SGP header size is 30 bytes. Network service (NS). This layer transports BSSGP PDUs. The NS is based on the frame relay connection between the BSS and the SGSN, and can multihop and traverse a network of frame relay switching nodes. The NS layer header size is 4 bytes while the frame relay header size is 6 bytes. For GSN-to-GSN traffic, i.e. traffic between SGSN and GGSN (own or other PLMN's GGSN), the protocol stack shown in Figure 4.47 is used. GTP, UDP and IP overheads must be considered in the Gn interface between 3G SGSN and the GGSN. In the case of the 2G SGSN, the Gb overhead will be considered in the dimensioning instead as it is higher than the Gn overhead in order to cater for the worst-case scenario. A proposed GPRS packet core network is dimensioned to support the total estimated or projected GPRS data traffic expected during the busy hour, including an appropriate overhead traffic level in addition to the actual data payload. Different applications and protocols generate packets of variable lengths. It is mainly the length of packet headers that drives the overhead size and it remains constant while the packet size varies depending on the application. Overheads as a percentage of total traffic therefore vary quite significantly for every packet length change and may be calculated as a function of the payload based on the average packet size. For example, for a packet size of 512 bytes, a 14.25 % protocol overhead is expected at the Gb interface while an 8.59 % protocol overhead is expected at the Gn interface. These overhead figures will be input to the GPRS data traffic to take into account the impact these overheads will have on the network dimensioning results. Dimensioning Inputs and Assumptions There are several parameters that can be taken as input from the operator based on their marketing requirements, e.g. the subscriber and traffic forecasts and the number of core sites. In case data are not provided by the operator, then some suitable figures can be assumed based on field experience of som