If the OLT (the central node of the EPON) were to be implemented as a PHY with a single MAC attached to it, this would cause serious trouble for any bridge (=switch) to which the MAC were connected. Consider a MAC frame coming into the bridge from an ONU (a subscriber node) via an EPON port. The bridge associates the source address of the frame with the port on which it came in, i.e. the EPON port. When at a later time another MAC frame comes in from an ONU, this time destined for the MAC address previously learned, it will not be transmitted back to the EPON port, because the bridge assumes that the frame was already received by all the stations on the attached “broadcast” LAN. However, this is not the case; upstream transmissions are not received by other ONUs. Standard bridging has no way of accomodating an attached LAN that behaves as a broadcast LAN in downstream and as a point-to-point LAN in upstream. The solution to this problem was designed in close cooperation with Working Group 802.1. Instead of one single MAC, the OLT would have a different dedicated MAC for every ONU attached to the EPON. As a result, higher layers can consider the EPON as a collection of logical point-to-point links. From the individual MACs down to the OLT PHY, the logical point-to-point links share a common GMII; hence, a way to identify data frames for/from the different ONUs is required. The Logical Link identifier (LLID) was created for this purpose. The LLID is carried along by the frame in the bytes of its preamble.