eptual network architecture planning. It is also useful in the definition of network development, roll-out, migration and network re-use strategies. Dimensioning results can be used to work out alternative network scenarios and to give technical support in pricing exercises. The number of core network elements and traffic per site can be two prerequisites for input to a dimensioning tool. 4.11.1 GPRS Protocol Stacks and Overheads Various protocols used at various interfaces add/remove data in the form of IP packets. These are called overheads. These overheads are considerable and have to be taken into account. They cannot be neglected during dimensioning. To enable the various overheads that have to be taken into account in the dimen- sioning of the packet core network to be understood, first the protocol stacks in the GPRS network need to be explained. Each layer (L) in the protocol stack will add additional bits to the original payload. In addition, depending on the interface, this will result in different overheads being used for network di- mensioning for different nodes. Figure 4.46 shows the different protocol stacks at the different interfaces for the GPRS network. Here is a brief description of each of the protocols shown in Figure 4.46: GPRS Tunnelling Protocol for the user plane (GTPU). This protocol tunnels user data between GPRS support nodes in the backbone network. The GTP-U header size is 16 bytes. Dimensioning GTP-U GTP-U Gn or Gp Figure 4.47 Protocol stack between two GSNs The UDP carries GTP packet data units for protocols that do not need a reliable transport link and provides protection against corrupted GTP packet data units. The UDP header size is 8 bytes. IP. This is the backbone network protocol used for routing user data and control signalling. Presently IPv 4 is being used in networks worldwide. The IP header size is 20 bytes. Base Station Subsystem GPRS Protocol (BSSGP). The BSSGP is used over the Gb interface between the BSC and the SGSN. BSSGP does not perform error correction. The BS