e GPON, different users are assigned different time-slots in a TDM frame and bidirectional transmission on the fiber infrastructure is realized using different wavelength SO that only a single fiber tree is needed, as shown in Figure 11.2 [11]. PON capacity 1:N split 1:N split OLT trunk capacity (north bound interface) 1:N split FIGURE 11.2 Topology of the set of GPON ODNs man- aged by a single OLT. Telecommunication Networks In this figure, a realistic OLT is shown where a set of PONs is managed by the same equipment. Hundreds of PONs can be terminated on the same OLT, optimizing on one side real estate and power consumption, and on the other, the packet multiplexing oper- ated by the OLT switch. Due to the topology of the ODN, a further issue arises: the different end users are at dif- ferent distances from the splitter at the tree branching point. In this condition, each end user device (the ONU), has to transmit with a certain delay with respect to the reference time-frame in order to allow correct reconstruction of the signal at the branching splitter. This delay is continuously updated by a particular PON control procedure that measures the optical distance between the ONU and the branching point and drives the ONU asyn- chronous transmission. This procedure is implemented into the physical layer PON processor that is generally a part of a single chip running all the physical layer protocols, comprising maintenance and critical alarms. Another important element of the logical architecture of the GPON is the GEM: the GPON enveloping method, allowing almost every packet-oriented format to be conveyed through the PON, without the inefficiencies that were characteristics of ATM, that were deputed to do this in APON and BPON [11]. The layering of a PON network transporting Ethernet over ATM, native Ethernet and Ethernet over GFP (generic framing) is shown in Figure 11.3, where the role of the GEM evident. Another important physical layer procedure implemented on all the GPONs is dynamic bandwidth allocation (DBA