The need for ever-faster data-transmission rates is insatiable. The good news about that is we have the ability to go faster with fiber-optic data transmission. The bad news is that fiber is expensive. With greater usage, however, that's beginning to change as newer and faster systems emerge. So while fiber has been at the core of our long-haul telephone and Internet networks, it's finally getting cheap enough to run to the home.
Fiber to the home (FTTH) has long been the ultimate goal of most telecom carriers, but cost has either slowed or stopped that desire. Passive optical networks (PONs), with their lower cost and higher speeds, are bringing the fastest speeds to the consumer.
The drive for FTTH is related to video. Telecom carriers want to offer Internet Protocol TV (IPTV) to compete with the cable companies, which are offering VoIP services and stealing wired phone customers from the telcos. The telcos also want to expand their own VoIP services.
By using fiber, the carriers can offer IPTV, video on demand (VOD), and other video services. Furthermore, with fiber, data rates for Internet access can rise to meet the growing Internet usage, where subscribers demand ever higher speeds. Especially galling to subscribers these days are the very low upload rates.
Though most subscribers seem happy with their download rates, the growing desire to upload photos and videos to sites like YouTube and FaceBook makes current upload services seem like the dial-up of old. Thus, carriers are looking to implement the triple-play (voice, video, and data) to keep subscribers healthy (see "100-Gbit Networks On The Horizon" at www.electronicdesign.com, Drill Deeper 15995).
Some experts even predict the end of plain-old telephone service (POTS) over twisted-pair cable by 2012 or so. While the likelihood of that happening seems remote, fiber will definitely play an expanding role for major telecom carriers like AT&T and Verizon.
As old POTS subscriptions decline and with DSL rates practically at their peak, the major telcos are essentially forced to go to fiber. But thanks to the improving technology of PONs, the costs are manageable—over time.
PON UPDATE
PONs are more affordable simply because they don't use the active circuits and equipment needed for high-speed, long-haul networks. They work over shorter distances up to about 20 km and use only the fiber plus some inexpensive passive optical splitters/combiners.
At one end of the network is the carrier's equipment, generally called the optical line terminal or OLT (Fig. 1). The fiber goes out to one or more splitters that divide the optical signal into four, eight, 16, 32, or up to 64 individual channels that go to the homes. At the home, an optical networking unit (ONU) or optical networking terminal (ONT) receives the signal and connects to the PC, TV, or other networking device through a router or gateway.
In some systems, the optical cable ends at an ONU in a multi-dwelling unit (MDU), where the signal is distributed to apartments, condos, or townhouses within the unit. This distribution is usually accomplished via the existing twisted-pair telephone wiring using ADSL2+, or in newer systems, the faster VDSL2. In other systems, the fiber reaches a neighborhood digital subscriber line access multiplexer (DSLAM), which then distributes the signal via the installed telephone twisted pair using ADSL2+ or VDSL2.
There are two major forms of PON. GPON, or gigabit PON, is the latest generation of PONs developed for telephone-system distribution based on the asynchronous-transfer-mode (ATM) protocol. The first version, called APON, was short-lived and replaced by BPON, which implemented 622 Mbits/s downstream and 155 Mbits/s upstream.
GPON implements 2.488 Gbits/s downstream and 1.244 Gbits/s upstream. The downstream data is carried on a 1490nm laser, while the upstream is carried on a 1310-nm laser. Sometimes, a separate 1550-nm laser also carries TV. GPON is an official ITU standard (G.984).
The other form of PON is Ethernet PON, or EPON. An IEEE standard (802.3aq), it uses the same carrier wavelengths downstream and upstream, but is symmetrical with a 1.25-Gbit/s rate. A faster 10-Gbit/s standard is being developed. EPON, which sometimes is called Gigabit Ethernet PON or GE-PON, is widely used in Japan and some other Asian countries as well as in some parts of Europe. It's not used in the U.S., where GPON is dominant.
The download and upload speeds are fast, but of course, subscribers don't get that much. Instead, the telcos parcel out speeds according to their policies and payment plans. No one gets more than about 100 Mbits/s. Most customers get lower than that. A rate of 30 Mbits/s is minimum for most HD IPTV applications. That tends to be the lowest rate available, though 50-Mbit/s plans are also available.
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