A Protocol Data Unit (PDU) is a unit of data specified in a protocol of a given layer of the OSI model. Each layer has its own PDU: at the Application layer, it's called a message; at the Transport layer, it's a segment (or datagram in UDP); at the Network layer, it's a packet; at the Data Link layer, it's a frame; and at the Physical layer, it's a bit or symbol. These PDUs help in managing and encapsulating data as it traverses through the layers of the OSI model.
The PDU at the Network layer is referred to as a packet. A PDU at the Data Link Layer is referred to as a frame.
In networking, PDU means "protocol data unit", and it is the generic name of the "packets" of data used at different levels of the network. Using the numbering of the OSI layers: At layer 2, the PDUs are called "frame". Example: An Ethernet frame, a frame-relay frame. At layer 3, the PDUs are called "packets". Example: An IP packet. At layer 4, the PDUs are called "segments". Example: A TCP segment, a UDP segment.
Datagrams TCP/IP layer 4 PDU's are called segments...
The PDU (protocol data unit) @ transport layer (e.g. TCP or UDP) is called as 'segment' .
Frames
A PDU (Protocol Data Unit) is defined by which layer it is in. In the physical layer and network layer, it is synonymous with the packet, in the data link layer, it is the frame. In the transport layer, it is a datagram for UDP. A datagram holds one or more PDU's, as it is the basic unit of transferring information via packet switching.
Segements
What is a PDU? corruption of a frame during transmission data reassembled at the destination retransmitted packets due to lost communication a layer specific encapsulation
In the OSI model, encapsulation occurs as data is passed down through the layers. At the Application layer (Layer 7), data is created and then passed to the Presentation layer (Layer 6) for formatting. The Session layer (Layer 5) manages sessions, while the Transport layer (Layer 4) adds headers for segmentation and reliability. As data moves down to the Network layer (Layer 3), it receives IP addressing, followed by the Data Link layer (Layer 2), which adds MAC addressing and framing, and finally, the Physical layer (Layer 1) transmits the raw bits over the physical medium. Each layer adds its own header (and sometimes footer) to the data, forming a protocol data unit (PDU) specific to that layer.
Application layer: Data Presentation layer: Data Session layer: Data Transport layer: Segment (TCP) or Datagram (UDP) Network layer: Packet Data Link layer: Frame Physical layer: Bit
Protocol layers may be defined in such a way that the communications within a layer is independent of the operation of the layer being being used. This is known as "peer-to-peer" communication and is an important goal of the OSI Reference Model. Each layer provides a protocol to communicate with its peer. When a packet is transmitted by a layer, a header consisting of Protocol Control Information (PCI) is added to the data to be sent. In OSI terminology, the packet data (also known as the Payload) is called a Protocol Data Unit (PDU). The packet so-formed, called a Service Data Unit (SDU) is passed via a service access point to the layer below. This is sent using the service of the next lower protocol layer.