PON Build Phase Testing Considerations
SUMMARY
This document discusses installation testing for the build phase of a typical FTTH
Passive Optical Network (PON) cable plant using a connectorised splitter with
particular emphasis on an external centralised splitter architecture.
We discuss the purpose of testing and the function of typical build phase test
instrumentation.
We posit that the PON network can be divided into several logical sections for testing
purposes and that the sectional testing solutions are dependant upon the required
Quality of Service.
Brian Crook, Applications Engineer,
Bruce Robertson, Technical Director
Kingfisher International, April 2008
www.kingfisher.com.au
1
DEFINITIONS ............................................................................................................................................ 2
2
WHY TEST? ............................................................................................................................................... 2
3
PON ARCHITECTURES ........................................................................................................................... 3
4
PON ACCESS PROTOCOLS..................................................................................................................... 5
5
WHAT QOS?............................................................................................................................................... 5
6
WHERE TO TEST ...................................................................................................................................... 7
7
BUILD PHASE TEST INSTRUMENTATION .......................................................................................... 8
7.1
7.2
7.3
SECTION LOSS ......................................................................................................................................... 12
SYSTEM ORL .......................................................................................................................................... 12
CABLE CHARACTERISATION ..................................................................................................................... 13
8
TEST WAVELENGTHS........................................................................................................................... 13
9
TESTING SOLUTIONS / INSTALLATION TESTS............................................................................... 14
9.1
9.2
PRE INSTALLATION TESTS ........................................................................................................................ 14
INSTALLATION TESTS – PRE POWER UP ..................................................................................................... 16
9.2.1
9.2.2
9.2.3
9.2.4
9.2.5
Head-end to LCP .............................................................................. 16
Optical Splitter ................................................................................. 17
LCP to NAP ..................................................................................... 18
Drop Cable ....................................................................................... 20
End to End Loss Testing / Verification ............................................. 23
10
CONCLUSION.......................................................................................................................................... 23
11
REFERENCES .......................................................................................................................................... 23
12
RECORD OF ISSUE ................................................................................................................................. 24
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1 DEFINITIONS
Within the industry there is a proliferation of terms and acronyms in use for a given
network element. Further subtle variances in definitions may exist between differing
administrations. The following generic definitions are used in this document.
APC
CAPEX
DSL
FDH
FIT
FTTH
FTTx
Head-End
ITU
LCP
MCSM
MTBF
MTTF
MTTR
NAP
NID
NMC
OLT
ONT
ONU
OPEX
ORL
OTDR
P2MP
P2P
PON
POTS
QoS
ROI
VLS
Angled Physical Connect
Capital Expenditure
Digital Subscriber Line
Fibre Distribution Hub. Aka LCP
Failure In Time. Typically 1 failure in 109 hours.
Fibre To The Home
Generic for several types of Fibre delivery systems.
Carrier building where the network signal distribution starts.
International Telecommunications Union
Local Convergence Point. Aka FDH
Matched Clad Single Mode
Mean Time Between Failure
Mean Time To Failure
Mean Time To Repair
Network Access Point
Network Interface Device
Network Maintenance Centre
Optical Line Terminal
Optical Network Terminal. Serves one customer. See also ONU.
Optical Network Unit. Serves several customers. See also ONT.
Operational Expenditure
Optical Return Loss
Optical Time Domain Reflectometer
Point to Multipoint optical system
Point to Point optical system
Passive Optical Network
Plain Old Telephone Service
Quality of Service
Return On Investment
Visible Light Source
Further information on terms relating to FTTx can be found in ITU-T
Recommendation G.983.1 Broadband optical access systems based on Passive Optical
Networks (PON) and the FTTH Council document Definition of Terms.
2 WHY TEST?
Optical cable plant typically has a planned design life of 20-40 years. Transmission
equipment life span may considerably less.
Transmission systems require installation testing to verify that variously:1. The network is correctly configured and is fit for use.
2. The network is built to specification.
3. Proposed transmission equipment performance limits are met.
4. The installation is proven to meet manufacturer warranty requirements.
5. Required information is gathered to facilitate future maintenance
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6. Construction contractual obligations have been met.
7. Operational reliability promises can be met
Failure to adequately test to an acceptable level during construction will randomise
CAPEX whilst maximising OPEX and customer dissatisfaction. Put another way:
inadequate testing is expensive.
To achieve “adequate” testing in this situation requires careful consideration.
It’s quite possible to achieve “adequate” testing at modest cost, however incorrect
planning will result in inadequate testing at a much higher initial cost.
Who Tests?
This is an important issue, since in the currently anticipated FTTH scenario, and for
lowest cost, it must be done on-site by relatively less skilled personnel than has
traditionally been the case, so testing has to be simpler to perform and interpret. Test
equipment requiring specialised skills can’t be used, except by a specialist 2nd tier
response crew.
Pre-Installation Testing
We introduce this concept here, because it’s an important part of a QA and risk
containment strategy, particularly if (when) some network element is found to have a
high as-delivered fault rate, and therefore requires extra testing, or some high level of
risk is associated, eg cable qualification prior to installation. Given the nature of PON
installation, it’s obviously a lot cheaper overall to ensure such testing is properly done
and documented prior to site delivery. This could be done by the network operator,
installation contractor, 3rd party, logistics supplier, vendor agent etc. The important
thing is that it is done, and as part of an overall QA plan. It will save money, time, and
boost the operator’s brand image.
3 PON ARCHITECTURES
A PON network is a Point to Multipoint (P2MP) architecture servicing many low
revenue (typically domestic) customers. Testing is more complex than in traditional
Point to Point (P2P) because a given customer’s circuit is composed of both shared
and dedicated fibre. Indeed, the increased test complexity, combined with greatly
increased cost constraints, means that testing needs to be very carefully evaluated and
implemented.
PON networks use passive splitters to distribute fibre to individual homes.
One fibre is optically split into 16, 32, or 64 fibres, (128 under
investigation) which are then distributed to residential or business
subscribers. The switching and routing is done at the carrier's central
office.
Once on the customer's premises, the fibre connects to a Network
Interface Device (NID), called variously an Optical Network Termination
(ONT) or an Optical Network Unit (ONU). This device converts the
optical signal into a format that can be used directly by the customer.
PON networks can be configured in one of three types of access topologies.
1. Head End splitter, also called Central Switch.
2. Single LCP splitter, also called Local Convergence.
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3. Multiple LCP splitters, also called Distributed Splitting or Cascaded Splitting.
At time of writing, there is no agreed upon naming nomenclature.
These configurations are shown in the Figures 1 below with Figure 1B being the most
common implementation and that which is used within this document.
OLT
NAP
ONT
Splitter
Figure 1A, Head-End Splitter
LCP
OLT
NAP
ONT
Splitter
Figure 1B, Single LCP Splitter
LCP-2
OLT
LCP-1
ONT
Splitter
NAP
Splitter
LCP-3
Splitter
Figure 1C, Multiple LCP Splitters
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4 PON ACCESS PROTOCOLS
There are several PON protocols and standards defined by the ITU organisation with
varying levels of maturity. The three main ones, at time of writing, are BPON, EPON
and GPON. Table 1 below illustrates their main features.
Standard
Upstream λ
Downstream λ
Protocol
Bandwidth
Max Distance
BPON
ITU-T-G.983
1310 nm
1490 & 1550 nm
ATM
EPON
IEEE 802.3ah
1310 nm
1550 nm
Ethernet
GPON
ITU-T G.984
1310 nm
1490 & 1550 nm
ATM, Ethernet, TDM,
GEM
Down ≤ 1.24 Gbps
Up
622 Mbps
20 Km
Down ≤ 1.25 Gbps
Up
≤ 1.25 Gbps
10 or 20 Km
Down ≤ 2.4 Gbps
Up
≤ 2.4 Gbps
10 or 20 Km
Table 1, PON Protocols
At time of writing, the ITU and IEEE are working on a ‘next generation’ 10Gbits/s
PON standard.
5 WHAT QOS?
What Quality of Service (QoS) is required? A simple question that doesn’t seem to
get discussed, and doing so yields some useful answers, so we attempt to tackle it
here:
The reliability performance of a transmission network can be calculated by the well
known unavailability formula:-
µ=
MTTR
(MTTF + MTTR)
≈
MTTR
MTTF
=
MTTR * FITs
109
Where MTTF is Mean Time To Failure, MTTR: Mean Time To Repair and FIT
is Failure In Time with 1 FIT = 1 failure in 109 hours.
Availability, α is simply 1-µ .
Traditional P2P networks may typically have an availability of circa 99.9% or higher.
That is, a down time of less than 8.8 hours/year. Where greater reliability is required
equipment duplication and ring topologies, often in conjunction with geographical
isolation of the cable path are employed, at of course, additional CAPEX.
PON networks however, are per unit customer, low revenue. A lower availability
may therefore be acceptable. But what should it be?
The reliability issue is usually also affected by 3 key factors:
• Performance of automated service fault reporting systems
• The Mean Time To Repair (MTTR) a fault
• Customer Service Level Agreements
One suggestion is that the acceptable availability should be not less than that for a
Plain Old Telephone Service (POTS) or DSL service. Another is that it has to be
higher due to video service delivery. What is the availability of your POTS/DSL?
What are your customers’ expectations?
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From surveys we have done on FTTH equipment and installed cable plant reliability,
the reliability of the customer’s ONT is likely to be a practical limiting factor, on
service reliability.
We arrive at this conclusion from the following data:• ONT manufacturers’ web sites typically quote an MTBF of 10 years.
We also make an assumption about fibre cable systems reliability as follows:
• A “traditional” fibre cable, using proven best practice materials & installation
practices, seems to achieve a typical intrinsic installed cable MTBF of 1
failure / year / 3,000 fibre-Km. This figure of course ignores the main causes
of failure, being human intervention, storm & tempest.
Consider what we arrive at if the cable intrinsic MTBF is allowed to be 30 times
worse and the average length of the PON is assumed to be 5 km:• New fibre MTBF=100 Years/Km
• PON fibre MTBF = 20 years/5 Km (FIT = 5,850)
Another way of expressing this is one fibre failure a year for every 100 Km of fibre,
which is significantly less than the rate of accidental cable damage in an urban
environment.
The importance of these simple assumptions is as follows:
• Given the widely varying MTBF’s, it can be seen that the ONT is the limiting
factor for service reliability.
• The fibre cabling doesn’t need to be very well installed to achieve a very
acceptable level of service reliability as the majority of cable faults are man
made. Table 2 below shows typical causes of fibre failures in an urban
environment.
Installed cabling reliability can be of the order of 30 times worse / km than
traditional best practice with minimal QoS impact in a PON environment.
70
60
% of Failures
50
40
30
20
10
O
th
er
Tr
ee
s
S
to
rm
E
xc
av
at
io
n
G
un
s
Fi
re
Er
ro
r
P
ow
er
Li
ne
Ro
de
nt
s
S
ab
ot
ag
e
H
um
na
n
Du
gu
p
V
eh
ic
le
0
Cable fault causes
Table 2, Causes of Installed OFC Fibre Failure – Crawford Study
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6 WHERE TO TEST
When the splitter is located at the Head-End, (Fig. 1A) traditional P2P testing
techniques are applicable. This architecture presents the simplest testing regime.
If one of the field based LCP splitter options is chosen, which is the norm, testing
must be performed at two or more locations to adequately characterise the
transmission link. This of course increases testing complexity and costs.
Let us consider the testing requirements of a PON configured with a single external
LCP. (Fig 1B)
To adequately characterise the installation, testing is required between the following
network components:• Head-End to LCP
• LCP to NAP
Possibly the Drop cable, which connects the NAP to ONT. More on this later. These
are shown in Figure 2 below.
20 Km Max as per ITU
Typically 5-15 Km
OLT
Typically 100 m- 5 Km
LCP
NAP
ONT
Splitter
Figure 2, PON Sectional Testing
As can be seen, unless care is taken the cost of testing can be quite expensive relative
to income. This is especially so when one factors in the potential for many installed
fibres to lie unused and a customer’s expectation of rapid service activation.
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7 BUILD PHASE TEST INSTRUMENTATION
Several instrument types may be considered to perform PON installation tests as
shown in Table 3.
Test
Measurement
Connector
Fibre
Identification
Section Loss
Instrument
Microscope
• VLS
• Fibre Identifier
• Source & Meter
Source & meter or
Loss Test Set
System ORL
ORL meter / LTS
Cable
characterisation
Splice Loss
Optical Power
PON OTDR
Triple Play Test
Splicer
λ Selective Power
Meter
NID Diagnostics
Spare NID
Other
Head Room
In line attenuator
Faulty NID
Purpose of Test
Test
Complexity
Easy
Price
Range USD
> 300
Easy –
Medium
600 -2,500
Section loss of the cable
Pass/Fail evaluation
Warranty
• Ensure ORL is within
design parameters. E.g.
ITU-G-983
•
Physical fault location
•
Loss & reflection at joints
•
Check Splice Loss
•
Check Transmission Power
Easy
> 3,000
Easy Medium
> 7,000
High to very
high
Easy
Easy
> 16,000
•
Determine if NID has a
fault
•
Check the 3 services
delivered to the customer
•
Determine if design
transmission margin met
Easy
Medium
0, but operator
costs at NMC
> 4,000
Easy
< 10
•
•
Check contamination
Check end face quality
• Identification of
designated fibre
•
•
•
7,000
> 1000
Table 3, Typical PON Build Phase Test Instruments
Some pertinent aspects of these test instruments are discussed below.
Microscope
Traditional optical microscopes with 200x magnification and an optical safety filter
are cheap, adequate and easy to use. However some users now favour electronic
video scopes since they allow recording of image data, are intrinsically optically safe
and can be used to inspect installed bulkhead connectors. They are also about 10
times the cost, less reliable and require more skill to use. The decision here is
probably about how much the operator is trusted. Simple training is needed to focus
and recognise good / bad connector condition (pictorial).
Figure 3, Connector – damaged & dirty
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Visual Light Source
Visible Fibre Light sources provide a red light of circa 635 nm wavelength. Useful
for finding open circuits in patch leads or splices, for locating fibre ends, and
generally checking continuity. Maximum range is dependant upon several factors but
is in the order of 5-7 Km.
This is a very favourite craft tool for both skilled and unskilled field staff. It’s very
easy to use, and very effective on these shorter links. It won’t do much on fibre
length over about 5 Km, for which another solution is needed. Most useful where the
fibre ends are accessible, which may make it of limited use when re-entering a live
system.
Simple hands-on training is needed to learn how to bend a fibre to make it glow. As
PONs use APC connectors, an APC-PC adaptor patch lead may be required to match
the VLS connector. –Refer Section 7.2 ORL for cautionary.
Fibre Identifier
Also known as a traffic identifier. Works over the full 20 Km PON length, and allows
the user to see if there is a live signal, test signal, or no signal in a fibre before reentering a live system, or as a long range fibre identifier. Typically used with a test
light source, which is used to produce an optical test tone. Simple hands-on training
is needed.
The use of a fibre identifier when working on live cable is mandated by many
network owners so as to minimise costly outages.
All fibre identifiers create a small loss on the fibre when in use. This loss is often
overlooked by some system designers who produce designs with minimal ‘head
room’ or ‘system margin’. Failure to allow sufficient ‘head room’ for foreseeable
maintenance activities can have disastrous results.
Table 4 below, shows some typical published insertion loss figures for a popular
brand of Fibre Identifier.
Wavelength Insertion
Loss
1310 nm
0.01 dB
1550 nm
0.11 dB
1625 nm
0.31 dB
Table 4, Typical fibre identifier, published insertion loss for 3 mm SMOF fibre
Be aware that there are brands on the market with lower insertion losses. These come
at the price of being rather insensitive!
Source and Meter / Simple Loss Test Set
Used to test end to end loss, and also as a continuity tester if equipped with a tone
detector function. The source can also be used as a tone source for a clip on fibre
identifier. The meter can also be used to measure transmitter power, however this
function is not so useful for this here, since a wavelength selective meter is needed.
Consideration may be needed as to how test results will be recorded, since in practice
data recording / manipulation can be over half of the testing cost. Some theoretical
and practical training is required.
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Two Way & ORL Tester
Does the job of a source and meter / simple loss test set, but it can greatly simplify
accurate testing if it can perform bi-directional loss and additional ORL testing in one
go. Consideration may be needed as to how test results will be recorded, since in
practice data recording / manipulation can be over half of the testing cost. Some
theoretical and practical training is required. A pass/fail function may be available.
This equipment is the simplest and fastest method of certifying the cabling system.
OTDR
OTDRs have limitations which are normally not of great concern. However in splitter
based PON network there are various significant problems as follows:
• The splitter is a cause of severe Event Dead Zone limitations.
Consider a 32 way splitter, then when operated in a mode which overcomes the
18 dB splitter loss, say to achieve about 25 dB dynamic range, a modern FTTX
OTDR will have a pulse length of 1 – 10 µsec, giving it a substantial dead zone
of 150 m – 1.5 Km. That’s a long way in an urban environment!
• They cannot properly characterise cable downstream from the splitter as any
discontinuities cannot be uniquely identified. Some limited level of
performance can be achieved with low split ratios.
• Experienced OTDR operators who really know how to optimise the OTDR
settings for dead zones etc, are uncommon. In a PON, this knowledge is critical.
• Another distinct problem is difficulty of analysing the trace. The automated
analysis doesn’t always work, and this is one of the hardest jobs in fibre
measurement, requiring substantial theoretical and hands-on training and
experience
To us, this expensive instrument looks best left to the specialist back-up crew, as an
aid to troubleshooting. In fact, this instrument is a major reason why you would have
such a team.
The essential limitations are well documented in available literature.
An early reference is Optical Time-Domain Reflectometry, by Duwayne Anderson &
Florian Bell, Chapter 10.
There is another issue here which is currently appearing: Cable and Fibre types are
being developed which are very insensitive to bending loss. Improved G.652D
compliant cable meeting the ITU’s recently introduced recommendation G.657,
Characteristics of a Bending Loss Insensitive Single Mode Optical Fibre and Cable
for the Access Network is available now.
The G.657 standard describes two types of bend-improved optical fibers. The first is
referred to as Class A singlemode fiber that is fully compliant with the embedded base
of G.652D low-water-peak fibre.
The second G.657 fiber type is referred to as Class B and was included in the standard
to target the niche application of wiring within residences, though some Class A fibers
can also support this application. The Class B recommendation encompasses existing
and proposed bend-insensitive fibers that may not be compliant with the embedded
base such as hole-assisted fibers and fibers with very small mode-field diameters.
These bending insensitivities are demonstrated in Figures 4 & 10 below.
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OFS Fibre
Cordage performance comparism
single wrap 20 mm diameter mandrel
OFS Fibre
Cordage performance comparism
single wrap 15 mm diameter mandrel
Figure 4, OFS G.652D compliant, Bend insensitive fibre performance
On the reasonable assumption that G.657A fibres will become common, then the
practice of using loss or OTDR testing to identify cable stress points will need reevaluating as it will be unlikely to show much.
Splicer
This isn’t usually regarded as test equipment, but when it’s an auto-alignment type, it
has a built in loss estimator that can form a useful part of an overall PON test strategy.
If a splice passes the loss estimation check, it would be good enough for use in a PON.
This is a zero-effort, zero-training test!
Most splicers used by cable crews are auto-alignment types, since they cope with field
conditions much better than fixed v-groove types.
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Wavelength Selective Power Meter / ONT Diagnostics / Spare ONT
A Wavelength Selective Power Meter is required for transmission power testing.
First, it is required to measure the 1490 / 1550 nm power transmitted at the head end.
Second, it is required to measure the much lower power 1490 / 1550 nm levels at the
ONT at service turn-up.
The currently topical requirement for an in-line meter for service turn-up and
maintenance seems problematic to us, based on the following logic:
• If there is a problem with the cabling plant, then an in-line meter isn’t needed
to find it.
• If the 1490 / 1550 nm power levels look OK, then either the ONT has a
problem or the fault is upstream from the ONT.
• The obvious thing is to examine available ONT diagnostics and if necessary
sectionalise the fault.
• If the fault is in the ONT, then exchange the ONT. In this case, a spare ONT
forms an integral part of the test/repair strategy, and a crew would presumably
have one anyway.
There are of course, some important issues with how the carrier’s operational support
systems interface to the field staff in a timely manner during maintenance and service
activation.
Triple Play Test
We assume that at service turn–up, the greatest challenge for the field crew will be to
get all of the customer’s “triple play” equipment (phone system, internet connection,
video) to work properly with the ONT, since it’s likely to be different with each
customer. However, this is outside the scope of this paper.
7.1 Section Loss
Physical loss over a section of fibre. This is measured with a source and meter at 2 or
more wavelengths, or preferably, with a bi-directional loss test set which incorporates
Optical Return Loss (ORL).
7.2 System ORL
The requirement to measure ORL in a PON seems to be open to debate. ORL
induced problems have diminished since the wide spread adoption of APC connectors,
isolated lasers, and digital transmission, all of which, if applied properly, tend to make
ORL something of a non-issue. However, in the event of a transmission problem, it
may need checking, and can be tested with negligible effort using a suitably equipped
two-way tester. The classic problem is that someone accidentally leaves an
unterminated PC connector somewhere on a system, which then mysteriously
misbehaves.
If a transmission system’s minimum ORL requirements are not met the transmitted
data may be corrupted resulting in increased noise and or system failure. A general
rule is that;
• Analogue transmission systems are more affected than digital.
• Digital systems become more prone at higher data rates.
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7.3 Cable Characterisation
In this situation an OTDR can be used to evaluate the as built characteristics of the
installed cable plant and locate faults. Typically testing is performed at 2 wavelengths
so as to be able to identify any bending violations
As previously discussed, with the advent of G.657 fibre, traditional OTDR testing
must be re-evaluated. The use of a higher wavelength pair than 1310/1550 nm will be
required to identify stress points in such fibre.
It must also be noted that inherent with the use of bend insensitive G.657 fibre is the
need to ensure that the cable construct itself also permits greater bending.
OTDRs also provide an estimate of the cable section loss and ORL. To measure the
actual section loss as will be seen by the transmission equipment, a light source and
power meter combination or Loss Test Set must be used. Similarly, an ORL meter is
required to measure the ORL that will be seen by the transmission equipment.
8 TEST WAVELENGTHS
The cable needs to be loss tested at all three PON wavelengths, 1310, 1490 & 1550
nm. Right? Well not necessarily. If an installation passes at 1490 & 1550 nm or
1550 & 1625 nm, how can it not pass at 1310 nm?
Recently, modestly priced testers at 1625 nm have become available. Many network
owners are now beginning to mandate testing at this wavelength due to the several
advantages that 1625 nm offers, such as:• Out of band in service monitoring and testing.
• More sensitive to fibre anomalies such as bending violations. (important for
G.657)
• Early detection of cable faults.
There is certainly a need to test at least two wavelengths, to identify serious bending
violations.
That said, modern loss test sets, generally incorporate an Autotest or automatic
wavelength toggling capability, thus the extra time to perform a loss test at all 3
wavelengths is insignificant.
It is strongly suspected that if loss testing is done at 1625 nm (or even 1550 nm), then
this is nearly as effective as OTDR testing at identifying if there is a serious cable
bending problem (remember, we can survive 25 times worse cable reliability than
traditional best practice). Such incidents can be expected to be relatively uncommon,
and therefore it is justified to not keep an OTDR and operator on-site during general
installation.
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9 TESTING SOLUTIONS / INSTALLATION TESTS
There are three main components of the build phase field testing. These are:1. Pre-installation tests
2. Installation tests – pre power up.
3. Installation tests – post power up.
Items 1 and 2 are discussed more fully below.
Once work practices and reliability have been stabilised and proven, a sample test
methodology could be considered, which could lower costs further.
Standard QA methodology includes various established statistical sampling methods,
a detailed discussion of which is outside the scope of this paper. .
We assume that any organisation that has reached the point of considering statistical
sampling methods has therefore achieved a proven a level of installed reliability,
which also implies that they have enough operational data to make proper decisions
without reference to this paper!
9.1 Pre Installation Tests
There is a tendency amongst many network builders to follow the ISO9001 paper trail
requirements in a simplistic fashion.
A wise network builder however, ensures that prior to installation, equipment and
cables are inspected and possibly tested (by someone, somewhere) to ensure that they
comply with the required specifications.
Failure to check as delivered equipment and cables can lead to costly rework and
litigious warranty claims.
Example of consequences
Figure 4 below shows an OTDR trace of an optical cable showing distributed micro
bending losses at 1550 nm due to a faulty cable manufacture.
• The cable manufacturer failed to test the cable after manufacture.
• The network owner failed to test the cable prior to field installation.
• The Field Project Managers failed to specify any loss testing prior to system
power up.
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dB
6.9339Km
6.6330km
1310nm
22
Faulty cable
Section
1550nm
21
20
Km
6.2
6.4
6.6
6.8
7.0
Figure 5, Distributed micro bending due to manufacture defect in OFC cable
The result of all these oversights was costly rework and many dissatisfied customers.
The following pre-installation acceptance methodology should be considered.
1. Manufacturer to be required to provide factory test data.
2. Clearly determine who is responsible for storage of ‘as supplied’ test data.
3. Clearly determine access methodology for ‘as supplied’ test data.
4. Network owner randomly test a selection of ‘as supplied’ product.
Especially:• When a different supplier is used.
• When a new or modified design is introduced.
• When a mission critical installation is involved.
For cable, the recommended testing is
• OTDR test 1 way on the, ‘as delivered’, cable drum.
• Testing at two wavelengths recommended.
• Test at least 1 fibre in every tube.
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9.2 Installation Tests – Pre power up
As previously discussed an optical fibre system must be tested so as to determine its
fitness for service. This requires that various individual link loss parameters be
measured.
9.2.1 Head-end to LCP
LCP
OLT
Splitter
2 Way LTS with ORL
and
2 Way OTDR
Figure 6, Testing Head-End to LCP
Typical characteristics of this section are;
• Low fibre count.
• Typically 5-15 Km in length.
• May be either FTTx based or traditional services
• Individually these fibres carry the greatest revenue in the PON.
• A failure of one PON fibre will affect all customers on its associated splitter,
that is, up to 64 customers.
It is recommended that full testing be performed to adequately characterise the
transmission link. .
• Both way link loss at a minimum of two wavelengths including ORL.
• Both way OTDR at two wavelengths.
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9.2.2 Optical Splitter
Joining the Head-End fibre to the NAP fibres and located at the LCP, the optical
splitter is a highly reliable device having undergone extensive manufacturer testing.
Indeed one manufacturer claims that of the 10’s of thousands shipped, none have ever
come back!
Figure 7, Optical Splitter
If we accept this claim, and certainly anecdotal industry feedback backs it up, with
FITs of between 1,000 and 775 being suggested, then a case can be made that the
splitter itself does not need to be field tested for correct transmission performance.
Some confirmation testing will of course be required to confirm that the correct
splitter has been connected to the correct head-end fibre. This can be as simple as
injecting light from the Head-End and confirming the optical splitter output level at a
specified port, say the first or last port is within specifications.
The risk is that of post production connector end face contamination. This is typically
caused by electrostatic dust attraction and plastic outgassing from the protective end
cap. It is easily mitigated by adherence to standard inspection and cleaning
procedures whenever the connector ports are accessed.
Typical splitter losses and output port uniformity are shown in Table 5 below.
Splitter
Ratio
4
8
16
32
64
Typical Loss dB
6.7
9.8
13.2
17.9
21
Output
Uniformity dB
0.5 – 0.9
0.8 – 1.2
1 – 1.7
1.3 – 2
2
Table 5, Typical Optical Splitter losses
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9.2.3 LCP to NAP
This cable runs from the Local Convergence Point (LCP) to all Network Access
points (NAP) in the PON network, whilst there are many ways it can be built. Typical
characteristics are;
• High fibre count. From 32 to over 2,000.
• Typically 100 m -5 Km in length. Presumably not much splicing / jointing, or
the splitter would be placed elsewhere?
• Individually these fibres carry the least revenue. A failure of one fibre will
normally affect one customer.
• Many fibres may be unused for a relatively long time, due to vacant blocks,
estate rollout timings, an all mobile phone household, etc
• Cable sections may be joined by traditional splicing, connectorised techniques
or a combination of both.
LCP
NAP
Splitter
2 Way LTS with ORL
Figure 8, Testing LCP to NAP
Traditional both way OTDR and LTS testing on every fibre may be considered to be
excessive when compared to individual revenue and designed QoS.
It is also expected practice that on odd occasions where a fibre is found to be not
working, an alternative fibre will be used, rather than repair. This implies that spare
access ports are to be provisioned in several of the NAPs. What is your acceptable %
of faulty fibres in the cable?
Possibly, a sample test methodology could be considered at a later stage. In which
case, should any test fail, testing can be escalated as required using standard
techniques.
Where both connector ends available:1. Using a LTS (possibly with ORL), both way link loss of all connectorised
NAP ports at a minimum of two wavelengths.
• Testing at two or more wavelengths preferred. Else test at one higher
wavelength.
Advantage
• Minimises future rework
• Section loss positively identified.
• Fibre transpositions will be found.
• Bad splices will be found.
• Faulty connectors and through-connects/adaptors will be found.
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•
•
•
Bending violations will be found.
Any cable sections not yet provisioned to NAPs will not be tested.
A matter of workforce timing. It is generally cheaper to fix faults whilst the
cable install crew are not yet redeployed rather than have to recall them.
Disadvantage
• Increased cost
Where only LCP connector end available:This may occur with for instance a delayed estate rollout.
1. OTDR test 1 way from LCP towards NAP
• Testing at one wavelength recommended.
Advantage
• Costs are minimised.
Disadvantage
• If there are any fibre transpositions in the distribution cable, they may not be
located.
• Continuity through to the NAP not positively identified.
• Section loss not positively identified.
• Restoration activities as a result of unidentified cable faults may adversely
affect customer services and brand name recognition.
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9.2.4 Drop Cable
This cable runs from the NAP to the customers Optical Network Terminal (ONT).
Deployment distances are small being typically less than 100m. Over such distances,
the fibre loss is negligible, connector losses dominate.
Loss, 1 way
Figure 9, NAP to ONT Drop Cable
In recent years, spurred by the desire to minimise OPEX & CAPEX, there has been
considerable development in drop cable design, including the introduction of a new
ITU category G.657 to cater for them.
Recent innovations include;
1. Bend violation resistant cable sheath designs.
2. Factory assembled & tested pre-terminated cable.
3. Bend optimised fibre. Refer Figures 10 below & 3 above.
4. Environmentally hardened optical connectors
5. Connector pulling shrouds
6. Blown/Pushed fibre
Figure 10, G.657B Bend Optimised Fibre
Figure 11, Typical FTTx Hardened Connector and Pulling Shroud
As a result of these innovations and of course the short distances involved, several
network owners are now using optically unskilled workers for the drop installation.
They are also asking ‘do we need to ‘test the drop cable?’ Unfortunately there is no
hard and fast answer.
From of a testing viewpoint, the several cable designs can be considered as preconnectorised and other styles.
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The relative advantages and disadvantages of the two types are discussed below.
Pre-terminated off-the-shelf;
Advantages
1. Factory assembled and tested prior to delivery.
2. Fast installation.
3. Minimal skill required.
4. Cost of tooling low.
5. Easily replaced, especially if underground installation in conduit.
6. Minimal testing.
Disadvantages
1. Increased cable inventory costs as must keep several standard lengths in stock.
2. Need for provision of excess drop cable storage space.
3. Connectors more prone to contamination/damage during cable installation.
4. Proprietary hardened connector styles may lock out preferred vendors.
5. Power meter adaptors and patch leads may not be readily available for some
proprietary connector styles.
Figure 12, Typical connectorised drop cable
Other styles;
These include, blown fibre, pushed fibre, factory terminated one end and of course the
traditional roll of drop cable on a drum.
Advantages
1. Minimises cable inventory costs.
2. Traditional termination technique.
3. Excess cable storage space not required.
4. Minimises connector contamination and damage issues, since the connector is
not present during hauling.
Disadvantages
1. Slower installation.
2. Higher skill required: splicing and testing
3. Increased tooling costs: splicer usually required
4. Longer to replace.
5. Testing required.
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Testing Options
1. Don’t test
The advantage here is that coupled with factory pre-terminated cable it permits
use of a lower skilled workforce who may not be able to afford, let alone know
how to use optical test equipment.
The risk is that a faulty installation may cause rework & potential customer
dissatisfaction. Rework costs can be minimised by ensuring drop cable
installation techniques which ease cable replacement.
2. Pre-PON Power up Test
It is recommended that the following testing methodology be considered.
• Light source power meter test 1 way
• Test at minimum of one wavelength.
• Test at 1550 nm or 1625 nm.
• Do not test at the upstream 1310 nm wavelength.
• Test in direction NAP to ONT
By testing at 1550 or 1625 nm, any major bending violations will be highlighted. By
testing in the downstream direction, the possibility of inadvertently injecting light in
the upstream direction on a live PON and potentially bringing down the PON is
minimised.
3. Post PON Power up test
As discussed earlier in Section 7, post power up, testing can be achieved using a
wavelength selective meter to check that the incoming 1490 & 1550 nm wavelengths
power levels are within system specification. If they are then it is sufficient. Testing
thus resolves to:• Power up ONT & check for green sync light
• Wavelength selective meter
• Power at 1490 & 1550 nm within specification?
4. PON Headroom verification
A quick and economical way to verify that there is sufficient transmission system
headroom to ensure reliable operation is to temporarily fit a fixed in-line attenuator to
the ONT.
For instance if the system designed headroom is 3 dB, then this can be readily verified
by inserting this amount into the circuit. If the ONT holds up then this is sufficient, if
not further investigation is required.
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9.2.5 End to End Loss Testing / Verification
If the sections of the systems were tested correctly, then end to end loss can be
deduced by adding the measurements together, with possibly a small allowance for an
untested drop cable.
End to end loss can also be deduced indirectly after system power up by using a
wavelength selective meter to measure the power delivered at any point, preferably
prior to customer connection.
10 CONCLUSION
Installed cable plant quality in the PON customer distribution infrastructure can be
less than that for traditional P2P services with minimal QoS impact.
With existing SMF 28 or similar fibre, insertion loss testing at higher wavelengths,
with a source and meter or loss test set, can be nearly as effective as OTDR testing in
identifying if there is a serious cable bending problem. Lasers at the out of band
wavelength of 1625 nm are particularly good in this application and are now modestly
priced.
OTDR’s are of limited use during the build phase of a PON network and this usage is
likely to decrease, in the customer cable distribution network, as the development and
rollout of G.652D / G.657 bend tolerant fibres for PON networks continues.
11 REFERENCES
The following sources are acknowledged in the writing this document.
1
2
3
4
5
6
7
8
9
10
11
12
13
FTTH Council – Definition of Terms
http://www.ftthcouncil.org/documents/678042.pdf
Guaranteeing service availability in optical network design. Dominic A Schupke, Siemens
ITU-T Recommendation G.652. Characteristics of a single-mode optical fibre and cable
ITU-T Recommendation G.657. Characteristics of a Bending Loss Insensitive Single Mode Optical Fibre
and Cable for the Access Network
ITU-T Recommendation G.983
ITU-T Recommendation G.984
ITU-T Recommendation L.40, Optical fibre outside plant maintenance support and testing system
Mesh-based Survivable Transport Networks: Options and Strategies for Optical, MPLS, SONET and
ATM Networking. By Wayne D. Grover. ISBN-13: 978-0-13-494576-7
Novel Redundancy Design Methodology for an Optimal PON Protection Architecture. Presentation at
2007 BICSI Winter Conference, U.S.A.
OFS. AllWave FKLEX ZWP Fiber White Paper
http://www.ofsoptics.com/resources/AllWaveFLEXFiberWP-web.pdf
Optical Time-Domain Reflectometry, by Duwayne Anderson & Florian Bell, Chapter 10.
Unavailability Analysis of Long-Haul Networks. IEEE Journal IEEE Journal on Selected Areas in
Communications, Vol 12, Jan 1994
Video-optimized fiber is all about the bends, Lightwave July 2007, Author(s) : John George David
Mazzarese ,
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12 RECORD OF ISSUE
If you have any suggestions for improvement to this document, please contact the
authors at Kingfisher International.
Issue No.
1
2
Issue Date
May 2007
April 2008
Nature of Amendment
Expanded information on G.657. Added use
of fixed attenuators for headroom verification.
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