PHOTONIC FRONTIERS: FIBER TO HOME - Fiber to the home takes aim at a ‘100-megabit nation’

Three decades after the first fiber to the home (FTTH) test bed began operating in Higashi-Ikoma, Japan, the technology has gone mainstream. In April, the Fiber to the Home Council reported 2.91 million fiber subscribers in the United States, nearly double the number a year earlier. The survey by Michael Render of RVA Market Research (www.rvallc.com; Tulsa, OK) found that 1.6 million customers received video through their fiber links, and that FTTH systems passed 11.8 million homes in North America. In June, a group of congressmen including the influential Ed Markey (D-MA) introduced a resolution into the House urging that broadband systems delivering 100 Mbit/s be made available throughout the country by 2015.

Meanwhile, the technology is evolving to meet those goals. One big push is in an area that consumers won’t notice directly improving installation economics. Another is in developing new architectures that upgrade transmission capacities without obsolescing earlier generations of hardware. But developers face a moving target as new services emerge with their own thirst for bandwidth, including high-definition television, video on demand, Internet-protocol TV (IPTV), telecommuting, and interactive gaming.

Improving installations

Scaling up FTTH systems has led to innovations to cut labor costs and eliminate cumbersome and complex equipment. Field technicians now screw preconnectorized fiber drop lines into terminals in home and pole-mounted boxes instead of splicing the fibers. “Within three or four years, we’ve gone from bulk fiber cable and regular boxes to pre-engineered interconnect systems,” says Mark Turner of Corning Cable Systems (Hickory, NC). “It’s amazing.”

Installation also has been aided by development of new fibers less sensitive to bends than standard types. Specifications set by the International Telecommunications Union (ITU) for type G.652 standard single-mode fiber call for loss of no more than 0.1 dB when wound 100 times around a mandrel with 30 mm radius of a 6 cm (2.75 in.) loop. But loss increases rapidly for tighter bends (see Fig. 1). The ITU G.657A standard calls for fibers to have loss no more than 0.75 dB in a single loop with 10 mm radius. The ITU G.657B standard specifies loss no more than 0.5 dB in a single loop with 7.5 mm radius. Developers have done better than current standards; Corning now offers a ClearCurve fiber that it specifies as having bend-induced loss no more than 0.1 dB in a full turnaround loop with 5 mm radius.

Bend-insensitive fibers originally were developed largely for use where fibers must be looped to store extra fibers or to make connections within terminal boxes. They also allow the use of smaller terminal boxes at homes, appreciated by homeowners who don’t like the looks of big boxes on their outside walls or in their garages. Most installations deliberately leave slack to allow for changes and repairs, and most preconnectorized drops require a slack box to hold excess cable. The push for fibers that can withstand even tighter bends comes from installations in apartment buildings and other existing housing, where cables may be run along molding and bent around tight corners.

Passive optical networks evolve

The architecture for the passive optical networks (PONs) that account for most FTTH systems was set in standards proposed by the Full Service Access Network group, (FSAN; www.fsanweb.org) and adopted by ITU. The PON design avoids putting any active elements between the local switching office and the subscriber; one single-mode fiber runs from the switching office to a distribution note, where the signal is divided among 16, 32, or sometimes 64 separate single-mode fibers serving individual customers.

The first widely used version is the Broadband PON (BPON) described in the ITU G.983 standard. It typically transmits a total of 622 Mbit/s downstream to subscribers at 1310 nm, and a total of 155 Mbit/s upstream from subscribers at 1490 nm. Signals are time-division multiplexed, with each customer getting a specific fraction of the total capacity. The 1550 nm window is reserved for downstream video transmission in the radio-frequency format used in cable-television networks.

An enhanced version called Gigabit PON (GPON) uses the same wavelengths and splitting ratios, but transmits at higher data rates—2.488 Gbit/s downstream and 1.244 Gbit/s upstream. Details are specified in the ITU G.984 standard. In practice, signals are packet-switched rather than time-division multiplexed. Verizon uses this format in its FiOS system, which accounts for roughly 70% of all FTTH connections in the U.S.

Ethernet PON (EPON, also called GEPON for Gigabit Ethernet PON) transmits a payload of 1 Gbit/s in variable-length packets, with a line rate of 1.25 Gbit/s because it uses Ethernet-style 8 bit/10 bit encoding. Details are specified in the IEEE 802.3ah standard.

An alternative approach is point-to-point or switched Ethernet transmission, with a dedicated fiber between an Ethernet switch and each subscriber. This requires distributing active Ethernet switches in the field, but has the advantage of using mass-produced Gigabit Ethernet hardware. According to Render, it provides connections to about a quarter of homes served by FTTH networks other than Verizon’s.

Cranking up the bandwidth