PDU and SDU Explained: Protocol Data Units and Service Data Units

PDU (Protocol Data Unit) and SDU (Service Data Unit) are key concepts in networking and telecommunications that describe how data is encapsulated and passed between different layers of a communication protocol stack.

Protocol Data Unit (PDU)

• Definition: A Protocol Data Unit (PDU) is a unit of data specified in a protocol of a given layer in the OSI (Open Systems Interconnection) or TCP/IP model. A PDU contains control information and user data.

• Role in Networking:

  • PDUs are the building blocks of data transmission, which are processed and modified by each protocol layer.
  • Each layer in the OSI model adds its own header (and sometimes trailer) to the PDU, which defines how data should be handled at that specific layer.

• Examples:

  • Physical Layer PDU: Bit or Symbol.
  • Data Link Layer PDU: Frame.
  • Network Layer PDU: Packet.
  • Transport Layer PDU: Segment (TCP) or Datagram (UDP).
  • Application Layer PDU: Message or Data.

• Encapsulation: As data moves down the layers, each layer adds its specific header (and possibly a trailer) to the data received from the upper layer, creating the PDU for that layer. This process is known as encapsulation.

Service Data Unit (SDU)

• Definition: A Service Data Unit (SDU) is the data passed from an upper layer to a lower layer that the lower layer uses for further processing. An SDU is a unit of data that has not yet been encapsulated with the lower layer’s protocol-specific headers and trailers.

• Role in Networking:

  • SDUs represent the raw data that needs to be transmitted to the lower layer, which then adds its own control information, forming the PDU.
  • At each layer, the SDU is treated as data to be encapsulated by that layer’s PDU.

• Examples:

  • At the Network layer, the SDU is a segment from the Transport layer, which is then encapsulated into a packet.
  • At the Data Link layer, the SDU is the packet from the Network layer, which is encapsulated into a frame.

Relationship Between PDU and SDU

• Encapsulation Process: The SDU of a higher layer becomes the payload of the PDU of the lower layer.

  • For example, the Transport layer SDU (application data) becomes the PDU when the Transport layer adds its headers to form a segment.

  • This segment is then passed as the SDU to the Network layer, where it is encapsulated into a packet (Network layer PDU).

• De-encapsulation Process: During the de-encapsulation process at the receiver’s end, each layer removes its header/trailer to extract the SDU and passes it up to the next higher layer until the original data is restored.

Summary

• PDU: The encapsulated form of data at a given layer, including headers/trailers.
• SDU: The data that a layer receives from the upper layer, which will be encapsulated into the PDU for transmission.
• Understanding the distinction and relationship between PDUs and SDUs is essential for grasping how data is formatted and managed as it moves through the different layers of network protocols.