A protocol data unit (PDU) is a layered data delivery system that either communicates with a network device or with protocols in the same machine. There are five named protocol data unit layers, but the fifth layer and above are all classified as the same thing, so there can be many more than five layers. The only physical layer used to communicate with hardware is the first layer, which transports information in binary, bit by bit. When data are transferred, they begin at the highest layer and stream down, with each layer adding a layer head. The second layer adds a footer and converts the data into bits for the first layer, and the information is sent.
Both networks and programs running on the same computer need to communicate and swap data to function. The protocol data unit is used as a packet to create and deliver these data between the two protocols or processes. There can be many layers to data packets; the more layers, the farther away the data are from the physical level.
While there can be a large number of layers transporting information, there are technically only five different layers. The first protocol data unit layer is Layer 1 and is known as the physical level. Layer 2 is the data link layer, which links data, and Layer 3 is the network layer that communicates between networks. The fourth layer is known as the transport layer, because it is where data get ready to send. Layers 5 and above are collectively called data and are high up in the computer hierarchy.
When information sends, it goes in descending order, starting from the top layer. For example, if the data sent need Layer 6, then the packet will begin here. Each layer under it will then add a header packet, which adds more data. At Layer 2, the layer adds a header and a footer, making the packet suitable for transport.
Layer 1 is special when compared to the other protocol data unit layers, because it neither uses a header or footer nor actually creates data packets. This layer transports information by bits, in binary, to the protocol, process or hardware requesting the data. If the packet needs Layer 2 or higher, then Layer 2 is responsible for converting all the headers, the footer and the packet itself into bits, so Layer 1 can deliver it.