July 30, 2019
EPON implements a point-to-point emulation protocol sublayer that makes the point-to-multipoint network topology a collection of point-to-point links for higher layers. This sublayer is implemented by adding an LLD (Logical Link Identification) logical link identifier in front of each datagram. The LLID will replace the two bytes in the preamble. Data is downlinked from the OLT to multiple ONUs. According to the IEEE802.3ah protocol, the header of each data frame contains the logical link identifier (LLID) of the specific ONU allocated at the time of registration, indicating that the data frame is Give the only one of the ONUs (ONU1, ONU2, ONU3...ONUn). In addition, some data frames set the broadcast mode bit (the first bit to the left of the LLID) to be a broadcast message to all ONUs or a special set of ONU multicast messages, unicast messages, and the broadcast mode position is zero. When the unicast data signal arrives at the ONU, the ONU makes a judgment on the physical layer according to the LLID, receives its own data frame, and discards the data frames that are sent to other ONUs. If the data frame in the broadcast mode position 1 arrives at the ONU, the ONU determines the acceptance mode according to the destination MAC address, and if the MAC address is the broadcast MAC, it is received. If it is a multicast MAC address, the ONU receives the data packet when it is in the working mode of transparently transmitting the multicast stream. If the ONU is in the working mode of filtering the multicast stream, the destination MAC address and the ONU multicast MAC address can be matched. Otherwise, the packet is discarded. . Generally, when the ONU is connected to the corridor switch, the multicast is set to transparent transmission mode. If the ONU is directly connected to the user PC, the multicast setting filtering mode is adopted.
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