PASSIVE OPTICAL LAN (FIBER-TO-THE-DESKTOP)
Following the FTTH trend to deliver more bandwidth to the consumers, a new technology is rising to provide more bandwidth, more services and future-proof networks to the enterprises: passive optical LAN (POL).
Also called OLAN or fiber-to-the-desktop, this GPON-based technology creates a very cost-effective local area network (LAN) with virtually unlimited capabilities.
Singlemode fibers all the way to the desktop and optical splitters replace the traditional CAT5/6 cables and active Ethernet switches providing considerable benefits to the network owners:
- CAPEX saving for cables and equipment
- OPEX saving from lower energy and cooling (Green technology)
- Space saving with fewer racks of equipment and no remote hubs
- Secure and reliable communications over fiber-optic cables
Passive optical LANs use a different architecture than LANs with electronic switches. Passive optical LANs use optical splitters to divide the optical signal to allow up to 32 devices (ONTs) to be connected to one port on the optical line terminal (OLT) that is the center of the LAN. Downstream, the splitter splits the signals to all the devices and upstream it combines them into one fiber, allowing bidirectional signals on one singlemode fiber. Each ONT is usually a 4-port gigabit Ethernet switch in a LAN, but it can also be a switch for multiple phone lines or even a FTTH triple play (phone/Internet/TV) converter.
A POL cable plant needs nothing but passive components – fiber optic cabling and a splitter – between the main equipment room and the work area. Signals are transmitted simultaneously in both directions at two different wavelengths on a single fiber using wavelength-division multiplexing (WDM) so each work area needs only a single singlemode fiber to connect to the network. FTTH passive optical networks use fiber optic splitters to split out a downstream signal to a number of users, usually 32, reducing the complexity of the electronics and the bulk of the cabling, plus it spits the cost of the downstream electronics by the number of users, making the cost per user much lower, even lower than a link using copper cables between Ethernet ports. Upstream, the POL uses the coupler as a combiner – combining the incoming signals into a single fiber to connect to the OLT.
Testing POLs is easy for insertion loss but requires special knowledge with OTDRs as they splitter can be confusing. Cable plants with splitters have different characteristics upstream and downstream so techs should be trained on the special needs of POL testing with OTDRs.
Once the design of the network has been completed, the lifecycle of a network generally consists of three main phases: construction, activation and maintenance.
- Fiber Inspection
What To Look
EXFO’s FIP-400B Fully Automated and Intelligent fiber inspection solutions are designed to speed up and simplify this critical step to any fiber testing and help you deploy faster, more reliable fiber networks, providing accurate and consistent test results, and preventing the reporting of false-positive results.
- 100% automated, one-step inspection process
- On-board connector endface analysis
- (IEC, IPC or custom standards) including MPO/MTP analysis.
- Pass/fail LED indicator for immediate diagnosis of connector health
- Optimal digital image quality with three levels of magnification
- Characterizing Insertion Loss and Optical Return Loss
In this step, both loss and fiber attenuation of the ODN elements must be measured to ensure that they meet supplier specifications. There are several testing approaches to characterized the ODN during construction.
– OLTS Approaches
Advanced automated OLTS solutions, such as EXFO’s FTB-3930 coupled with the FOT-930, can allow multiple technicians to access the same unit. As the FTB-3930 can manage up to 10 references, it can be placed in the CO, connected to the link under test. The technicians in the field, equipped with FOT-930s can use the unit at the CO to perform the test. Note that once the referencing is performed, no technician is required to operate the FTB-3930, and all the test results can automatically be saved in this main unit at the CO.
An OTDR identifies and specifically locates individual events in a fiber-optic span, which typically consists of sections of fiber joined by connectors and splices. The reflectometry test is single-ended and is performed by one technician. The unit transmits pulsed light signals along a fiber span in which light-scattering occurs due to discontinuities such as connectors, splices, bends and faults. The OTDR then detects and analyzes the parts of the signals that are returned by Fresnel reflections and Rayleigh backscattering. The OTDR method is extremely accurate, yet can be complex and time-consuming.
Characterized ODN using OTDR Approaches
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– iOLM Approaches
The most recent test tool for this application is the intelligent optical link mapper (iOLM). This tool uses the same method as the OTDR, but performs the test procedure automatically. It does this by using different pulse widths to fully characterize the various sections of an FTTH network—each section being characterized with the optimal pulse. Then, the iOLM consolidates all this information into a single comprehensive Link View; the operator does not have to manually compare results at different pulses like a traditional OTDR. The iOLM provides the link’s loss and ORL, in addition to identifying all network elements such as splices, splitters and connectors. It also provides the loss and reflectance values of the identified elements. And when a specific element or the link itself gets a “fail” verdict, it offers a diagnosis to help the operator solve the problem. The whole routine takes 30 to 60 seconds, depending on network complexity.
iOLM Test Result
(For more information about iOLM, Click here)
2. Service Activation Phase
Service activation is associated with what is known as “home-connect” service turn-up. This process includes the connection between the fiber drop point (FDP) and the optical network terminal (ONT) at the customer’s premises. It is therefore critical that accurate, PON-specific power meters with good track records are used for measurement and documentation purposes.
3. Maintenance Phase
When service is activated on a passive optical network (PON), telephony, high-speed Internet and video signals are sent from the optical line terminal (OLT) at the central office (CO) to various optical network terminals (ONTs) at different residential customer locations. In this situation, if one of the ONTs goes down and cannot restart its synchronization with the OLT, this branch of the PON becomes inactive and the customers associated with this branch lose service. The result is that a technician is called in to troubleshoot and restart the service. The faulty zone can be isolated using EXFO PON Power Meter PPM-350C.
Even though a PON power meter can help isolate the affected zone, it cannot provide the exact location of the fault. In order to physically pinpoint the location of a fault, the technician must use what we refer to as a PON-optimized OTDR or iOLM with a dedicated port for testing at 1625 or 1650 nm and incorporate a filter that rejects all unwanted signals (1310, 1490 and 1550 nm) that could contaminate the OTDR-iOLM measurement.
Live Testing with OTDR or iOLM
(For more information about Live testing with OTDR or iOLM, Click here)