we have to take into account a gain equal approximately to 4-5 from the ONU and a further gain of about 2 coming from the DBA. VolP 288 kbit/s Web browsing 280 Mbit/s Free band mbit/s FIGURE 11.4 896 Mbit/s Occupied and free bandwidth in our example of GPON installed in a populated area. Telecommunication Networks At the end, assuming that the ONU gains a bit less than the OLT and that an SR DBA is used, the user has the same system experience in terms of service quality and speed that he would have with deterministic TDM multiplexing and service activity equal to one with a reserved bandwidth of about 320 Mbit/s available. This calculation seems strange, but it is a fact that generally, GPON users do not perceive the transmission capacity unbalance of the network. 11.2.2.4 Functional Structure of a GPON OLT and ONU In order to implement, with a suitable hardware and software structure, the functionalities we have seen, the GPON OLT and ONU have rich functional structures that assure both control and management functionalities and quality of service (QoS) implementation [15]. The functional structure of the OLT is shown in Figure 11.5. From a functional point of view, the OLT is divided into three areas that are often referred to as shells. The core shelf is the functional area terminating the physical layer on the side of the ODNs. We have to remember that each OLT generally terminates several GPONs, also a few hundred in the case of big OLTs. Thus, it is important to design this shell in a compact and functional way. Besides the physical interfaces with transceivers and all the transmission and detection electronics, the multiplex and demultiplex functions that are needed to create and terminate the GPON frame are also located in the core shell. interface Tributary function interface Digital function Transmission cross multiplex connect function function interface function Tributary interface Core shell function Common shell Signaling processing function Power supply function function Service shell FIGURE 11.5 