y a packet-switching network that transfers the (packet) data in packets of variable lengths. These packets are known as frames. The network service (NS) layer controls the frame-relay layer. There are two hardware units associated with the Gb interface or the frame-relay network: PCU and PAPU. The packet control unit (PCU) is located on the BSS side, while the packet processing unit (PAPU) is located on the SGSN side. For Gb interface dimensioning there are three major aspects: BSC (or PCU) dimensioning, SGSN dimensioning, and frame relay planning. We have already seen the inputs that are required for PCU dimensioning. For BSC dimensioning from the perspective of packet core network planning, the only input required is the BSC's PCU handling capacity. SGSN Dimensioning SGSN dimensioning basically results in the number of SGSNs that may be required in a GPRS network. Major inputs are the SGSNs own capacity in terms of handling the amount of data, subscribers and the number of PCMs. The data handling or the processing capacity will determine the number of SGSNs required in a given network based on the amount of traffic. Calculations should not be done based on the traffic generated by the total number of BSCs, because at a given moment all the BSCs will not be fully loaded. Similarly, not all subscribers will be using the GPRS at the same time. So, a reduction factor (i.e. what percentage of users will be using the GPRS network at the same time) should be planned carefully. Apart from this, every SGSN (depending on the vendor) has its own capacity for handling the number of frame-relay links. Frame-relay Planning There are many ways in which to transfer data from the BSC to the SGSN. This may include techniques such as sending voice and data traffic separately, or multiplexing both the voice and data traffic. In the initial phases of network launch, the PS data might be negligible in terms of the amount of data handled by each PCU. Thus, the traffic from some PCUs is then multiplexed and transmitted to the SGSN. T