The development and evolution of PON technology

                    1. Early Narrowband PON and BPON     

Passive optical network (PON) is a technology that uses point-to-multipoint tree fiber distribution network for information transmission.The point-to-multipoint physical topology is especially suitable for wired access networks.A PON system consists of an OLT device at the local end, an ONU device at the user side, and a passive optical distribution network connecting the two.In PON system, because multiple ONU devices share the same optical medium to communicate with OLT, the media sharing problem between

different ONUs needs to be solved.The main ways to solve media sharing in optical fibers include time division multiplexing/multiple access, wavelength division multiplexing and orthogonal frequency division multiplexing (OFDM).Therefore, the main PON technologies can be divided into tDM-PON, WDM-PON and OFDM-PON.At present, EPON and GPON, which are widely used, mainly adopt TDM-PON technology.

       The earliest PON system is mainly used to solve the interconnection of multiple narrowband access network (digital subscriber loop) remote devices, transmission of N ×64 kbit/s voice timeslot. But the technology failed because it could not compete with digital subscriber loop devices with ring topology in terms of price and service protection.In the 1990s, with the rise of ATM/B-ISDN, broadband became an important direction of telecommunication technology development for the first time, and optical fiber technology with huge bandwidth potential also became the darling of information transmission technology.Therefore, in 1995, seven major operators in the world set up the Full Service Access Network Organization (FSAN), committed to promoting the standard and application of optical fiber access network.Thanks to the joint efforts of FSAN and ITU-T, the first international standard on PON systems, Broadband Optical Access Systems Based on Passive Optical Networks (PON) (ITU-T G.983.1), was published in 1998 and is also commonly known as BPON standard.

       BPON environment at the time of the technology adopted in ATM, for the kernel design train of thought, and limited to the device level and price factors, PON equipment cost is relatively high, optical fiber access network external supporting conditions is not mature, so BPON only in the north American telecom operators have a certain scale of deployment, has not obtained widespread application in the world.

        EPON/GPON

With the decline of ATM technology and the rapid rise of Internet IP technology, following BPON, the industry hopes to develop a new TYPE of PON system to replace the outdated BPON technology.In this context, IEEE and ITU-T successively started the standardization work of EPON and GPON in 2000 and 2001, and released the completed standards in 2004 respectively, laying a foundation for the extensive application of EPON and GPON in the live network todayEPON standard is completed by Ethernet in the First Mile (EFM) working group of IEEE and approved by IEEE 802.3AH standard in September 2004.Many contents of EPON standard inherit the design idea of Ethernet, reuse the rate and physical layer coding of gigabit Ethernet, and modify the MAC layer protocol and Ethernet frame leading code sequence to adapt to the PON point-to-multipoint network topology.

        GPON standards are standardized by ITU-T Study Group 15. Gpon-related standards include G.984.1~G.984.6, which respectively cover the GPON system architecture, physical media related layer, transmission convergence layer, ONU control management protocol, and the provisions on enhanced wavelength use and range extension.The GPON standard is designed to fully consider the business and operation and maintenance requirements of operators. The standard system is complete and comprehensive, but the content is relatively complex.

        The EPON system adopts single-fiber bidirectional transmission with a nominal upstream wavelength of 1 310 nm and a nominal downstream wavelength of 1 490 nm.According to the maximum transmission distance, EPON interface optical transceiver specifications are divided into 10 km (PX10) and 20 km (PX20). In practical networks, PX20 interfaces are mostly used to obtain a larger optical power budget, which can achieve a transmission distance of 20 km and a 1:32 shunt ratio.The actual bandwidth of each PON port in the EPON system ranges from 800 Mbit/s to 950 Mbit/s

        GPON also uses single-fiber bidirectional transmission, with a nominal upstream wavelength of 1310 nm and a nominal downstream wavelength of 1490 nm.GPON uses GEM encapsulation to adapt various services. GEM encapsulation can directly carry Ethernet, ATM, and TDM services.Unlike epON-like Ethernet, which transmits varied-length frames, GPON uses a fixed frame length of 125 μs, which helps transmit clock signals accurately.The GPON channel code uses the NRZ code. The downstream rate is 2.488 Gbit/s, and the upstream rate is 1.244 Gbit/s. After excluding the system overhead, the actual effective bandwidth of each PON port is about 2.45 Gbit/s for the downstream and 1.1 Gbit/s for the upstream.At present, the mainstream GPON system uses B+ optical devices, which can achieve a transmission distance of 20 km, 1∶64 shunt ratio, and support a maximum logical distance of 60 km.

　     The EPON and GPON provide 1 GBIT /s and 2.4 GBIT /s downstream bandwidth respectively. In FTTH scenarios, each user with the maximum shunt ratio (32 Mbit/s and 64 Mbit/s) can obtain about 30 Mbit/s downstream bandwidth without considering concurrency.However, in China's current network, carriers use FTTB networking, that is, 16 to 32 users are connected to each ONU. The number of users connected to each PON port may reach 1 000 (32 x 32).In this way, the bandwidth available to each user cannot meet the requirements of the live network speed increase.

GPON system can evolve to NG-PON2 in three ways: sequential evolution, jump evolution, and flexible evolution.

The existing GPON system needs to be evolved to xG-PON system first, and GPON and XG-PON coexist in the same ODN for a period of time.When nG-PON2 needs to evolve, only THE XG-PON system can evolve to NG-PON2 and coexist with NG-PON2 in the same ODN. Therefore, ensure that the GPON system has been removed from the ODN.

         Evolves directly from GPON to NG-PON2.Based on service and network development, this mode skips the XG-PON phase and directly upgrades GPON to NG-PON2. Therefore, GPON and NG-PON2 must coexist in ODN

Flexible evolution Supports the evolution from XG-PON to NG-PON2 or from GPON directly to NG-PON2. GPON, XG-PON, and NG-PON2 can coexist on the same ODN.In this way, the spectrum planning is the most difficult because all the three systems need to occupy the spectrum resources in the optical fiber.At present, the nG-PON2 physical layer specification considers the requirement of flexible evolution mode, and the spectrum used by NG-PON2 is basically determined to use C- (1 530 ~1 540 nm) band and L+ (1 595~1 625 nm) band. The specific band boundary value remains to be further discussed.

How EPON system evolves smoothly to NG-PON2 system has not been studied.From the perspective of ODN coexistence, as the C- and L+ bands selected by NG-PON2 are different from those used by EPON/10G-EPON, they can co-exist in the same ODN through WDM by referring to GPON/XG-PON.

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