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EPON, a long-haul Ethernet access technology based on fiber optic transport network

May 31, 2023

EPON is a long-range Ethernet access technology based on fiber optic transport network. EPON adopts a point-to-multipoint architecture, where a single fiber carries upstream and downstream data signals, and after 1:N splitter, the optical signal is divided into N channels, covering multiple access points or access users with optical branches.

EPON Network Architecture

EPON is also traditionally called GEPON, without any misleading intention. The early EPON devices in the industry were based on FE(Fast Ethernet) bus. However, after the introduction of EPON devices based on GE(Gigabit Ethernet) bus, they were called GEPON in order to distinguish them. Currently, the majority of EPON devices in the industry are basically based on GE bus. At present, they are generally referred to as EPON.

A typical EPON system consists of OLT, ONU, and ODN, and the network structure of EPON is shown in Figure 1.

EPON's network structure

OLT is placed in the central server room, which can be regarded as an L2 switch or L3 routing switch. In the downstream direction, the OLT provides fiber optic interfaces for passive optical networks (ODN); in the upstream direction, the OLT will provide GE optical/electrical interfaces, and in the future, when the 10Gbit/s Ethernet technology standard is finalized, the OLT will also support similar high-speed interfaces. In order to provide multi-service access, OLT can also support E1 and OC3 interfaces to realize traditional voice access or circuit trunking services.

In terms of EPON network management, the OLT serves as the main control center with built-in OAMP Agent, which can manage the ONU terminal devices under its control and perform the five functions of network management. It can also provide effective user isolation.

ODN is an optical distribution network, which consists of passive fiber optic splitters and optical fibers. A passive fiber optic splitter is a passive device that connects OLT and ONU, and its function is to distribute downstream data and centralize upstream data. The deployment of passive splitter is highly flexible and can be adapted to almost all environments due to their passive nature. The general passive fiber optic splitters have splitting ratios of 1:2, 1:4, 1:8, 1:16, 1:32, 1:64, etc. It is generally recommended to use one level of splitting, and at most, no more than two levels of splitting.

The ONU is a terminal device placed on the subscriber premises side. The ONU in EPON utilizes the Ethernet protocol to achieve cost-effective Ethernet Layer 2 switching functionality. By employing the Ethernet protocol, there is no longer a need for protocol conversion during the communication process to achieve transparent transmission of user data from one ONU to another. Additionally, and encryption protocol is used between OLT and ONU to ensure the security of user data.

The advantage of EPON-based FTTH lies in its strong coverage capability, covering up to 20 km (1:32 split ratio) from the end office and connecting each optical access point through the ODN. In the traditional optical access networks, fiber optic extension range is typically limited to the network access point. To achieve fiber to the home, a large number of expensive optical port access layer switches need to be configured at the access point. With the emergence and maturation of passive optical network technology, especially the current EPON technology, it can provide an economically viable solution for directly delivering optical fiber to the end of the user, making FTTH an efficient access method. In EPON-based FTTX solutions, solving the introduction of fiber optic cable to the buildings, OLT planning in the community, ODN deployment, and fiber connectivity to indoor user terminals (OUN) has become crucial.

EPON’s upstream and downstream technologies

EPON uses a single fiber between OLT and ONU to provide symmetric 1.25Gbps bandwidth, due to limitations of the physical interface, it actually provides 1Gbps bandwidth to transmit data, voice and video services. EPON employs wavelength division multiplexing technology on a single-core fiber, allowing the upstream and downstream data streams to be transmitted in different frequency bands. Among them, the

Downlink 1490nm ;

Uplink 1310nm ;

1550nm is optional for CATV.

The downlink data stream is broadcasted, and the OLT pushes the Ethernet frame data stream in 802.3 format to all ONUs through unicast replication. The ONUs determine whether to receive the frames by judging the LLID (Logical Link ID) assigned by the OLT in the Ethernet frame header. They receive the data frames that belong to them and discard the ones that do not. This is shown in Figure 2.

EPON Downlink Data Stream

The uplink data stream adopts Time Division Multiple Access (TDMA) technology, which divides the uplink time into multiple time slots, Based on the allocated bandwidth and service priority of each ONU,different time slots are assigned to their respective uplink data streams. At each time slot, only one ONU’s uplink data stream is transmitted over the fiber. Through negotiation between OLT and ONU, conflicts between uplink data streams from different ONUs are avoided, ensuring no data loss occurs. As shown in Figure 3.

EPON Uplink Data Stream

Comparison of EPON and ADSL

After several years of vigorous development, ADSL has become the most popular broadband access method for China’s fixed network operators. By transmitting broadband data, and ADSL makes full use of the copper wire resources of fixed network operators , making it one of the best choices for operators like China Telecom and China Netcom in the early stage of broadband access.

ADSL/ADSL2+ service is an asymmetric transmission broadband access technology with a limited uplink bandwidth of less than 1Mbps and a maximum downlink bandwidth of up to 26Mbps. In actual commercial use, it covers a distance of no more than 3km, generally providing a downlink bandwidth ranging from 512Kbps to 2Mbps and is mainly applied for public Internet access.

However, with the rise of various new services, especially video services, users’ bandwidth requirements are increasing. The rapid growth of applications such as blogging, online gaming, instant messaging, broadband telephony, video telephony, and personal photo album sharing has led to an increased demand for uplink bandwidth from users.

In the broadband network planning of China Telecom and China Netcom, the future two-way bandwidth for individual users will reach 10M~20M. The bandwidth of ADSL is strictly limited by the transmission distance, and the higher bandwidth can only be achieved over short distances, Even with the transformation of “fiber to copper” and shortening the coverage of ADSL, it can only meet the bandwidth requirements to a certain extent and for a certain period of time. Even after the transformation of “fiber to copper” and the reduction of ADSL coverage area, it can only meet the bandwidth requirement within a certain period of time and to a certain extent.

The bandwidth of fiber-based access network is theoretically infinitely scalable. Therefore, with the maturity of EPON technology, its high bandwidth and long-distance coverage make EPON an inevitable choice for technology development, replacing ADSL technology.

Compared to ADSL, EPON has higher initial construction expenses, including the preliminary equipment cost and fiber laying cost. However, due to the use of passive optical network technology for network connectivity, fiber-based PON technology has significantly lower operation and maintenance costs compared to ADSL and copper wire.

The ability to provide higher bandwidth and longer-distance service coverage through lower post-operation and maintenance costs, allows for the provision of additional new services, bringing more business revenue, can relatively offset the investment in equipment and line costs. The cost of fiber is already low, FTTX has entered a period of rapid growth, and the cost of equipment will continue to decrease during construction. Therefore, by deploying EPON, operators can enhance the competitiveness of the full range of services, including broadband access, thereby stabilizing subscriber resources and even regaining lost subscribers, which will bring more business revenue to operators and benefit them in the long run.

Technical advantages of EPON

With the maturity of EPON technology, mainstream carriers in the industry have begun large scale deployment of EPON systems to launch FTTX applications. They aim to realize Triple Play (triple play, providing voice, data and video services) on this basis to build a triple-play access platform.

Since 2004, EPON technology has been widely deployed in mature FTTH markets such as Japan, Korea, as well as the US and Europe, further driving the the prosperity of IPTV business. In the Chinese market, EPON products have been piloted and commercially available in all provinces.

EPON technology uses wavelength division multiplexing (WDM) technology to achieve a symmetric 1 Gbps bandwidth on a single fiber, and it enables downstream splitting in close proximity to the customer segment, saving a lot of backbone fiber resources. Another advantage of EPON system is its strong coverage capability, with a maximum coverage of 20 km at 1:32 split ratio and 10 km at 1:64 split ratio. This ensures reliable equipment converge. Under 1:32 split ratio, the average bandwidth of each ONU user can reach more than 30M, providing sufficient bandwidth guarantee for video services.

The use of passive optical splitters in EPON systems saves significant maintenance resources, including saving server rooms and power supply packages, thereby reducing the overall construction and maintenance costs of FTTX networks. The decreasing cost of optical fiber in recent years has contributed to the cost reduction of cable infrastructure in FTTX deployments.

At present, the broadband access services provided by the operators are mainly two types: ADSL and LAN access. Additionally, with the increasing demand for bandwidth, VDSL is also gradually becoming an option. Other access methods, such as Cable Modem, power line access, have limited market share due to resource constraints in the industry.