Gigabit speeds outrace traffic needs

Gigabit speeds outrace traffic needs

paul palumbo

Transmission at the OC-192 rate of 10 gigabits per second is the next level of capacity for synchronous optical network, or Sonet, protocols in the backbones of interexchange networks. And long-haul providers see gigabit speeds as a way to relieve bottlenecks in data-communications traffic.

However, judging from the evolution of similar Sonet protocols, it is possible that the higher-speed Sonet protocol may eventually migrate closer to residential areas as high-bandwidth applications such as video-on-demand are realized.

Operating at OC-192 provides four times the capacity of OC-48, which operates at 2.5 gigabits per second. It, therefore, allows operators to maximize existing fiber by using electronics rather than tearing up cabling plant.

According to Greg Mumford, senior vice president of Sonet network technology at Northern Telecom near Toronto, "The other side of the value proposition is that OC-192 may actually result in a simpler network architecture because bandwidth may be available in bigger aggregations. Operators can drive up utilization, which in turn, lowers costs."

George Gilder, a technology consultant based in Tyringham, MA, refers to the general lack of fiber utilization as a "bandwidth scandal." He says that of the 10 million route miles of fiber in the ground or strung on poles, almost one third is "dark" or inactive. Gilder adds that the remaining two-thirds is only being used "to one millionth of its capacity."

Underutilization is likely to change, however, as OC-192 begins to play a larger role in backbone networks. And considering the transfer rates obtainable with wavelength-division multiplexing, or WDM, this next level of Sonet can reach 40 Gbits/sec. At that rate, the entire United States Postal Service`s national listing of names and addresses can be transmitted from New York to Los Angeles in approximately 4 seconds; similarly a single strand of fiber can carry almost 500,000 simultaneous Internet conversations.

Ironically, although OC-192 may eventually grow into a market worth as much as $1.8 billion at maturity, according to Northern Telecom, there may be reduced demand for the next level of Sonet because operators are looking to increase the useful life of OC-48. Carriers are telling vendors that because they have already made significant investments in OC-48, there has to be a way to re-use those network elements. WDM can offer them a way to incrementally move up to the 10-Gbit/sec level.

WDM technology lets lasers operating at a pre-determined set of frequencies be "packed" around the 1550-nanometer range; therefore, multiple wavelengths can be shipped through one or two fibers. For example, one laser designed to operate at narrowband output levels at 1561 nm can be coupled with another laser operating at 1557 nm to increase the throughput of a single fiber pair.

According to Greg Wortman, director of marketing at Fujitsu Network Transmission Systems Inc. in Richardson, TX, OC-192 is not currently needed by the regional Bell operating companies, particularly in local-loop situations. This has led some suppliers, such as AT&T, "to stick with WDM solutions instead of developing an OC-192 product."

In September, MCI announced that it would be the first long-haul provider to install OC-192 technology, using Northern Telecom`s transport components along a 125-mile stretch running from Dallas to Longview, TX. According to Shawn O`Vonnel, Sr., MCI`s senior manager of transmission engineering and implementation, "MCI may deploy more OC-192 components in 1996 and possibly move to widespread rollouts by 1997, particularly in areas where there is considerable DS-3 [44.74-Mbit/sec] traffic." He says that "capacity evolutions typically start in the backbone locations and work up toward the edges of the network, closer to residential customers."

Rich Moran, director of product marketing for NEC, says that "back in the early 1980s, OC-3 [rates of 155.52 Mbits/sec were] considered a high bandwidth, but demand has typically exceeded forecasts, and that has driven Sonet levels forward." And those technology evolutions are also getting faster. OC-48 came to market in 1991 and still remains concentrated in high-traffic, business application areas. US West is deploying NEC`s OC-3 multiplexers and plans "several hundred million dollars worth of additional Sonet upgrades throughout its network to serve the needs of larger business customers," according to company spokesperson Dave Banks.

Banks adds that "OC-192 is not likely to be installed throughout US West`s network, but only in limited areas." US West currently has two Sonet vendors--NEC and Fujitsu.

MCI has an investment advantage, however, by upgrading to OC-192. MCI began incorporating Pirelli Cable Corp.`s T-31 bidirectional line amplifiers earlier this year (see Lightwave, May 1995, page 1). O`Vonnel says that OC-48 systems being deployed throughout the network this year are using those bidirectional amplifiers. "That allows the company to upgrade sections of the network to OC-192, and only the electronics at network ends have to be changed because the line amplifiers are bit-rate independent."

Although OC-192 may substantially increase the available bandwidth in networks and offer better error-correction and management capabilities, the faster transport speeds also tighten up performance criteria. At 10 Gbits/sec, any signal loss or dispersion can be critical, and operators will have to pay closer attention to the types of connections and splices being made.q

Paul A. Palumbo writes from Seaside, CA.