, which the lower layer has not yet encapsulated into a protocol data unit. A service data unit is 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. The protocol data unit at a layer N is the service data unit of layer N 1. In fact, the service data unit is the payload of a given protocol data unit. That is, the process of changing a service data unit to a protocol data unit consists of an encapsulation process, performed by the lower layer. All data contained in the service data unit becomes encapsulated within the protocol data unit. The layer N - 1 adds headers/subheaders and padding bits (if necessary to adjust the size) to the service data unit, transforming it into the protocol data unit of layer N. The added headers/subheaders and padding bits are part of the process used to make it possible to send data from a source node to a destination node. 72 Chapter 1 transport network congestion. Nevertheless, due to performing flow control in the CU and the RLC transmit side in the DU, double buffering is needed for transmission. Option 4: In this case, MAC sublayer, physical layer, and RF functions are processed in the DUs, whereas PDCP and RLC protocols are processed in the CU. No particular advantage was shown for this option. Option 5: In this option, the RF, physical layer, and some part the MAC sublayer func- tions (e.g., HARQ protocol) are implemented in the DUs. The upper protocol stack is implemented in the CU. Therefore by splitting the MAC sublayer into two entities (e.g., upper and lower MAC), the services and functions provided by the MAC sublayer will be implemented in the CU and/or the DU. As an example, the centralized scheduling function located in the upper MAC will be in charge of the control of multiple lower MAC sublayers. The inter-cell interference coordination located in the upper MAC will be responsible for interference coordination. Time-critical functions in the lower MAC may include functions with str