Testing Fibre To The Home

   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

(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.

(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!)

(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.

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)

Step 6. Loss Margin Calculation

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.