Service Data Unit

Service Data Unit (SDU ) and Protocol Data Unit (PDU ) are concepts from the field of network protocols.

Network protocols are usually built up in layers, for example in accordance with the OSI reference model. Different hierarchical layers have different tasks. They each implement a protocol according to their task. The layers are usually numbered, with a number N.

Typically, each layer adds the data to be transmitted own management information. The complete set of data and management information of a layer N is called a Protocol Data Unit (PDU ) of this layer. The PDU is the complete message, which implements the protocol of layer N.

If this PDU now passed to a lower protocol layer N- 1, it represents the view of the layer N-1 represent only the data which is to transfer the layer N-1. The layer N-1 provides the service (service) ready to transfer just those PDUs of layer N. Therefore, the data to be transmitted is called ( PDU N) on the layer N-1, the service data unit (SDU) of layer N -1.

In summary, what is on layer N PDU, called on the layer N-1 SDU - the part transferred from the layer N-1 data set, which was formed on the N layer. Nothing else means the simple formula

PDU (N) = SDU (N - 1 )         PCI = (N ) (PDU N 1) footer (N)         = PCI ( N ) SDU (N ) Footer (N) Where Protocol Control Information - PCI ( N) ( header) and Footer (N ) represent the added by the protocol in layer N management information.

From the formula follows by implication also

SDU (N) = (PDU N 1) What is called on a level N, the SDU is called N on the level above 1 PDU.

In practice PDU and SDU are often confused, or both referred to as PDU. This is because it alone of the layer on which you ( mentally ) is located, depends on whether the same record is a PDU or SDU.

Likewise, the PDU is often confused with the term Payload Data Unit. The latter is only blurred defined and referred to in accurate form, the ' payload ' (payload ) of a data packet - so what is, strictly speaking, the SDU on a layer. If then Payload Data Unit still abbreviated to PDU, the likelihood of confusion is particularly large.

PDUs in relation to each of the first four layers of the OSI model:

From Layer 5 content broadcasted are called data.