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The features or services of the network are implemented in distinct ''layers''. The physical layer sends ones and zeros across a wire or fiber. The data link layer then organizes these ones and zeros into chunks of data and gets them safely to the right place on the wire. The network layer transmits the organized data over multiple connected networks, and the transport layer delivers the data to the right software application at the destination.
Between the layers (and between the application and the top-most layer), the layers pass service data units (SDUs) across interfaces. The higher layer understands the structure of the data in the SDU, but the lower layer at the interface does not; moreover, the lower layer treats the SDU as the [[Payload (computing)|payload]], undertaking to get it to the same interface at the destination. In order to do this, the ''protocol'' (lower) layer will add to the SDU certain data it needs to perform its function
The SDU and metadata added by the lower layer can be larger than the maximum size of that layer's PDU (known as the [[maximum transmission unit]]; MTU). When this is the case, the PDU must be split into multiple payloads of a size suitable for transmission or processing by the lower layer
The significance of this is that the PDU is the structured information that is passed to a matching protocol layer further along on the data's journey that allows the layer to deliver its intended function or service. The matching layer, or "peer", decodes the data to extract the original service data unit, decide if it is error-free and where to send it next, etc. Unless we have already arrived at the lowest (physical) layer, the PDU is passed to the peer using services of the next lower layer in the protocol "stack". When the PDU passes over the interface from the layer that constructed it to the layer that merely delivers it (and therefore does not understand its internal structure), it becomes a service data unit to that layer. The addition of addressing and control information (encapsulation) to an SDU to form a PDU and the passing of that PDU to the next lower layer as an SDU repeats until the lowest layer is reached and the data passes over some medium as a physical signal.
The above process can be likened to the mail system in which a letter (SDU) is placed in an envelope on which
==Examples==
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Protocol data units of the [[OSI model]] are:<ref>{{Cite web |url=http://www.tcpipguide.com/free/t_DataEncapsulationProtocolDataUnitsPDUsandServiceDa.htm |title=Data Encapsulation, Protocol Data Units (PDUs) and Service Data Units (SDUs) |work=The TCP/IP Guide |access-date=2012-04-05}}</ref>
*The Layer 4: [[transport layer]] PDU is the [[Packet
*The Layer 3: [[network layer]] PDU is the [[Network packet|packet]] or the [[datagram]].<ref>{{Cite web|url=https://www.cloudflare.com/learning/network-layer/what-is-a-packet/|title=What is a packet? {{!}} Network packet definition|work=Cloudflare}}</ref>
*The Layer 2: [[data link layer]] PDU is the [[Frame (networking)|frame]].
*The Layer 1: [[physical layer]] PDU is the [[bit]] or, more generally, [[Symbol (data)|symbol]].
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===Media access control protocol data unit===
A '''media access control protocol data unit''' ('''MAC PDU''' or '''MPDU''') is a message that is exchanged between [[media access control]] (MAC) entities in a communication system based on the layered
In systems where the MPDU may be larger than the MAC service data unit (MSDU), the MPDU may ''include'' multiple MSDUs as a result of [[packet aggregation]]. In systems where the MPDU is smaller than the MSDU, then one MSDU may ''generate'' multiple MPDUs as a result of [[packet segmentation]].<ref name=":0" />
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