1-5Bandwidth AllocationOnce the extended OAM connection is established, downlink data transmission can begin. Uplink datatransmission can begin only after uplink bandwidth is allocated.In bandwidth allocation, mainly two types of MPCP messages: GATE and REPORT, are used:z A GATE message is sent by an OLT to assign a transmission timeslot to an ONU.z A REPORT message is sent by an ONU to feed back the local status information, such as bufferoccupancy, to the OLT, helping the OLT assign timeslots intelligently.An OLT allocates bandwidth to an ONU as follows:1) The OLT sends a GATE message to notify the ONU of the timeslot for sending a REPORTmessage.2) The ONU sends a REPORT message within the assigned timeslot to report its local statusinformation to the OLT.3) Upon receiving the REPORT message from the ONU, the OLT, based on the current bandwidth ofthe system, assigns the ONU a data transmission timeslot, which contains the start time and lengthfor transmitting data by the ONU.4) The ONU receives the GATE message and waits for the arrival of the start time contained in theGATE message. Once the start time is reached, data transmission begins.5) The bandwidth allocation is complete.Data TransmissionDownlink data transmissionDownlink data is broadcast to the ONUs, with each ONU receiving only the packets destined to it anddiscarding other packets, as shown in Figure 1-3.Figure 1-3 Downlink data transmission in an EPON systemUplink data transmissionAs shown in Figure 1-4, each ONU buffers the data frames received from users and sends the buffereddata frames at the full wire-speed (1000 Mbps) once the timeslot for the ONU arrives.
