The transmission layer of FTTH is divided into three categories: Duplex (dual fiber bidirectional) loop, Simplex (single fiber bidirectional) loop and Triplex (single fiber three-way) loop.The dual-fiber loop uses two optical fibers between the OLT end and the ONU end, one way is downstream, and the signal is from the OLT end to the ONU end; the other way is upstream, and the signal is from the ONU end to the OLT end.Simplex single-fiber loop is also called Bidirectional, or BIDI for short. This solution uses only one optical fiber to connect the OLT end and the ONU end, and uses WDM to transmit upstream and downstream signals with optical signals of different wavelengths.Compared with Duplex dual-fiber circuits, this single-fiber circuit using WDM transmission can reduce the amount of fiber used by half and reduce the cost of the ONU user end. However, when the single-fiber method is used, a splitter and combiner should be introduced in the optical transceiver module.It is a little more complicated than the optical transceiver module using the dual fiber method. The BIDI upstream signal uses the laser transmission in the 1260 to 1360nm band, and the downstream uses the 1480 to 1580nm band. In the dual-fiber loop, both the upstream and downstream use the 1310nm band to transmit signals.
FTTH has two technologies: Media Converter (MC) and Passive Optical Network (PON). MC is mainly used to replace the copper wires used in traditional Ethernet networks, and adopts a point-to-point (P2P) network topology to transmit 100Mbps services to users’ homes through optical fibers.The architecture of PON is mainly to split the optical signal from the optical line terminal (OLT) downstream through an optical fiber through an optical splitter to transmit the optical signal to each optical network terminal (ONU/T), thereby greatly reducing the network equipment room And the cost of equipment maintenance, saving a lot of construction costs such as optical cables,so it has become the latest hot technology of FTTH. FTTH currently has three solutions: point-to-point FTTH solution, EPON FTTH solution and GPON FTTH solution.
P2P is a point-to-point optical fiber connection Ethernet transmission technology. It also uses WDM technology to achieve two-way communication. Compared with EPON, it has the characteristics of simple technology implementation, low price and easy access for a small number of users.
The P2P FTTH network transmits the upstream and downstream wavelengths on one optical fiber between the central office switch and the user equipment through WDM, and each user only needs one optical fiber. The upstream wavelength is 1310nm, and the downstream wavelength is 1550nm.Through the use of optical fiber transmission, the Ethernet is directly extended from the central office to the user desktop. While providing a high-bandwidth and economical access method, it eliminates the difficulty of power supply and maintenance of the corridor switch in the traditional Ethernet access method,and avoids Difficulty in investment recovery caused by low opening rate, flexible opening and high security. In the P2P solution, users can truly enjoy 100M bandwidth exclusively, and easily support high-bandwidth services such as videophone, video on demand, telemedicine, and distance education. While supporting high-speed data applications, it can provide E1 interface and POTS interface, so that a variety of services that originally required independent wiring can be resolved through a single fiber.
EPON adopts a point-to-multipoint structure and a passive optical fiber transmission method. The downstream rate can currently reach 10Gb/s, and the upstream sends data streams in bursts of Ethernet packets. In addition, EPON also provides certain operations, maintenance and management (OAM) functions.EPON technology has good compatibility with existing equipment. The newly developed Quality of Service (QoS) technology makes it possible for Ethernet to support voice, data and image services. These technologies include full-duplex support, priority and virtual local area network (VLAN).
EPON uses an optical fiber to connect between the central office equipment and the ODN optical coupler. After splitting through the optical coupler, up to 32 users can be connected. The upstream wavelength is 1310nm, and the downstream wavelength is 1490nm. The optical fiber from the PON port of the OLT combines the 1550nm analog or digital CATV optical signal to the optical fiber through the multiplexer, and then connects to the ONU after being split by the optical coupler. The ONU separates the 1550nm CATV signal and converts it into a radio frequency signal that can be received by an ordinary TV. The ONU also processes the data signal sent by the OLT and sends it to the user interface.The user interface can provide FE and TDM interfaces to meet the user’s service requirements for broadband access, and is compatible with the existing operators’ TDM service requirements. EPON uses WDM technology to realize point-to-multipoint two-way communication on a single optical fiber. It has the characteristics of transparent format and low price, and conforms to the development trend of IP-based next-generation networks. Considering that the future “three networks in one” will use IP as the core protocol, most experts believe that EPON is the best solution for realizing FTTH in the future.
GPON is the latest optical access technology launched by ITU-T after A/BPON. In 2001, FSAN started another standard work aimed at standardizing PON networks (GPON) with operating speeds higher than 1Gb/s. In addition to supporting higher speeds, GPON also supports multiple services with high efficiency, providing abundant OAM&P functions and good scalability. The main features of GPON are:
1) Support all services.
2) The coverage distance is at least 20km.
3) Support multiple rates under the same protocol.
4) Provide OAM&P function.
5) According to the broadcast characteristics of PON downstream traffic, a security protection mechanism at the protocol layer is provided.
The GPON standard provides the most efficient transmission rate for different services, while taking into account OAM&P functions and upgrade capabilities. GPON not only provides high bandwidth, but also supports various access services, especially in data and TDM transmission, supporting the original format without conversion.GPON adopts the new transmission convergence layer protocol “General Framing Protocol (GFP)” to realize the encapsulation of multiple service streams; meanwhile, it maintains many functions in G.983 that are not directly related to the PON protocol, such as OAM and DBA.