PDF of this tutorialThe economic case for EPONs is simple: fiber is the most effective medium for transporting data, video, and voice traffic, and it offers virtually unlimited bandwidth. But the cost of running fiber “point-to-point” from every customer location all the way to the CO, installing active electronics at both ends of each fiber, and managing all of the fiber connections at the CO is prohibitive. EPONs address the shortcomings of point-to-point fiber solutions by using a point-to-multipoint topology instead of point-to-point in the outside plant; by eliminating active electronic components, such as regenerators, amplifiers, and lasers, from the outside plant; and by reducing the number of lasers needed at the CO.
| Point-to-Point Fiber Access | EPON |
| Point-to-Point Architecture | Point-to-Multipoint Architecture |
| Active electronic components are required at the end of each fiber and in the outside plant. | Eliminates active electronic components, such as regenerators and amplifiers, from the outside plant and replaces them with less-expensive passive optical couplers that are simpler, easier to maintain, and longer lived than active components |
| Each subscriber requires a separate fiber port in the CO. | Conserves fiber and port space in the CO by passively coupling traffic from up to 64 optical network units (ONU) onto a single fiber that runs from a neighborhood demarcation point back to the service provider’s CO, head end, or POP |
| Expensive active electronic components are dedicated to each subscriber | Cost of expensive active electronic components and lasers in the optical line terminal (OLT) is shared over many subscribers |
Table 1. Comparison of Point-to-Point Fiber Access and EPONs
Unlike point-to-point fiber-optic technology, which is optimized for metro and longhaul applications, EPONs are tailor-made to address the unique demands of the access network. Because they are simpler, more efficient, and less expensive than alternative access solutions, EPONs finally make it cost-effective for service providers to extend fiber into the last mile and to reap all the rewards of a very efficient, highly scalable, low-maintenance, end-to-end fiber-optic network.
The key advantage of an EPON is that it allows carriers to eliminate complex and expensive asynchronous transfer mode (ATM) and SONET elements and to simplify their networks dramatically. Traditional telecom networks use a complex, multilayered architecture, which overlays IP over ATM, SONET, and WDM. This architecture requires a router network to carry IP traffic, ATM switches to create virtual circuits, add/drop multiplexers (ADM) and digital cross-connects (DCS) to manage SONET rings, and point-to-point DWDM optical links.
There are a number of limitations inherent to this architecture: (1) it is fiendishly difficult to provision because each network element (NE) in an ATM path must be provisioned for each service; (2) it is optimized for time division multiplex (TDM) voice—not data—so its fixed bandwidth channels have difficulty handling bursty data traffic; (3) it requires inefficient and expensive optical-to-electrical-to-optical (O–E–O) conversion at each network node; (4) it requires installation of all nodes up front (because each node is a regenerator); and (5) it does not scale well because of its connection-oriented virtual circuits.
In the example of a streamlined EPON architecture in Figure 2, an ONU replaces the SONET ADM and router at the customer premises, and an OLT replaces the SONET ADM and ATM switch at the CO.
Figure 2. Streamlined EPON Architecture
This architecture offers carriers a number of benefits. First, it lowers up front capital equipment and ongoing operational costs relative to SONET and ATM. Second, an EPON is easier to deploy than SONET/ATM because it requires less complex hardware and no outside plant electronics, which reduces the need for experienced technicians. Third, it facilitates flexible provisioning and rapid service reconfiguration. Fourth, it offers multilayered security, such as virtual LAN (VLAN) closed user groups and support for virtual private network (VPN), IP security (IPSec), and tunneling. Finally, carriers can boost their revenues by exploiting the broad range and flexibility of services offerings available over an EPON architecture. This includes delivering bandwidth in scalable increments from 1 to 100 Mbps up to 1 Gbps and value-added services, such as managed firewalls, voice traffic support, VPNs, and Internet access.
