As new equipment magically multiplies the capacity of fiber, engineers need to test and monitor the equipment and services on those networks to ensure the promises of the new optical network are more than just smoke and mirrors.
Of course, multiplying bandwidth through such relatively new technologies as dense wavelength-division multiplexing (DWDM), which greatly expands the transmission capabilities of fiber by layering signals at different wavelengths, means more, higher-capacity connections with a greater number of services on each link.
At the same time, of course, synchronous optical network (SONET) rates are growing ever higher. In addition, SONET equipment is sometimes used in tandem with DWDM.
So test vendors are answering the call of DWDM and higher-rate SONET equipment--and the services they support--with new equipment.
A Quick Trick
"Lots of DWDM vendors have OC-192 and OC-768 systems under development. The trend toward high-speed interfaces is clear," says Kevin Smith, vice president of engineering for the loop portion of the transmission test business at TTC, Germantown, Md. The company sells a line of optical testing equipment under the T-BERD name.
"You can get more speed over fiber just by having more channels," Smith says. "As you get more channels you need better resolution. That resolution can be ensured by optical spectrum analyzers."
Dispersion becomes more significant at the higher OC rates, he adds. So test systems try to analyze how much dispersion is in the fiber. That's something equipment manufac-turers deal with in various ways, by having the laser change the shape of the pulse or making other adjustments.
Smith notes the shift over to optical information means a need for higher-speed bit error rate testing (BERT).
"We have OC-48 transmitters/receivers," he says. "Most DWDM [systems are] interfaced to SONET at OC-48 rates. It's not inherently different, it's just at a higher speed."
Upgrading the OC rates on the T-BERD simply requires installing a new card in the T-BERD 310, he says.
The new OC-48 capability is relatively new to TTC's product line, as is an optical channel monitor, says Smith. The channel monitor looks at a DWDM signal to measure optical power, wavelength and signal-to-noise ratio. It can drop out one optical channel and do BERT in that channel at the SONET, DS-3 or T1 layer. The unit also performs tests on insertion loss (how much power is lost in the fiber) and optical return loss (for which it transmits a signal through one end of a fiber to see how much signal is returned).
"That way they can test the whole network at one point rather than having to go to different points in the network," he says.
Smith adds that forward error correction is also something new being used in SONET up to OC-192 and OC-768. "The idea is to keep error rates low--10 to the minus 12 or lower. That's in the transmission equipment protocol. It can be encapsulated in SONET equipment," he says.
Abracadabra
One of the more interesting test vendors answering the call of new DWDM networks is Digital Lightwave Inc., Neptune, N.J. The company recently came out with its Network Access Agent IV (NAA IV), which it claims is the first centralized DWDM test and monitoring system. It can be used for automated performance management or as an on-demand test tool.
NAA IV, which sells for less than $10,000, can automate and conduct on-demand testing of multiple DWDM fibers and embedded OC-48 signals, including optical power measure-ments, signal-to-noise ratio, graphical spectrum analysis and SONET overhead analysis. It allows network managers to view signals down to the DS-0 level.
It quickly can pinpoint problems or potential problems, reducing trouble-shooting time. It can centralize and remotely manage networks, increasing technician efficiency and reducing costs associated with training and equipment. It monitors DWDM networks, reducing risk of low revenue and service level agreement (SLA) penalties. It accommodates future DWDM bandwidth growth with a flexible and scalable system architecture. In addition, it will easily integrate into existing network management systems via standards-based interfaces.
"Some of [the existing DWDM equipment and testing product] does rudimentary stuff, but not all of it can do higher order stuff," says Danny Briere, with consulting firm TeleChoice Inc., Verona, N.J. "Digital Lightwave lets you track those higher order issues.
"Digital Lightwave is a very technologically oriented company," he says. "They're always trying to be a step ahead."
Networks can be built with lengths of up to 500 kilometers (km) between DWDM nodes, explains Reuben Prichard, Digital Lightwave's director of systems en-gineering and architecture. A run can go 80km to 125km before requiring erbium-doped fiber amplifiers (EDFAs). So a length may have seven to eight EDFAs, he says.
However, when the receiving node gets an alarm, it's for the whole link. "At amps, most vendors provide optical power measurements, which look at peak power and signal-to-noise ratio. So they're looking for low peaks or low signals," says Prichard. "So you need big failure for it to show up.
"Our product reaches inside light at amps, analyzes power measurements and also extracts SONET signals for SONET analysis," says Prichard. Another unique aspect of the equipment is its ability to adjust to different vendors' DWDM wares, he says.
Different WDM vendors means different network management systems--and amps and nodes may be from different vendors in one network, Prichard says. "Our product sequences through all fibers, extracts channels and does overhead analysis." Prichard adds that spacing between wavelengths in various WDM systems may vary by vendor, but Digital Lightwave's equipment will automatically adjust to different spacing schemes.
Behind the Curtain
The NAA IV can work with any vendors' equipment, but the product already is embedded in equipment from Lucent Technologies Inc., Murray Hill, N.J., through an original equipment manufacturer (OEM) agreement.
Basic testing features are included with many vendors' DWDM equipment. Many DWDM vendors are looking to add new testing features in their equipment through OEMs with specialized test vendors, says Briere.
Raleigh, N.C.-based competitive carrier BTI Telecom Corp. is using WDM equipment from Nortel Networks, Richardson, Texas, in its long-haul stretch from New York to Atlanta. The 16-wavelength system presents no major new testing challenges for BTI, according to Mel Patterson, vice president of network facilities at the carrier.
"We're using Nortel integrated network management. You can look down at the DS-3 level. You can look at performance monitoring. Once WDWM is installed, we just treat it as though it's fiber," he says.
What the Audience Sees
Both Digital Lightwave and TTC emphasize that ensuring services live up to the carrier's promise is the most important aspect of optical testing and monitoring.
"What people are really interested in is the service that comes out of it. Our focus is making sure customers can get the T1, verify ATM (asynchronous transfer mode) or frame relay, look at the SONET payloads, that kind of thing," says TTC's Smith.
With all the growth in data services, technologists have been looking at new ways to make data payloads fit more snugly into SONET, which was designed specifically for voice traffic. Such new equipment is typically described as data-aware SONET.
"Data is 50 percent of traffic and growing from 30 to 50 percent or more," says Smith.
Smith believes that network managers could probably use the same test and measurement equipment for data-aware SONET connections they use today, as long as it's encapsulated in SONET.
Briere of TeleChoice says it may simply be too early to see specialized test and monitoring equipment for data-aware SONET, since the concept is so new.
"Most of that stuff is neglected until later," says Briere. "[Test equipment for new technologies] usually comes in generation two and generation three."
Some of the primary test and monitoring concerns of carriers overall--particularly newer, competitive carriers--are portability and ease-of-use.
"The biggest issue for CLECs (competitive local exchange carriers) is they're less likely to put equipment everywhere, and Digital Lightwave is portable," says Briere.
Smith of TTC says the company's T-BERD 2310 can go to OC-12 in a handheld package. The T-BERD 3110 is more a central office (CO) piece of equipment, mounted with SONET equipment, he says.
"With 3110 you don't have to buy a whole new mainframe, and there are a lot of 3110s out there," adds Smith. Going to OC-48 is half the cost of buying a new piece of equipment, he says.
In addition, the ability to do a variety of tests through T-BERD's single, familiar interface makes it easier to train on and remember how to use, says Smith. Digital Lightwave also emphasizes the ease-of-use and training with its product.
