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EPON — An Ideal Optical Access Network Solution

In recent years, the telecommunications backbone has experienced huge growth. The tremendous growth of Internet traffic has far surpassed the network capacity. The “last mile” still remains the bottle neck between high-capacity local area networks and the backbone network. So a new technology for optical access network, which is simple, scalable but not expensive, is needed. And that is Ethernet passive optical network (EPON).

EPON Definition

EPON, unlike other PON technologies which are based on the ATM standard, is based on the Ethernet standard. This lets you utilize the economies-of-scale of Ethernet, and provides simple, easy-to-manage connectivity to Ethernet-based, IP equipment, both at the customer premises and at the central office. As with other Gigabit Ethernet media, it is well-suited to carry packetized traffic, which is dominant at the access layer, as well as time-sensitive voice and video traffic.

EPON Network

An EPON network includes two parts: an optical line terminal (OLT) and an optical network unit (ONU).

The OLT resides in the central office (CO). This could typically be an Ethernet switch or Media Converter platform. OLT is mainly designed for controlling the information float across the optical distribution network (ODN). OLT has two float directions: upstream (getting an distributing different type of data and voice traffic from users) and downstream (getting data, voice and video traffic from metro network or from a long-haul network and sending it to all ONU modules on the ODN.

The ONU resides at or near the customer premise, in a building, or on the curb outside. It uses optical fiber for connecting to the PON on the one side, while interfacing with customers on the other side.

EPON Upstream and Downstream Traffic

In an EPON, the process of transmitting data downstream from the OLT to multiple ONU is fundamentally different from transmitting data upstream from multiple ONUs to the OLT.

In the downstream direction (from network to user), Ethernet frames transmitted by OLT pass through a 1:N (N represents the number of subscribers each fiber can serve) passive splitter and reach each ONU. Splitting ratios are usually between 4 and 64. At the splitter, the traffic is divided into separate signals, each carrying all of the ONU–specific packets. When the data reaches the ONU, it accepts the packets that are intended for it and discards the packets that are intended for other ONUs.

downsteam

In the upstream direction (from user to network ), due to the directional properties of a passive combiner (optical splitter), data frames from any ONU will only reach the OLT, not other ONUs. Frames in EPON from different ONUs transmitted simultaneously may collide. Thus, ONUs need to share the trunk fiber channel capacity and resources.

upstream

EPON Advantages

First, EPON uses a point-to-multipoint topology instead of point–to-point in the outside plant. Thus it saves much of the cost of running fiber from every customer to the CO, installing active electronics at both ends of each fiber and managing all of the fiber connections at the CO. And EPON also eliminates active electronic components, such as regenerators and amplifiers, and replaces them with passive optical couplers that are less-expensive, simpler, and longer lived than active ones. As to the cost of expensive electronic components and lasers in the OLT, it will be shared over many subscribers not paid by each subscriber.

Second, EPON offers high bandwidth to subscriber. The traffic rates of 1Gbps in downstream and return traffic of 800 Mbps have been achieved already. Compared with point–to-point technology, EPON is specially made to address the unique demands of the access work. So more bandwidth can be got by each subscriber.

At last, the main advantage of EPON is that it can eliminate complex and expensive asynchronous transfer mode (ATM) and SONET elements and simplify the networks dramatically. Traditional telecom networks use a complex and multilayered architecture. While this architecture requires a router network to carry IP traffic, ATM switches to create virtual circuits, add/drop multiplexers (ADM) and digital cross-connects to manage SONET rings, and point-to-point DWDM optical links.

Summary

EPON is suitable for Fiber-to-the-Home/Building/Business applications, including voice, data and video services. EPON is a shared network but with much higher bandwidth. It’s a highly attractive access solution for service providers to extend fiber into the last mile because of low cost and good performance, resulting from their nature as passive networks, point-to-multipoint architecture, and native Ethernet protocol.

A Guide for PON

Nowadays, there is a growing popularity of Video-on-Demand (VoD), VoIP and increased IPTV deployment. Providers aim to offering fiber-to-the-home (FTTH), (fiber-to-the-building) FTTB and fiber-to-the-curb (FTTC) solutions through advancing passive optical network (PON) technology. The term “PON” may confuse you for its complexity and extensiveness. Details are as followed.

PON is a single, shared optical fiber that uses inexpensive optical splitters to divide the single fiber into separate strands. It can build up a point-to-point topology supporting 1Gbps transmission to home and business typically within 20km. PON system is called “passive” because that there are no active electronics within the access network. It uses optical splitters to separate and collect signals rather than electrically powered switching equipment.

PON consists of an Optical Line Terminal (OLT) connected to multiple Optical Network Units (ONUs) via an Optical Distribution Network (ODN).

OLT: it is a device at the service provider’s central office, performing conversion between the electrical signals used by the service provider’s equipment and the fiber optic signals used by the passive optical network and coordinating the multiplexing between the conversion devices on the other end of that network.

ODN: it is used for distributing signals to users in a telecommunications network by optical fiber. ODN has been made up entirely of passive optical components particularly singlemode optical fibers and optical splitters.

ONUs: they are devices near end users, delivering traffic-load information provided by OLTs to each end user.PON System

PON system has achieved significant deployment in today’s FTTx networks especially in FTTH networks as the development of Gigabit passive optical network (GPON) and Ethernet passive optical network (EPON). Nowadays, GPON and EPON are the mostly widely used types of PON for their low cost, high bandwidth, great flexibility and easy management, etc.

GPON: it is defined by ITU-T recommendation series G.984.1 through G.984.6. It can transport not only Ethernet, but also ATM and TDM (PSTN, ISDN, E1 and E3) traffic. It supports services like carrying video and delivering video on single fiber distribution, allowing low-consuming transmission, more efficient maintenance, cabling and overall performance.

EPON: it is defined by the Ethernet standard rather than by the ATM standard, making you utilize the economies-of-scale of Ethernet. It can provide simple and easy-to-manage connectivity to Ethernet-based, IP equipment both at the customer premises and at the central office. It is perfect for voice and video traffic solution as with other Gigabit Ethernet media.GPON and EPON

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