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Table Of Contents  The TCP/IP Guide
 9  The Open System Interconnection (OSI) Reference Model
      9  Key OSI Reference Model Concepts

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Protocols: Horizontal (Corresponding Layer) Communication
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Indirect Device Connection and Message Routing
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Data Encapsulation, Protocol Data Units (PDUs) and Service Data Units (SDUs)
(Page 3 of 3)

Use of PDU and SDU Terminology

The term “protocol data unit” is rather formal. You will see it used in standards and sometimes in discussions, but more often, the “plain” message terms, such as “frame” and “datagram”, are encountered, as discussed in the networking fundamentals topic on messages. Similarly, data encapsulated by these messages is not normally called a “service data unit” but rather simply the message body or payload, as discussed in the topic on message formatting. There are cases, however, where knowing the difference between an SDU and a PDU is important to understanding the technology. One example is the IEEE 802.11 physical layer; the 802.11 standards talk about SDUs and PDUs constantly.

Related Information: See the OSI Reference Model analogy if you want to see an example that compares networking encapsulation to a type done in a real-world, non-networking context.


Key Concept: The message used to communicate information for a particular protocol is called its protocol data unit (PDU) in OSI model terminology. That PDU is passed down to the next lower layer for transmission; since that layer is providing the service of handling that PDU, it is called the lower layer’s service data unit (SDU). The SDU is encapsulated into that layer’s own PDU and in turn sent to the next lower layer in the stack, proceeding until the physical layer is reached. The process is reversed on the recipient device. In summary: a layer N PDU is a layer N-1 SDU, which is encapsulated into a layer N-1 PDU.



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Protocols: Horizontal (Corresponding Layer) Communication
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Indirect Device Connection and Message Routing
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