Protocol data unit
Encyclopedia
In telecommunication
s, the term protocol data unit (PDU) has the following meanings:
PDUs are relevant in relation to each of the first 4 layers of the OSI model
as follows:
(Layer 5 and above are referred to as data
.)
Given a context pertaining to a specific layer, PDU is sometimes used as a synonym for its representation at that layer.
, 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". For example, sending ones and zeros across a wire, fiber, etc. is done by the physical layer, organizing the ones and zeros into chunks of data and getting them safely to the right place on the wire is done by the data link layer, passing data chunks over multiple connected networks is done by the network layer and delivery of the data to the right software application at the destination is done by the transport layer. Between the layers (and between the application and the top-most layer), the layers pass service data units across the interfaces. The application or higher layer understands the structure of the data in the SDU, but the lower layer at the interface does not; it treats it as payload, undertaking to get it to the same interface at the destination. In order to do this, the protocol layer will add to the SDU certain data it needs to perform its function. 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 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 that 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, decide 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 (which is called 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 is written an address (addressing and control information) making it a PDU. The sending post office might look only at the post code and place the letter in a mail bag so that the address on the envelope can no longer be seen, making it now an SDU. The mail bag is labelled with the destination post code 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 labelled 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.
Telecommunication
Telecommunication is the transmission of information over significant distances to communicate. In earlier times, telecommunications involved the use of visual signals, such as beacons, smoke signals, semaphore telegraphs, signal flags, and optical heliographs, or audio messages via coded...
s, the term protocol data unit (PDU) has the following meanings:
- InformationInformationInformation in its most restricted technical sense is a message or collection of messages that consists of an ordered sequence of symbols, or it is the meaning that can be interpreted from such a message or collection of messages. Information can be recorded or transmitted. It can be recorded as...
that is delivered as a unit among peer entities of a networkComputer networkA computer network, often simply referred to as a network, is a collection of hardware components and computers interconnected by communication channels that allow sharing of resources and information....
and that may contain control information, addressNetwork addressNetwork address may refer to:*Base address*Classful address*IP address*IPX address*Logical address*Network layer address,*X.25/X.21 address*MAC address-See also:*Autonomous system *Host address*Link layer*Subnet mask...
information, or dataDataThe term data refers to qualitative or quantitative attributes of a variable or set of variables. Data are typically the results of measurements and can be the basis of graphs, images, or observations of a set of variables. Data are often viewed as the lowest level of abstraction from which...
. - In a layered system, a unit of data which is specified in a protocolCommunications protocolA communications protocol is a system of digital message formats and rules for exchanging those messages in or between computing systems and in telecommunications...
of a given layerLayerLayer may refer to:* A layer of archaeological deposits in an excavation* A layer hen, a hen raised to produce eggs* Stratum, a layer of rock or soil with internally consistent characteristics...
and which consists of protocol-control informationProtocol-control informationIn telecommunication, the term protocol-control information or PCI has the following meanings:# The queries and replies among communications equipment to determine the respective capabilities of each end of the communications link....
and possibly userUser (telecommunications)In telecommunications, a user is a person, organization, or other entity that employs the services provided by a telecommunication system, or by an information processing system, for transfer of information....
data of that layer. For example: Bridge PDU or iSCSIISCSIIn computing, iSCSI , is an abbreviation of Internet Small Computer System Interface, an Internet Protocol -based storage networking standard for linking data storage facilities. By carrying SCSI commands over IP networks, iSCSI is used to facilitate data transfers over intranets and to manage...
PDU
PDUs are relevant in relation to each of the first 4 layers of the OSI model
OSI model
The Open Systems Interconnection model is a product of the Open Systems Interconnection effort at the International Organization for Standardization. It is a prescription of characterizing and standardizing the functions of a communications system in terms of abstraction layers. Similar...
as follows:
- The Layer 1Physical layerThe physical layer or layer 1 is the first and lowest layer in the seven-layer OSI model of computer networking. The implementation of this layer is often termed PHY....
(Physical Layer) PDU is the bitBitA bit is the basic unit of information in computing and telecommunications; it is the amount of information stored by a digital device or other physical system that exists in one of two possible distinct states... - The Layer 2Data link layerThe data link layer is layer 2 of the seven-layer OSI model of computer networking. It corresponds to, or is part of the link layer of the TCP/IP reference model....
(Data Link Layer) PDU is the frame - The Layer 3Network LayerThe network layer is layer 3 of the seven-layer OSI model of computer networking.The network layer is responsible for packet forwarding including routing through intermediate routers, whereas the data link layer is responsible for media access control, flow control and error checking.The network...
(Network Layer) PDU is the packet - The Layer 4Transport layerIn computer networking, the transport layer or layer 4 provides end-to-end communication services for applications within a layered architecture of network components and protocols...
(Transport Layer) PDU is the segment (e.g. TCP segment)
(Layer 5 and above are referred to as data
Data
The term data refers to qualitative or quantitative attributes of a variable or set of variables. Data are typically the results of measurements and can be the basis of graphs, images, or observations of a set of variables. Data are often viewed as the lowest level of abstraction from which...
.)
Given a context pertaining to a specific layer, PDU is sometimes used as a synonym for its representation at that layer.
Packet-switched data networks
In the context of packet-switched data networksPacket switching
Packet switching is a digital networking communications method that groups all transmitted data – regardless of content, type, or structure – into suitably sized blocks, called packets. Packet switching features delivery of variable-bit-rate data streams over a shared network...
, a protocol data unit (PDU) is best understood in relation to a service data unit
Service Data Unit
In Open Systems Interconnection terminology, a service data unit is a unit of data that has been passed down from an OSI layer to a lower layer and that has not yet been encapsulated into a protocol data unit by the lower layer...
(SDU). The features or services of the network are implemented in distinct "layers". For example, sending ones and zeros across a wire, fiber, etc. is done by the physical layer, organizing the ones and zeros into chunks of data and getting them safely to the right place on the wire is done by the data link layer, passing data chunks over multiple connected networks is done by the network layer and delivery of the data to the right software application at the destination is done by the transport layer. Between the layers (and between the application and the top-most layer), the layers pass service data units across the interfaces. The application or higher layer understands the structure of the data in the SDU, but the lower layer at the interface does not; it treats it as payload, undertaking to get it to the same interface at the destination. In order to do this, the protocol layer will add to the SDU certain data it needs to perform its function. 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 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 that 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, decide 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 (which is called 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.
See also
- Smart card application protocol data unit
- Frame (networking)
- MAC protocol data unit
- MAC service data unit
- Distributed Interactive Simulation protocol data unit
External links
- comp.protocols.iso FAQ (search for "PDU")