How Much Do You Know About PON?

Definition

PON (Passive Optical Network) is a telecommunication network that uses point-to-multipoint fiber to the end-points in which unpowered optical splitters are used to enable a single optical fiber to serve multiple end-points. A PON consists of an optical line terminal (OLT) at the service provider's central office and a number of optical network units (ONUs) or Optical Network Terminals (ONTs) near the end users.The optical distribution network (ODN) between the OLT and ONU contains fiber and passive optical splitters or couplers.

PON system mainly consists of the optical line terminal (OLT: Optical Line Terminal) in central office. And the optical distribution network (ODN: Optical Distribution Network) includs passive optical devices, the client's optical network unit/optical network terminal (ONU/ONT Optical Network Unit/Optical Network Terminal). The difference is that ONT directly lies in the client, and between the ONU and the user there are other networks, such as Ethernet, as well as Element Management System (EMS) usually using multipoint tree topology.

The outstanding advantage of PON network is the elimination of outdoor active equipment, all signal processing functions are performed at the switch and the user equipment. Meanwhile, the initial investment of this access method is small, and most of the money can be deferred to invest after the users truly get the access. Its transmission distance is shorter than that of the active optical access system, and the coverage is smaller. But the cost is low and they don't need a separate room, which is easy to maintain. Therefore, this structure can be economical for home users.

Brief Introduction of PON

Fiber is so "cheap and easy to use", so for the next-generation broadband solution, FTTx (Fiber To The X, optical access) is widely used to provide users with high-bandwidth and full-service access platform. And FTTH (Fiber To The Home, FTTH, optical fiber directly connected to the user's home). It is known as the best business transparent network, which is the ultimate way of access network development.

How does the FTTx achieve it? In a variety of solutions, for the multipoint (P2MP) fiber access PON (Passive Optical Network, Passive Optical Network) is the best choice. PON is used in the access network, the central network and a plurality of CPE (ONU / ONT). It's composed of the passive optical cable, optical branching device/optical combiner.

ODN (Optical Distribution Network)
OLT (Optical Line Terminal)
ONU (Optical Network Unit)
ONT (Optical Network Terminal) (as the picture below)

ONU and ONT both belong to the CPE, the difference between which lies directly at the client ONT. Between the user and ONT there are other networks, such as Ethernet.

The key "passive" is that ODN between OLT and ONU is the optical access network without any active electronic devices. Because of this "passive" features, this pure media PON network equipment can avoid external electromagnetic interference and lightning effects, reducing lines and external equipment failure rate, and improving system reliability, and reducing maintenance costs at the same time.

PON technology is developed from the beginning of the 1990s, ITU (International Telecommunication Union) from APON (155 M), and then BPON (622 M), as well as to GPON (2.5 G); And in the early century, due to the wide use of the ethernet technology,

IEEE also developed EPON technology based on Ethernet technology. Currently PON technology for broadband access mainly are EPON and GPON both of which use two different standards. The future development should pay attention to higher bandwidth. For example, EPON / GPON technology developed 10GEPON/10GGPON, which made the higher bandwidth.

Feature

The complexity of PON lies in the signal processing techniques. In the downstream direction, the signal sent by the switch is the broadcast that sent to all users. In the upstream direction, each ONU must use some kinds of multiple access protocols. For example, time division multiple access TDMA (Time Division Multiple Access) is the protocol to complete the channel access to information. Currently PON technology for broadband access are mainly: EPON and GPON.

Standard

ITU-T G.983

APON (ATM PON, ATM Passive Optical Network) is the first passive optical network standard, which is based on ATM, mainly used for commercial applications.

BPON (broadband PON, Broadband Passive Optical Network) is a standard based on APON. This standard adds support for WDM and dynamic bandwidth allocation, linking the high-speed, and supporting the durability. BPON also has created a management interface standard OMCI---mixed supplier network authorized by the OLT and ONU / ONT.

IEEE 802.3ah

EPON or GEPON (Ethernet PON Ethernet Passive Optical Network) is a use Ethernet packet data IEEE / EFM standard. 802.3ah standard is now a part of the IEEE 802.3 standard. Now there are about fifteen million EPON ports being used. In 2008, China vigorously developed EPON technology. It is estimated that, in the end of 2008, China had a total of 2 million EPON installing users.

ITU-T G.984

GPON (Gigabit PON, Gigabit Passive Optical Network) is developing from the BPON standard. GPON supports higher rates, enhanced security, and an optional second layer protocol (ATM, GEM, Ethernet). In mid-2008, Verizon Telecom has installed 800,000 lines. BT (British Telecom) and the American Telephone and Telegraph Company (AT & T) had advanced tests. Other companies such as Separate Optical Network Company (Independent fiber networks LTD) is working with the service supplier. They see the potential business chances to build higher-speed GPON access and fiber to the home (FTTH-fiber to the home).

IEEE P802.3av

10G-EPON (10 Gigabit Ethernet PON) is an IEEE specialized project. In order to achieve 10Gbit/s rate, it is backward compatible with 802.3ah standard EPON. 10GigEPON will use separating wavelengths for 10G and 1G downstream. 802.3av will continue to use TDMA isolation of individual wavelengths, which makes the use in the uplink between 1G and 10G possible. 10G-EPON will be compatible with WDM-PON (according to the definition of WDM-PON), which will make multi-wavelengths change between the two directions possible.

SCTE IPS910

RFoG (RFoverGlass) is sub-committee standard of interface practice which is applied to the wavelength plan compatible multipoint data (P2MP) operations, such as EPON, GEPON or 10GigEPON. RFoG provides a FTTH PON ( FTTH PON) and we just do not have to select the MSOs or deploy PON architectures.

Background

From the whole network, due to the large number of laying, Many new technologies such as DWDM made a breakthrough in fiber-optic backbone networks. At the same time due to advances in Ethernet technology, its dominant LAN bandwidth was also developing from 10M, 100M to 1G or 10G.

And now everyone's attention is the network backbone and LAN connection as well as home users, it is often said as "the last one kilometer" which is a bottleneck. Only breaking this bottleneck can we usher in a new world of cyberspace. It is as if the state highway systems, trunk and regional roads have been built with high grade, but the door leading to the family and business are still narrow pass, thus the road network can not act effectively.

After years' development, APON is still not really entering the market. The main reason is the complicated ATM protocol. With respect to the access network equipment market it is still relatively expensive. At the same time due to the rapid development of Ethernet technology, ATM technology is completely out of the local area network. So as to how to combine the simple and economical Ethernet technology and PON transmission structure has caused great attention by the technical communities and network operators since 2000. Meanwhile, the industry generally believe that there are many shortcomings of ATM PON, such as the lack of video transmission capabilities, limited bandwidth, system complexity and high costs, and so on. These shortcomings will not exist in EPON.

The first target of OAN evolution are FTTB (Fiber To The Building) and FTTC (Fiber To The Curb) system, then the development of FTTH (Fiber To The Home) which includes data by providing a simple platform for the user. There are comprehensive services, including video and voice. EPON can provide higher bandwidth and more comprehensive services than APON , and the cost is very low, while EPON architecture is also in line with most of the requirements G.983 standard.

Advantages

1)relatively low cost, easy to maintain, easy to expand, and easy to upgrade. PON structure can transit without power and electronic parts. Thus it is easy to lay, and basically no maintenance, which can save much long-term operating costs and management cost.

2) passive optical network is a pure media network,completely avoiding electromagnetic interference and lightning effects, very suitable for using in areas of poor natural conditions.

3) PON system takes up very little resources for infrastructure, the initial system investment is low, easy expansion, high return on investment.

4) EPON currently can apply for the down the line symmetrical 1.25Gb / s of bandwidth, and with the development of Ethernet technology it can be upgraded to 10Gb/s. GPON is up to 2.5Gb/s bandwidth.

5) As a multipoint network, PON save resources of CO and service large numbers of users in a fan-shaped structure. The fact that mode users can share infrastructure equipment and fiber is saving the users' investment.

6) The quality of service (QoS) is guaranteed. There is a complete system for G/EPON system to guarante the bandwidth, which can realize the user-level SLA.

PON Technological Development

Conventional PON system downstream data uses the broadcast technology, and the upstream uses TDMA technology to solve complex problems of multiple users in each direction of the signal. Conventional PON technology uses WDM technology, and realizes single fiber bidirectional transmission over fiber to solve the multiplex transmission of 2 direction signals. PON is usually composed of optical line terminal (OLT), splitter (ODU), and 3 parts subscriber terminal (ONU).

Currently widely used PON technologies in the existing network include two mainstream technologies: EPON and GPON. EPON uplink and downlink bandwidth are 1.25 Gbit/s. The GPON downstream bandwidth is 2.5Gbit/s and upstream bandwidth is 1.25 Gbit/s.

Currently in the applications of FTTx , most EPON / GPON are configured with only the Ethernet interface, optional POTS and 2M interface. However, from technical standards, both EPON / GPON can achieve IP and TDM services such as multi-service access, and achieve QoS classification.

EPON/GPON can transmit clock synchronization signal and can be extracted frequency synchronization signal from an external circuit through the OLT STM-1 interface or GE interface. Then OLT should support synchronous Ethernet; You can also enter from an external BITS on the OLT clock signal as a common clock source of the PON. ONU should keep pace with the clock source to maintain frequency synchronization.

Although PON and 10G EPON are not commercial in large scale, 10 Gbit/s rate PON technology over the last two years is the research focus of FSAN and ITU-T. And related technical standards of XG-PON1 are mature now, such as NG-PON2 standard XG-PON1 after ITU-T on GPON, XG-PON1 standards and NGPON2 ITU-T on GPON, XG-PON1 standards and NGPON2. The framework has been basically completed. The focus now is the extension to multi-wavelength technology. FSAN has made it clear that TWDM-PON will be the choice of future technology NG-PON2, but a variety of technical specifications in the ITU-T SG15 G.multi standard hasbeen basically completed, which means that a plurality of multi-wavelength extended technical schools battle are far from over.