Introduction to PON Technology
1.Basic structure of PON
PON (Passive Optical Network)
PON is a single-fiber bidirectional optical access network using a point-to-multipoint (P2MP) structure. The PON system is composed of an optical line terminal (OLT), an optical distribution network (ODN), and an optical network unit (ONU) on the user side of the central office, and is a single-fiber bidirectional system. In the downstream direction (OLT to ONU), the signal sent by the OLT reaches each ONU through the ODN.In the upstream direction (ONU to OLT), the signal sent by the ONU will only reach the OLT and will not reach other ONUs.In order to avoid data collision and improve network efficiency, the uplink direction adopts TDMA multiple access mode, and manages data transmission of each ONU. The ODN provides optical channels between the OLT and the ONU. The reference structure of the PON is shown in the figure below.
PON system reference structure
The OLT is located on the network side and placed at the central office. It can be an L2 switch or an L3 router, providing network concentration and access, enabling optical/electrical conversion, bandwidth allocation, and control of each channel connection, with real-time monitoring and management. And maintenance functions. The ONU is located on the user side to implement processing and maintenance management of various electrical signals, and provides a user-side interface. The OLT and the ONU are connected by a passive optical splitter, and the optical splitter is used to distribute downlink data and aggregate uplink data. In addition to the terminal equipment, the PON system does not require electrical components and is therefore passive.
The PON adopts a wavelength division multiplexing (WDM) technology with a downlink 1490 nm/uplink 1310 nm wavelength combination on a single fiber. The uplink direction is a point-to-point mode, and the downlink direction is a broadcast mode. The figure below shows the basic structure of the PON.
Basic network structure of PON
In the downstream direction, the OLT transmits the data packets to all ONUs in a broadcast manner, each packet carrying a header with a transmission to the destination ONU identifier. When the data packet arrives at the ONU, the MAC layer of the ONU performs address resolution, extracts the data packet belonging to itself, and discards other data packets.
The uplink direction uses Time Division Multiplexing (TDM) technology, and the uplink information of multiple ONUs constitutes a TDM information stream to be transmitted to the OLT.
2.Optical line terminal (OLT)
The optical line terminal (OLT) functions to provide an optical interface between the service network and the ODN, and provides various means for transmitting various services. The OLT is internally composed of a core layer, a service layer, and a public layer. The service layer mainly provides service ports and supports multiple services; the core layer provides cross-connection, multiplexing, and transmission; and the public layer provides power supply and maintenance management functions.
The presence of the OLT can reduce the tight coupling between the upper-layer service network and the specific interface, the bearer, the networking, and the device management of the access device, and can provide a unified optical access network management interface.
The core functions of the OLT include: aggregation distribution function and DN adaptation function.
The OLT service interface functions include: service port function, service interface adaptation function, interface signaling processing, and service interface protection.
The OLT common functions mainly include OAM functions and power supply functions.
The optical power emitted from the OLT is mainly consumed in the following places.
Splitter: The greater the number of shunts, the greater the loss.
l Fiber: The longer the distance, the greater the loss.
l ONU: The greater the number, the greater the OLT transmit power required. In order to ensure that each power reaching the ONU is higher than the receiving sensitivity and has a certain margin, the budget should be based on the actual quantity and geographical distribution.
3.Optical distribution network
The optical distribution network (ODN) is a means for providing optical transmission between the OLT and the ONU. Its main function is to complete the information transmission and distribution between the OLT and the ONU, and establish an end-to-end information transmission channel between the ONU and the OLT.
The ODN configuration is usually a point-to-multipoint mode, that is, multiple ONUs are connected to one OLT through one ODN, so that multiple ONUs can share the optical transmission medium between the OLT and the ODN and the optoelectronic device of the OLT.
(1) Composition of ODN
The main passive components that make up the ODN are: single-mode fiber and fiber optic cable, connectors, passive optical splitters (OBD), passive optical attenuators, and fiber optic connectors.
(2) Topological structure of ODN
The topology of an ODN network is usually a point-to-multipoint structure, which can be divided into a star, a tree, a bus, and a ring.
ODN network structure
(3) Settings for active and standby protection
The active/standby protection setting of the ODN network is mainly to set up two optical transmission channels for the optical signals transmitted by the ODN network. When the primary channel fails, it can automatically switch to the alternate channel to transmit optical signals, including optical fibers, OLTs, ONUs, and The primary and backup protection settings of the transmission fiber.
The main and standby transmission fibers can be in the same optical cable or in different optical cables. The main and backup optical cables can be installed in different pipelines, so that the protection performance is better.
(4) Optical transmission characteristics of ODN
ODN’s design features should ensure that any currently foreseeable service can be provided without major changes, a requirement that has a major impact on the characteristics of various passive components. The requirements that may directly affect the optical characteristics of the ODN are as follows.
l Optical wavelength transparency: Various optical passive components should not affect the transparency of the transmitted optical signal. The optical signal required by the designed optical network should be transparently transmitted, thus providing future WDM system applications. The foundation.
l Reversibility: When the output and input of the ODN network are interchanged, the transmission characteristics of the ODN network should not change significantly, that is, the change of transmission bandwidth and optical loss characteristics should be minimal. This simplifies the design of the network.
l Consistency of network performance: The ODN network should maintain consistent optical signals. The transmission characteristics of the ODN network should be consistent with the entire OFSAN and the entire communication network. The transmission bandwidth and optical loss characteristics should be suitable for the entire OFSAN.
(5) ODN performance parameters
The parameters that determine the optical channel loss performance of the entire system are mainly as follows.
l ODN optical channel loss: the difference between the minimum transmit power and the highest receive sensitivity.
l Maximum allowable channel loss: the difference between the maximum transmit power and the highest receive sensitivity.
l Minimum allowable channel loss: the difference between the minimum transmit power and the lowest receive sensitivity (overload point).
(6) Reflection of ODN
The reflection of the ODN depends on the return loss of the various components that make up the ODN and any reflection points on the optical channel. In general, all discrete reflections must be better than −35 dB, and the maximum discrete reflection of fiber access should be better than −50 dB.
4. Optical Network Unit (ONU)
The optical network unit (ONU) is located between the ODN and the user equipment, and provides an optical interface between the user and the ODN and an electrical interface with the user side to implement processing and maintenance management of various electrical signals. The ONU is composed of a core layer, a service layer, and a public layer. The service layer mainly refers to user ports; the core layer provides multiplexing and optical interfaces; and the public layer provides power supply and maintenance management.
5. PON application mode
PON’s business transparency is good, and in principle can be applied to any standard and rate signal. Compared with point-to-point active optical networks, PON technology is characterized by simple maintenance, low cost (saving fiber and optical interfaces), high transmission bandwidth and high performance price ratio. These characteristics will make it maintain a competitive advantage for a long time, and PON has always been regarded as the future development direction of the access network.
The most suitable application for PON is: the part of the access network close to the end of the customer; the customer of the ONU service does not emphasize the need for redundancy or bypass protection; the OLT can be set up at a node with good survival performance (for example, a node with roundabout protection). A place where users are geographically concentrated. PON mainly has three application modes.
(1) Replace the existing two-layer aggregation network: PON can replace the existing Layer 2 switch and optical transceiver, and direct the access network of the LAN to the IP metropolitan area network, as shown in the figure:
PON replaces existing Layer 2 network
(2) Replace the access cable of the relevant paragraph: PON system can replace the existing part of the optical cable and optical switching equipment, thus saving the access cable of the relevant paragraph, as shown:
PON replaces relevant segments to access optical cable
(3) Multi-service access mode (implementing FTTH): The PON system can provide multi-service and multi-rate access that meet different QoS requirements, and can adapt to the diversity of users and the uncertainty of business development, as shown in the following figure:
Multi-service access