01-08-2012, 04:53 PM
Ethernet Passive Optical Network (EPON)
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Abstract
An Ethernet passive optical network is an emerging local subscriber access architecture that combines low-cost point-to-multipoint fiber infrastructure with Ethernet. EPONs are designed to carry standard Ethernet frames at standard Ethernet rates. An EPON uses a single trunk fiber that extends from central office to passive optical splitter, which then fans out to multiple optical fibers connected to subscriber nodes. Other than the end terminating equipment, no component in the network requires electrical power, hence the term passive. Local carriers have long been interested in passive optical networks for the benefits they offer: minimal fiber infrastructure and no powering requirement in the outside plant. With Ethernet now emerging as the protocol of choice fro carrying IP traffic in metro and access networks, EPON has emerged as a potential optimized architecture for fiber to the building fiber to the home.
Introduction
While in recent years telecommunication backbone has experienced substantial growth, little has changed in access networks. The tremendous growth of Internet traffic has accentuated the aggravating lag of the access network capacity. The “last mile” still remains the bottleneck between high-capacity local area networks (LANs) and the backbone network. The most widely deployed broadband solutions today are digital subscriber line (DSL) and cable modem (CM) networks. Although they are an improvement over the 56kbps modems, they are unable to provide enough bandwidth for emerging services such as IP telephony, video on demand (VoD), interactive gaming, or two-way video conferencing. A new technology is required; one that is inexpensive, simple, scalable, and capable of delivering bundled voice, data, and video services to an end-user subscriber over a single network. Ethernet passive optical networks (EPONs), which represent the convergence of low-cost Ethernet equipment and low-cost fiber infrastructure, appear to be the best candidate for the next-generation access networks.
Topic Details
Evolution of the first mile
Once called the last mile, the Ethernet community has renamed this network section to the first mile, to symbolize its priority and importance. The first mile connects the service provider central offices to business and residential subscribers. Also referred to as the subscriber access network or local loop, it is network infrastructure at the neighborhood level. Residential subscribers demand first mile access solutions that are broadband, offer Internet media-rich services, and are comparable in price to existing networks.
Incumbent telephone companies responded to Internet access demand by deploying DSL technology. DSL uses the same twisted pair as telephony lines and requires a DSL modem at the customer premises and a digital subscriber line access multiplexer (DSLAM) in the central office. The data rate provided by DSL is typically in a range of 128kbps-1.5Mbps. While this is significantly faster than an analog modem, it is well shy of being considered broadband, in that it cannot support full-service voice, data and video. In addition, the physical area one central office can cover with SDL is limited to distances less than 5.5 km, which constitutes approximately 60 percent of end-user subscribers. As a result, network operators are now deploying remote DSLAMS closer to subscribers; however, in general, service providers do not provide DSL services to subscribers located more than a few miles from a local exchange office due to costs. Cable television companies responded to Internet services by integrating data services over their coaxial cable networks, which were originally designed for analog video broadcast. Typically, these hybrid fiber coax (HFC) networks have fiber running between a video head-end or hub to curbside optical node, with the final drop to the subscriber being coaxial cable, repeaters, and tap couplers. The drawback of this architecture is that each shared optical node has less than 36 Mbps effective data throughput, which is typically divided between 2000 homes, resulting in frustrating slow speed during peak hours. To alleviate bandwidth bottlenecks, optical fibers, and thus optical nodes, are penetrating deeper into the first mile.
The next wave of local access deployment promises to bring fiber to the building (FTTB) and fiber to the home (FTTH). Unlike previous architectures, where fiber is used as a feeder to shorten the lengths of copper and coaxial networks, these new deployments use optical fiber throughout the access network. New optical fiber network architectures are emerging that are capable of supporting gigabit per second speeds, at costs comparable to DSL and HFC networks.
Traffic Growth
Data traffic is increasing at an unprecedented rate. Sustainable data traffic growth rate of over 100 percent per year is observed since 1990. There were periods when a combination of economical and technological factors resulted in even larger growth rates (1000 percent increase per year in 1995 and 1996). This trend is likely to continue in the future. Simply put, more and more users are getting online and those who are already online are spending more time online. Market research shows that after upgrading to a broadband connection user spend about 35 percent more tie online than before. Voice traffic is also growing, but at a much slower rate of 8 percent annually. According to most analysts, data traffic has already surpassed voice traffic. More and more subscribers telecommunicate, and require the same network performance as seen on corporate LANs. More services and new applications will become available as bandwidth per user increases.
Neither DSL nor CMs can keep up with such demand. Both the technologies are built on top of existing copper communication infrastructure not optimized for data traffic. In CM networks, only a few RF channels are dedicated to data, while the majority of the bandwidth is tied up servicing legacy analog video. DSL copper networks do not allow sufficient data rates at required distances. Most network operators have come to the realization that a new data-centric solution is necessary. Such a technology would be optimized for Internet Protocol (IP) data traffic. The remaining services, such as voice and vide, will converge into a digital format, and a true full-service network will emerge.