The EPON network is a FTTB-based network. The basic unit of the networking is the OLT and the ONU. The OLT provides a rich PON port for the central office device to connect to the ONU device. The ONU provides the user equipment to provide corresponding data and voice interfaces to implement user service access. For the access implementation of different services, different VLAN tags of different services are used to transparently transmit to the corresponding service access server, and the corresponding VLAN tags are stripped and sent to the IP bearer network for transmission.
1. EPON network introduction
EPON (Ethernet Passive Optical Network) is an emerging fiber access network technology that uses point-to-multipoint architecture, passive optical fiber transmission, high-speed Ethernet platform and TDM time-based MAC (Media Access Control) media access control. Provide broadband access technologies for a variety of integrated services. The so-called "passive" means that the ODN does not contain any active electronic devices and power supplies, and all consist of passive components such as splitters. It uses PON technology in the physical layer, Ethernet protocol in the link layer, and Ethernet access using the PON topology. Therefore, it combines the advantages of PON technology and Ethernet technology: low cost, high bandwidth, scalability, flexible and fast service reorganization, compatibility with existing Ethernet, easy management, and so on.
EPON can realize the fusion of voice, data, video and mobile services. The EPON system is mainly composed of OLT (optical line terminal), ONU (optical network unit), ONT (optical network terminal) and ODN (optical distribution network). It is at the access network level of the network and is mainly suitable for fiber optic connection of broadband services. In.
Active network devices include central office rack equipment (OLT) and optical network units (ONUs). An Optical Network Unit (ONU) provides users with an interface between data, video, and telephone networks and PONs. The initial role of the ONU is to receive the optical path signal and then convert it to the format desired by the user (Ethernet, IP broadcast, telephone, T1/E1, etc.). The OLT device is connected to the IP core network through fiber optics. The introduction of the optical access network has a coverage of 20km, which ensures that the OLT can be upgraded to the traditional metro aggregation node from the initial stage of the optical access network construction, thereby simplifying the network structure of the access network convergence layer and saving The number of end offices. In addition, the characteristics of large capacity, high access bandwidth, high reliability, and multi-service QoS level support capability of the optical access network also make the access network evolve into a unified, converged, and efficient bearer platform.
2. The basic principle of EPON network
The EPON system adopts WDM technology to realize single-fiber bidirectional transmission, transmitting data and voice with uplink 1310nm and downlink 1490nm wavelength, and CATV service using 1550nm wavelength carrier. The OLT is placed at the central office, which allocates and controls the connection of the channel, and has real-time monitoring, management and maintenance functions. The ONU is placed on the user side, and the OLT and the ONU are connected by a passive optical distribution network in a 1:16/1:32 manner.
In order to separate signals from multiple users on the same fiber, the following two multiplexing techniques can be used.
1) The downlink data stream uses broadcast technology. In EPON, the process of transmitting data from the OLT to multiple ONUs is transmitted by means of data broadcasting. The data is broadcasted from the OLT to multiple ONUs in the form of variable length packets. Each packet carries an EPON header that uniquely identifies whether the packet is addressed to ONU-1, ONU-2, or ONU-3. It can also be identified as a broadcast packet sent to all ONUs or to a specific ONU group (multicast packet). When the data arrives at the ONU, the ONU receives and identifies the packet sent to it by address matching, and discards the packet sent to other ONUs. A unique LLID is allocated after the ONU is registered. When the OLT receives data, the LLID registration list is compared. When the ONU receives data, it only receives frames or broadcast frames that match its own LLID.
2) The upstream data stream uses TDMA technology. Before the OLT receives the data, the LLID registration list is compared; each ONU sends a data frame in a time slot uniformly allocated by the central office device OLT; the allocated time slot (by ranging technology) compensates for the gap between the ONUs and avoids each ONU. The collision between.
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