The concept of passive optical network (PON) has a long history, which has the characteristics of saving fiber resources and transparent protocol, and plays an increasingly important role in optical access network. At the same time, Ethernet (Ethernet) technology after 20 years of development, with its simple and practical, inexpensive features, almost completely dominate the LAN, and in fact proved to be the best carrier of IP packets. With the increase of the proportion of the IP service in the metropolitan area and trunk transmission, the Ethernet is gradually penetrating into the access, the metropolitan area and even the backbone network through the improvement of transmission rate and manageability. The combination of Ethernet and PON produces an Ethernet passive optical network (EPON). It also has the advantages of Ethernet and Pon, and is becoming a hot technology in the field of optical access network.
The properties of Epon medium are the combination of the shared medium and the point-to-point network. In the downward direction, it has the connectivity of the shared medium, and its behavioral characteristics in the uplink are like a point-to-point network.
Downlink direction: The Ethernet datagram sent by Olt passes through a 1: N Passive optical splitter or a few levels of taps are transmitted to each ONU. The typical value of n is between 4~64 (limited by the available optical power budget). This behavior feature is the same as the shared media network. In the downlink direction, because Ethernet has broadcast characteristics, and EPON structure and matching: Olt broadcast packets, the purpose of ONU selective extraction.
Uplink direction: Because of the directional characteristics of the passive optical path, any packet sent by a ONU can only reach the Olt, not the other ONU. The behavior characteristics of epon in the uplink direction are the same as the point-to-point network. However, unlike a true point-to-point network, in Epon, all ONU belong to the same conflict domain ―― packets from different ONU can still be conflicting if transmitted simultaneously. Therefore, in the uplink direction, Epon needs to adopt some kind of arbitration mechanism to avoid data conflict.