Electronic Design

There's A Bright Light At The End Of The Fiber

Down in the dumps for so long, the fiber-optics industry nearly became a forgotten entity. But after years of low sales, bankruptcies, and consolidation, life is returning. All sectors are seeing a major upturn in sales, and new developments make optical even more attractive (Fig. 1). No other media offers the high speed, security, and lack of noise of fiber optics. And while its inherent higher cost has held it back in some applications, volume could build as costs moderate.

Fiber optics is the medium of choice for all of the nation's long-distance telephone networks as well as the Internet backbone. It's also widely used in metropolitan area networks, where Sonet and ATM dominate. Most of these networks carry data at the OC-48 rate (2.488 Gbits/s).

Because 10-Gbit/s systems were just arriving at the beginning of the downturn in 2001, few have been deployed. Since then, the technology has matured, and it's now ready for action in upgraded long-distance and backbone networks. China and Japan lead in new fiber deployments, but U.S. growth is encouraging.

The Internet continues to explode, creating more demand for faster networks. More importantly are the ever-more massive music and video downloads that tax and ultimately slow down existing systems. Increased usage of the Internet for Voice over Internet Protocol (VoIP), video on demand (VOD), video security, and machine-to-machine (M2M) applications adds to the burden. And some experts say we haven't seen anything yet. Already, cable TV companies and telecom carriers are restricting their broadband-connection bandwidth to various customers. Thus, many carriers now see the need for the 10-Gbit/s upgrade. During the dot-com boom, thousands of miles of fiber were laid. Much of that remains unused. It's time to light it up and increase the bandwidth, which in turn should increase revenues.

Dense wavelength-division multiplexing (DWDM) increases bandwidth on a fiber by transmitting simultaneous data streams on different wavelengths of light. It's a less expensive way to get higher data rates with existing lower-cost components.

Four OC-48 data streams on existing fiber get you to the OC-192 rate of 10 Gbits/s (9.952 Gbits/s actual). With today's components, it's possible to put several hundred wavelengths of light on a fiber for amazing capacity at a reasonable cost. Terabit (1012) rates are within reach, with petabits (1015) doable in the near future.

The fastest currently defined fiber-optic system is Sonet OC-768, at 39.8 Gbits/s. Yet aside from some real exceptions, few adoptions have occurred. The cost is very high, especially for systems transmitting 40 Gbits/s in a single stream over single-mode fiber. With 10-Gbit/s components and WDM methods, it's now possible to do 40 Gbits/s for a reasonable cost.

While the need isn't quite there yet, 40-Gbit/s systems eventually will be necessary. Development is continuing not only for Sonet systems, but also for the next generation of Ethernet. Ethernet systems have had speed upgrades by a factor of 10, but there may be a place for an intermediate speed between the current 10-Gbit/s maximum and the seemingly impossible 100-Gbit/s level required by a tenfold increase. Again, WDM will no doubt be the path to an affordable 40-Gbit/s system.

Local-area networks (LANs) regularly required major upgrades. That need has cooled somewhat. The most recent upgrade was from the ubiquitous 100-Mbit/s Ethernet to the 1-Gbit/s version. Many PCs now feature a 1-Gbit/s connection (most often UTP) to the LAN—and if they don't, they will soon. But to consolidate all of these connections into a larger LAN, a 10-Gbit/s backbone is necessary. In this case, optical is the only way to go.

The place to be today in fiber-optic systems is 10 Gbits/s. Declining prices and more demand are driving growth in all sectors. This is especially true for Ethernet optical solutions and Ethernet over other optical standards. The Resilient Packet Ring (RPR) option for optical Ethernet also makes 10 Gbits/s more attractive. Increased growth can be seen in Europe and China.

On top of that, the new XPF transponders are more than affordable, making this speed more attractive than ever. And don't forget developments like electronic dispersion compensation, which make it easier to implement 10 Gbits/s on existing fiber links.

Many companies now offer 10-Gbit/s Ethernet over CAT6 twisted pair. While it seems impossible, 10 Gbits/s can be achieved on copper by combining four parallel data streams, multilevel modulation (PAM), and heavy DSP for equalization, filtering, and compensation.

Though the transmission is limited to a maximum distance of about 50 m, it works for fast connections in server farms and collections of switches, routers, and security appliances. CAT6 is still cheaper than a 10-Gbit/s optical connection, but it runs out of steam at greater distances.

This niche technology will find its place, but fiber is still king for reliable, fast connections with considerable upgrade capability. Fiber-optic systems are an investment for the future. They're sure to be the technology of choice in a growing number of data centers, server clusters, grid computing networks, and other short-range interconnect applications.

Another promising optical system is the passive optical network (PON). Some carriers are adopting it for their new fiber-to-the-home (FTTH) broadband systems. These completely passive (no electronic-optical-electronic conversion repeaters) metro networks are low in cost and super fast.

Japan and South Korea already have such systems with 100-Mbit/s and higher data rates. In the U.S., Verizon and AT&T are building PONs across the country to support even faster broadband connections for Internet access, the growing base of VoIP subscribers, and their forthcoming Internet Protocol TV offerings.

EPON, an Ethernet version of PON, is popular in Asia. In the U.S., the GPON system has become accepted as the standard. It provides a maximum download speed of 2.5 Gbits/s and an upload rate to 1.25 Gbits/s. That should be fast enough for virtually any current or future services or combination thereof.

Only a few installations will bring the fiber all the way into the home, as in Verizon's Fois system. Most systems like those from ATT/SBC, especially in apartment buildings and condos, run fiber up to a neighborhood gateway. Then, they use the installed base of a telephone twisted pair and a fast digital subscriber line (DSL) technology like ADSL2+ or VDSL to deliver up to 100 Mbits/s to the subscriber.

Storage-area networks (SANs) are another major growth opportunity for fiber. Storage needs are off the charts today, thanks to huge databases, search engines, and other Internet needs. The parallel SCSI-connected RAIDs of yesteryear have given way to massive storage systems that can only be accessed at a reasonable speed by fiber-optic systems.

Most SANs employ a fiber system called Fibre Channel (FC). It started out at a 100-Mbit/s rate. But today, most FC systems operate at 1, 2, or 4 Gbits/s. A 10-Gbit/s FC system was defined, but it has seen limited use.

A new system called iSCSI ("eye skuzzy") is a serial version of the older parallel SCSI command set that operates over Ethernet. Most systems use 1-Gbit/s Ethernet, but a 10-Gbit/s fiber connection greatly enhances performance. Look for this sector to keep fiber optics healthy in the coming years.

Some developments are beginning to pay off, like the series of multi -source agreements (MSAs) that define physical and electrical standards for optical transceivers or transponders.

These units house a transmit laser, an optical receiver, interfaces, and monitor and control circuits. Proprietary designs have ceded to these standards accepted by the major manufacturers. This has yielded volume and lower costs, greatly benefiting routers, switches, and other high-speed systems. Standards include SFF, SFP, XPF, XENPAK, X2, and 300-pin.

Silicon light-processing circuits also are growing. Many companies are designing light waveguides, array waveguide gradings, splitters/combiners, switches, and other light-processing components in silicon using standard IC processes. Some companies even integrate the light-processing components with the high-speed CMOS digital circuits to create fully integrated packages at lower costs.

One important development, the vertical-cavity surface-emitting laser (VCSEL), permits lasers to be formed on integrated circuits. The 850-nm VCSELs are now very popular in fiber-optic networks. Older lasers still dominate just because VCSELs haven't been able to operate at the higher wavelengths of 1310 and 1550 nm.

However, look for 1310-nm lasers in the near future, which will help to lower fiber-optic costs further (Fig. 2). Affordable tunable lasers are also now available for sophisticated DWDM systems.

For more, go to www.elecdesign.com: "Broadband—The Fast Track To Triple-Play Services And IPTV" by Kenneth Madison, Centillium Communications, Drill Deeper 11780

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