Passive Optical Network (PON) is a network applied to the access network, in which the central office equipment OLT (optical line terminal) and multiple client equipment ONUs (optical network unit) are connected by an optical distribution network (ODN) composed of passive optical cables, optical splitters/combiners, etc. It is an optical access technology developed to support point-to-multipoint applications.
Ethernet Passive Optical Network (EPON) downlink transmission adopts time division multiplexing, uplink transmission adopts TDMA, transmission medium layer adopts Gigabit Ethernet as transmission protocol, and data link layer adopts Ethernet protocol. EPON is a single-fiber bidirectional optical access network that adopts a point-to-multipoint structure, and its uplink and downlink transmissions work in full duplex.
1. EPON system composition
(1) Optical line terminal OLT. In an EPON system, OLT is not only a switch or router, but also a multi-service provisioning platform (MSPP), which provides optical fiber interfaces for passive optical networks. It is the core component of the entire EPON system, and its main functions include:
a) Send Ethernet data to ONU in broadcast mode;
b) Initiate and control the ranging process, and record ranging information;
c) initiate and control ONU power control;
d) Allocate bandwidth to the ONU, that is, control the start time of the ONU sending data and the size of the sending window;
e) other related Ethernet functions.
(2) Optical network unit ONU. The ONU in the EPON adopts a mature and economical Ethernet protocol, and the low-cost Ethernet Layer 2 and Layer 3 switching functions can be implemented in the ONU. Its main functions are:
a) Select to receive broadcast data sent by OLT;
b) Respond to ranging and power control commands issued by the OLT, and make corresponding adjustments;
c) Buffer the user’s Ethernet data and send it to the upstream direction in the sending window allocated by the OLT.
d) other related Ethernet functions.
(3) Passive Optical Distribution Network ODN. ODN is a passive device connecting OLT and ONU. Its function is to distribute downlink data and concentrate uplink data.
2. EPON principle
(1) Single-fiber bidirectional transmission mechanism. The EPON system adopts WDM technology to realize single-fiber bidirectional transmission, as shown in Figure 1. In order to separate the incoming and outgoing signals of multiple users on the same fiber, the following two multiplexing techniques are used:
a) The downlink data stream adopts broadcasting technology;
b) The upstream data stream adopts TDMA technology.
(2) The working process of the system.
Operation of OLT:
a) generate timestamp messages for system reference time;
b) assign bandwidth through MPCP frames;
c) Perform ranging operation;
d) Control ONU registration;
Operation of ONU:
a) The ONU synchronizes with the OLT through the timestamp of the downstream control frame;
b) The ONU waits for the discovery frame (gate);
c) The ONU performs discovery processing, including ranging, specifying physical ID and bandwidth;
d) The ONU waits for authorization, and the ONU can only send data at the authorization time.
3. EPON technical features
EPON technology can support tree-shaped, chain-shaped hand-in-hand, ring and other topological structures, which are consistent with the distribution network structure. It has the characteristics of high bandwidth and high QOS guarantee, and the minimum uplink and downlink transmission rate is not less than 1.25Gbit/s. Using optical fiber as the transmission medium, the operation is reliable and stable. It has effective isolation guarantee mechanism, VLAN isolation, protection port, MAC address binding, IP address binding, port speed limit, queuing technology, flow control technology, etc, which has played a technical guarantee for the development of multi-service integration, and can smoothly upgrade and expand. The disadvantage is that network construction requires re-wiring.