A protocol data unit is any of a number of terms used to describe pieces of data that are being transmitted over a network. They are:
- Data
- Segments
- Packets
- Frames
- Bits
Data
Data refers to information that has not yet entered the transport layer, and so hasn’t undergone any of the processes to prepare for its transmission over the network.
Segment
A segment is a piece of data that has entered the transport layer and has had a layer 4 header appended to it.
Packet
A packet is what a segment becomes after it hits the network layer, where a layer 3 header is appended to it.
Frame
A frame is what a packet becomes after it hits the data link layer, where a Ethernet header and Ethernet trailer are added to it. At this point, it’s ready to be transmitted over the physical layer toward its final destination.
Bit
A bit is what the data is called when it hits the physical layer and is being transmitted to its destination. An entire frame does not become a single bit, but is encoded as many bits.