EPON implements a point-to-point simulation protocol sublayer, which makes the point-to-multipoint network topology a set of point-to-point links for high-level. This sub-layer is realized by adding an LLD (Logical Link Identification) logical link identifier to the front of each datagram. The LLID replaces two bytes in the preamble. Data is downloaded broadcasting from OLT to multiple ONUs. According to the IEEE802.3ah protocol, the header of each data frame contains the logical link identifier (LLID) assigned to a specific ONU at the time of previous registration, which indicates that this data frame is the only one for ONU (ONU1, ONU2, ONU3... ONUn). In addition, some data frames set the broadcast mode bit (the first bit on the left of LLID) to 1, which can be a multicast message for all ONUs or a special group of ONUs, a unicast message and a zero broadcast mode position. When unicast data signals arrive at ONU, ONU judges on the physical level according to LLID, receives its own data frames, and discards those to other ONUs. If the data frame of broadcast mode location 1 arrives at ONU, ONU decides the mode of reception according to the destination MAC address, and if this MAC address is broadcast MAC, it receives. If it is multicast MAC, when ONU is in the working mode of transmissive multicast stream, it receives data packets; if ONU is in the working mode of filtering multicast stream, the destination MAC address and ONU multicast MAC table can match, then it will be accepted, otherwise it will be discarded. Generally, when ONU connects to the corridor switch, multicast is set as the transmission mode. If ONU directly connects to the user PC, multicast is set as the filtering mode.
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