Fiber optical networks are what we would deploy today for combined telephone, TV, and Internet access if we were building a broadband communications system from scratch. Optical networks have the bandwidth to handle any current application and anything we may want in the future. However, optical networks are expensive and have even greater installation cost.
Optical fiber and component prices have come down over the years but copper cable still wins. Such high costs have forced us to make do with the ancient existing copper twisted-pair telephone infrastructure and the newer hybrid fiber-cable TV network. But those systems also have limits that we are reaching gradually. The conventional pots twisted pair has just about come to the end of its useful life despite its amazing success with DSL. True broadband requires something faster. The cable TV people currently hold the U.S. lead with broadband access because the coax cable will take signals out to nearly 1 GHz. The solution for the RBOCs (regional Bell operating companies) appears to be a less expensive version of the traditional optical network known as the passive optical network (PON).
A PON is a point-to-multipoint tree or star-like topology network using a fiber with no expensive optical-electrical-optical (OEO) intervening electronics. The electronics only appear at each end of the link, one at the carrier's central office and the other at the subscribers end. Passive splitters (couplers) distribute the signals to as many as 32 nodes (64 maximum in some systems) at distances up to 20 km, in some cases farther.
These connections are also referred to as the Optical Distribution Network (ODN) and the "last mile" or the "first mile." They can also be called fiber to the premises (FTTx) where x means the home, business, curb, or user. It is one more attempt to get broadband into the homes and small office/home office (SOHO) businesses. Now, the telecom carriers are finally getting serious about broadband by beginning what is expected to be the gradual replacement of the old twisted pair with PONs.
The survival and continuing competitiveness of the regional Bell operating companies (RBOCs) is at stake. These companies have been losing subscribers slowly for the past decade as more of them move to wireless or alternative telephone connections such as VoIP on a cable TV system. As VoIP rolls out, the traditional carriers stand to lose even more, not to mention the inability to take advantage of the many exciting and profitable broadband applications like video-on-demand (VOD) and gaming. While DSL has kept many of the carriers viable, it has almost run its course. The newer ADSL2+/2++ is helping to speed up the networks but the range is very limited. Fiber provides all the bandwidth for whatever services may be needed now and in the future.
The U.S. has a pattern of falling behind the rest of the world and missing major opportunities in electronic technology. Asia already took away consumer electronics. Cell phones are another loss for the U.S., again with Asia leading the way and even Europe being farther ahead. Now it's broadband. With its ultra restrictive and even stupid telecommunications regulations and policies, the U.S. is again falling behind. But now with the major carriers like SBC, Bell South and Verizon recognizing their own survival is at stake, maybe we can catch up. With PONs on the verge, the affordable triple play (voice, video and data) may be within reach.
PON TECHNOLOGY
The PON concept has been around for decades with the first PONs built during the early 1980s in Europe. High costs prohibited much use in the U.S. and the carriers turned to DSL and cable TV systems for broadband connections. Yet, development has continued over the years and several standards have emerged.
The first, APON or ATM-based PONs, was originally developed by the Full Service Access Network (FSAN), an organization of telecom service providers. This standard uses the mature asynchronous transfer mode (ATM) packet transmission system employed by virtually all telephone carriers. Data is transmitted in 53 byte packets with 48 bytes plus a 5-byte overhead. The base speed is 155.52 Mbits/s but that is scaled to 622.08 Mbits/s in most systems. A more advanced version, called broadband PON (BPON), is now the primary target standard for most US PON systems. It includes video transmission and can handle Ethernet. ITU-T standard G.983.x cover both APON and BPON.
Figure 1 shows the major elements of a BPON system. At the central office (CO) of the carrier is the Optical Line Terminal (OLT). This handles communications with the remote sites, known as the Optical Networking Unit (ONU) or Optical Networking Terminal (ONT). In between the OLT and ONU is the ODN, made up of passive optical devices used as splitters. Also known as combiners because of their dual bidirectional nature, splitters simply divide the light power between two or more outputs.
Passive splitters are available in 1:2, 1:4, 1:8, 1:16, 1:32 and even higher level versions. Usually power is split equally among the outputs but there are asymmetrical splitters that can divide the power in an uneven ratio as demanded by the system. Splitters are cheap, highly reliable, and require zero maintenance. In most existing or proposed systems a 1:4 splitter is used at the OLT and then 1:8 splitters are used in the field to get up to 32 users on a fiber. Although BPON supports up to 64 users, 32 will probably be the normal upper limit in early systems.
At the CO, the OLT uses wavelength-division multiplexing (WDM) for transmitting and receiving. Data and voice is broadcast to all users on a 1490-nm laser at a download rate of 622 Mbits/s. In COs where video is distributed, a separate 1550-nm laser path is provided. It is multiplexed onto the fiber with the voice and data in a combiner. The video transmits in the standard analog format used by cable TV systems. Upstream traffic for the user to the CO is handled by time-division multiplexing users on to a 1310-nm channel back to the OLT. Careful timing is controlled by the OLT to ensure that each user gets a time slot regardless of the distance differences that will be inherent. The aggregate upload speed is 155 Mbits/s.
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