) [12,13]. DBA allows upstream timeslots to grow and shrink based on the distribution of upstream traffic loads and operates on a timescale of milliseconds. Of course, the total of all times- lots cannot be greater than the length of a single upstream frame. Layer 5 T1/E1 T1/E1 or more Video Video Video Layer 4 TCP+UDP, etc. TCP+UDP, etc. TCP+UDP, etc. Layer 3 Ethernet Ethernet Layer 2 AAL1/AAL2 ATM cell Generic frame Ethernet frame Layer 1 GPON encapsulation method Equipment and transmission infrastructure FIGURE 11.3 Layering of an access network based on GPON transport. The New Access Network Systems and Enabling Technologies GPON Downstream frame is divided in transmission containers (T-CONTs), that are the upstream timeslots. There are much more T-CONTs than ONUs and DBA works exploit- ing this property. An ONU must have assigned at least one T-CONT, but most have several T-CONTs whose number is managed by the DBA following the bandwidth needs of the different users. Besides a drastic enhancement in the upstream bandwidth exploitation, DBA also allows a possible service level agreement (SLA) with a PON customer to be managed and controlled. This is a very important characteristic, allowing a shared medium-based transmission system to be used to serve business customers with high requirements. The ITU-T standard does not prescribe the specific DBA algorithm to be used, but defines two classes of possible algorithms. If one of the so-called status reporting (SR) DBA is used, all ONUs report their upstream data queue occupancy, to be used by the OLT calculation process. Among the T-CONT assigned to the same ONU, each individual element may have its own traffic class, cor- responding to the SLA of the conveyed service. By combining the queue occupancy infor- mation and the provisioned SLA of each T-CONT, the OLT can effectively optimize the upstream bandwidth allocation. If, on the other hand, a non-status reporting (NSR) DBA is used, ONUs do not provide explicit queue occupancy information. Instead, the OLT est