Electronic Design

Optical Primed To Handle Data-Speed Demand

The Internet is not fast enough. Actually, you could say that about any network. The slow response to net surfing isn’t just browser latency, but the network interconnections themselves. Simply put, increased viewing of TV and downloading of videos further burdens the Internet.

Despite the fact that 4G smart phones are faster than ever, carrier backhaul networks slow everything down. On top of that, the cloud computing utopia proposed by Google and others will require considerably faster networks.

Coming to the rescue, however, are fiber-optic networks. Moreover, we’re at the onset of a major update and overhaul for the Internet backbone.

Most of the build-out of long-haul and metro fiber networks in the late 1990s and early 2000s used 2.488-Gbit/s SONET (OC-48) technology. Since then, many systems underwent upgrades to 10 Gbits/s (9.95328-Gbit/s SONET or OC-192) and a precious few to SONET’s peak rate of near 40 Gbits/s (39.81312-Gbit/s SONET or OC-768).

These synchronous systems connect to asynchronous Ethernet networks, creating the need for interface solutions such as Ethernet-over-SONET protocols. With faster Ethernet and Internet Protocol (IP) systems growing in number and the emergence of all-IP 4G wireless networks, SONET has struggled to keep pace.

Add to that the need for speeds beyond 40 Gbits/s, and suddenly a faster IP-based system is no longer a luxury, but rather a necessity. That new system, called the Optical Transport Network (OTN), is already in place in some locations and starting to expand.

OTN Surpasses SONET

Known as the ITU standard G.709, OTN is called the “digital wrapper” or “optical channel wrapper.” It can carry IP traffic with ease, much like SONET and SDH (Synchronous Digital Hierarchy). OTN features line rates of 2.66 Gbits/s (OTU1) to carry SONET OC-48, 10.7/11.09 Gbits/s (OTU2/2e) to carry 10 Gigabit Ethernet or SONET OC-192, and 43.01 Gbits/s (OTU3) to carry SONET OC-768 or 40 Gigabit Ethernet. A line rate of 112 Gbits/s (OUT4) is also defined to carry 100 Gigabit Ethernet. Overall, OTN is gradually replacing SONET/SDH in most long-haul and metro networks.

The Cortina Systems CS6041 Optical Transport Processor and forward error correction (FEC) IC represents one the devices now available to support OTN (Fig. 1). The device transparently transports 10- and 40-Gbit/s clients and aggregates and deaggregates 10-Gbit/s clients to 40 Gbits/s. It integrates a 40G Ethernet media access controller (MAC) that enables monitoring and mapping of 40GE clients to OTN, compliant with latest G.709 standard for the mapping of various bit-rate clients into OTN. The device also offers four integrated jitter attenuation and clean-up phase-locked loops (PLLs) and four 10G multi-rate (OTL/STL/XLAUI) serial interfaces that make single-chip/single-card design possible for CFP/QFSP to CFP/300-pin MSA transponder applications.

Another expanding optical networking technology is the ubiquitous Ethernet. The latest IEEE standard 802.3ba defines both 40- and 100-Gbit/s optical versions for local-area network (LAN) use as well as data centers and long-haul backbone networks. It’s expected that OTN will form most of the Internet backbone, while Ethernet will serve the LANs, datacenters, and some metro areas.

With the optical standards defined, next comes the eventual deployment. That, in turn, depends on the availability of the hardware for its implementation. Such hardware is emerging, but it’s expensive. Volume use will see costs decline over time, though.

Fiber-Friendly DP-QPSK

The technology generally agreed upon for OTN is a modulation scheme called dual-polarization quadrature phase-shift keying (DP-QPSK). It uses orthogonal light polarizations and the encoding of two bits per symbol in QPSK to achieve a data rate that’s four times the line baud rate of 25 to 28 Gbits/s to achieve the peak of 100 Gbits/s.

Designed for today’s speed requirements, DP-QPSK also helps to resolve the fiber deficiencies inherent in most of the available in-place fiber. Specifically, the technique helps overcome chromatic dispersion as well as polarization-mode dispersion common on the installed fiber base. Only now are chips and optical modules becoming available. In fact, the Multi-Source Agreement (MSA) organization has defined a DP-QPSK standard module that should arrive in 2012.

For Ethernet, the industry seems to have come to the consensus on a 10- by 10-Gbit/s interface for 802.3ba. It uses 10 single-mode fibers carrying 10-Gbit/s signals at a reach of up to 2 km. A standard MSA module defined by the 10x10MSA alliance is just now becoming available, and a 10-km-reach version is in the works. Various 40-Gbit/s versions are on the market, too.

Mobile backhaul networks are also going through upgrades using fiber Ethernet. The adopted technology is Carrier Ethernet employing 10-Gbit/s hardware, which is upgradable to 40 or 100 Gbits/s as needs warrant.

Other optical networking technologies, such as passive optical networks (PONs), are slowly replacing standard coax cable in some cable TV systems. The standard hybrid fiber coax (HFC) now in place is being stretched to its limit due to increased video consumption and demands for faster Internet access.

Consequently, U.S. Gigabit PON (GPON) and Ethernet PON (EPON) are finding their way into cable systems. Also helping this cause is the availability of the Data Over Cable Service Interface Specification (DOCSIS) standard to provision Ethernet over PON.

On this front, Cortina Systems developed the CS8032 single-chip optical networking unit (ONU), which incorporates a 500-MHz processor with embedded memory (Fig. 2). It supports the IEEE Service interoperability EPON (SIEPON) standard as well as the IEEE 802.3az Energy Efficient Ethernet standard.

Hardware Market On The Rebound

After years of decline, the fiber-optical hardware sector is again experiencing growth thanks to the multitude of network upgrades. Most market-research firms project at least single-digit growth in the optical hardware business in the coming years. With less than 50% of fiber-optical cable buried in the U.S. being used, according to TeleGeography, an international telecommunications market research firm, the potential for growth is enormous.

In a recent Infonetics Research survey, 74% of the respondents say that they plan to adopt OTN switching in their core networks. Of the carriers interviewed, 71% said they have or will deploy OTN in their core or metro networks by the end of 2011. Infonetics projects that by 2014, 40- and 100-Gbit/s OTN will exist in more than 50% of metro networks.

Other market research firms tracking optical hardware also forecast growth. Ovum estimates a 2011 level 7% higher than last year. Dell’Oro estimated a 10% growth rate in the first quarter of 2011 over last year. It also projects a 40% growth spurt in mobile backhaul deployments over the next five years and total worldwide optical transport equipment sales of as much as $17.3 billion over the same time span. This estimate includes the continued decline of SONET/SDH revenue and OTN and related 40- and 100-Gbit/s equipment rising at a compound annual growth rate (CAGR) of 40%.

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