The development history of PON, a technology node of network traffic

Dojammer 2021-12-02

The story starts with the origin of PON.

In the 1980s, with the rise of computer and multimedia technologies, more and more people began to own computers and began to contact the Internet (local area network or the Internet). As a result, data communication services began to rise, and the demand for Internet access emerged.

In the early days, Internet services provided by operators were basically copper cables such as telephone lines, twisted pair (network cables), and coaxial cables. Technologies include ADSL (Asymmetric Digital Subscriber Line) and ISDN dedicated line (Integrated Services Digital Network), etc.

ADSL Modem

These methods are either very slow or costly, and cannot meet the needs of users.

As a result, optical fiber, as a new type of communication medium, began to enter people's field of vision.

Optical fiber

Since Mr. Gao Kun, a Chinese scientist in 1966, established the theoretical foundation of optical fiber communication through a paper, this technology has entered a stage of rapid development. The optical fiber manufacturing process continues to mature, and the industrial chain is also increasingly taking shape.

By the end of the 1980s, in order to apply optical fiber to broadband access services, manufacturers successively introduced their own narrowband PON technology. At that time, the speed of this technology was very low, not exceeding 2Mbps. Moreover, because manufacturers are fighting each other, there has been no unified norms and standards.

In 1995, seven network operators including BELLSOUTH, BT, and France Telecom jointly initiated the establishment of the Full Service Access Network Alliance (FSAN), hoping to propose a unified optical access network equipment standard.

Soon after, in 1997, in accordance with FSAN's recommendations, ITU-T (International Telecommunication Union Telecommunications Standards Branch) introduced the APON technology system, which is the G.983.1 standard.

APON is ATM PON. ATM is not an automatic teller machine, it is an abbreviation for Asynchronous Transfer Mode. The essence of ATM is a transmission protocol. The older generation of communication people must be very familiar with ATM. It was once a competitor of the IP protocol and was once very popular.

In 2001, FSAN and ITU-T upgraded and revised the APON specification, and by the way changed its name to BPON (Broadband PON, Broadband Passive Optical Network). The reason for the name change is that they do not want APON to be misunderstood as only providing ATM services.

In order to further improve the PON rate standard, in 2002, FSAN started a new work to standardize PON networks above 1Gbps.

In March 2003, on the basis of FSAN's proposal, ITU-T promulgated the G.984 standard, which is GPON (Gigabit-capable PON, Gigabit Passive Optical Network).

While FSAN and ITU-T were doing enthusiastically, another standardization organization was not idle, and started to play PON technology. It is the same famous IEEE (Institute of Electrical and Electronics Engineers).

IEEE is the creator and backer of the Ethernet standard.

After the IEEE released the Gigabit Ethernet standard in 1998, it was thinking about developing a PON standard based on Ethernet.

In 2000, the IEEE established the EFM working group and officially launched related standardization work. The full name of the EFM working group is very interesting. It is called the Ethernet for the First Mile (Ethernet for the First Mile), which belongs to the IEEE 802.3 group that develops the Ethernet standard.

In April 2004, the EFM working group was completed and officially launched the IEEE 802.3ah standard, which is EPON (Ethernet PON, Ethernet-based PON).

With the passage of time, ATM has gradually lost its momentum in the competition with IP. APON (BPON) has also been abandoned by operators due to cost, efficiency and other reasons, and has withdrawn from the stage of history. Therefore, I won't introduce APON (BPON) more, and you don't need to know too much.

We will focus on EPON and GPON, which have become the mainstream of the industry at that time.

EPON and GPON are different technical systems launched by different standards organizations. There is no relationship of upgrading, evolution or substitution between the two, which can be regarded as parallel development.

The key difference between EPON and GPON lies in the second layer (that is, the data link layer).

EPON, at the second layer is to replace ATM with Ethernet. GPON uses a newly defined encapsulation structure GEM (GPON Encapsulation Mode, GPON encapsulation mode) at the second layer to encapsulate and map multiple services such as Ethernet, TDM, and ATM.

We make a simple comparison through the following table:

In short, EPON and GPON have their own advantages and disadvantages. Simply put, GPON has larger bandwidth, more users, and higher efficiency, but it is also more complicated to implement, so the cost is higher.

From the perspective of domestic market share, EPON was widely adopted by China Telecom at that time, while GPON was more popular with China Unicom and China Mobile.

02. 10G-EPON and XGS-PON

EPON and GPON are both 1Gbps PONs. Note that this 1Gbps is not the rate on the user side. EPON and GPON can only provide users with a rate of 100Mbps.

Obviously, with the development of the times, this rate cannot meet the needs of home and business users. As a result, PON began to evolve to the 10Gbps level.

In 2006, the IEEE began to establish a project to formulate a 10Gbit/s rate EPON system standard, which was later IEEE 802.3av, 10G-EPON.

In this standard, 10G EPON is divided into two types: one is asymmetrical, that is, the downstream rate is 10 Gbps, and the upstream rate is 1 Gbps; the other is the symmetrical method, that is, both the upstream and downstream rates are 10 Gbps.

The GPON of ITU-T is also evolving.

In 2008, the ITU initiated the study of the next-generation GPON standard. In 2010, the XG-PON standard was born, which is the ITU-T G.987 series.

At the very beginning, XG-PON also had two modes, one is the asymmetric mode XG-PON1, the downlink rate is 10 Gbps, and the uplink rate is 2.5 Gbps; the other is the symmetric mode XG-PON2, the uplink and downlink rates are both 10. Gbps.

Later, around 2013, because the symmetrical solution of XG-PON2 was difficult to realize, it was suggested to cancel. XG-PON1 was directly renamed XG-PON.

Later, in 2015, the symmetric solution was restarted and adopted a new name called XGS-PON (S stands for symmetric, symmetric).

In 2017, ITU officially passed the G.9807 XGS-PON international standard.

Through the following figure, you should see more clearly:

It is worth mentioning that there is also a Combo-PON, which has been collected by operators.

Combo-PON integrates GPON optical module, 10G PON optical module and WDM1r multiplexer through a three-in-one optical module. In this way, whether the user side is connected to a GPON ONT or a 10G PON ONT, it can work normally, which is conducive to the upgrade transition.

After 2010, Internet applications such as video and games have developed rapidly, and users have a strong demand for network bandwidth. This has further stimulated the maturity of the 10G PON industry chain.

Since 2013, domestic operators have carried out large-scale centralized procurement and mass deployment of 10G PON. In recent years, with the promotion and popularization of "Gigabit Broadband", the centralized procurement of 10G PON has reached its peak.

For 10G PON, domestic operators also have some differences in their upgrade strategies. Mainly China Telecom. Telecom has a large EPON inventory before, so upgrade and replacement inevitably still adopted the 10G EPON solution, but the new ones have basically switched to GPON.

From a technical point of view, the upgrade and replacement of 10G PON is actually very convenient. Basically, it can be directly upgraded on the basis of the existing hardware architecture.

Among the three major parts of OLT, ODN, and ONU, ODN (Optical Distribution Network) is almost completely unchanged. Anyway, it is also passive, just a pipeline. It is estimated that it will be no problem for decades. For the central office equipment OLT, the original GPON/EPON platform can remain unchanged, and only need to upgrade and replace the large-capacity interface board. The user-side ONU replacement is relatively simple, according to user needs (when changing the broadband package), gradually phase out the replacement.

All in all, the upgrade and transformation of 10G PON is not too difficult, but the user experience has improved significantly, and it can make users pay more. Isn't it beautiful? This is also one of the reasons why operators are vigorously promoting.

03. The future of PON

For standards organizations, 10G PON is not the end of PON technology. 25G/50G/100G PON has already been put on the agenda.

First look at FSAN and ITU-T.

After the formulation of the XG-PON standard was completed, FSAN started standard research on NG-PON2 (NG-PON1 is the previous 10G PON, including 10G GPON and 10G EPON). The key requirements of NG-PON2 are mainly 40G downlink and 40G/10G uplink, achieving 20km transmission distance and 1:64 optical splitting.

At that time, mainstream alternative technical solutions for NG-PON2 included high-speed TDMA-PON, TWDM PON, OFDM-PON and WDM-PON. After analysis and comparison, in April 2012, FSAN decided to adopt TWDM PON technology as the implementation plan of NG-PON2, and started to formulate the G.989.x series of standards, which was finally completed in 2015.

TWDM PON adopts 4/8 wavelength superposition method, single wavelength 10G TDM.

NG-PON2 system architecture

Let's take a look at the IEEE side.

In 2013, IEEE started the research of NG-EPON and established IEEE ICCOM to analyze the market demand and technical solutions of NG-EPON. In March 2015, IEEE released the NG-EPON technical white paper. In July 2015, the establishment of the 100G-EPON standard began, named IEEE 802.3ca. According to the latest information, it will be released in August 2020.

100G-EPON system architecture

Also draw a picture for everyone to understand:

On the basis of 10G TDM, ITU-T experts are also considering further increasing the single-wave capacity. The consideration directions are mainly focused on 25G, 50G and 100G.

Among them, the difference between 25G and 10G is too small, and 100G is too difficult. Therefore, 50G is generally considered to be a more appropriate evolution direction.

In February 2018, the domestic optical access network industry successfully promoted the establishment of the 50G TDM-PON standard, marking a key step taken by ITU-T in the field of next-generation PON standard research and further clarifying the future technology evolution route of PON ( At least it is clear domestically).

However, the IEEE did not accept the establishment of a single-wave 50G PON project. They have a big disagreement with ITU-T on the choice of the next-generation PON technology route, and they cannot reach a consensus in a short time.

In summary, 10G PON can provide 100M~1G bandwidth per user, while 25G/50G/100G PON can provide users with 1G~10Gbps bandwidth. The large-scale deployment time of broadband access platforms is usually about 7-8 years apart. From 2016 to 2018, we entered a period of large-scale deployment of 10G PON.

In other words, it is estimated that in 2025, we will usher in the large-scale deployment of 5Gbps and 10Gbps optical fiber access broadband.

04. Convergence of PON genres

In addition to studying the next-generation PON technology, the industry is still paying close attention to a very important issue, which is the integration of the two technical schools of ITU-T and IEEE.

Since the birth of EPON and GPON, the two factions have coexisted for a long time, which is actually very unfavorable for the industry.

On the one hand, it brings difficulties to the technical decisions of operators and equipment (multiple-choice questions are difficult to do), on the other hand, it also increases the cost of the industrial chain (industrial chain enterprises need to invest in two lines).

For such a huge optical access network market in China, the differentiation of PON technology factions has a greater impact and more waste of resources. Therefore, Chinese operators and the industry chain have a strong desire for PON integration and have done a lot of work to promote it.

Under the active promotion of domestic and foreign industries, ITU-T and IEEE also have some positive "shows", including issuing joint statements, forming working groups, and establishing a liaison letter mechanism. However, it will take a long time to truly realize the final integration.

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