PDF of this tutorialPassive optical networks (PONs) address the last mile of the communications infrastructure between the service provider’s CO, head end, or point of presence (POP) and business or residential customer locations. Also known as the access network or local loop, the last mile consists predominantly, in residential areas, of copper telephone wires or coaxial cable television (CATV) cables. In metropolitan areas, where there is a high concentration of business customers, the access network often includes high-capacity synchronous optical network (SONET) rings, optical T3 lines, and copper-based T1s.
Typically, only large enterprises can afford to pay the $3,200 – $4,300 per month it costs to lease a T3 (45 Mbps) or optical carrier (OC)–3 (155 Mbps) SONET connection. T1s at $375 per month are an option for some medium-size enterprises, but most small and medium-size enterprises and residential customers are left with few options beyond plain old telephone service (POTS) and dial-up Internet access. Where available, digital subscriber line (DSL) and cable modems offer a more affordable interim solution for data, but they are difficult and time-consuming to provision. In addition, bandwidth is limited by distance and by the quality of existing wiring; and voice services have yet to be widely implemented over these technologies.
Even as the access network remains at a relative standstill, bandwidth is increasing dramatically on longhaul networks through the use of wavelength division multiplexing (WDM) and other new technologies. Recently, WDM technology has even begun to penetrate metropolitan-area networks (MAN), boosting their capacity dramatically. At the same time, enterprise local-area networks (LAN) have moved from 10 Mbps to 100 Mbps, and soon many LANs will be upgraded to gigabit Ethernet speeds. The result is a growing gulf between the capacity of metro networks on one side and end-user needs on the other, with the last-mile bottleneck in between.
Figure 1. Sweet Spot for Passive Optical Networks
PONs aim to break the last-mile bandwidth bottleneck by targeting the sweet spot between T1s and OC–3s that other access network technologies do not adequately address.
The two primary types of PON technology are asynchronous transfer mode PONs (APONs) and Ethernet PONs (EPONs).
APONs
APONs were developed in the mid 1990s through the work of the full-service access network (FSAN) initiative. FSAN was a group of 20 large carriers that worked with their strategic equipment suppliers to agree upon a common broadband access system for the provisioning of both broadband and narrowband services. British Telecom organized the FSAN Coalition in 1995 to develop standards for designing the cheapest, fastest way to extend emerging high-speed services, such as Internet protocol (IP) data, video, and 10/100 Ethernet, over fiber to residential and business customers worldwide.
At that time the two logical choices for protocol and physical plant were ATM and PON: ATM because it was thought to be suited for multiple protocols, PON because it is the most economical broadband optical solution. The APON format used by FSAN was accepted as an International Telecommunications Union (ITU) standard (ITU–T Rec. G.983). The ITU standard focused primarily on residential applications and in its initial version did not include provisions for delivering video services over the PON. Subsequently, a number of start-up vendors introduced APON–compliant systems that focused exclusively on the business market.
EPONs
The development of EPONs has been spearheaded by one or two visionary start-ups that feel that the APON standard is an inappropriate solution for the local loop because of its lack of video capabilities, its insufficient bandwidth, its complexity, and its expense. Also, as the move to fast Ethernet, gigabit Ethernet, and now 10-gigabit Ethernet picks up steam, these start-ups believe that EPONs will eliminate the need for conversion in the wide-area network (WAN)/LAN connection between ATM and IP protocols.
EPON vendors are focusing initially on developing fiber-to-the-business (FTTB) and fiber-to-the-curb (FTTC) solutions, with the long-term objective of realizing a full-service fiber-to-the-home (FTTH) solution for delivering data, video, and voice over a single platform. While EPONs offer higher bandwidth, lower costs, and broader service capabilities than APON, the architecture is broadly similar and adheres to many G.983 recommendations.
In November 2000, a group of Ethernet vendors kicked off their own standardization effort, under the auspices of the Institute of Electrical and Electronics Engineers (IEEE), through the formation of the Ethernet in the First Mile (EFM) study group. The new study group aims to develop a standard that will apply the proven and widely used Ethernet networking protocol to the access market. Sixty-nine companies, including 3Com, Alloptic, Aura Networks, CDT/Mohawk, Cisco Systems, DomiNet Systems, Intel, MCI WorldCom, and World Wide Packets, have indicated they will participate in the group.
