Tài liệu Look both ways before crossing the network doc

alue-added modules
(VAMs), sometimes called
splitter modules, are find-
ing tremendous acceptance
at fiber demarcation or hand-
off points, which are proliferating in
today’s collocation scenarios. Although
VAMs can perform a variety of network
functions, including splitting, multi-
plexing, and providing access within
fiber networks, the most popular appli-
cation today is non-intrusive monitor-
ing, which lets providers proactively
troubleshoot their networks without
forcing a disruption of service on cus-
tomers.
The driver behind this trend is collo-
cation. Competition has never been
keener in the telecommunications mar-
ketplace. In this era of deregulation
and nonstop technology advances,
competitive local-exchange carriers
(CLECs) are pushing incumbent local-
exchange carriers (ILECs) to provide
better service, lower rates, and
enhanced offerings. But CLECs still
rely on incumbents. Because of high
startup costs, staffing, and logistical
issues, many CLECs collocate equip-
ment and hook into local-exchange
fiber networks from an ILEC’s central

office. This competition, however
friendly, can lead to difficulty when
one party needs access for network
troubleshooting, especially when it’s
not clear whose equipment is at fault.
To alleviate some of this conflict, the
concept of “collocation hotels” has
become fashionable. Owned and oper-
ated by independent entities, these car-
rier-neutral dwellings provide central-
office floor space in a single building
for many different flavors of service
providers. With potentially hundreds
Look both ways before
crossing the network
Value-added modules allow competitive carriers with collocation
agreements to non-intrusively monitor
network performance.
Todd Duberstein
ADC Telecommunications
V
Figure 1. Value-added modules, sometimes called splitter modules, are proliferating in
today’s collocation scenarios.
Fiber termination bay Optical equipment bay
Fiber
cable-management
system
Value-added
module
chassis

Collocation Cage
Photo 1. A value-added module
housed in a fiber frame.
Reprinted with revisions, from the October 2000 edition of LIGHTWAVE
Copyright 2000 by PennWell Corporation
of cages separating the equipment and
providing security, carriers swap back
and forth a multitude of different sig-
nals and services through this single
office. In theory, cooperation should be
improved since everybody is still just
leasing space and more or less on
equal footing, but ownership of net-
work problems can still lead to territo-
rial disputes.
Point of demarcation
In either environment, a service
provider selling a DS-1 (1.544
Mbits/sec), a DS-3 (44.736 Mbits/sec),
or any SONET rate to a customer is
responsible for that circuit up to the
agreed-on demarcation point. Handoff
in the optical domain can be accom-
plished in a number of ways.
Generally, fiber-optic patch cords are
simply connected to fiber panels or
frames, with one provider granted
access to the rear, the other to the front.
For example, an ILEC hands off OC-12
(622-Mbit/sec) optical bandwidth to

an Internet service provider (ISP); the
ISP will then split that signal out, and
convert it to electrical signals before
sending Internet services to various
homes.
It goes without saying that service
integrity has become extremely impor-
tant to all parties. As e-mail and other
forms of data services have become
critical to individuals and
corporate enterprises, any
significant signal loss or
extended downtime cannot
be tolerated. Finding a
problem before it becomes
a hard failure and affects
customers does more than
prevent loss of business. It
gives the service provider
flexibility in choosing the
best way and best time to
address the problem. Thus,
many service providers are
implementing proactive
maintenance practices by
using test access points on
every fiber at the demarca-
tion point prior to handoff.
Testing these handoff
points on today’s fiber net-

works provides some
unique challenges. A fiber
network is by nature a closed system
and, unlike its copper cousin, not easy
to tap. The first indications of trouble
are typically the failure of a signal to
show up at its intended destination or
when its arrival is either
corrupted or attenuated.
Even though the current
service quality may still
be adequate for cus-
tomer needs, these types
of errors often are a fore-
warning of equipment
failure down the road.
As a result, an operator
may be forced to con-
duct a labor-intensive
hunt to find the source
within a buried or oth-
erwise inaccessible loop
to avoid future prob-
lems.
One approach is to
disconnect the connec-
tors from the fiber frame
at each suspected point
of failure and plug them
into an external test

device. This procedure
is time-consuming and
requires that part of the
network be taken out of service.
Convenient access to the equipment
is another issue. A collocated CLEC in
one cage may suspect it is not receiv-
ing good signals from the ILEC equip-
ment in another cage. But there’s no
easy way to log into the ILEC’s equip-
ment to make sure. Usually, a few
phone calls are required and, with
luck, the CLEC can find the one or two
people who are familiar with the
equipment, then wait for them to go
test it. This cost delay and interruption
of service while a line is tested is more
than an inconvenience; it is a critical
obstacle to the evolution of network
service and support.
To avoid these types of issues, net-
work operators need a passive, non-
intrusive testing solution. Non-intru-
sive handoff contracts—which specify
that if there are non-catastrophic net-
work test issues, the carrier must con-
tinue transmitting—is a major trend
in this area.
Window to the optical layer
VAMs are finding tremendous

acceptance at these critical fiber
demarcation points. The modules
Photo 2. A value-added module with removable retainers
allows a technician to access individual connectors for
service.
Photo 3. If an open-platform value-added module (VAM) is
installed, the technician requires only a single test box.
Regardless of what brand of transmission equipment is on
the network, a technician can plug into the VAM to obtain
test results.
slide into the fiber frame, and fiber
patch cords are installed from each to
the network equipment (see Figure 1
and Photo 1). The VAM is equipped
with separate ports for local testing.
Within the module, each transmit and
receive signal passes through a 90/10
splitter; 90/05 splitters are also com-
monly used. While 90% of the signal
is allowed to proceed to its destina-
tion, 10% is routed to the local moni-
tor port for use by an external test
device. This routing allows local test-
ing of either signal without interrup-
tion of service, with test devices hav-
ing access to the full optical signal—
exactly what the customer is getting
(see Figure 2).
When commissioning a network,
test equipment uses this signal to gen-

erate a known pattern, which simu-
lates real customer traffic. The pattern
is carried on the network or circuit
under test, then test equipment at the
receive point determines whether the
transmission is successful (without
errors). At this point, the technician
knows that it is functioning correctly
and what the levels are supposed to
be. Periodic checks will show if power
levels are deviating from this norm or
if bit-error rates are increasing, indi-
cating a signal degradation associated
with the future failure of a laser, con-
nector, splice, or some other network
device.
Coupler quality
Several vendors supply VAMs, but
operators should consider some key
differentiators when choosing a mod-
ule. The first relates to the design and
production of the coupler components,
which are mounted within the VAM
and provide the means for splitting
signals with low loss and high reliabil-
ity. The coupler devices used in the
network must have high reliability to
guarantee network availability. The
machinery that makes these devices is
very expensive, and the techniques

used to manufacture with precision are
difficult to master.
On top of that, significant pressure
exists for fiber component vendors to
bring products to market as quickly as
possible in high volume. Because of
time and technology constraints, many
systems vendors do not have the luxu-
ry to develop capabilities in-house and
are willing to purchase from outside
sources or co-develop components
with established vendors. As a result,
these vendors sacrifice internal control
over quality and are sometimes subject
to the irregular manufacturing sched-
ules of their suppliers.
Vendors such as ADC that develop
the technology and perform produc-
tion in-house generally are immune to
these types of problems. Producing
millions of couplers a year allows an
in-house vendor to fine-tune the man-
ufacturing equipment, software, and
processes to guarantee very high yield,
quality, and high reliability in field
environments. Compliance with in-
dustry standards, for high tempera-
ture, high humidity, and accelerated
aging, such as required by Telcordia
Technologies (Bellcore) GR-1209 and

GR-1221, British Telecom, Deutsche
Telekom, and others, are a standard
part of the production process. The
Table on page 86 depicts meantime to
failure (MTTF) in years for 1x2 cou-
plers, identified by service condition.
Connector access
An important design consideration
when choosing a VAM is the access to
the connectors. Dirty connectors are
the highest cause of poor performance
and the most likely failure points in the
network. Thus, periodic cleaning of
the connectors is a mandatory require-
ment. But connector cleaning can pres-
ent some unique challenges.
Most VAMs have at least two mon-
itor ports, one each for the transmit
and receive circuits. When a connec-
tor is cleaned or new circuits are
turned up, there is the potential for
dust or dirt to enter other connectors
within the fiber frame. One solution
is an adapter design with removable
retainers, which allows access to indi-
vidual connectors (see Photo 2). This
design lets a technician remove the
connector, clean it, and put it back in
without exposing the other fiber to
the risk of breaking and taking down

service. In some VAM designs, the
adapters are fixed to the sheet metal
of the fiber frame, and the only way
to get to the connectors is by remov-
ing the cover and exposing all circuits
to contaminants and potential fiber
damage.
Performance-monitoring limitations
Many vendors build performance-
monitoring functions into transmis-
sion equipment. However, there is a
mistaken notion that this function
eliminates the need for external test
equipment. Performance monitoring
is optimized to report what is going
on at the transport rate, for example,
OC-48 (2.5 Gbits/sec) or OC-192 (10
Gbits/sec). It is very good at detecting
major problems such as the complete
failure of a transmitter or a fiber cut.
But there are a number of failure con-
Figure 2. The value-added module is equipped with separate ports for local testing. Each
transmit and receive signal passes through a 90/10 splitter—90% is allowed to proceed to its
destination, while 10% is routed to the local monitor port for use by an external test device.
Local Testing of Signals
Customer
receives 90%
Demarcation point
Service-provider transmits signal
10% routed

Service-provider receives 90% from customer
10% routed
Customer
transmits signal
90/10 splitter
90/10 splitter
ditions not detected by per-
formance monitoring:
• Input/output port failure—
since this is a physical inter-
face, it is an area with one of
the highest failure rates.
• Faulty cabling or dirty fiber
connections on long-haul
and tributary equipment.
• Sectionalizing or localizing
trouble—most monitoring of path
errors and alarms is done at the path
terminating points within long-haul
or metro routes, making it difficult to
locate the exact point of the problem.
VAMs located at strategic points in
the network simplify this process.
• Ownership of only one end of the
path terminating equipment. In col-
location, a network provider may
own only the transmitter or the
receiver equipment, not both ends.
This makes it impossible to test the
end-to-end network. A VAM located

at the optical handoff solves this
problem.
There are also interoperability issues
between multivendor equipment to
consider. No regulations exist for stan-
dardization on the testing portion of
the signal. Therefore, manufacturers
may look at bit-error rates in different
ways and not communicate this test
information to other vendors.
Open-platform test boxes, which can
plug into a receiver from brand X or a
transmitter from brand Y and look at
the overhead, are available. But with-
out a VAM, it is still an intrusive solu-
tion and doesn’t provide a true picture
of the network, only the specific point
under test.
There is also a notion that testing can
be done using the digital-crossconnect
systems (DCSs) employed by some car-
riers. While DCS equipment does
include some testing features, these
boxes are geared to the telephony
switched network and optimized for
DS-1, DS-3, and other electrical signals.
Even though the DCS may have an
optical input and output, it still only
tests a signal comprising electrical sig-
nals, and all-optical crossconnect sys-

tems are not yet commercially avail-
able. Thus, carriers handing off pure
optical bandwidth cannot test it
through the DCS.
Cost issues
Sometimes network operators don’t
see the benefit of VAMs until after a
serious network failure. That’s under-
standable, especially in the case of
startup carriers. VAMs may be per-
ceived as an operations department
luxury that doesn’t contribute to rev-
enue generation—it costs money and
doesn’t really do anything except pro-
vide an opportunity to test.
There are several value propositions
to counter these perceptions. There are
many ways to look at operations cost,
one way being, what is the cost of serv-
ice going down? If a circuit fails and
can be discovered in minutes with a
VAM and repaired, customers may not
be upset. When service is down for a
day or two, as sometimes is the situa-
tion with intrusive methods, cus-
tomers may consider switching to
another provider that will guarantee
quality of service.
In addition, maintenance costs are
reduced. With an open-platform VAM

installed, the technician has to carry
only a single test box. Regardless of
what brand of transmission equipment
is on the network, a technician can
plug into the VAM to obtain test
results (see Photo 3).
Moreover, the technician
doesn’t have to learn the
specifics of each piece of
transmission equip-
ment—its proprietary
software, different testing
protocols, etc.
Keeping technicians
trained and up-to-date on
a variety of vendor-specific equipment
is expensive. Once technicians are
well-trained, their market value
increases and retention becomes an
issue. As a result, many carriers are
adopting modular products that are
easy to learn and easy to service. This
approach not only brings down opera-
tional training costs, it makes it possi-
ble to hire less-skilled personnel to
work on these networks.
The bottom line is that the combined
cost of a VAM and other associated
passive termination equipment is less
than 1% of the infrastructure service

cost needed to deliver a fiber circuit.
This one-time cost is miniscule com-
pared to the millions of dollars spent
on transmission elements. And yet, the
cost savings in problem prevention and
customer retention gained through
VAMs can be enormous.
With competition at an all-time high
in the telecommunications market-
place, network reliability, efficiency,
and performance are critical to acquir-
ing and retaining customers.
Throw collocation into the mix and
five-nines (99.999%) operability is
mandatory, even with limited man-
power and time. The VAM, when used
for non-intrusive access for network
testing and monitoring, helps ensure
networks are up and running and pro-
viding revenue. ◆
Todd Duberstein is a product manager for
the fiber-optic division of ADC
Telecommunications (Minneapolis).
Splitter Central Unpowered Unpowered Powered
type office underground street cabinets street cabinets
50/50 145 377 205 57
5/95 250 653 355 98
1/99 450 1,170 630 180
MTTF in Years for 1×2 Couplers