NEBS 101
Ten
Things You Must Know About NEBS
Dave
Lorusso
Designing for NEBS is complicated. Understanding
these facts is critical to starting the process.
 Network
Equipment–Building System: that's the long name for NEBS.
GR-63-CORE is the Telcordia document number. But, there's more to
NEBS than a name and a number. There are hundreds of requirements
that must be met before products can be sold to Verizon, SBC, Qwest,
Bell South, AT&T, and even competitive local exchange carriers
(CLECs). These telecom companies are among the largest in the world,
with multi-billion-dollar capital equipment budgets. To get a piece
of that budget, products must be designed for NEBS. To aid in this
task, here are 10 things you must know about NEBS.
1. NEBS is Here
to Stay
The Regional Bell Operating
Companies (RBOCs) are the customer. They control their own networks,
and they dictate the requirements. The RBOCs require that products
in their networks be NEBS Level 3 compliant. It's up to you as the
designer to give the RBOCs what they want.
An RBOC's central office
is a specialized minienvironment. Major cities have grown around
these central offices, and the RBOCs are unlikely to give up this
valuable real estate.
Telcordia recently released
Issue 2 of GR-63-CORE, "Network Equipment–Building System
(NEBS)." The reissue mainly addresses up-to-date test methods
for gaseous contaminants, hygroscopic dust, fire resistance, and
altitude. New products, or products previously not tested, must
be tested to the new requirements for resistance to airborne contaminants.
2. NEBS is Expensive
A full NEBS Level 3 test
program can cost as much as $100,000. With that kind of investment,
it's imperative to do it right the first time. If testing is done
incorrectly, retesting adds to the cost. Delays caused by retesting
in turn delay market entry.
Because NEBS must be designed
into the product, there also are development costs that must be
factored in. However, designing NEBS into the product reduces overall
cost because fixes are not being added to the end of the design.
3. NEBS Takes
Time
How long the process takes
depends on how well the product has been designed to NEBS. The number
of samples that can be tested is also a factor. The more samples
you can spare, the more quickly you can get through the testing
process. However, it is important to keep in mind that providing
more samples increases the cost of the test cycle. Cost and schedule
must be balanced.
Plan on a minimum of three
units for the NEBS test cycle: one unit for GR-63-CORE, one for
GR-1089-CORE, and one for second-level lightning, ac power fault,
and fire resistance tests.
Each unit follows a logical
test path to minimize the time necessary to complete a NEBS Level
3 test program. Problems and questions are inevitable during testing.
Having a knowledgeable person in the test lab to respond on the
spot ensures a smooth NEBS test program. A technical member of the
staff or a NEBS consultant should be present during testing.
4. NEBS is Complicated
There are hundreds of
requirements to satisfy before a product can become Level 3 certified.
If you miss just one, you cannot claim NEBS Level 3. Each requirement
needs to be compared with the overall design. Having a NEBS design
review at critical points in the development is critical to ensuring
that a product is NEBS compliant.
Designers who are uncertain
of all of the requirements should use a NEBS consultant. Considering
the time and money required to develop a product for this specialized
market, the investment is worth it. A NEBS consultant can help design
the product to meet NEBS requirements. However, getting through
all of the requirements takes a team effort.
5. NEBS Takes
Careful Planning
As in anything complex,
planning makes the task manageable. The process includes concept
reviews, design reviews, precompliance testing, scheduling external
lab time, preparing the unit for testing, conducting actual testing,
problem resolution, coordination between the test lab and the manufacturer,
and preparing for other minute details. Each is crucial to a successful
program. Without careful planning, minor problems can turn into
major disasters. NEBS certification will be a milestone in the development
plan, and it needs its own schedule to ensure success.
6. NEBS Requires
Fire Testing
The fire resistance test
must be done first. It's as simple as that. If you're trying to
save a sample by burning it last, you're the one who will be burned.
The ability of fire to spread in a unit is extremely difficult to
predict. Factors such as air flow, location of the line burner,
and heat build will influence fire spread. The only way to find
out whether the product will pass the fire test is to test it early.
The secret to passing
this test is containing fire spread within the first 6 minutes.
If the fire is not contained within this time, the chances of passing
the test are slim to none. Some successful design techniques include
but are not limited to:
• Adding a metal
back to circuit packs.
• Using perforated
metal fire barriers at the top and bottom of the card cage.
• Minimizing fuel sources to the line burner.
• Controlling air flow.
Verizon has a unique perspective
on fire resistance. While GR-63-CORE allows the equipment under
test to produce smoke indefinitely, Verizon requires no smoke after
15 minutes. In a central office it's not so much the fire that causes
damage, but, rather, the smoke. The corrosive nature of smoke causes
widespread damage to electronic equipment. For this reason, Verizon
has its own requirements for fire resistance. A product could pass
the GR-63-CORE fire resistance test, but for acceptance at Verizon,
it must meet the company's more-stringent requirements.
7. NEBS Requires
EMC Testing
EMC precompliance testing
is a must for a product being designed for NEBS. Unlike products
required to meet less-rigorous FCC requirements, a NEBS product
must pass EMC tests from 10 kHz to 10 GHz. It must pass radiated
and conducted emissions tests as well as radiated and conducted
immunity tests over this broad range of frequencies.
It's not
unusual to encounter radiated emissions problems in the 20-30 MHz
region. Because this frequency range is not considered when doing
FCC testing, it is often overlooked when designing for NEBS. It
is critical to design with EMC in mind. Considerations include:
• Correct layer
stackup.
• Careful selection of clocks to avoid harmonic addition.
• Proper component placement.
• Filtering and hooks for filtering in the design.
• Manual routing
of critical nets.
It is also important to
consider ESD testing at 4 kV and 15 kV, and testing for lightning
surges and power-cross events. Even if the product doesn't have
any signals going to outside plant, there are still intrabuilding
surge and power-cross events that must be addressed.
8. NEBS Requires
Seismic Testing
This is another area that
requires careful planning. As with the fire resistance test, doing
the seismic test early pays off.
Pay careful attention
to how cables are routed for this test. Cables that are too taut
can result in a false failure if the cable loosens or breaks during
the test. It's a NEBS objective for the equipment to operate
during the test. It's a NEBS requirement for the equipment
to operate after the test.
About 80% of products
pass the seismic test the first time with little or no preparation.
Despite these excellent odds, designers should not be over-confident.
The product must be prepared to be hit with an 8.2 earthquake.
9. NEBS Improves
Reliability
Reliability is greatly
improved with NEBS designed into the product. NEBS is the cornerstone
of five-nines (99.999%) uptime. When considering the environment
the product will be used in, the tests a product must pass ensure
that it will be highly reliable.
With increased reliability
comes a lower warranty cost. In some ways, NEBS pays for itself
over the life of the product.
10. The NEBS Customer
is Always Right
Meeting the strict requirements
of GR-63-CORE and GR-1089-CORE is just the beginning. Each RBOC
has its own requirements in addition to the requirements in these
two standards. Verizon, for example, has a stricter pass criteria
for fire resistance. Verizon also has its own guidelines for some
of the EMC requirements. SBC has unique dc power requirements. Because
of Qwest's geographic locations, it is particularly interested in
altitude requirements. These special requirements are detailed in
each RBOC's NEBS checklist.
RBOC customers may request
that a product meet NEBS requirements because they know the received
product will be Carrier Class. Carrier Class means the product will
be around for the long haul. These customers want an excellent return
on their investment; they can get this with a product that meets
NEBS Level 3 requirements.
NEBS Resources
• Lorusso Technologies
LLC, http://www.lorusso.com.
• NEBS compliance information, http://www.nebs-faq.com.
• Verizon's NEBS Compliance Web Page, http://www.verizonnebs.com.
• Telcordia, the creator and keeper of NEBS documents, http://www.telcordia.com.
Dave Lorusso is a consultant for Lorusso Technologies LLC (Austin,
TX). He can be reached at 512-695-5871 or via e-mail at dave@lorusso.com.
|
