Optics and Photonics Journal, 2013, 3, 179-182
doi:10.4236/opj.2013.32B043 Published Online June 2013 (http://www.scirp.org/journal/opj)
Automatic Detection of Optical “Faults” in
Communications Networks
——An Efficient, Fast, Physical Layer Monitoring Approach for Access Networks
Meir Bartur
Optical Zonu Corporation, Delano Street, Van Nuys, CA, USA
Received 2013
ABSTRACT
An efficient, fast, Physical Layer monitoring approach is needed to instantaneously identify, locate and report (via
SNMP) optical fiber cuts, breaks or other faults (open or dirty optical connectors) in the optical fiber cable plant.
Keywords: Optical; Fiber; Fault; Monitoring; Physical; Layer; Sing le; Wavelength
1. Introduction
Today there is no proven method for automated moni-
toring of the optical fiber cable plant in the aggregation
and data center segments of private campus or public
communications networks. Metrics at the higher network
layers may identify that a problem exists, but they canno t
quickly isolate the location of an optical fiber fault nor
can they automatically trigger the immediate dispatch of
repair technicians.
An efficient, fast, physical layer monitoring approach
is needed which can instantaneously identify, locate and
report (via SNMP) an y optical fiber cuts, breaks or other
faults (as well as any open, damaged or dirty optical con-
nectors) in the optical fiber cable plant. New technology
is now available to accomplish this, which is backward
compatible with legacy networks, and also forward com-
patible with new DWDM, 10G (and beyond) emerging
networks, and may be easily integrated into SNMP moni-
toring of existing Switch/Router Equipment with mini-
mal software/firmware upgrades.
2. Micro-OTDR (uOTDR)
2.1. Structure of the Access Network
The Aggregation and Data Center Segments of Private
Campus or Public Communications Networks may be
considered as the “edge” of the overall Access Network.
Considering the breakdown of optical link distances in
the Access Network [1], about 80% of the links consist
of distances of 10 Km or less, while virtually all of the
Access Network links fall within the range of 40 Km and
below. While the data throughput of any particular Ac-
cess link might not match that of its Long Haul “cousins”,
the fact that there are so many splices, connections,
patch-panels and kilometers of optical fiber in this seg-
ment makes it particularly vulnerable to a host of optical
“faults”. Optical cables may be cut, optical connectors
left open or become “dirty”, unexpected power outages
may occur…a broad range of potential problems which
are magnified by the sheer number of potential failure
points.
2.2. Fault Detection Must be Fast, Distributed
and Pervasive
To materially improve the reliability of the Access Net-
work, fault detection must be fast, distributed and perva-
sive.
The detection of a fault needs to be fast so that the op-
eration of the network, particularly an Ethernet-based
one, is minimally impacted. A fault should be detected,
located and reported to the Host Switch within a fraction
of a second. If the fault is momentary, then the link
should instantly recover. If not, then there ou ght to be the
capability to initiate a series of diagnostic and/or repair
options. Such responsiveness to a service disruption may
translate directly into improved Service Level Agree-
ments (SLAs) and the competitive advantages they may
offer the Communications Service Provider (CSP).
Having a sophisticated piece of equipment monitoring
a small number of optical fibers (thereby “consuming”
these fibers, making them unavailable for revenue gen-
eration), is not very efficient in the extremely high link
density environment found at the “edge” of the Access
Network. We know that the link distances are relatively
short (under 40 Km) and that the performance require-
ments of an OTDR to operate in this segmen t are modest.

M. BARTUR
180
Perhaps a large number of moderate performance moni-
tors, and their associated OTDRs, distributed throughout
the Access Network, makes more sense than fewer,
higher performance (and much more costly) systems.
In order to have the highest level of effectiveness,
these distributed moderate performance monitors and
OTDRs need to be virtually everywhere. Near 100%
fault detection may be achieved when large numbers of
(or preferably all) optical links incorporate this perform-
ance monitoring. Fiber fault detection is based upon the
reflected signal from the point of fault.
Fiber faults and intermittent connections present opti-
cal reflections of varying intensities. The reflection in-
tensity of a fiber break has a known statistical distribu-
tion. The distribution of the return signal from a fiber
break/cut has been empirically determined and is pre-
sented in Figure 1.
As indicated in Fiber Fault Histogram of Figure 1 [2],
to achieve a detection probability of greater than 95%,
the detection circuit must be better than 51 dB.
Since the Fiber Fault Histogram indicates that the de-
tection probability of an optical fiber fault follows a
Normal Distribution, modeling to predict outcomes is
straightforward. In fact, from the modest number of
equations shown in Figure 2, a series of curves for the
likely optical fiber cable deployments encountered in the
Access portion of the Communications Network may be
developed.
Figure 1. Fiber fault (Cut and Break) histogram.
Figure 2. Equations underpinning predictive modeling of fiber faults.

M. BARTUR 181
Monitoring multiple fibers in the same cable trunk in-
creases the detection probability (each fiber breaks dif-
ferently). The optical fiber link may be monitored from
one side, or from both sides, when a ring net work topology
is utilized. Monitoring both link ends, the detection prob-
abilities of an optical fiber break which may be achieved
are shown in the chart Figure 3.
Where is the most logical place to put this perform-
ance monitoring? It is in the optical transceivers, which
by necessity, must reside at either side of every optical
link in the Access Network. With performance monitor-
ing distributed throughout the Access Network, essen-
tially at both ends of every optical link, the Service Pro-
vider gains the highest probability of catching optical
fiber faults in any portion of the Access Network. But is
this approach feasible given the state of technology to-
day?
2.3. Single Fiber, Single Wavelength (SFSW)
Transceivers
In order for any transceiver to operate as an OTDR (as it
must in order to fulfill the role of an optical fiber fault
monitor), it must both transmit and receive at the same
wavelength. The transceivers most widely deployed to-
day are not of this type. However, recent advances in
single fiber, single wavelength transceiver design, make
the incorporation of OTDR functionality within the
transceiver pra ctical.
SFSW transceivers transmit and receive at the same
wavelength (actually doubling the optical fiber plant ca-
pacity). Upon disruption of data link, or failure to con-
nect, the transceivers could be designed to switch into a
“Micro-OTDR” mode, emitting optical power pulses (>
+13 dBm) and detecting the reflected pulses at least
down to -42 dBm optical power, for a dynamic range of
at least 55 dB, in order to detect reflections generated by
physical faults in optical fiber link.
While not new, single fiber, single wavelength trans-
ceivers have in the past suffered from performance and
operational issues related to their sensitivity to reflec-
tions…the exact thing whic h m a kes them work as OTDRs.
Reflections within the optical subassembly of the trans-
ceiver itself, as well as reflections returning from the
optical cable plant, all serve as noise contributors, ad-
versely impacting optoelectronic performance. In certain
situations, like an open Blue UPC (non-angled polish)
connector, legacy single fiber, single wavelength trans-
ceivers were subject to “false locks” onto their own re-
flected signals. These concerns of the past have been re-
solved and robust single fiber, single wavelength trans-
ceivers are, in fact, available today.
Advancements in the design of single fiber, single
wavelength optoelectronics, including near removal of
internal optical crosstalk, allow these transceivers to more
than make up for the optical budget loss penalties. New
“smart” desig ns with micro co n tro llers ad d “sta te ma ch ine”
firmware which makes the single fiber, single wavelength
transceivers “immune” to reflections, for example from
Blue UPC connector cable plants, while adding the func-
tionality required for the transceiver to double as a mi-
cro-OTDR.
The additional functionality of an SFP transceiver with
OTDR is depicted in Figure 4. Key aspects are the very
high pulsed Laser Diode Driver which can generate 15
dBm peak pulse power, and the specialized threshold
receiver that detects the reflections with higher sensitivity
than the data receiver. Combined dynamic range of at
least 55 dB is achievable.
Figure 3. Probability of multi-strand detection depicted for 1, 2, 4 and 8 optical fibers monitored per bundle.

M. BARTUR
182
Figure 4. Functional block diagram of Micro-OTDR SFSW transceiver.
Table 1. Comparative benefits.
Benefit Legacy SFP+OTDR
Fast No Yes – Instantaneous.
Distributed No Yes – In the SFP.
Pervasive No Yes – At the end of every Link.
Cost High Low
Revenue No Yes – Carries Data.
3. Conclusions
There are available in the marketplace today, new tech-
nology single fiber, single wavelength transceivers which
incorporate the functionality required to deploy fast, dis-
tributed and pervasive optical link monitoring with near
100% detection efficiency. These transceivers do not
require specialty equipment but may reside in standard
Host Switches which may be managed through industry
standard SNMP systems, offering many benefits over
legacy Remote Fiber Testing Systems.
The new single fiber, single wavelength transceivers
do not suffer from any of the performance or operational
drawbacks which plagued these designs in the past, and
with their new capabilities provide the basis for robust
automatic detection, localization and reporting of optical
fiber cable plant “faults” in the Aggregation and Data
Center Segments of Private Campus or Public Commu-
nications Networks.
REFERENCES
[1] Michael J. Hartmann, Unpublished Working Papers:
Compendium of Multiple Industry Sources and Trade
Press References. (2007-2012).
[2] Meir Bartur, Unpublished Working Papers: Summary of
Empirical Data from Optical Fiber Cut Experiments.
(2012).