science

Protocol data unit

Protocol data unit is a science topic covered in the lgStudy science library. This page brings together a partial reference excerpt, illustrations, worked examples, real-world applications and a short study plan, so you can understand Protocol data unit rather than just read about it. In short: In telecommunications, a protocol data unit (PDU) is a single unit of information transmitted among peer entities of a computer network. It is composed of protocol-specific control information and user data.

Key takeaways

Protocol data unit belongs to science; place it in that map before memorising details.
Learn the definition first, then one example that makes the definition concrete.
Connect Protocol data unit to a quantity you can measure, compute or draw — that is where exam questions come from.
Reproduce the core statement of Protocol data unit from memory before moving on to harder problems.

Reference excerpt

In telecommunications, a protocol data unit (PDU) is a single unit of information transmitted among peer entities of a computer network. It is composed of protocol-specific control information and user data. In the layered architectures of communication protocol stacks, each layer implements protocols tailored to the specific type or mode of data exchange. For example, the Transmission Control Protocol (TCP) implements a connection-oriented transfer mode, and the PDU of this protocol is called a segment, while the User Datagram Protocol (UDP) uses datagrams as protocol data units for connectionless communication. A layer lower in the Internet protocol suite, at the Internet layer, the PDU is called a packet, irrespective of its payload type.

Packet-switched data networks

In the context of packet switching data networks, a protocol data unit (PDU) is best understood in relation to a service data unit (SDU). The features or services of the network are implemented in distinct layers. The physical layer sends ones and zeros across different communication media such as copper wiring or fiber optic cable. The data link layer then organizes these ones and zeros into chunks of data and gets them safely to the right place on the wire. The network layer transmits the organized data over multiple connected networks, and the transport layer delivers the data to the right software application at the destination. Between the layers (and between the application and the top-most layer), the layers pass service data units (SDUs) across interfaces. The higher layer understands the structure of the data in the SDU, but the lower layer at the interface does not; moreover, the lower layer treats the SDU as the payload, undertaking to get it to the same interface at the destination. In order to do this, the protocol (lower) layer will add to the SDU certain data it needs to perform its function, which is called encapsulation. For example, it might add a port number to identify the application, a network address to help with routing, a code to identify the type of data in the packet and error-checking information. All this additional information, plus the original service data unit from the higher layer, constitutes the protocol data unit at this layer. The SDU and metadata added by the lower layer can be larger than the maximum size of that layer's PDU (known as the maximum transmission unit; MTU). When this is the case, the PDU must be split into multiple payloads of a size suitable for transmission or processing by the lower layer, a process known as IP fragmentation. The significance of this is that the PDU is the structured information that is passed to a matching protocol layer further along on the data's journey, which allows the layer to deliver its intended function or service. The matching layer, or peer, decodes the data to extract the original service data unit, decides if it is error-free and where to send it next, etc. Unless we have already arrived at the lowest (physical) layer, the PDU is passed to the peer using services of the next lower layer in the protocol stack. When the PDU passes over the interface from the layer that constructed it to the layer that merely delivers it (and therefore does not understand its internal structure), it becomes a service data unit to that layer. The addition of addressing and control information (encapsulation) to an SDU to form a PDU and the passing of that PDU to the next lower layer as an SDU repeats until the lowest layer is reached and the data passes over some medium as a physical signal. The above process can be likened to the mail system in which a letter (SDU) is placed in an envelope on which an address (addressing and control information) is written, making it a PDU. The sending post office might look only at the postcode and place the letter in a mailbag so that the address on the envelope can no longer be seen, making it now an SDU. The mailbag is labeled with the destination postcode and so becomes a PDU until it is combined with other bags in a crate, when it is now an SDU, and the crate is labeled with the region to which all the bags are to be sent, making the crate a PDU. When the crate reaches the destination matching its label, it is opened and the bags (SDUs) removed only to become PDUs when someone reads the code of the destination post office. The letters themselves are SDUs when the bags are opened but become PDUs when the address is read for final delivery. When the addressee finally opens the envelope, the top-level SDU, the letter itself, emerges.

Examples

OSI model Protocol data units of the OSI model are:

The Layer 4: transport layer PDU is the segment. The Layer 3: network layer PDU is the packet or the datagram. The Layer 2: data link layer PDU is the frame. The Layer 1: physical layer PDU is the bit or, more generally, symbol. Given a context pertaining to a specific OSI layer, PDU is sometimes used as a synonym for its representation at that layer.

Internet protocol suite Protocol data units for the Internet protocol suite are:

The transport layer PDU is the TCP segment for TCP, and the datagram for UDP The Internet layer PDU is the packet. The link layer PDU is the frame. On TCP/IP over Ethernet, the data on the physical layer is carried in Ethernet frames.

Internetwork packet exchange (IPX) For IPX, the IPX packet is the network layer PDU for IPX, and can simultaneously be used in the transport layer as well. The SPX packet on top of IPX, is a transport layer PDU. The link-layer PDU is the frame.

ATM The data link layer PDU in Asynchronous Transfer Mode (ATM) networks is called a cell.

Media access control protocol data unit A media access control protocol data unit (MAC PDU or MPDU) is a message that is exchanged between media access control (MAC) entities in a communication system based on the layered OSI model. In systems where the MPDU may be larger than the MAC service data unit (MSDU), the MPDU may include multiple MSDUs as a result of packet aggregation. In systems where the MPDU is smaller than the MSDU, then one MSDU may generate multiple MPDUs as a result of packet segmentation.

See also Distributed Interactive Simulation § Protocol data units IP fragmentation Smart card application protocol data unit Transaction Protocol Data Unit

References

… excerpt ends here. Continue reading the full article.

Illustrations

Worked examples

Example 1 — a first encounter with Protocol data unit

Start with the simplest possible case. Write down what Protocol data unit claims or describes in one sentence, then invent the smallest concrete situation in which that sentence is true. In science, the smallest case is usually a single object, a single equation or a single measurement. Check that every symbol or term in your sentence has a meaning in that case.

Example 2 — changing one variable

Take the situation from Example 1 and change exactly one quantity: double it, halve it, or set it to zero. Predict what should happen to Protocol data unit before you calculate. Comparing your prediction with the result is the fastest way to find out whether you understand the idea or only the words.

Example 3 — an exam-style question

Typical questions about Protocol data unit ask you to (a) state it precisely, (b) apply it to given data, and (c) explain a limitation. Practise writing all three answers in under five minutes; the third part is what separates a full-mark answer from an average one.

Applications of Protocol data unit

In research: Protocol data unit appears in science research whenever the underlying quantities have to be modelled precisely. Papers usually cite it as a starting assumption and then explore where it breaks down.
In technology and industry: Engineering practice reuses Protocol data unit in design rules, simulations and safety margins. Knowing the idea lets you read a specification sheet and understand why the numbers look the way they do.
In the classroom: Protocol data unit is common in secondary-school and first-year university syllabi. It links to neighbouring topics Data transmission, Packets (information technology), so understanding it makes those chapters shorter.
In everyday life: Look for Protocol data unit outside the textbook — in sport, cooking, traffic, electronics or the sky above you. An example you found yourself is remembered far longer than one you were given.

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How to study Protocol data unit in 20 minutes

Read the reference excerpt below once, without taking notes.
Close the page and write down what Protocol data unit means in your own words.
Compare your version with the excerpt and mark what you missed.
Work through the three examples above with pen and paper.
Explain Protocol data unit out loud to somebody else — or to Teacher Smith in the lgStudy chat.

Frequently asked questions

What is Protocol data unit in simple terms?

Why does Protocol data unit matter?

Because it connects several science ideas at once: it gives you a definition you can apply, a quantity you can calculate, and a way to check whether a result is plausible.

How should I study Protocol data unit?

Read the excerpt, restate it from memory, then work through the examples and applications listed on this page. The five-step study plan above takes about twenty minutes.

What does this page cover?

It gives you a compact reference excerpt plus original lgStudy explanations, examples, applications and study material on Protocol data unit.