Detailed Introduction of Passive Optical Network (PON) Transceiver
PON is a point-to-multipoint passive optical network, which mainly consists of optical line terminal (OLT), optical network unit (ONU) and passive optical splitter (POS). PON transceiver is a kind of high-performance optical module for PON system, which complies with ITU-T G.984.2 standard and Multi-Source Agreement (MSA), and it uses different wavelengths to send and receive signals between OLT and ONT (Optical Network Terminal).The wide application of PON Ethernet passive optical network cannot be achieved without PON transceiver, because It is an important component of the PON system.
PON Working Principle Block Diagram
What are the types of PON transceiver?
- SFF vs. SFP vs. SFP+ vs. XFP transceiver
According to different package types, PON transceiver can be divided into SFF (small form factor), SFP (small form pluggable, 4G), SFP+ (enhanced small form pluggable, Ethernet 10G) or XFP (standard small form pluggable, serial 10G). Among them, SFP + with its advantages of miniaturization and low cost meets the demand of equipment for high-density optical modules, and has gradually replaced XFP as the mainstream of 10G market.
- OLT vs. ONU transceiver
According to different plug-in devices, PON transceivers include OLT optical modules and ONU optical modules, and they are all in SFP form factor. OLT optical modules are more complex than ONU optical modules, because each OLT optical module must transmit signals with 64 pieces ONU optical modules.
- APON vs. BPON vs. GPON vs. EPON
According to different technical standards (i.e. transmission protocols), PON optical modules can be divided into APON (ATM PON), BPON (broadband passive optical network), EPON (Ethernet passive optical network) and GPON (gigabit passive optical network).
At present, the latter two are widely used:
The EPON optical module has the packaging form of SFP, XFP and SFP +, and its transmission distance can reach 20km. The receiver part has a sealed preamplifier and a limiting amplifier with LVPECL compatible differential output. The advantages of EPON technology include low cost, high bandwidth, high scalability, compatibility with existing Ethernet, and easy management.
The common EPON transceiver in the market today are:
- EPON OLT PX20 + / PX20 + + / PX20 + + + : these three types of optical modules are applicable to optical network units(ONU) and optical line terminals(OLT). The transmission distance of 10G EPON OLT transceiver is 20km, with single-mode, SC interface and supports DDM.
- 10G EPON ONU SFP + : this optical module suitable for ONU and OLT. Its transmission distance is 20km, single-mode, SC interface, and supports DDM. 10G EPON can be divided into two types by rate: asymmetric mode and symmetric mode. The downlink rate of asymmetric mode is 10Gbit/s, the uplink rate is 1Gbit/s, and the uplink rate and downlink rate of symmetric mode are 10Gbit/s. With the development and popularization of large traffic and high bandwidth services such as 4K video and virtual reality, the large-scale use of 10G EPON has become a general trend.
However, the GPON adopts WDM(wavelength division multiplexing) technology in single fiber bi-directional transmission, and can transmit data through downlink and uplink. First, the downlink side adopts a wavelength of 1490 nm, and the OLT sends out data by broadcasting, then after the data passing through the splitter, it reaches different ONUs. Next, on the uplink side, the wavelength of GPON is 1310nm and is transmitted through TDMA(Time Division Multiple Access). The users(OLT) send different data in a specific map field (allocate different time slots) and reaches the OLT after photosynthetic wave. The transmission principle is shown in the following figure:
GPON transceiver is a kind of high-performance and economical optical module for 2.5Gpbs serial optical data communication. The advantage of this module is that it can ensure no damage to eyes in any fault condition. GPON transceiver also has the advantages of longer transmission distance (the coverage radius of the access layer is more than 20km), higher bandwidth efficiency, flexible spectral characteristics and so on.
The common GPON SFP transceiver in the market today are:
- GPON OLT Class B +, the channel insertion loss is 28dB, and the maximum optical split ratio is 1:64. At present, GPON devices are generally equipped with GPON OLT class B + optical modules;
- GPON OLT Class C + , allows 32dB channel insertion loss and supports a maximum optical split ratio of 1:128. At present, GPON OLT class C + optical modules are mature and will soon be popularized.
- GPON OLT Class C + +, allows 32dB channel insertion loss and supports a maximum optical split ratio of 1:128. GPON OLT Class C + + optical module has not been widely used.
Comparing GPON OLT class B + / C + / C + + optical modules, the main difference lies in the transmission power and reception sensitivity:
Although GPON transceiver has better performance than EPON transceiver, EPON optical module has more advantages in cost. They have their own advantages and disadvantages and can coexist and complement each other. For more information, please refer to what is GPON and EPON. For the customers who have high requirements for bandwidth, multi-service, QoS and security, or the customers who are using ATM technology as the backbone network, it is recommended to choose GPON optical transceiver; For the customers who want to save costs and have low requirements for QoS and security, EPON optical transceiver may be more suitable.
- PON transceiver vs. Conventional transceiver
According to the development time, there are two types of optical modules: PON optical module and conventional optical module.
When using conventional optical modules, the optical signal transmission mode is point-to-point(P2P), the modules are used in pairs, and one optical fiber or two optical fibers are used for transmission (duplex or simplex). The optical fiber link loss includes attenuation, dispersion, fiber connection insertion loss, etc. And it has a long transmission distance, up to 200km. Nevertheless, due to the high cost, it is less used in the access network but mainly used in the backbone network.
When PON optical module is used, the transmission mode of optical signal is point to multipoint(P2MP), and the modules are not used in pairs. Optical fiber link loss includes attenuation, dispersion, frequency division ratio loss, fiber connection insertion loss, etc. Then, about the transmission distance, it is shorter than the conventional module which only up to 20km. The PON transceiver is mainly used in access network.
What are the application scenarios of PON transceivers?
The growing demand for Internet access applications such as video conferencing and cable TV has led to the popularity of fiber-to-the-home(FTTH) deployments. PON, the leading technology for fiber-to-the-home applications, is also widely used in access networks for PDH SONET, ATM, SDH, data communications, and telecom networks. Today, the commercial GPON rate is up to 10 Gbit/s, which can meet the needs including 4K, primary VR, etc. However, with the further development of VR technology, together with 4K, 8K HD video and other services, each user’s demand for bandwidth is around 0.5G~1G, and the system capacity should be more than 50G under 1×64 multi-use, while the network of ultimate VR needs more than 100G dat rate. In addition, with the accelerated development of 5G technology, the discussion of higher speed PON technology standards such as 25G, 100G, and even 400G is gradually on the agenda. At present, the ITU and IEEE organizations have set up discussions on the evolution of standards for next-generation optical network technology. IEEE has already issued a white paper on NG-EPON technology, and the organization is currently developing a standard for 100G-PON ( containing 25G, 50G, 100G three rates ).
The shortcomings of PON transceiver
Although 10G PON optical modules provide satisfactory performance in FTTH(fiber to the home) applications, there are still some challenges in the following areas:
- The burst mode optical transmission technology for uplink is insufficient;
- The need for high output optical power and high sensitivity OLTs at the CO (central office) for connecting optical splitters and fiber introduction losses at subscriber premises;
- Optical line terminal (OLT) RXs need to be able to receive packets with large differences in optical power and phase alignment to minimize protection time during transmission.
Summary: a brighter future for PON technology
Due to the rapid development of the Internet, users’ consumption needs and consumption level of communication services have also been greatly improved. Users have reduced their attention to service charges and increased their attention to bandwidth and information transmission rate, which further promotes the development of optical fiber services. Therefore, telecommunication operators have accelerated the upgrading and transformation of the network. For areas lacking copper cable resources, because the passive characteristics can reduce costs, it is very beneficial to improve the return on investment of operators. Therefore, in the process of community broadband construction, OLT and ONU technologies are mainly used to ensure the stability of the system and the stability, practicability and extensibility of the standard. GPON technology architecture is suitable for the construction of community access network, which is of practical significance to accelerate the expansion of the network. In the future development, GPON will be more widely used. On the basis of improving the traditional Ge level, it can ensure the diversity of technology types, and the advantages of GPON technology will be more prominent, so as to provide guarantee for access to greater bandwidth, realize high-speed data transmission and expand the coverage area, which can more meet the diversified needs of current users.
At present, 10G PON is in commercial use, and the standards of the next generation PON are being formulated (25G PON and 100G PON). It can be predicted from the hot discussion of the current standards that the evolution rate of the next generation PON will start with 25G, which is likely to be a fusion scheme. However, the current 25G optical module industry chain is not mature and the cost is expensive, Later, the development of high-speed PON requires corresponding digital processing technology in order to obtain higher reception sensitivity and longer transmission distance.