1.1 Passive optical splitter
Passive optical splitter is an important component of PON network. The function of the passive optical splitter is to divide the optical power of one input optical signal into multiple outputs. Typically, the splitter achieves light splitting from 1:2 to 1:32 or even 1:64. The characteristic of passive optical splitter is that it does not need power supply and has strong environmental adaptability. Since the EPON upstream channel is time-division multiplexed by all ONUs, each ONU can send data within a specified time window. Therefore, the EPON upstream channel transmits burst signals, which requires the use of optical devices that support burst signals in ONUs and OLTs.
Passive optical splitters in PON networks are generally divided into two types: the traditional fusion taper splitter and the newly emerging planar optical waveguide splitter.
1.2 Physical topology
The EPON network adopts a point-to-multipoint topology structure instead of a point-to-point structure, which greatly saves the amount of optical fiber and management costs. PON OLT equipment reduces the number of lasers required by the central office, and the OLT is shared by many ONU users. In addition, EPON uses Ethernet technology and standard Ethernet frames to carry the current mainstream service—IP service without any conversion.
1.3 Burst synchronization of EPON physical layer
In order to reduce the cost of ONU, the key technologies of the EPON physical layer are concentrated on the OLT, including: rapid synchronization of burst signals, network synchronization, power control of optical transceiver modules, and adaptive reception.
Since the signal received by the OLT is a burst signal of each ONU, the OLT must be able to achieve phase synchronization in a short time, and then receive data. In addition, because the uplink channel adopts the TDMA mode, and the 20km optical fiber transmission delay compensation technology realizes the time slot synchronization of the entire network, the data packets arrive at the time slot determined by the OBA algorithm. In addition, due to the different distances of each ONU from the OLT, for the receiving module of the OLT, the power of different time slots is different. In DBA applications, even the power of the same time slot is different, which is called the near-far effect. Therefore, the OLT must be able to quickly adjust its “0″ and “1″ level decision points. In order to solve the “far-near effect”, a power control scheme has been proposed, and the OLT informs the ONU of the transmit power level through operation and maintenance management (OAM) packets after ranging. Because this scheme will increase the ONU cost and the complexity of the physical layer protocol, and limit the line transmission performance to the ONU level farthest from the OLT, it has not been adopted by the EFM working group.