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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
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 layers service data unit (SDU). The SDU is encapsulated into that layers 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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Version 3.0 - Version Date: September 20, 2005
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