Each layer has a Protocol Data Unit (PDU). The name changes as data descends.
OSI PDU names:
- Layer -: Data
- Layer : Segment (TCP) or Datagram (UDP)
- Layer : Packet
- Layer : Frame
- Layer : Bit
When you send "Hello," Layer creates data. Layer wraps it in a segment. Layer wraps that in a packet. Layer makes a frame. Layer transmits bits.