In
Open Systems Interconnection
The Open Systems Interconnection model (OSI model) is a conceptual model that 'provides a common basis for the coordination of SOstandards development for the purpose of systems interconnection'. In the OSI reference model, the communications ...
(OSI) terminology, a service data unit (SDU) is a unit of data that has been passed down from an
OSI layer or
sublayer to a lower layer. This unit of data (SDU) has not yet been
encapsulated into a
protocol data unit
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 c ...
(PDU) by the lower layer. That SDU is then encapsulated into the lower layer's PDU and the process continues until reaching the PHY,
physical, or lowest layer of the OSI ''stack''.
The SDU can also be thought of as a set of data that is sent by a user of the services of a given layer, and is transmitted semantically unchanged to a peer service user.
SDU and PDU
It differs from a PDU in that the PDU specifies the data that will be sent to the peer protocol layer at the receiving end, as opposed to being sent to a lower layer.
The SDU accepted by any given layer (n) from layer (n+1) above, is a PDU of the layer (n+1) above. In effect the SDU is the '
payload' of a given PDU. The layer (n) may add
headers or
trailers
Trailer may refer to: a
Transportation
* Trailer (vehicle), an unpowered vehicle pulled by a powered vehicle
** Bicycle trailer, a wheeled frame for hitching to a bicycle to tow cargo or passengers
** Full-trailer
** Semi-trailer
**Horse trail ...
, or both, to the SDU and may do other kinds of reformatting, recoding, splitting or transformations on the data, forming one or more layer (n) PDUs. The added headers or trailers and other possible changes are part of the process that makes it possible to get data from a source to a destination. Layer (n) may also generate additional layer (n) PDUSs. Each unit of data that layer (n) gives to layer (n-1) below is in turn handed down as a layer (n-1) SDU.
When the PDU of layer (n+1), plus any metadata layer (n) would add; would exceed the maximum size a layer-n PDU can be (called layer (n)
's
maximum transmission unit
In computer networking, the maximum transmission unit (MTU) is the size of the largest protocol data unit (PDU) that can be communicated in a single network layer transaction. The MTU relates to, but is not identical to the maximum frame size tha ...
); the SDU must be split into multiple payloads for layer (n); a process known as
fragmentation.
MAC SDU
MAC SDUS or MSDUS are data units transmitted between other
Media access controllers on a lower OSI Layer. The PDU counterpart
MAC PDU that does the same thing but on the same OSI Layer. When there are larger MAC PDU's as MAC SDU's in the system, the MAC PDU includes more MAC SDU's, because of
packet aggregation
{{Use mdy dates, date = March 2019
In a packet-based communications network, packet aggregation is the process of joining multiple packets together into a single transmission unit, in order to reduce the overhead associated with each transmissi ...
. If the MAC PDU's are smaller then the MAC SDU's includes more MAC PDU's, because of
packet segmentation
In data communications networks, packet segmentation is the process of dividing a data packet into smaller units for transmission over the network. Packet segmentation happens at layer four of the OSI model; the transport layer. Segmentation ma ...
.
See also
*
Federal Standard 1037C
Federal Standard 1037C, titled Telecommunications: Glossary of Telecommunication Terms, is a United States Federal Standard issued by the General Services Administration pursuant to the Federal Property and Administrative Services Act of 1949, a ...
References
Telecommunications standards
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