Downstream frames are layering maps data using GTC (GPON Transmission Convergence). GPON frames have different amount of bytes with different transmission rates. The higher the rate, the bigger is the frame. ONE 1.24416 Gbps frame is 19440 bytes long, while one 2.48832 Gbps frame is 38880 bytes.

Figure 1. GPON downstream frame format

Figure 1 shows the detailed PCBd (Physical Control Block downstream) explanation. PCBd is located in the beginning of downstream frame, followed by the payload containing GEM frames or ATM cells. Sometimes payload might contain both types, if so the amount of ATM cells is specified in the ALen field of PCBd. In the payload ATM cells always go first and GEM frames go second. PCBd fields are explained in the Table 1 below.

Table 1.

BW map field allows ONU/ONT to coordinate and send upstream data in specific time slot that does not interference with other ONUs/ONTs.