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Testing Fibre To The Home

Testing FTTH

    New network architectures (PONs or passive optical networks) have been developed that allow sharing expensive components for FTTH. A passive splitter that takes one input and broadcasts it to as many as 32 users cuts the cost of the links substantially by sharing, for example, one expensive laser with up to 32 homes and only requiring an inexpensive laser at each home. However, this architecture changes the methodology of testing the complete installed cable plant and links for proper operation. Of course, individual links are tested as usual, it is the PON coupler that creates the difference.



Each home needs to be connected to the local central office with a single singlemode fibre, through a local PON splitter (or maybe two if the PON splitters are cascaded.) Every home will have a singlemode fibre link pulled or strung aerially to the phone company cables running down the street and a network interface device containing fibre optic transmitters and receivers will be installed on the outside of the house. The incoming cable needs to be terminated at the house, tested, connected to the interface and the service tested. See FTTH Architectures  for more information on typical FTTH installations.

FTTx Testing Issues
    Testing FTTH network is similar to other OSP testing but the splitter and WDM add complexity. FTTP PON networks can be more complicated than simple OSP links, with WDM couplers, PON splitters, etc. in a single link, so complete testing can include some components and installation issues not familiar to the usual OSP tech. PON couplers add high loss, WDM couplers have different performance at different wavelengths and connector reflectance, not a problem in most systems, can be a problem in short links typical in FTTx. Many FTTx systems use APC (angled PC) connectors to reduce reflectance so test cables for both OLTS and OTDR need to have matching connectors.

    However, once installed, users on a live network means testing cannot disrupt service. Thus testing may be as simple as checking power at the ONT on the subscriber’s house with a calibrated fibre optic power meter or just seeing if the ONT has a “green” connection light! The ONT at the home usually has some intelligence that can be accessed from a remote location, allowing a service tech to initiate a loopback test to verify connections at any user. If only one user has a problem, a service tech is then sent there, while if all users are down, the tech is sent to the central office.


   As with most fibre optic links, troubleshooting requires knowing the architecture of the system, expected link losses and optical signal levels and typical problems that may be encountered. As always, we emphasize the importance of having documentation on the system before testing and troubleshooting.

Link Testing
    A link is a single run of fibre, e.g.: from CO to FDH or from FDH to ONT. The fibre run may have connectors or not, depending on whether the links are spliced or use connectors for terminations. Quite a few now use pre terminated cables to speed installation. The loss of the PON splitter must be included in the
loss budget for the link. See FTTH Architectures  for more information on PON splitter losses.

   You must measure loss with OLTS at all wavelengths and bidirectional to check all operational modes - similar to how the transmission equipment will use the fibre.


    The installer may need to characterize each fibre with an OTDR, verifying fibre attenuation, termination losses and reflectance and splice quality. The OTDR will also show any bending losses caused during installation. OTDR traces should be filed for future reference.


    Optionally, the installer may test splitters at the FDH or the WDMs at the CO. If these are pretested, as they should have been, this may not be necessary or advisable, especially since it is time-consuming and costly. WDMs also require specialized test equipment.




    After the link is installed, it needs testing from end to end. The end-to-end loss includes the connectors on each end, the loss of the fibre in each link, the connectors or splices on the splitter and the loss of the splitter itself. Since the fibres are being used bi-directionally and connector or splice loss may be different in each direction if the fibre core diameter (mode field diameter for SM fibre) is different, testing in both directions is important too. Special FTTx PON OLTS are available that test the proper wavelengths in each direction, simplifying testing logistics.

    Since PON links are generally short (<20km) chromatic dispersion (CD) and polarization mode dispersion (PMD) are not concerns. CD and PMD are generally only issues on very long links.

BPON
    Let’s consider the most complex version of PON testing, BPON. It’s similar to OSP testing but splitter and WDM add complexity as well as more loss and there are three wavelengths in use. Tests include each coupler, each link and end-to-end loss. Loss and reflectance are especially important if systems are using an AM video transmission system at 1550 nm, as it has a maximum tolerable loss and reflectance before signals are noticeably affected. Tests need to be done at all three wavelengths of operation: 1310 nm for upstream digital data, 1490 for downstream digital data and 1550 nm for AM video downstream (BPON).




    Insertion loss of the cable plant including the loss of the coupler is tested using an optical loss test set (special test sets for FTTH PONs are available that cover all 3 wavelengths of interest.) OTDRs can be used if length is adequately long, to determine connection reflectance, fibre attenuation and troubleshoot problems. Many systems will take OTDR traces and store for troubleshooting. The splitters can confuse the OTDR so one generally reverses OTDR test, taking traces from the subscriber upstream. 

OTDR Testing PONs
    Using an OTDR to test every fibre in  an OSP link is traditional, as the OTDR provides a snapshot of the losses in the fibre, locates loss events (connectors, splices and bending losses from improper installation), aids installation troubleshooting and provides a trace which can be stored for later troubleshooting and restoration. On FTTH PON networks, the PON splitter causes some unusual traces on OTDRs, with the traces looking totally different when tested from each direction. Here are two traces from an actual system taken in two directions.

This trace is taken downstream from the CO to the subscriber:


This trace is taken upstream from the subscriber toward the CO.


In both traces, you can see the large loss of the PON coupler, best seen in the upstream trace at the bottom, on the left side of the trace. On the downstream trace, it is the large loss preceding the multiple peaks of the subscriber fibres, marked with the dashed marker line. Below we will show a simpler coupler and explain what you are seeing here.

OTDR Testing From CO
  PON systems create problems for OTDRs. Shooting from the input of a PON splitter at the CO, the OTDR sees and adds together the backscatter traces from all the fibres. As a result, it becomes impossible to see detail on individual fibres, and an event (connector, splice of bending loss) cannot be easily assigned to any individual fibre unless the cable plant is carefully documented at installation.

   Consider the “X” shown in the network diagram below. If it was a loss or reflective event, it would show on the OTDR trace, but the operator would not know if  were in fibre 1,2,3 or 4. The only unambiguous part of the OTDR trace shown is the end of fibre 4, the longest fibre, beyond the length of the next longest fibre, #3.


   It should be noted that FTTH links, because of their short lengths and the use of some high power transmitters, usually have APC connectors or fibres prepared to have minimal reflectance. That can make analysing downstream OTDR traces very difficult when no reflective end is available to mark the fibre end and there are 32 fibres in the system.



Here is an illustration of how a real trace can become very complex to analyse. This is an enlargement of the coupler to subscriber section of the downstream trace above which is outlined in red on the trace.



    As a result of the complexity of downstream traces, OTDRs are generally used on PONs from the subscriber end toward the CO to characterize the fibre path. However, the OTDR may also be used from the CO end, because, as you can see from the diagram above, it allows the operator to quickly characterize the length of each fibre link, providing actual fibre length to add to network diagrams for future troubleshooting.

    Special PON OTDRs will test at 1310, 1490 and 1550 nm. Some also test  “out of band”  at 1650 nm, which is more sensitive to bending losses and allows in-service testing with a filter to remove signal wavelengths. Since PONs are short, the OTDR needs very high resolution, usually obtained by having the shortest test pulse that will give adequate range.


    Testing PONs in the downstream direction is helped with launch and receive cables. The launch cable allows testing the initial connector on the link as well as allowing the initial overload of the OTDR to settle down as with any OTDR test. But on the receive end, if a cable of known length is used, say 100m or 500m, one can look back exactly that distance from the reflective end to see the loss of the end connector.

OTDR Testing From Subscriber
    Testing from the subscriber end is easier. The fibre path will show events on just one fibre, like the “X” shown on fibre 3, and a high loss for the coupler. Here a 1:4 coupler will have 6 dB of splitting loss plus perhaps 1dB excess loss for a total of 7 dB loss.
Using launch and receive cables allow testing connectors on both ends and measuring end to end loss.




   Here is a detailed trace from the upstream example above, showing how much simpler the trace is when the other subscriber links are not shown.


Other FTTx Testing Issues
    Network equipment will be tested as the system is turned on or for troubleshooting. Will the network equipment transmit and receive properly? If the cable plant is installed correctly and tests within specifications for loss and reflectance, it should. Most FTTx equipment has extensive self-testing capability and that may prove sufficient for most testing. PON couplers may have a second port on the upstream side just for testing or unused downstream connectors may be useful for testing, especially with OTDRs.


    The network equipment should be tested for optical power. The transmitter output should be within specifications, as should the receiver input, when tested with a calibrated optical power meter set at the proper wavelength(s). If testing is done while all three systems are operating at their respective wavelengths, a power meter with wavelength selective input is required.  Power at the receiver is critical. Too low and the signal-to-noise ratio will be too low; too high and the receiver will saturate. Both conditions will cause transmission errors. High power is not uncommon, so attenuators may be used in these links to reduce power to acceptable levels.


    Data transfer testing with a protocol analyser is the final test. It will be done using specific protocol testers for the data formats being transmitted. Personnel doing these tests are probably not the same that test the cable plant as each have specific training and test equipment needs.


    Remember that ONTs are generally capable of loopback testing under remote control. This may mean more sophisticated testing is unnecessary for troubleshooting.

Cable Plant Link Loss Budget Analysis

Loss budget analysis is the calculation and verification of a fibre optic system's operating characteristics. This encompasses items such as electronics, transceiver wavelengths, fibre type, and link length. Attenuation and bandwidth/dispersion are the key parameters for budget loss analysis.

FOA has a free app for smartphones and tablets that will calculate loss budgets for the cable plant you are designing or testing. See the app store for your device for details.

 

Analyse Link Loss In The Design Stage

Prior to designing or installing a fibre optic system, a loss budget analysis is recommended to make certain the system will work over the proposed link. Both the passive and active components of the circuit have to be included in the budget loss calculation. Passive loss is made up of fibre loss, connector loss, and splice loss. Don't forget any couplers or splitters in the link. Active components are system gain, wavelength, transmitter power, receiver sensitivity, and dynamic range. Prior to system turn up, test the circuit with a source and FO power meter to ensure that it is within the loss budget.

The idea of a loss budget is to insure the network equipment will work over the installed fiber optic link. It is normal to be conservative over the specifications! Don't use the best possible specs for fibre attenuation or connector loss - give yourself some margin!

The best way to illustrate calculating a loss budget is to show how it's done for a 2 km multimode link with 5 connections (2 connectors at each end and 3 connections at patch panels in the link) and one splice in the middle. See the drawings below of the link layout and the instantaneous power in the link at any point along it's length, scaled exactly to the link drawing above it.

 

Cable Plant Passive Component Loss

Step 1. Fibre loss at the operating wavelength

Cable Length

2.0

2.0

Fibre Type

Multimode

Singlemode

Wavelength (nm)

850

1300

1310

1550

Fibre Atten. dB/km

3 [3.5]

1 [1.5]

0.4 [1/0.5]

0.3 [1/0.5]

Total Fibre Loss

6.0 [7.0]

2.0 [3.0]

(All specs in brackets are maximum values per EIA/TIA 568 standard. For singlemode fibre, a higher loss is allowed for premises applications. )

 

Step 2. Connector Loss

Multimode connectors will have losses of 0.2-0.5 dB typically. Singlemode connectors, which are factory made and fusion spliced on will have losses of 0.1-0.2 dB. Field terminated singlemode connectors may have losses as high as 0.5-1.0 dB. Let's calculate it at both typical and worst case values.
Remember that we include all the components in the complete link, including the connectors on each end.

Connector Loss

0.3 dB (typical adhesive/polish conn)

 0.75 dB (TIA-568 max acceptable)

Total # of Connectors

5

 5

Total Connector Loss

1.5 dB

 3.75 dB

(All connectors are allowed 0.75 max per EIA/TIA 568 standard)

 Remember that we include all the components in the complete link, including the connectors on each end. In our example above, the link includes patchcords on each end to connect to the electronics. We need to assess the quality of these connectors, so we include them in the link loss budget and if we test the link end to end, including the patchcords, these connectors will be included in the test results when connected to launch and receive reference cables. On some links, only the permanently installed link, not including the patchcords, will be tested. Again, we still need to include the connectors on the end as they will be included when we test insertion loss with reference test cables on each end.

Step 3. Splice Loss

Multimode splices are usually made with mechanical splices, although some fusion splicing is used. The larger core and multiple layers make fusion splicing about the same loss as mechanical splicing, but fusion is more reliable in adverse environments. Figure 0.1-0.5 dB for multimode splices, 0.3 being a good average for an experienced installer. Fusion splicing of singlemode fibre will typically have less than 0.05 dB (that's right, less than a tenth of a dB!)

Typical Splice Loss

0.3 dB

Total # splices

1

Total Splice Loss

0.3 dB

(All splices are allowed 0.3 max per EIA/TIA 568 standard)

 

Step 4. Total Passive System Attenuation

Add the fibre loss, connector and splice losses to get the link loss.

Typical

 TIA 568 Max

 

 850 nm

 1300 nm

 850 nm

1300 nm

Total Fibre Loss (dB)

6.0

2.0

 7.0

3.0

Total Connector Loss (dB)

1.5

1.5

 3.75

 3.75

Total Splice Loss (dB)

0.3

0.3

 0.3

 0.3

Other (dB)

0

0

 0

0

Total Link Loss (dB)

7.8

3.8

 11.05

 7.05

Remember these should be the criteria for testing. Allow +/- 0.2 -0.5 dB for measurement uncertainty and that becomes your pass/fail criterion.

Equipment Link Loss Budget Calculation: Link loss budget for network hardware depends on the dynamic range, the difference between the sensitivity of the receiver and the output of the source into the fibre. You need some margin for system degradation over time or environment, so subtract that margin (as much as 3dB) to get the loss budget for the link.

Step 5. Data From Manufacturer's Specification for Active Components (Typical 100 Mb/s link)

Operating Wavelength (nm)

1300

Fibre Type

MM

Receiver Sens. (dBm@ required BER)

-31

Average Transmitter Output (dBm)

-16

Dynamic Range (dB)

15

Recommended Excess Margin (dB)

3

 

Step 6. Loss Margin Calculation

Dynamic Range (dB) (above)

15

 15

Cable Plant Link Loss (dB)

3.8 (Typ)

 7.05 (TIA)

Link Loss Margin (dB)

11.2

 7.95

As a general rule, the Link Loss Margin should be greater than approximately 3 dB to allow for link degradation over time. LEDs in the transmitter may age and lose power, connectors or splices may degrade or connectors may get dirty if opened for rerouting or testing. If cables are accidentally cut, excess margin will be needed to accommodate splices for restoration.

Fibre To The Home Architectures

   New network architectures have been developed to reduce the cost of installing high bandwidth services to the home, often lumped into the acronym FTTx for "fibre to the x". These include FTTC for fibre to the curb, also called FTTN or fibre to the node, FTTH for fibre to the home and FTTP for fibre to the premises, using "premises" to include homes, apartments, condos, small businesses, etc. Recently, we've even added FTTW for fibre to wireless.

Let's begin by describing these network architectures.

FTTC: Fibre To The Curb (or Node, FTTN)
   Fibre to the curb brings fibre to the curb, or just down the street, close enough for the copper wiring already connecting the home to carry DSL (digital subscriber line, or fast digital signals on copper.)



 
  FTTC bandwidth depends on DSL performance where the bandwidth declines over long lengths from the node to the home. There are many types of DSL (ADSL, HDSL, RADSL, VDSL, UDSL, etc.) that offer varying performance over length and some "bond" more pairs of wires to improve the bandwidth.



   Newer homes that have good copper and are near where the DSL switch is located can expect good service. Homes with older copper or longer distances away will have less available bandwidth.
   FTTC is less expensive than FTTH when first installed but since performance depends on the quality of the copper wiring currently installed to the home and the length to reach from the node to the home, the level of service may be obsoleted quickly by customer demands. In older areas where the copper wires are of poorer quality or have degraded over time, DSL is difficult or impossible to implement. The good news it that FTTC is ready to upgrade to FTTH.

FTTW: Fibre to Wireless
   Of course today's mobile device users depend on wireless connections for their laptops, smartphones and tablets. Even many homes and businesses are now using wireless connectivity, especially those outside areas where FTTH or FTTC are not available or considered economical for future installations. Options for wireless include cellular systems which are the most widely available wireless solution around the world, WiFi which has become available inside many businesses and even outdoors in areas served by municipal networks and satellite wireless, used in many rural areas where distances are so large that cabling or WiFi is unfeasible. Future options include WiMAX and Super WiFi, land-based wireless with longer ranges and higher bandwidth capability than most cellular systems and smaller cellular antennas with more localized coverage like this LightCube Radio from Alcatel-Lucent that can be placed anywhere and connected with fibre and power. All these options are aimed at providing more bandwidth to users more efficiently.



    All these wireless systems depend on the same fibre optic communications backbones that everyone else does. As they grow, higher bandwidth demands means more traffic to local antennas which makes fibre more attractive. Most cellular users are converting older antenna towers connected by copper cables or line-of-sight wireless over to fibre. Fibre is even being used for connections up towers to wireless antennas as it is smaller and lighter than the coax cables previously used.
Read more on how wireless depends on fibre here.

FTTH Active Star Network
   The simplest way to connect homes with fibre is to have a fibre link connecting every home to the phone company switches, either in the nearest central office (CO) or to a local active switch.




The drawing above shows a home run connection from the home directly to the CO, while below, the home is connected to a local switch, like FTTC upgraded to fibre to the home.



   A home run active star network has one fibre dedicated to each home (or premises in the case of businesses, apartments or condos.) This architecture offers the maximum amount of bandwidth and flexibility, but at a higher cost, both in electronics on each end (compared to a PON architecture, described below) and the dedicated fibre(s) required for each home.

FTTH PON: Passive Optical Network
  A PON system allows sharing expensive components for FTTH. A passive splitter that takes one input and splits it to broadcast to many users cuts the cost of the links substantially by sharing, for example, one expensive laser with up to 32 homes. PON splitters are bi-directional, that is signals can be sent downstream from the central office, broadcast to all users, and signals from the users can be sent upstream and combined into one fibre to communicate with the central office.


    Because of all the splitters and short links, plus since some systems are designed for AM video like CATV systems, non-reflective connectors (like the SC-APC angle-polished connector) are generally used.




The splitter can be one unit in a single location as shown above or several splitters cascaded as shown below. Cascaded splitters can be used to reduce the amount of fibre needed in a network by placing splitters nearer the user. The split ratio is the split of each coupler multiplied together, so a 4-way splitter followed by a 8-way splitter would be a 32-way split. Cascading is usually done when houses being served are clustered in smaller groups. Splitters are sometimes housed in the central office and individual fibres run from the office to each subscriber. This can enhance serviceability of the network since all the network hardware is in one location at only a small penalty in overall cost for either dense urban areas or long rural systems.



        Most PON splitters  are 1X32 or 2X32 or some smaller number of splits in a binary sequence (2, 4,8, 16, 32, etc.). Couplers are basically symmetrical, say 32X32, but PON architecture doesn't need but one fibre connection on the central office side, or maybe 2 so one is available for monitoring, testing and as a spare, so the other fibres are cut off. Couplers work by splitting the signal equally into all the fibres on the other side of the coupler, Splitters add considerable loss to a FTTH link, limiting the distance of a FTTH link compared to typical point-to-point telco link. When designing a fibre optic network, here are guidelines on loss in PON couplers.

Splitter Ratio

1:2

1:4

1:8

1:16

1:32

Ideal Loss / Port (dB)

3

6

9

12

15

Excess Loss (dB)

1

1

2

3

4

Typical Loss (dB)

4

7

11

15

19


    Each home needs to be connected to the local central office with singlemode fibre through an optical splitter. Every home will have a singlemode fibre link pulled into underground conduit or strung aerially to the phone company cables running down the street. Verizon has pioneered installing prefabricated fibre links that require little field splicing.



   Here is a fibre distribution system that has been spliced into cables connected to the local central office. The pre-terminated drop cable to the home merely connects to the closure on the pole in the red circle and is usually lashed to the aerial telephone wire already connected to the home.



   If the cable is underground, it will usually be pulled through conduit from connection to the distribution cable or the splitter to the home. Here a pre-terminated systems has two home drops connected to the distribution cable.

The splitter can be housed in a central office or a pedestal in the neighbourhood near the homes served. Here is a typical pedestal that has connections to the CO, splitters and fibres out to each home in a sealed enclosure. The advantage of PONs is that this pedestal is passive - it does not require any power as would a switch or node for fibre to the curb.



   A network interface device containing fibre optic transmitters and receivers will be installed on the outside of the house. The incoming cable needs to be terminated at the house, tested, connected to the interface and the service tested.


Below is the layout of a typical PON network with the equipment required at the CO, fibre distribution hub and the home. This drawing shows the location of the hardware used in creating a complete PON network and defines the network jargon.



Triple Play Systems
   Most FTTH systems are "triple play" systems offering voice (telephone), video (TV) and data (Internet access.) To provide all three services over one fibre, signals are sent bidirectional over a single fibre using two or three separate wavelengths of light. Three different protocols are in use today, BPON, shown below, uses a third wavelength for AM video, while EPON and GPON use digital IPTV transmission.

 Read more on PON protocols.
   Downstream digital signals from the CO through the splitter to the home are sent at 1490 or 1550 nm. This signal carries both voice and data to the home. Video on BPON systems uses the same technology as CATV, an analogue modulated signal, broadcast separately using a 1550 nm laser which may require a fibre amplifier to provide enough signal power to overcome the loss of the optical splitter. Upstream digital signals for voice and data are sent back to the CO from the home using an inexpensive 1310 nm laser. WDM couplers separate the signals at both the home and the CO.




Powering FTTH
    Traditionally, telephone services, at least what are called "POTS" or plain old telephone service, have been self-powered from the central office. POTS phones were on a current loop powered from batteries or some other type of uninterruptible power in the CO. When a subscriber had an electrical power outage, they expected to be able to still use their phone, to call the electrical utility to report the outage, of course! Obviously, FTTH is not going to operate the same way. Fibre does not easily deliver electrical power, although systems have been developed to power sensors over light in the fibre, it is inefficient and expensive. Many FTTH systems provide a battery backup at the customer premises powered from the customer electrical system to keep the system operational during power outages. Some systems use the old copper wires replaced by the fibre to deliver power to keep the backup charged, so that the FTTH system provider pays for the power needed by the system. And some systems, recognizing that most people have a mobile phone, do not address the issue of backup power at all.