The protocol data unit (PDU) is a layered data delivery system used to communicate between network devices or protocols. There are technically five layers, with the first layer being the physical layer. Each layer adds a header packet, with layer 2 adding a header and footer. Layer 1 carries information in binary and doesn’t use headers or footers.
A protocol data unit (PDU) is a layered data delivery system that communicates with a network device or protocols in the same machine. There are five levels of named protocol data units, but the fifth and higher levels are all classified as the same thing, so there can be many more than five levels. The only physical layer used to communicate with hardware is the first layer, which carries information in binary, bit-for-bit format. As data transfers, it starts at the top level and flows down, with each level adding a level head. The second level adds a footer and converts the data into bits for the first level and the information is sent.
Both networks and programs running on the same computer need to communicate and exchange data to work. The protocol data unit is used as a packet to create and deliver this data between the two protocols or processes. There can be many layers in data packets; the more layers, the farther the data is from the physical layer.
While there may be a large number of layers that carry information, there are technically only five different layers. The first data unit layer of the protocol is layer 1 and is known as the physical layer. Layer 2 is the data link layer, which connects data, and layer 3 is the network layer, which communicates between networks. The fourth layer is known as the transport layer because it is where data prepares to be sent. Levels 5 and above are collectively called data and are at the top of the computer hierarchy.
When information is submitted, it goes in descending order, starting at the top level. For example, if the data being sent requires level 6, the packet will start here. Each layer below will then add a header packet, which adds more data. At layer 2, the layer adds a header and footer, making the package suitable for transport.
Layer 1 is special compared to the other data unit layers of the protocol, because it doesn’t use a header or footer or actually create data packets. This layer carries information in bits, in binary, to the protocol, process, or hardware requesting the data. If the packet needs Layer 2 or higher, Layer 2 is responsible for converting all headers, footer, and the packet itself to bits so Layer 1 can deliver it.
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