FTTP Measurements Introduction

FTTP Measurements Introduction
White Paper
FTTP Measurements
An in-depth review of Fiber-to-the-Premises technology and its associated optical testing
Introduction
Broadband, including high-speed internet, always-on video, always-on data,
and voice connectivity, represents the next phase in the evolution of telecommunications requiring new network technologies. The most relevant example
of new network technologies is Fiber-to-the-Premises (FTTP). In most cases,
FTTP requires the implementation of Passive Optical Networks (PONs). By
eliminating the need for regenerators and active equipment components of
typical fiber networks, PONs significantly reduce the costs associated with
both installation and deployment. Therefore, the deployment and usage of
Passive Optical Networks (broadband) will significantly impact the global
competitiveness of nations and businesses in the future.
With the enormous interest of consumer broadband technologies such as
ADSL and cable modem, residential broadband deployment has grown steadily
over the past few years. However, the most powerful physical media enabling
unlimited bandwidth capacity is the singlemode fiber, making its use
mandatory for broadband services. Therefore, Fiber- to-the-Premises (FTTP)
technology has been developed and deployed in response to several residential
access market drivers, including the following:
– The internet explosion.
– The increased competition in the market as well as the actions of federal
and state regulators.
– The declining costs of optical equipment, making the cost of FTTP
similar to alternative available options.
– The significant savings in maintenance costs associated with FTTP as
compared to other technologies.
– A technology life cycle that dictates both a need to deploy the right
technology at the right time and the need to future-proof existing
networks. Fiber has a life expectancy of about 100 years compared to
copper with a life expectancy of 20 to 30 years.
In order to make FTTP a viable solution, the cost of the optical network
structure must be as low as possible. It must also make the use of active
components such as regenerators unnecessary because they require power,
adding to the complexity of the network and increasing maintenance issues. In
most cases, a FTTP network is a Passive Optical Network (PON), consisting of
only fiber and passive components such as splitters. Once splitters are installed,
they do not require any further maintenance.
The purpose of this document is to describe FTTP technology as well as its
requirements as far as physical layer testing is concerned.
WEBSITE : www.jdsu.com
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Section 1: The Standards and Status of Passive
Optical Networks
Although the concept of FTTP was invented by British Telecom in 1982, tremendous effort has been
made in the past 10 years to standardize this technology. In the early 1990s, initial trials were conducted
using FTTP, and JDSU actively collaborated with key providers including France Telecom, Deutche
Telecom (OPAL system), and British Telecom (OTIAN system) to develop testing procedures for FTTP.
At that time, though, the components required for the ONTs and OLTs were too expensive for a large
scale deployment. Therefore, the project was halted even though it showed great promise.
In parallel, a group of seven telecom providers called Full Services Access Networks (FSAN) met in 1995
to develop common system specifications to accelerate the deployment of optical access systems. They
were responsible for developing the specifications that are now known as the ITU-T G.983
recommendations.
While BellSouth (US) had an FTTP-like offering, NTT (Japan) was the first company, in 1999, to
provide a full-scale FTTP deployment in Japan. Presently, Verizon and SBC (US) are heavily investing
research and development dollars to provide FTTP to their customers as well. In Europe, providers,
who currently provide point-to-point networks close to the customer premises, are still in the
evaluation phase before embarking on a large-scale deployment using a point-to-multipoint network
architecture.
There are several different types of Passive Optical Networks (PONs) including Broadband Passive
Optical Network (B-PON) using mainly ATM, Gigabit Passive Optical Network (G-PON), and
Ethernet Passive Optical Network (E-PON). While B-PON technology is the current offering, G-PON
and E-PON are under investigation. The specifications and characteristics of each technology are listed
in Table 1. The International Telecommunications Union (ITU) and the Institute of Electrical and
Electronic Engineers (IEEE) are two organizations who are currently in the process of defining the
standards for PONs.
Span
Maximum insertion loss
Maximum number of branches
Bit rate (Mbps)
Wavelengths
Traffic mode
Architecture
Video overlay
Applicable standard/status
Chipset support
Upstream burst time
B-PON
G-PON
E-PON
20 km
60 km max, 20 km differential
15/20/25 dB
64
Down: 1244, 2488
Up: 155, 622, 1244, 2488
Down: 1480-1500 nm
Video at 1550 nm
Up: 1260-1360 nm
ATM, Ethernet,TDM
Asymmetric or Symmetric
Yes
ITU-T G.984.x/will be completed in 2004
First prototypes
Guard: 25.6 ns
Preamble: 35.2 ns (typical)
Delimiter: 16.9 ns
10 km today, 20 km planned
15/20 dB
16
Down: 1244
Up: 1244
Down: 1490 nm
Up: 1300 nm
32
Down: 155, 622, 1244
Up: 155, 622
Down: 1480-1500 nm
Video at 1550 nm
Up: 1260-1360 nm
ATM
Asymmetric or Symmetric
Yes
ITU-T G.983.x/complete
Available
Table 1: The specifications and characteristics of B-PON, G-PON, and E-PON technologies
Ethernet
Ethernet
No
IEEE 802.11/will be completed in 2004
First prototypes
Laser turn on/off: 512 ns (max)
AGC setting and CDR look: 400 ns
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The major recommendations and standards for PON technology are listed in Table 2.
As PON technology is in its infancy, new standards will most likely be defined in the coming years. In
addition, other technologies are being reconstructed using PON technology and architecture. They
include:
– The use of Video over IP (IP Video) at the same wavelength for data and voice, making the
requirement for video at 1550 nm no longer relevant.
– The use of WDM technology on a PON network. In the case of Wavelength Division Multiple
Access (WDMA) technology, each customer will be accessed using their own wavelength.
This requires the addition of a WDM element at the splitter location.
Standards
Description
ITU-T G.982
Optical access networks must support
services up to the ISDN primary rate or
equivalent bit rate.
A broadband optical access system
based on Passive Optical Networks
(PONs).
ITU-T G.983.1
ITU-T G.983.2
ITU-T G.983.3
ITU-T G.983.4
ITU-T G.984.1 (in progress)
IEEE 802.3ah (in progress)
An ONT management and control
interface specification for ATM PON.
A broadband optical access system
with increased service capability by
wavelength allocation.
A broadband optical access system
with increased service capability
using dynamic bandwidth
assignment.
General characteristics of Gigabitcapable Passive Optical Networks
(G-PON).
Ethernet in the First Mile (EFM)
Table 2: Major recommendations and standards for PON technology
Content
Specifies an optical access system
with symmetrical line rates of 155.520
and 622.080 Mbps and asymmetrical
line rates of 155.520 Mbps upstream
and 622.080 Mbps downstream (B-PON).
Adds an additional wavelength band
to G.983.1 to enable the distribution
of unidirectional or bidirectional video
broadcast or data services.
Specifies enhancements to G.983.1
using dynamic bandwidth assignment.
Specifies an optical access system with
up to 2.488 Gbps symmetrical line rates.
E-PON
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Section 2: FTTP Technology
PON Signals and Network Structure
The following section describes a typical passive optical network (PON). A PON can also be called an
Optical Access Network (OAN).
In a standard FTTP system (Figure 1), as defined by ITU-T G.983.1, the Central Office (CO) is
interfaced to the public switched telephone network (PSTN) using DS-X/OC-Y signals and is
connected to ATM or Ethernet interfaces. Data and voice signals use the 1490 nm wavelength for
downstream signals and the 1310 nm wavelength for upstream signals. Video services enter the system
from a cable television headend or from a satellite feed. Video signals currently use the 1550 nm
wavelength (downstream signals only).
Central Office or Remote Terminal
Fiber Distribution
Packet
Optical
Line
Termination
Passive Outside Plant
IP Data and Voice
Upstream and Downstream
Voice
IP Data
WDM
Video
Splitter
Video
Optical
Line
Termination
Video Wavelength
Optical
Network
Termination
•
•
•
•
Passive outside plant – signal is passively split
Coarse WDM supports three wavelengths – 1490/1310/1550 nm
Two wavelengths for IP data and voice – separate 1490 nm downstream and 1310 nm upstream wavelengths
Optional 1550 nm third wavelength – one way broadcast for analog and digital RF video
Figure 1: A standard FTTP system
In a typical FTTP network (Figure 2), the signals are combined into a single fiber using WDM
techniques at the CO with an optical line terminal (OLT). At the CO, a fiber distribution frame (FDF)
integrates a number of OLTs together with splicing trays and connectors, which, in turn, connect the
OLTs to the fiber network. Because the CO may use old optical networks, the type of connectors at the
OLTs is most often Ultra Polished Connectors (UPC).
Because customers may be located far apart along the feeder, there may be splice cases located in aerial
or underground environments. In this case, one cable may be divided into a number of fibers going to
different directions. A feeder distance is typically 30,000 ft (10 km).
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Terminal
Terminal
FDF
OLT
FDH
Splitter
Central Office
Terminal
ONT
ONT
Customer
Premises
ONT
ONT
Figure 2: A typical FTTP network
The signals are then transmitted along the complete feeder to fiber distribution hubs (FDH), where the
signal distribution occurs through the use of passive optical splitters. There is a maximum of 32
branches per splitter and usually a maximum of 10 splitters per FDH. Therefore, the FDH houses
multiple splitters as well as splicing trays and connectors. No power is required at this location because
all of the components are passive components. This hub is also called the Primary Flexibility Point
(PFP) or the Fiber Cross Connect (FCC). If there are different splitters along the cable, then the other
splitters are located at a Fiber Distribution Panel (FDP).
After the FDH in the network, there is a distribution cable. One fiber is dedicated to each customer.
Once a customer has been designated, the splitter is connected to the fiber of this specific customer.
Before a customer is designated, the fibers coming from the distribution cable and from the splitters are
left open. They are positioned in a ‘‘parking lot’’ and are not connected together. Parking lot is an
industry term used to define a staging or waiting area for connectors. In order to satisfy the constraints
of the ORL, the type of connectors used here is Angled Polished Connectors (APC).
The cable is then distributed along to the customer locations. Access terminals close to the customer’s
premises enable some fibers of the cable (usually 4 to 12) to branch out of the distribution cable and
be terminated with the use of a splice tray. The type of connectors used here is APC. Most of the time,
the other fibers go through the splice tray using slack loops. A distribution distance is typically of 20,000
ft (6 km).
From the access terminals, a drop cable of a maximum of a several hundred feet (10-300 m) spans to
the customer premises where the Optical Network Terminal (ONT) is located. The type of connectors
used here is APC.
At the ONT, the optical signal is converted into an electrical signal using an optical-electrical converter
(OEC). This converter then splits the signal into the services required by the end user. The various
interfaces include RJ-11 twisted pair jacks for Plain Old Telephone Service (POTS), Category 5/6 RJ-45
10/100/1000 Base-T Ethernet jacks for high-speed data (IP interface), and 75 ohm coaxial ports for
CATV and Digital Broadcast Services (DBS). The 75 ohm coaxial port is connected to the set top box,
which is connected to the TV monitor.
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FTTP Optical Structure
While providing fiber directly to the home (FTTH) is a very attractive offering for customers, it is not
always the most cost-effective solution for providers. Therefore, for cost-saving alternatives, providers
will try to utilize their existing plants to deploy different types of fiber architectures. Fiber-to-thePremises (FTTP) is a summary of several different offerings. The various types of FTTP fiber
architecture are shown in Figure 3.
Operating
System
Q3
Service Mode
Optical Distribution Network
Passive Optical Splitter (G.671)
Internet
ONT
OLT:
ONU:
ONT:
SNI:
NT:
Optical Line Termination
Optical Network Unit
Optical Network Termination
Service Node Interface
Network Termination
FTTH:
Fiber-to-the-Home
Optical Fiber (G.652)
Leased Line
FTTB:
Fiber-to-the-Building
ONT
Frame/Cell
Relay
OLT
ONU
Telephone
Interactive
Video
ONU
SNI
B-PON
NT
NT
FTTC:
Fiber-to-the-Curb
FTTCab:
Fiber-to-the-Cabinet
VDSL
Figure 3: The various types of FTTP fiber architecture
For new homes and new residential areas where there is no network available yet, providers will provide
complete Fiber-to-the- Home (FTTH) technology. These new areas are termed ‘‘greenfield’’
applications.
For ‘‘brownfield’’ or overlaid/overbuilt applications, many providers will end the fiber before the
customer premises in order to continue their attempt to leverage their existing network infrastructure.
Stopping just short of the customer premises allows providers to bypass the cost of pulling fiber under
driveways. Therefore, the following technologies have been defined:
(1) Fiber-to-the-Cabinet (FTTCab) - The fiber goes to a street-side cabinet or digital loop carrier
(DLC) and uses ADSL2 technology to access customers. Typical distance to the customer
premises is up to 12,000 feet (4 km).
(2) Fiber-to-the-Node/Neighborhood (FTTN) - The fiber goes to a large street-side cabinet or
optical network unit (ONU) and uses ADSL2 or ADSL2+ technology to access customers. FTTN
typically serves about 200 residential or small business customers with a radius of 3,000 to 8,000
feet (1 to 2.5 km).
(3) Fiber-to-the-Curb (FTTC) - The fiber goes to an outdoor cabinet beside homes or office
buildings approximately 1,000 to 2,000 feet (300 to 600 m) from the customer premises. FTTC
may use VDSL technology to access customers.
(4) Fiber-to-the-Building/Basement/Business (FTTB) - The fiber goes to a building. FTTB is the
same as FTTH except that it serves multiple customers. It can also serve multi-dwelling units as
opposed to single-family units for FTTH.
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Figure 4 shows the maximum downstream speed as a function of loop length for ADSL2, ADSL2+, and
VDSL technologies.
50
VDSL
45
ADSL2+
Max. Downstream Speed (Mbps)
40
ADSL2
35
30
25
20
15
10
5
0
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
Loop Length (k ft)
Figure 4: Maximum downstream speed as a function of loop length
For large residential areas, as is the case for the first installations of FTTP in the United States, the
optical distribution network will use only 1:32 splitters. For smaller residential areas, the splitters can
be spread out in order to be closer to other smaller areas using a single FTTP. There may be a 1:4 splitter
at one FDH followed by four 1:8 splitters at other FDHs, or there may be a 1:8 splitter followed by eight
1:4 splitters.
An FTTP network can follow a star/tree (standard), ring, or bus topology, with the possible use of active
components depending on the locations of the customers. However, an FTTP network can also be a
simple point-to-point network (P2P). This is the case in Japan, for example, where there is a highdensity population and where Ethernet switches or active components are used instead of splitters.
A Point-to-Multipoint Network
FTTP, as opposed to a standard P2P optical fiber network, is usually a point-to-multipoint network
(P2MP). This means that although there is one fiber at the OLT, the other end of the distribution
network can have up to 32 fibers under the same optical network. This is because FTTP uses a passive
optical component called a splitter (Figure 5).
Input
Optical Splitter
02
I1
Outputs
03
Figure 5: A schematic diagram of a splitter
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The splitter allows for one port at one end and up to 32 ports at the other end. Therefore, a signal sent
at one end can be possibly seen by 32 customers simultaneously. This application is ideal for video
distribution. For data and voice, the use of Time Division Multiple Access (TDMA) enables customers
to receive and send exactly what they want, without knowing what other customers are receiving and
sending. In addition, other technologies are currently under investigation. The most attractive alternate
technology is Wavelength Division Multiple Access (WDMA) in which a particular wavelength is
dedicated to each customer.
The main benefit of a splitter is that it is a passive component, requiring no maintenance and no power
activation. The main drawback of a splitter is its high rate of insertion loss. The insertion loss is defined
as 10log(1/n), where n is the number of ports (2 to 32). Table 3 shows the expected insertion loss that
corresponds to a specific number of ports.
Number of ports
Insertion loss
2
4
8
16
32
3 dB
6 dB
9 dB
12 dB
15 dB
Table 3: Expected insertion loss based on number of ports
This insertion loss factor has a high impact on the distances associated with FTTP, limiting them to a
distance of approximately 20 km (30 dB maximum network insertion loss). This distance limit also
takes into consideration that low-cost components are often used at the OLT and ONT.
Video Signal Evolution
Because the quality of a video image is key, video signals are very sensitive. Video signals are, therefore,
amplified with the use of an optical amplifier. Power levels are in the range of +17 dBm to +23 dBm
compared to +0 dBm to +5 dBm for data/voice signals. Analog signals are also very sensitive to
reflections coming from the network, requiring the use of APC connectors with a low reflectance value
of -60 dB.
Although the ITU-T G.983.1 standard defines the use of analog and digital video using the 1550 nm
wavelength, one alternative solution is to offer Video over IP. This new technology is currently under
evaluation and may simplify network topology. In this case, video signals will be integrated into the data
structure transmitting at 1490 nm. This technology may be ready for large-scale deployment in 2005.
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Section 3: Fiber Installation and Testing
Fiber installation and testing is usually performed by fiber construction crews.
Fiber Installation
For an FTTP application, the optical cable, containing the fibers, is laid using one of three methods.
(1) Direct burial installation - The optical cable is placed underground in direct contact with the
earth and rocks that make up the surrounding soil. It is inserted into the ground by either
creating a ditch or by simultaneously plowing a slot and inserting the cable. This is the most
expensive method and is only used for high-density population areas.
(2) Duct installation - The optical cable is placed inside a pre-installed duct that runs between
access points.
(3) Aerial installation - The optical cable is placed on poles or towers, allowing routing of the optical
transmission path above ground. This method is usually used for network overlaid (brownfield)
applications.
Once the cable is laid on the feeder, splices can be performed on the fiber, either to join two fibers or to
divide one large cable into multiple smaller cables going to different locations. Splicing is performed on
a splice enclosure that can be located either on an aerial vault or underground in a manhole. In order
to test and qualify the splicing, an OTDR is used from the central office (Figure 6).
The cable, containing the fiber, is laid (aerial or underground). The fiber is spliced and tested with an OTDR. This procedure can be sub-contracted.
Central Office
OTDR
OTDR
32 fibers/splitter
up to 8 splitters
Splice
points
4, 6, or 12 fibers
per terminal
16-50 kft
6-30 kft
5-15 km
2-10 km
Customer
Premises
Figure 6: The fiber installation process
For a distribution network, splices are performed in the same way, and an OTDR is also used to qualify
the splices.
Cables and fibers can be of any type, depending on the application and the density, including loose tube
or ribbon fiber.
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Fusion splicers have been specifically designed for FTTP applications. They are cost-effective
instruments that are much lighter, battery-operated, accurate, and quite simple to use. However, fiber
installation is still a complex and time-consuming process. It requires managing the fiber in the splice
tray, cleaning and cleaving the fiber, splicing the fiber, and then closing the splice enclosure. Table 4 lists
the tasks involved in fiber installation.
1
2
3
Task
Objective
Lay the cable, containing the fibers,
for the feeder and distribution network.
Position splice enclosures and splice
fibers for the feeder.
Position splice enclosures and splice
fibers for the distribution network, if
necessary.
Provide cable between the CO
and the customers.
Perform continuity along the feeder.
Test Tool(s)
Fusion splicer
and an OTDR
Fusion splicer
and an OTDR
Perform continuity along the
distribution network.
Table 4: Tasks involved in fiber installation
Frame Installation and Connection
The second step of the FTTP process is frame installation. There are several different types of frames
installed along the FTTP network (Figure 7).
The first frame usually installed is the Fiber Distribution Hub (FDH). The FDH is where the splitters
will be installed. This frame consists of an outside cabinet. At this location, all of the fibers coming from
the CO are spliced to the input of the splitters. An OTDR measurement is then performed from the CO
to verify the splice quality. All connectors coming from the output of the splitters are then put into the
parking lot. Next, all of the fibers going to the distribution network are also spliced to a pigtail to be
terminated. An OTDR measurement is then performed from the FDH to verify the splice quality. All of
the distribution network connectors are also put into the parking lot.
The second frame usually installed is the terminal. The terminal is located close to the customer
premises. This frame consists of a splice enclosure located either on an aerial vault or in a manhole. A
set of fibers (usually 4, 6, 8, or 12) coming from the cable is extracted and spliced in order to be
connected. An OTDR measurement is then performed from the FDH to verify the splice quality.
The Fiber Distribution Hub (FDH) is installed first. Then, the terminal and the Fiber Distribution Frames (FDFs) are installed. The splitter is spliced, and
the CO and terminal are spliced and connected. Testing is performed with an OTDR during splicing. Connectors at the splitter location are left open.
4, 6, or 12 fibers
Terminal 4, 6, or 12 splices
Terminal
FDH
Central Office
Splitter
OTDR
FDF
OTDR
Splice
points
Splice points
Customer
Premises
32 fibers/splitter
1 splice/FTTP
Terminal 4, 6, or 12 fibers
Figure 7: The frame installation and connection process
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The third frame usually installed is the Fiber Distribution Frame (FDF). It is located at the CO. This
frame consists of a cabinet. Like the other frames, the fiber is spliced to a pigtail in order to be connected
to the patch panel. An OTDR measurement is then performed from the CO to verify the splice quality.
For large networks, other FDFs may be located on the feeder in order to distribute the different cables.
The same process applies to those frames.
At the end of the frame installation and connection process, the feeder and the distribution network are
complete and are ready to provide the Optical Access Network (OAN). Table 5 lists the tasks involved
in frame installation and connection.
Task
1
Objective
2
Install the FDH and the splitter
at the hub.
network.
Install the terminals.
3
Install the FDF at the CO.
Test Tool(s)
To prepare for continuity between
the feeder and the distribution
Fusion splicer
and an OTDR
To prepare for continuity between
the distribution network, the drop
cable, and the ONT.
To prepare for continuity between
the OLT and the feeder.
Fusion splicer
and an OTDR
Fusion splicer
and an OTDR
Table 5: Tasks involved in frame installation and connection
Acceptance Testing of the Optical Access Network
This process serves to perform an acceptance test of the complete OAN. Insertion loss and Optical
Return Loss (ORL) measurements of the feeder and the distribution network are obtained. Most
technicians use a bidirectional loss and ORL test set to perform this acceptance test. For simplification
purposes, this test set is termed “tester” throughout the remainder of the document. Alternatively, an
OTDR can also be used.
The technician can perform automatic insertion loss and ORL measurements with the tester. ORL
testing is mandatory if analog video at 1550 nm is used due to the sensitivity of this technology to
reflectance. In order to test the feeder, one tester is positioned at the CO location and is connected to
the network by the technician. Then, the same technician leaves this tester connected and goes to the
FDH. There, a second tester is connected to one connector of the splitter. An automatic measurement
can then be obtained (Figure 8). The benefit of the tester is that it can both identify the fiber and also
perform both bidirectional loss and ORL tests, increasing the accuracy of the measurements. Some
technicians test all of the connectors going out of the splitter. Others only test a set of connectors (four,
for example). Optionally, an OTDR may be used for testing at the CO as well as testing from the FDH.
Acceptance testing between the CO and the splitter is performed with a bidirectional loss test set
(LTS) to measure insertion loss and ORL. An OTDR may also be used for this testing.
Terminal
OTDR
FDF
LTS
FDH
Terminal
Splitter
Central Office
Customer
Premises
OTDR
LTS
Terminal
Figure 8: Acceptance testing between the CO and the splitter
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The same tests are then performed for the distribution network. For this process, it is critical to
accurately identify all of the fiber numbers because the terminal locations may be scattered. Typically,
one technician stays at the FDH to connect one tester to one fiber while a second technician travels
along to the terminals to connect the other tester to the other end of the fiber (Figure 9). A Visual Fault
Locator (VFL), which can display a visible red light up to 15,000 ft, is used to easily identify the tested
fiber at the other end. Once the two testers are connected, the required measurements can be obtained.
This process is repeated for all the fibers on the network. Optionally, an OTDR may be used for testing
the distribution network.
Acceptance testing between the splitter and the terminals is performed with a
bidirectional loss test set (LTS) for all of the fibers
Terminal
LTS
LTS
FDH
Splitter
Central Office
Terminal
FDF
Customer
Premises
LTS
Figure 9: Acceptance testing between the splitter
and the terminals
LTS
Terminal
Table 6 lists the tasks involved in the acceptance testing of the OAN.
Task
Objective
1
Insertion loss/ORL testing of the feeder.
Acceptance testing of the feeder.
2
Insertion loss testing of the distribution network.
Acceptance testing of the distribution network.
Test Tool(s)
Two bidirectional loss and ORL test
sets (or an OTDR)
Two bidirectional loss and ORL
test sets (or an OTDR)
Table 6 lists the tasks involved in the acceptance testing of the OAN.
Why Isn’t Overall Acceptance Testing Performed?
For a traditional fiber network, an overall end-to-end acceptance test is always performed. This is not
the case with FTTP because, until a customer has been established, the connectors at the splitter
location are not connected. An overall end-to-end acceptance test would only be possible if all of the
customers were connected at exactly the same time for a given FTTP network. But most of the time,
only a selection of customers are activated initially, and others are added afterwards. This makes overall
acceptance testing impossible because it would require disconnecting the OLT for the existing
customers.
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Section 4: FTTP Equipment Installation and Testing
FTTP equiment tests are usually performed by CO personnel for optical line terminals (OLTs) and by
technicians in charge of the customer premises for optical network terminals (ONTs).
Once fiber construction is complete, all of the active equipment (OLTs and ONTs) can be installed.
OLTs are installed at the CO, and ONTs are installed at the customer premises once customers have
been established (Figure 10).
At the customer premises, the drop cable connecting the terminal and the ONT is also installed, using
either aerial or underground technologies. The drop cable is usually a pre-defined connected cable that
does not require further testing during its installation. Once the drop cable is installed, the technician
goes to the terminal and connects the drop cable to the terminal. He then goes to the FDH to connect
the fiber, which was previously stored in the parking lot, for this specific customer.
When this procedure has been completed, technicians perform bidirectional loss and ORL testing again
by connecting one tester to the FDH and the other tester to the ONT location. Although more costly,
an alternative solution is to use an OTDR from the ONT location. Table 7 lists the tasks involved in
FTTP equipment testing.
Once customers have been established, ONTs and OLTs are installed. Next, the drop cable is installed. When the ONTs and OLTs are activated,
testing with a selective power meter (PM) is performed at the ONT and OLT locations. At the OLT location, ORL testing may be performed.
Terminal
Central Office
Splitter
FDF
Terminal
FDH
OLT
Drop cables
<1900 ft (<30 m)
LTS
Terminal
ONT
Figure 10: FTTP equipment installation and testing
ONT
ONT
Customer
Premises
PM
ONT
Photonic Layer Testing of the OLT and ONT Signals
Once the OLT and ONT can be activated, basic photonic layer testing is performed. This testing is
accomplished by first activating the 1490 and 1550 nm wavelengths of the OLT as well as the 1310 nm
wavelength of the ONT and then by measuring the power level (usually at the ONT location). Both the
1490 and 1550 nm wavelengths can be easily activated by the OLT, making measurements at those
wavelengths easy to obtain.
1
Task
Objective
Insertion loss testing of the distribution
network with the drop cable.
Acceptance testing for the
distribution network with drop cable.
Table 7: Tasks involved in FTTP equipment testing
Test Tool(s)
Two bidirectional loss and ORL
test sets (or an OTDR)
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This is not the case for the 1310 nm wavelength, though, because most FTTP equipment providers do
not allow the activation of the 1310 nm signal only. The 1310 nm wavelength, provided by the ONT, is
effectively activated by the 1490 nm signal coming from the OLT. Therefore, a specific power meter,
called a ‘‘through mode’’ power meter, is used (Figure 11). The through mode power meter is a twoconnector power meter where the fiber coming from the OLT is connected to one port of the power
meter and the fiber coming from the ONT is connected to the other port of the power meter. The
through mode power meter connects the OLT to the ONT and, at the same time, performs
measurements of all of the wavelengths. In addition, because 1310 nm signals are emitted in a burst
mode most of the time, specific power measurements are performed. For example, burst measurement
is performed as opposed to CW measurement, which is what a classical power meter measures.
Three Wavelength Through Mode Power Meter
1490/1550 nm
1490/1550 nm
OLT
ONT
1310 nm
1310 nm
Meter 1
Meter 2
1310 nm
Selective
1490/1550 nm
Figure 11: Physical layer testing using a through mode power meter
Some technicians only measure the 1490 and 1550 nm wavelengths, making the assumption that these
measurements validate the fiber quality. If the 1310 nm wavelength is not transmitting to the CO, then
the ONT is in failure and requires changing.
Additionally, if video is using the 1550 nm transmission with analog signals, then ORL measurements
of the feeder are performed from the CO following the OLT connector.
Table 8 lists the tasks involved in physical layer testing of the OLT and ONT signals when 1550 nm video
is used.
Task
Objective
1
Power level measurements at the OLT location.
Verify the levels at the ONT with pass/fail indication.
2
ORL measurements from the OLT.
3
Power level measurements at the ONT location.
Verify the ORL in the case of analog video transmission
with pass/fail indication.
Verify the levels at the ONT with pass/fail indication.
Test Tool(s)
1.4/1.5 µm selective power meter
ORL meter
1.3/1.4/1.5 µm power meter with through
mode or 1.4/1.5 µm selective power meter
Table 8: Tasks involved in physical layer testing of the OLT and ONT signals with 1550 nm video
If IP Video technology is used exclusively, then 1550 nm testing becomes irrelevant. This simplifies the
testing tools drastically, as a broadband power meter calibrated at 1490 nm would be the only meter
required. Table 9 lists the tasks involved in physical layer testing of the OLT and ONT signals when IP
Video is used.
Task
Objective
Test Tool(s)
1 Power level measurements at the OLT location.
Verify the levels at the ONT with pass/fail indication.
1.49 mm power meter
2 Power level measurements at the ONT location.
Verify the levels at the ONT with pass/fail indication.
1.3/1.49 mm power meter with through
mode or 1.49 mm power meter
Table 9: Tasks involved in physical layer testing of the OLT and ONT signals with IP Video
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Section 5: FTTP Troubleshooting
Troubleshooting is usually performed by the technicians in charge of the customer premises. If there
are any fiber network issues, then they are handled by the fiber construction crew (Figure 12).
When there is a physical layer failure at a single ONT, the ONT is disconnected and a power measurement is performed.
Terminal
Central Office
Terminal
Splitter
FDF
FDH
OLT
Terminal
ONT
Figure 12: FTTP troubleshooting
ONT
Customer
Premises
ONT
PM
OTDR
ONT
Analyzing ONT Outage When Not All of the ONTs Are Out-of-Service
When few, but not all, of the ONTs are working, this indicates that the OLT and the feeder network are
working properly. The problem, in this case, is probably either along the distribution network or at a
specific ONT location. Therefore, the technician goes to the specific ONTs and tests the power levels at
those locations. There are three possible scenarios:
(1) If the wavelength power levels are correct, then the distribution network is working properly.
Therefore, the ONT is in failure and must be changed.
(2) If the wavelength power levels are incorrect (even after the connectors are cleaned), then the
distribution network is in failure. In this scenario, wavelength power levels are then measured at
the terminal. If the wavelength power levels are correct at the terminal, then the drop cable must
be changed.
(3) Then, if the wavelength power levels are still not correct at the terminal (even after the
connectors are cleaned), the connector at the FDH of this fiber link is disconnected and OTDR
measurements are performed (either from the FDH or from the ONT location). These
measurements will identify the location of the break/bend. Repairs may require the cooperation
between the technicians and the fiber construction crew.
Table 10 lists the tasks involved in analyzing ONT outage when not all of the ONTs are out-of-service.
Task
Action (Pass)
1
Measure power levels at the ONTs and verify connections.
Go to task 2.
2
Measure power levels at the terminaland verify connections.
Change the drop cable.
3
Disconnect the connector on the FDH, perform OTDR
measurements of the distribution network, and verify connections.
N.A.
Table 10: Tasks involved in analyzing ONT outage when not all of the ONTs are out-of-service
Action(Fail)
The ONT is in failure. Change the ONT.
Go to task 3.
Identify the break/bend and repair the fiber.
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Analyzing ONT Outage When All of the ONTs and the OLT Are Out-of-Service
When all of the ONTs and the OLT are out-of-service, the technician goes to the CO first to check that
the OLT is transmitting with the correct wavelength power levels.
If the OLT is not transmitting the correct power levels (even after the connectors are cleaned), then the
OLT is in failure and must be changed.
If the OLT is transmitting the correct power levels, then the fiber network is in failure. First, an OTDR
measurement is performed from the OLT connection to identify the location of the break/bend at the
feeder network. (Note: This is the portion of the OTDR trace just before the splitter trace drop.) Then,
any necessary repairs are made.
If the feeder network measurements are correct, then the problem may be within the FDH. In this case,
the technician goes to the FDH and verifies the connectors and splitters at this location. OTDR testing
is performed to clearly identify the break/bend in the event that is not visible. Then, any necessary
repairs are made.
Table 11 lists the tasks involved in analyzing ONT outage when all of the ONTs and the OLT are outof-service.
Task
Action (Pass)
Action(Fail)
1
Disconnect and measure power levels at the OLTs.
Go to task 2.
The OLT is in failure. Change the OLT.
2
Measure the feeder from the OLT location with an OTDR.
Go to task 3.
The feeder network is in failure. Repair the fiber.
3
Go to the FDH and verify the connections.
Go to task 4.
Modify the connections.
4
Test the distribution network with an OTDR.
N.A.
The distribution network is in failure. Repair the fiber.
Table 11: Tasks involved in analyzing ONT outage when all of the ONTs and the OLT are out-of-service
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Section 6: P2MP Reflectometer Trace Analysis
Because OTDRs are the main tools used to troubleshoot a classical fiber link, one would think that an
OTDR can also be used to test the complete FTTP optical network. Unfortunately, due to the use of
point-to-multipoint (P2MP) networks, the resulted OTDR traces are much more difficult to interpret.
Traditional OTDR analysis uses both the Rayleigh backscatter effect and the Fresnel back reflection
effect to characterize events and fiber ends. The OTDR then displays a trace that is the signature of the
fiber link. The Rayleigh backscatter effect takes into consideration that part of the transmitted light is
reflected back to the transmitter due to fiber impurities. The Fresnel back reflection effect relies on
index discontinuities. It has the same effect as Rayleigh backscatter effect, but its effect is due to fiberto-air connections.
These effects usually give an easy-to-understand signature of the link as long as there is only one branch
along the link. Because a PON network provides multiple branches along the link, the P2MP OTDR
signature becomes very complex to interpret. This is because the OTDR is not able to differentiate the
reflected light coming from a default of one branch location to another branch location, since they are
located at the same distance to the OTDR. The result is an aggregate trace that does not display the
effect of each branch individually; instead, it displays the effect of all of the branches at the same time.
P2MP Reflectometer Trace Analysis takes into consideration the backscatter response after the splitter
that is used in P2MP networks. It allows for the identification of the optical branch, the localization of
faults, and attenuation estimation.
Therefore, in order to analyze a P2MP network, a specific test procedure, described below, is required.
Typical Reflectometer Traces
This section compares two typical scenarios, a core network and a distribution network, to explain the
importance of P2MP Reflectometer Trace Analysis.
For the core network, the OTDR trace shows the splices and connectors along the link. The analysis of
this trace is simple, and it is done automatically by most of the OTDRs currently on the market. The
OTDR provides a signature of the link and a table of events, which can be stored for maintenance
purposes (Figure 13).
Figure 13: An OTDR trace of a core network
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For the P2MP network, the OTDR trace shows the splitter, splices, and connectors along the link. Prior
to the splitter, the analysis is simple and is similar to the analysis of the core network. But following the
splitter, the analysis becomes more complex. With only the trace and the table of events given directly
by the OTDR, it is impossible to locate and measure the different events of the different branches that
are located after the splitter. The OTDR trace does not take into account the fact that there are different
branches. It only analyzes the light that reflects from the entire network (Figure 14). All the information
is available on the trace itself, though. It is just necessary to decode the information coming from the
OTDR. Further software analysis is required for this decoding procedure.
Figure 14: An OTDR trace of a P2MP network
Theoretical Method of P2MP Reflectometer Trace Analysis
The difficulty in analyzing a P2MP network is directly linked to the use of the splitter. When a splitter
is used, analysis of the backscatter response after the splitter is necessary. Figure 15 shows a P2MP
reflectometer trace analysis of a simulated network with one splitter. The splitter can have any number
of branches with fiber of different lengths.
Figure 15: A simulated P2MP network with one splitter
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Figure 16 shows the P2MP reflectometer trace given by the OTDR of a simulated network with one
splitter.
Figure 16: An OTDR trace of a
simulated P2MP network with
one splitter
Displayed Attenuations on a P2MP Reflectometer Trace
There are two types of attenuation on a multi-branch backscattered signal. The first type, Ac, is located
at the splitter. The second type, AFEi, is located where fiber branch ‘i’ ends, corresponding to a fiber end.
These attenuations are closely dependent on the adjacent fiber parameters (backscatter coefficients) as
well as the splitter parameters (forward and reverse insertion losses).
In order to more fully understand the analysis, a simplified version of the theoretical formula is defined.
It requires the following assumptions:
(1) The different fibers connected to the splitter have very similar characteristics, including
bidirectional symmetry behaviors, backscatter coefficients, and attenuation.
(2) The splitter has the same characteristics for both directions.
Taking into account these assumptions, the different types of attenuation are calculated by the
following formulas:
Ac = 5xlog(m) + E.L.
AFEi = 5xlog[(m-i+1)/(m-i)]
where E.L. is the excess loss of the splitter, m is the number of outputs, and i is the analyzed branch.
Analysis Method of P2MP Reflectomer Trace Analysis
This example uses a 1:8 splitter with the same assumptions as in the theoretical method. A standard
OTDR is used to obtain the signature of the link. The analysis method requires a learning acquisition
phase. This phase uses an optical network OTDR measurement simulator. This innovative simulator,
developed by JDSU’s Optical Transport Division, can be used with both point-to-point and point-tomultipoint networks. In addition to classical fiber, connector, splice, and attenuator simulation, the
software algorithms integrate n to m splitter synthesis based on an attenuation behavior formula
similar to the calculations of Ac and AFEi.
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Learning Acquisition Phase
The learning acquisition phase can be divided into three main steps:
(1) OTDR acquisition simulation with construction data (Figure 17).
(2) OTDR acquisition in the field under real conditions (Figure 18).
(3) Comparison between the two acquisitions, including distance tuning and locking.
Figure 17: OTDR acquisition
simulation with construction data
Figure 18: OTDR acquisition in the
field under real conditions
This learning acquisition phase allows for the generation of a Reference Pattern (RP) and an Event
Reference Table (ERT). The RP corresponds to the trace, and the ERT includes the reflective and nonreflective events list. The analysis does not require all of the non-reflective events in order to run. After
this phase, the analysis can be performed using real conditions in the field.
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Analysis Phase
Analysis can be performed during installation, monitoring, or maintenance applications. During inservice monitoring, if the system detects an OTDR signature deviation, then analysis can be performed.
During maintenance, if a failure occurs during traffic, then the fault location can be determined. In
addition, the failure level is established, if possible. In both cases, the analysis method consists of a
comparison between the current pattern (CP) and the reference pattern (RP).
The following sections discuss the different events that can occur during analysis.
(a.) Optical Branch Identification
During optical branch identification, two different possibilities can appear:
1. Fresnel reflection extinction or attenuation
When analyzing the pattern comparison, if a fiber end reflective event deviation is detected,
then the affected optical branch can be directly identified (Figure 19).
Figure 19: Reflective event deviation
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2. Attenuation deviation
When analyzing the pattern comparison, if an attenuation deviation along a fiber section is
detected, then the affected fiber is the one that ends at the same distance that the deviation
stops appearing (Figure 20). In this example, branch number six is affected.
Figure 20: An attenuation deviation
(b.) Fault localization and attenuation estimation
After locating the distance where the deviation begins to appear, the location of the fault can be identified.
Eventually the fault can be associated with an event recorded in the reference table.
Due to the mixing of multiple branches, fault attenuation cannot be directly calculated from the
attenuation deviation between the two curves (Figure 21).
Figure 21: Identifying the location and attenuation of a fault
Using a pattern simulator, though, a virtual attenuator can be inserted into the affected branch and the
attenuation can be increased until the same deviation from the reference pattern is achieved. This
technique allows for an approximation of the attenuation level of the fault.
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Accuracy and Limitations of the Method
Because this type of analysis involves measurements, it is necessary to point out its accuracy and
limitations. As far as accuracy is concerned, the main source of error lies in the reality of the simulator
models and data entry uncertainties. For example, optical splitters reverse and forward parameters may
need to be entered. The fiber backscatter coefficient deviations add some uncertainty on the
attenuation estimation and could be integrated into the theoretical formula.
Moreover, when P2MP networks are measured with a high level of splitting, the sensitivity depends on
the amount of backscatter contribution that is lost. Therefore, sensitivity depends on the location of the
fault compared to the fiber ends.
As in other OTDR measurements, the signal-to-noise ratio (the dynamic range, for example) can
disturb the acquisition. If the network has a large total loss, then the dynamic range of the OTDR may
not be enough to provide a backscatter trace along the entire link. Therefore, it is important to have
enough dynamic range to go through a splitter with the OTDR.
If the network has branches of similar distances, then the OTDR may not differentiate the different
events and branches. For this reason, new OTDRs are being developed, providing event spatial
resolutions of 1 m or better.
In any case, if the concept of resolution is taken into consideration during the construction of a
network to avoid reflection coincidence, then the maintenance of the network will be easier to perform.
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All statements, technical information and recommendations related to the products herein are based upon information believed to be reliable or accurate. However, the accuracy or completeness thereof is not guaranteed, and no
responsibility is assumed for any inaccuracies. The user assumes all risks and liability whatsoever in connection with
the use of a product or its application. JDSU reserves the right to change at any time without notice the design,
specifications, function, fit or form of its products described herein, including withdrawal at any time of a product
offered for sale herein. JDSU makes no representations that the products herein are free from any intellectual
property claims of others. Please contact JDSU for more information. JDSU and the JDSU logo are trademarks of
JDS Uniphase Corporation. Other trademarks are the property of their respective holders. ©2005 JDS Uniphase
Corporation. All rights reserved. 10143197 500 0905 PON.WP.FOP.TM.AE
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