Development of PON
Due to its considerable advantages over copper access in terms of transmission media, passive nature and point-to-multipoint topology, PON has been widely used in the last decade.
In the development of PON technology, the standards organizations ITU-T/FSAN and IEEE have played a significant role. At present, XG(S)-PON and 10GE-PON standards have been formulated and relevant products have entered the large-scale deployment phase, and the technical research of the next-generation PON has been put on the agenda. The next-generation PON has two research trends: one is to improve the single-wavelength rate and the other is to improve the total rate through multi-wavelength multiplexing. IEEE employs multi-wavelength stacking and channel bonding to provide two 25 Gbps channels to achieve 50 Gbps. ITU-T/FSAN focuses on the single-wavelength 50G PON technology to meet the requirements for next-generation PON in homes, businesses, and mobile backhaul/fronthaul scenarios.
50G PON Key Technology Analysis
The 50G PON standard (G.9804 series) has currently three sub-series, among which, G.9804.1 defines the general requirements for higher speed passive optical networks, G.9804.2 the common transmission convergence (ComTC) layer specifications, and G.9804.3 the physical medium dependent (PDM) layer specifications.
G.9804.1: Overall System Requirements
The overall requirement is to provide a bandwidth capacity five times higher than 10G PON, a maximum fiber distance of 20 km, a downstream rate of 50 Gbps, and flexible upstream rate options of 12.5 Gbps, 25 Gbps, and 50 Gbps.
G.9804.2: ComTC Layer Specifications
When the rate of the 50G PON line is increased, the receiver sensitivity will decline. The performance of the receiver needs to be improved so that the massively-deployed ODN networks can be reused. To lower the requirements for high-speed optical components, 50G PON introduces the low density parity check (LDPC) scheme for FEC. For latency reduction, 50G PON applies dedicated activation wavelength (DAW), cooperative DBA (CoDBA) and bandwidth allocation interval reduction.
—DAW: A DAW can be a wavelength newly defined for 50G PON or one deployed for a PON system prior to 50G PON. It can be an independent upstream wavelength or a pair of upstream and downstream wavelengths. The DAW technology avoids opening a quiet window on the upstream wavelength, thereby eliminating the delay caused by the quiet window.
—Co-DBA: The OLT learns about the upstream service transmission requirement of the ONU through the upstream equipment, and then allocates bandwidth to the ONU in advance, hence reducing as much as possible the time for which the data is buffered in the ONU.
—Shortening the bandwidth allocation interval: The interval between bandwidth allocations for the ONU is reduced, thus reducing the time for which the data is buffered in the ONU. Each T-CONT supports a maximum of 16 bursts per 125 μs frame.
G.9804.3: PDM Layer Specifications
—Wavelength design: Currently, 50G PON can only use a small portion of wavelengths in the O band, which are not enough. After many discussions, ITU-T has made it clear that 50G PON will not coexist with GPON and 10G PON at the same time. ITU-T has decided on some wavelengths and is still discussing the other wavelengths.
—Line coding: ITU-T has considered PAM4, duobinary and NRZ line coding schemes in its early discussions. Because the PON system has stringent requirements for the optical power budget, ITU-T finally selected the NRZ with the highest signal receiving performance.
—PHY components: The PHY components of 50G PON mainly include key optoelectronic devices such as optical transmitter modules, optical receiver modules, laser device drivers (LDDs), burst-mode transimpedance amplifiers (TIAs), and clock-data recovery (CDR) chips. Experiments and simulations show that, by using a 50G EML transmitter and an APD receiver, 50G PON can attain a single-wavelength rate of 50 Gbps.
Features and Application Scenarios of Precise 50G PON
According to the current technical features of 50G PON, ZTE launched the industry's first precise 50G PON technology that optimizes the traditional best-effort transmission mechanism into a more reliable transmission mechanism over hardened pipes, which delivers precise bandwidth, precise latency and precise jitter.
First, the precise 50G PON technology, which optimizes the timeslots and changes the multiplexing of the traditional PON technologies, can not only provide 50 Gbps per wavelength but also deliver access rates ranging from 2 Mbps to 10 Gbps based on hard slicing. Second, ZTE uses the DAW technology to eliminate the latency caused by the quiet window and the single-frame multi-burst technology to reduce the transmission timeslots and unnecessary timeslot overhead, thereby reducing the latency in a 10 km transmission from 1.5﹣2.5 ms to within 200 μs. Finally, it provides precise jitter to optimize the traditional priority-based scheduling into fixed latency scheduling and reduce the jitter from the millisecond to nanosecond levels.
Take the comprehensive campus network access as an example, operators only need to build one feeder fiber to provide network coverage to the office, monitoring, manufacturing, and wireless access scenarios. In addition to coverage, campus networks usually need to provide precise latency and jitter. For example, telemedicine requires the latency to be less than 50 ms and the jitter to be less than 200 μs, grid relay protection requires the latency to be less than 200 ms and the jitter to be less than 50 μs, and underground mine communications require the latency to be about 30﹣100 ms. In the above application scenarios, precise 50G PON can be used to connect different terminals, and then access the public cloud or the operator's network to provide time-sensitive services.
At MWC 2022, ZTE unveiled the world's first precise 50G PON prototype, which is a milestone in the exploration of 50G PON application scenarios, the development of 50G PON technology and the maturation of the 50G PON industry chain. The precise 50G PON prototype featuring ultra-large bandwidth, low latency and low jitter can meet the demands for ultra-high bandwidth access in the home and enterprise scenarios, and support mobile xHaul and deterministic campus networks. In terms of bandwidth, the prototype supports up to 50 Gbps per wavelength, which is five times higher than the speed of the already massively deployed 10G PON technology.
The release of the 50G PON standard has received wide attention in the industry, and the maturity of the industry chain has also been put on the agenda. It is expected that the 50G PON industry chain will gradually mature by 2025, and 50G PON will be commercially ready by 2025.
As an ITU-T member, ZTE is committed to the maturity of 50G PON standards and industry chain improvement. The company has made breakthroughs in the technologies of some key components, and has submitted more than 30 50G PON standard proposals with those on physical layer parameters, low latency and FEC already adopted by the standards organizations. In the future, ZTE will develop key optical component chips, achieving win-win cooperation with its peers, and explore the full-scenario and full-service access capability enhancement, access network architecture integration and innovation, multi-network integration, and engineering O&M to help operators reduce overall investments and costs, and improve efficiency and value.