Undefined Sonet specifications pose test equipment challenges

dana cooperson and mehrdad givehchi

Tektronix

Manufacturers of Synchronous Optical Network test equipment strive to provide instruments that keep ahead of multigigabit fiber-optic network technology demanded by network providers and that satisfy undetermined dispersion and spectral width requirements

Increasing the carrying capacity of installed fiber-optic Synchronous Optical Network (Sonet) networks, without the costly overbuilding of spans, enables public communications network operators to compete effectively by maximizing the value of their infrastructure investment. Many network operators--the interexchange carriers, in particular--are looking for fast, efficient and cost-effective ways to increase the capacity of fiber already in place.

Test equipment vendors can help ease the transition, but only if they provide optics that surpass Sonet specifications for 2.5-Gbit/sec OC-48 time-division-multiplexed and wavelength-division-multiplexed-based (TDM and WDM) systems, as well as 10-Gbit/sec OC-192 systems. In doing so, though, test vendors have encountered a few problems, one of which entails the measurement of chromatic dispersion on high-capacity system performance. A second involves the effects of a second transport impairment: reflectance.

Historically, network operators have increased system backbone capacity by moving up the digital TDM hierarchy--for example, from 622-Mbit/sec OC-12 to OC-48 or OC-192 Sonet systems. Recently, wavelength-division multiplexing, a second method of increasing Sonet network capacity, has gained favor. It uses narrowband, closely spaced wavelengths of light to transmit multiple channels of information over a single fiber pair.

Both TDM and WDM high-capacity, long-haul (that is, over 40-km) transmission systems have moved networks away from 1310-nm optics, which are not optimized for long-distance, high-bit-rate transmission, to 1550-nm optics, which are optimized. However, imposing a new network design, particularly for OC-48-based WDM systems, on an existing network presents some interesting challenges for network operators.

Until recently, most installed Sonet networks have been designed around network elements using 1310-nm optics to transport a single optical channel at a maximum of 2.5 Gbits/sec. The standard singlemode fiber used to carry the Sonet signals was optimized by the manufacturer for 1310-nm transmission. At this wavelength and line rates over the optimized cable, signal attenuation characteristics were acceptable, and the effects of dispersion were minimized (see Fig.1).

As the optical signals move through the fiber, the light pulses` power is attenuated. The operational length of a fiber span, therefore, is limited by the total signal attenuation along the length of the cable (including losses from connectors and splices), the system`s laser output power and the receiver sensitivity. Regenerators are employed to rebuild the optical signal approximately every 40 to 60 km to counter the effects of attenuation.

Chromatic dispersion is the tendency of an optical signal`s wavelength components to spread out as they travel over a fiber. A major reason is that all lasers have some nonzero spectral width; they do not emit light at one pure wavelength. The wider a laser`s transmitted optical signal, the more that signal spreads out as the light travels through the fiber. Dispersion, like attenuation, impairs an optical receiver`s ability to discriminate between light pulses (representing information "ones" and "zeros") at the far end of the span. Because existing spans were designed to control dispersion, maximum transport distance tended to be limited by attenuation only.

New challenges

To achieve higher-capacity, longer-haul transport, the transmitting wavelength must be in the region where the optical fiber attenuation is at a local minimum--1550 nm. Unfortunately, fiber designed to limit dispersion at 1310 nm does not provide the same characteristic at 1550 nm. Table 1 illustrates the relationship of attenuation, wavelength, dispersion and span length for typical embedded fiber bearing OC-48 traffic.

Therefore, the attenuation limited distance is extended but at the expense of increased dispersion. Longer span lengths are preferable because regenerators add cost, complexity and possible points of failure to a network.

Once network operators decide to evolve a network to 1550 nm, they can employ erbium-doped fiber amplifiers (Edfas) to further increase repeater spacing and network reliability (see Fig. 2).

Because the optical signals are not being regenerated by the Edfas as they are amplified, dispersion effects accumulate along the span until the optical signal is regenerated or terminated at a multiplexer. Still, Edfas permit longer span lengths for lower cost and higher reliability when using only regenerators.

Long-haul traffic tests

To ensure that transmission services are carried error-free after fiber span capacity is increased, network operators must perform tests to safeguard that attenuation, dispersion and reflectance are not affecting system performance.

Initially, baseline requalification tests on the fiber span are recommended to check that reflectance is within the stringent specifications that apply at OC-48 rates and higher. Excessive reflectance from fiber connectors and splices on a span contributes to degradation of system performance by causing an increase in system jitter or laser chirp, that is, quick wavelength shifts due to 1-to-0 and 0-to-1 modulation transitions. Reflectance testing is done with an optical time-domain reflectometer (Otdr). Typical solutions to unacceptable reflectance include resplicing cables, cleaning dirty connections, repolishing fiber ends and replacing connectors.

After the fiber has been requalified, the TDM or WDM network upgrade can be installed. At this stage, three types of testers can further qualify the system upgrade: optical power meters, Sonet bit-error rate testers (Berts) and optical spectrum analyzers.

To rule out the simplest problems, a test technician should use an optical power meter to verify that there is no problem with receiver overload. This type of overload usually occurs over short spans, because according to Sonet specifications:

Output power levels of long-haul OC-48 systems must be at least +3 dBm (but can run beyond +10 dBm).

OC-48 receivers may overload at -10 dBm.

Sonet system attenuation loss budget is specified at 10 to 24 dBm.

An inline attenuator can be added to lower the power level at the affected receiver.

If optical power parameters are within specification and error problems still occur, an OC-48 Bert should be used to verify that the new system supports the maximum error rate -- for example, 10-10 -- that customers expect and that Sonet specifications impose. If end-to-end tests imply problems, error sectionalization can be done by moving the tester around the network to pinpoint the questionable network element. To be effective, the OC-48 tester must provide the level of optical performance needed for the application, which might be beyond what is needed to meet Sonet specifications. Because typical networks undergoing an upgrade are expected to contain a mix of 1310- and 1550-nm spans, a Sonet OC-48 Bert that provides dual transmitters furnishes more flexibility than testers that do not.

Finally, if a WDM system is being installed and the OC-48 Bert testing has established a problem in a WDM network element, an optical spectrum analyzer can be used to verify that the expected wavelengths are present at the transmit end, Edfas along the span have produced flat gain across all channels, and expected wavelengths are present at the receive end of the span.

At this test stage, if all system components seem to be working but the end-to-end system is still error-prone, then total system dispersion might be the problem.

Critical Sonet specifications

A 1550-nm system`s dispersion characteristic is critical, particularly in WDM systems, where narrowband filters might need to discern between channels spaced as close as 1.6 nm apart. Unfortunately, present Sonet specifications do not clearly state system-dispersion requirements. Because a typical network is difficult to characterize, OC-48 long-reach optical-dispersion requirements are left open as "under study" in Sonet standards specifications. This situation creates a scenario in which network and test equipment vendors can meet Sonet specifications but not a network`s transmission requirements (see Table 2).

The center wavelength parameter shows that Sonet specifications provide a less-stringent limit than some network applications require. A center wavelength as close as possible to 1550 nm is critical for Edfas, for example, which provide maximum amplification at 1550 nm and do not work well below 1540 nm or above 1560 nm. To properly check test spans using Edfas, an OC-48 Bert test set must provide a similarly narrow center wavelength.

The system`s dispersion and spectral width parameters affect the ability of a system and the equipment used to test it to provide error-free signal transmission and detection over the distances required of long-haul OC-48-based TDM and WDM systems. Maximum allowable system dispersion, according to Bellcore GR-253-Core, is the limit of the dispersion "that can be accommodated by a transmitter-receiver pair to meet the 10-10 bit-error rate performance objective" for Sonet systems. This parameter is inversely proportional--other things being equal--to the spectral width of the system`s lasers.

Despite the fact that the Sonet specification leaves the dispersion question unanswered, network planners can estimate the maximum allowable dispersion and spectral width by using two equations adapted from the Sonet specifications.

First, the maximum allowable dispersion (DSRmax , in psec/nm-km) is related to the length of a span (L, in km) and the worst-case dispersion coefficient of the fiber cable (Dmax , in psec/nm-km) as shown in the following equation:

DSRmax = L*Dmax

Using the 17 psec/nm-km coefficient typical of nondispersion-shifted singlemode fiber and a span length of 100 km, network planners can calculate the maximum allowable system dispersion as 1700 psec/nm. Substituting this result into the following equation relates the maximum allowable dispersion to spectral width (dl, in nm):

dl = (e*106)/(B*DSRmax)

where e is the allowable pulse broadening (0.491 for OC-48 long-reach applications) and B is the system bit rate in Mbits/sec (2488 Mbits/sec for OC-48).

Using this relationship, network planners would calculate a required spectral width of 0.12 nm--less than the 1 nm suggested in the Sonet specifications. To achieve the maximum allowable dispersion characteristics needed, network element and OC-48 Bert designers must take this result into account and provide high-performance optics with narrow spectral widths u

Dana Cooperson is marketing manager and Mehrdad Givehchi is senior hardware engineer at Tektronix` microwave logic product line in Chelmsford, MA. The authors thank Yukio Nakano of Hitachi Telecom (USA)`s Dallas Advanced System Center and Kinichiro Ogawa of Lucent Technologies Microelectronics for their assistance in gathering information for this article.