FiOS TV: The Light Fantastic Page 3


To understand why many analysts believe that Verizon's approach of running fiber to the home (FTTH, or FTTP, for fiber-to-the-premises) is a better, longer-term solution than either cable's hybrid fiber coaxial (HFC) or AT&T's fiber-to-the-node (FTTN) systems, it's helpful to first understand how it works.

At the heart of any fiber-optic distribution system is the actual fiber-optic "cable" itself, which consists of a bundle of single optical fiber strands, each barely the size of a human hair. At the center of each of these strands is the core, made of glass silica. A laser, which encodes data as pulses of light, sends the pulses through the fiber-optic cable's core. Surrounding the core is a mirror-like optical material, called the cladding, which reflects the light back into the core. Because the cladding doesn't absorb light, the light pulses are able to travel long distances. Surrounding the cladding is the buffer - sometimes both a primary and secondary buffer - which is a hard plastic coating applied to the core and the cladding for protection against damage and moisture. To give the cable added strength, especially for pulling runs, the buffer is generally surrounded by a sheath of aramid fibers (such as Kevlar). The cable is then finally wrapped in a flame resistant protective jacket, typically made of PVC.

Both transmission lasers and LEDs can be used as the light source, but lasers, which are more powerful, are more commonly employed. To understand how the system works, think of the laser as an extremely powerful flashlight that blinks billions of time each second. When the laser is turned on, the equivalent of a digital "1" is transmitted, and when it's off, a digital "0" is represented. The high level of internal reflectivity of the cladding allows the light to be transmitted down the length of the cable, even through the twists and turns of its run. Sometimes a pumping laser and "doped" fibers - which act to amplify the light - are used in the system to increase the maximum distance the light pulses can travel.

Both Verizon and AT&T employ passive optical networks (PONs), which allow multiple homes to share a single fiber. Rather than running individual fibers for each user (as would be the case in a point-to-point network), a PON uses point-to-multi-point architecture where splitters placed at junctions within the fiber loop divide the signal, so a single optical fiber can serve 32 customers. At the customer's home, the light pulses enter a receiver, called an optical network terminal (ONT), where they're converted back into common electrical signals. Video is typically sent to a Verizon set-top box (or DVR) via standard RG6 coax cable, while Internet connectivity is generally provided via Cat5e (Ethernet) cables and a supplied router. Verizon's set-top boxes have the capability of receiving both RF and IP (Internet Protocol) video.

Unlike Verizon's FTTH network, which runs fiber directly to the customer's home, AT&T's U-verse service is largely "fiber to the node" (FTTN), where programming is delivered via fiber from the head end only to neighborhood nodes - usually within 3,000 feet of customers' homes - then sent the rest of the way using existing copper wiring. Cable's HFC works similarly, but uses coax cable between the neighborhood nodes and homes. Running fiber all the way to the home can provide up to four times the bandwidth of FTTN networks, but there are other advantages to Verizon's FiOS service.

In all fiber-based networks, lasers can produce different wavelengths of light to transmit data. For example, remember the Verizon ad where a precocious kid explains to his dad that Verizon brings fiber-optics on three different frequencies of light ("You got your 1310, your 1490, and your 1550...")? He's right! In the FiOS system, downstream voice and data (such as Internet downloads and interactive services) are sent from the central office's optical line terminal (OLT) using the 1490 nanometer (nm) wavelength, while upstream data sent from the customer travels along the 1310 nm wavelength. The 1550 nm wavelength is reserved for optional overlay services - in FiOS TV's case, video.

That makes Verizon's approach different from AT&T's. In the latter's U-Verse service, everything, including video, is delivered via Internet Protocol. Verizon uses a hybrid approach where broadcast channels are sent as RF (QAM) video, while IP is used to deliver interactive services such as video on demand, the interactive program guide, and the widgets you can create that deliver local news and weather onscreen. As a result, AT&T has to reserve a portion of its IP bandwidth for video, limiting Internet access speeds to 10Mbps. Verizon doesn't, enabling it to offer Internet customers up to 50Mbps speeds. Verizon modulates the RF video onto the 1550 nm wavelength, then combines it with the 1490 IP wavelength (via optical couplers and a process called wave division multiplexing, or WDM), and sends it out through the PON. By using the optical splitters, 32 customers are served by a single fiber. WDM is also used to direct the returning 1310 nm traffic back from the customer's ONT to the head-end OLT.

Both Verizon and AT&T currently use what are called broadband PONs (BPONs), which are capable of 622-Mbps downstream, and 155Mbps upstream, data rates. However, Verizon has already started deploying an even faster network, called GPON (gigabit passive optical network) that offers a fourfold increase - to 2.4 Gbps - in downstream speeds, while upstream speeds jump from 155 Mbps to 1.2Gbps, and eightfold increase in upstream bandwidth back to the network. The latter will likely become important, given the growing amount of user-generated content being uploaded to the Internet. Verizon is already rolling out GPON in parts of California, Maryland, Massachusetts, New Jersey, New York, Rhode Island, Pennsylvania, Virginia, and Texas.

But Verizon is also looking beyond GPON; in its labs, it's testing an even more robust technology called wavelength-division multiplexing networks (WDM-PON), which could give each customer his or her own 1-Gbps (or more) wavelength into the home. However, the company believes that GPON will serve its - and its customers' - needs well into the future. But the extensibility of its network is another Verizon advantage.