What is the difference between PDU and SDU?

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.

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