Let’s talk about DBA technology. The DBA technology is used to dynamically allocate upstream bandwidths within microseconds or milliseconds. Why is DBA required? DBA is used to transmit the data as much as possible on conditions that not congestion occurs on the upstream optical path to increase the upstream bandwidth utilization. What is the advantage of DBA? Compared with the static bandwidth allocation, DBA helps fully utilize upstream line bandwidths, specifically, when some users do not need bandwidths, idle timeslots are allocated to users who require large bandwidths. Therefore, DBA applies to services that require burst bandwidths. DBA has two implementation modes: status-report DBA (SR-DBA) and non-status-report DBA (NSR-DBA).
The GPON system uses DBA only in the upstream direction because data is transmitted using TDMA mode. Each upstream GEM frame is put in to the T-CONT of specified type to form a GTC frame and then the GTC is sent in the timeslot specified by the previous downstream frame. How the OLT allocates timeslots to ONTs? In the SR-DBA mode, the ONU reports bandwidth requirements to the OLT. After receiving the status report of the ONU, the OLT performs a DBA calculation, updates BWmap, and uses the next downstream frame to deliver calculation result to the ONU. After receiving the BWmap from the OLT, the ONU sends upstream data based on the timeslots allocated by the OLT. This is the main mode used by Huawei to implement DBA in the GPON system. In the NRS-DBA mode, the ONU does not actively report to the OLT the information about its data to be sent. Instead, the ONU sends data whenever there is data. The OLT monitors and analyzes the received ONU data in real time. If the OLT finds that the data traffic sent by a certain ONU is always large in a period the OLT adjust the transmission period of this ONU, allocates longer period to this ONU, and dynamically allocates timeslots to the ONU based on the specific data traffic sent by the ONU. This mode is seldom used currently. T-CONT is the basis for DBA implementation. The T-CONT dynamically receives grants delivered by the OLT. These grants are used to manage the upstream bandwidth allocation at the transmission convergence layer of the PON system and improve the upstream bandwidth utilization in the PON system.
