stream is the physical layer's PDU.

The Physical Layer PDU is the raw bits sent over the physical medium.

Real World Analogy

Sending a message through the mail involves putting a letter inside an envelope, then placing that envelope inside a box, and finally handing it to a delivery truck. Each step adds a layer of packaging and information to ensure the message reaches the right person.

Application Layer PDU → The letter containing the actual message.

Transport Layer PDU → The envelope that directs the letter to the correct recipient within a building.

Network Layer PDU → The box with the address of the building to guide delivery.

Data Link Layer PDU → The label on the box that helps the local delivery person find the right door.

Physical Layer PDU → The truck carrying the box as a stream of packages.

Diagram

Diagram

┌─────────────────────┐
│ Application Layer    │
│ PDU: Data           │
└──────────┬──────────┘
           │
┌──────────▼──────────┐
│ Transport Layer      │
│ PDU: Segment/Datagram│
└──────────┬──────────┘
           │
┌──────────▼──────────┐
│ Network Layer        │
│ PDU: Packet         │
└──────────┬──────────┘
           │
┌──────────▼──────────┐
│ Data Link Layer      │
│ PDU: Frame          │
└──────────┬──────────┘
           │
┌──────────▼──────────┐
│ Physical Layer       │
│ PDU: Bits           │
└─────────────────────┘

This diagram shows the layers of a network stack with their corresponding Protocol Data Units (PDUs) from top to bottom.

Key Facts

Protocol Data Unit (PDU) → A unit of data specified at a particular layer of the network model.

Segment → The Transport Layer PDU used in TCP communication.

Packet → The Network Layer PDU that contains IP addresses for routing.

Frame → The Data Link Layer PDU that includes MAC addresses for local delivery.

Bits → The Physical Layer PDU representing raw data sent over the medium.

Common Confusions

Thinking the same PDU name applies to all layers.

Thinking the same PDU name applies to all layers. Each layer has a specific PDU name reflecting its role, such as segment at Transport and frame at Data Link.

Believing data is sent as one big chunk without layering.

Believing data is sent as one big chunk without layering. Data is wrapped and processed layer by layer, each adding its own header or information.

Summary

Data sent over networks is wrapped in layers, each with its own Protocol Data Unit (PDU) name.

The PDU names from top to bottom are Data, Segment/Datagram, Packet, Frame, and Bits.

Each PDU adds information to help deliver the message correctly through the network.

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