Open Systems Interconnection The Open Systems Interconnection (OSI) model is a reference model developed by the International Organization for Standardization (ISO) that "provides a common basis for the coordination of standards development for the purpose of systems inter ...

(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 Payload is the object or the entity that is being carried by an aircraft or launch vehicle. Sometimes payload also refers to the carrying capacity of an aircraft or launch vehicle, usually measured in terms of weight. Depending on the nature of t ...

' of a given PDU. The layer (n) may add headers or

trailers Trailer may refer to: Transportation * Trailer (vehicle), an unpowered vehicle pulled by a powered vehicle ** Baggage trailer, a large flatbed baggage trolley ** Bicycle trailer, a wheeled frame for hitching to a bicycle to tow cargo or passen ...

, 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 Packet may refer to: * A small container or pouch ** Packet (container), a small single use container ** Cigarette packet ** Sugar packet * Network packet, a formatted unit of data carried by a packet-mode computer network * Packet radio, a form ...

. If the MAC PDU's are smaller then the MAC SDU's includes more MAC PDU's, because of

packet segmentation Packet may refer to: * A small container or pouch ** Packet (container), a small single use container ** Cigarette packet ** Sugar packet * Network packet, a formatted unit of data carried by a packet-mode computer network * Packet radio, a form ...

References

Telecommunications standards {{telecom-stub

SDU and PDU

MAC SDU

See also

References