If only FTTx could transport cold liquid refreshment along with five simultaneous TV channels.
The development of ubiquitous, low-cost, wideband communications is constrained by the existing plain old telephone service copper infrastructure. Innovative versions of digital subscriber line (DSL) technology have used those copper wires to provide megahertz data rates, but applications continue to demand even more speed. To achieve a truly high bandwidth network, copper must be replaced by optical fibers in the local loop.
The transition from copper to fiber is not going to happen immediately, nor is it going to be accomplished uniformly throughout the country. Implementation details also depend on the nature of the end user. Table 1 lists many of the fiber-to-the-location (FTTx) terms that describe the growth of optical fibers within the local loop.
Some terms, such as fiber to the antenna and fiber to the radio, simply have taken advantage of the generic FTTx jargon. However, most of the others deal with bringing a high-capacity data pipe closer to the end user. The important distinction is how close.
Verizon's FiOS• is an example of a point-to-multipoint FTTH system that actually runs optical fibers into individual homes as depicted in Figure 1. FiOS is a passive optical network (PON) that uses a passive optical splitter to distribute data from the nearby optical line terminal (OLT) to up to 32 residential and small business premises. In each home or office, an optical network unit (ONU), equivalent to an optical network terminal (ONT), connects the optical fiber to local computers and phones.
In the downstream direction from the OLT to the ONUs, a data rate of 30 MHz is achievable per end user. The system has an overall reach limitation of 20 km. How the 20 km is divided between the OLT-to-splitter distance and the splitter-to-ONU distance depends on the distribution of subscribers within an area. In the upstream direction, only one ONU may transmit at a time at a 5-MHz rate, so the network must accurately monitor and control ONU synchronization.
FiOS is an Ethernet PON (EPON). Generally, EPON specifications are covered by IEEE 802.3ah and stem from the Ethernet in the First Mile initiative. In the downstream direction, all the ONUs receive the broadcast data stream and accept information packets according to the embedded addresses. Upstream transmissions can occur only at specific times associated with the 802.3ah multipoint control protocol signaling extension.
A few years ago, PONs appeared to be the only practical approach to wideband access because they used few active, and at that time expensive, components. Today, proponents of active, point-to-point Ethernet systems claim the costs of these and the PON approach are nearly equal. Active Ethernet does use many more active components, but because Ethernet is so popular in LAN applications, component cost is very low.
Active Ethernet also can offer true 100-Mb/s duplex service, and its deployment scales well. In a recent World Wide Packets white paper, these points were discussed in detail. The company claimed that an EPON only becomes cost-effective after 26 of the possible 32 users have been connected. This means that addressing 35 or 69 users, for example, is expensive because at least one of the required PON OLTs is very underutilized. In contrast, active Ethernet deployments scale more linearly.1
Nevertheless, many types of PONs as well as active systems are possible. For example, BellSouth is building FTTC networks with high-speed DSL links to individual homes and offices. This network architecture is claimed to offer predictable high-speed performance because the FTTC nodes are within a few hundred feet of the served premises.
The choice of FTTH or FTTC deployment also has been affected by the recent FCC ruling that the two fiber network architectures are equivalent: Neither must be shared with competing local exchange carriers. This ruling removed a major disincentive to the large fiber plant investment required by the incumbent local exchange carriers such as BellSouth.
Regardless of what FTTx approach is used, carriers must test the fiber networks. For EPON architectures, 802.3ah provides a new operation and maintenance (OAM) sublayer within the data link layer. OAM allows a network operator to remotely set up a loopback test as well as collect and access customer-specific statistics.
Troubleshooting an EPON is done with an 802.3ah-compliant Ethernet tester. At the physical level, optical fiber testers are needed. And, there are combined optical/Ethernet testers as well. However, given the wide variation in the types of networks, available technician skill level, and the different ways in which carriers approach testing, many kinds of instruments are being applied to FTTx.
Having a low cost of test obviously is important, and according to Peter Schweiger, Americas business development manager of the Photonics Measurement Division at Agilent Technologies, Agilent's focus is cost of test.
Agilent is ensuring that the instrument lowers the cost of test by incorporating technology to reduce test time and eliminate training requirements. Two themes that help accomplish this in spite of the market's lack of experience with FTTx,• Mr. Schweiger continued, are mechanical modularity and instrument firmware upgrades. By including an upgrade tool with every FTTx product shipped, we ensure fast and correct software upgrades. Also, instrument module changes require no tools.
Recently introduced Agilent testers fall into two categories: those based on optical time domain reflectometer (OTDR) physical layer technology and those targeting Ethernet test needs. In the former group, the Model E6020B Spark FTTx Color Mini-OTDR provides both 1,310- and 1,550-nm measurements with one-button certification. The Model N3950A Gemini Optical Loss Test Set determines loss in both directions at 1,310, 1,490, and 1,550 nm. It features a one-button autotest capability.
Both copper and fiber Ethernet testing are handled by the Model N2620A FrameScopePro that measures the performance of 10/100/1000 Mb/s networks. All three new products as well as earlier Agilent FTTx test equipment support remote control, typically via an embedded web server.
For installation and commissioning, OTDRs that operate at 1,310 and 1,550 nm for single-mode fiber often are used because these are the wavelengths for which the fiber was designed. However, there are several reasons why they are not necessarily the best wavelengths for troubleshooting a network in service.
Faults other than a complete fiber break may disrupt end-user communications but still allow an OTDR's pulse output to reach the OLT. It is possible that the OTDR may corrupt service to the other ONUs downstream from the splitter because of interference with the OLT's laser. Also, even if interference with the OLT does not occur, the OLT's laser certainly will effectively blind the OTDR's detector.
A solution that conforms to the ITU-T L.41 recommendation of a 100-nm difference between a test OTDR wavelength and that of a live PON is to use a 1,650-nm signal. This approach is discussed in detail in an EXFO application note.2
Figure 2 highlights the critical parts of an FTTH asynchronous transfer mode PON (APON) that carry voice and data downstream via the 1,490-nm (red) path and upstream at 1,310 nm (green). This system also supports downstream video, separately carried on a 1,550-nm (blue) wavelength and 1,650-nm (purple) testing through a separate port.
The Model FTB-7300D PON OTDR continues the features of the 7000D and 7200D but with the addition of a 1,650-nm capability used for troubleshooting live PONs. For installation, activation, and maintenance, EXFO provides the hand-held Model FOT-930 MaxTester Multifunction Loss Tester and the Model PPM-350B PON Power Meter.
Long active in optical fiber-based broadband communications, Acterna also has introduced products to address PON applications. The OLT-55 SMART Optical Loss Test Set is available with two or three wavelengths and features high-accuracy power measurements over a wide dynamic range. The instrument's built-in USB port extends its usefulness to the production floor.
In operation, the ONU at the premises end of the PON must distinguish actual data bits from background noise. When installing and activating a network, the threshold set in the test instrument mimics this function.
Acterna's Model OLP-57 SMART FTTx/PON Selective Power Meter simultaneously measures the combined power of the 1,490- and 1,550-nm downstream and 1,310-nm upstream wavelengths. Correction is made for the bursty nature of the upstream transmission, and the meter user can set the threshold as required to determine pass/fail performance.
The expertise a technician brings to an optical network test situation clearly can be a great asset. However, the massive deployment implied by FTTx means that many installers and test personnel may have little optical background, and the ease with which test equipment can be applied could be critical. As a result, the trend is to make more intelligent instruments, whether hand-held or benchtop, to minimize personnel training requirements and improve test result accuracy.
The hand-held Model CMA 50 line of test equipment features power meters, light sources, and loss test sets. The power meter provides a useful mix of capabilities by combining a light source and an optical power meter with LAN access. This configuration supports measurement of network-level continuity and frame time delay through the TCP/IP protocol over the built-in 10/100 Mb/s network interface card.
NetTest addresses the need for OTDR measurements through the Model CMA 5000 OTDR Platform. Various instrument models feature 850-, 1,310-, 1,550-, and 1,625-nm wavelengths with standard or enhanced dynamic range. All models typically resolve 0.001-dB loss, acquire up to 256,000 data points, have less than 15-nm spectral width, and achieve initial reflective or nonreflective dead zones less than 10 m.
FTTx is either close to where you live or will be soon. With it will come much greater flexibility in the way that you can access and use information of all kinds from Internet downloads to video and audio. As a marketing department might express it, the idea is to create a much richer consumer experience.
Achieving widespread broadband optical network access is as much an economic undertaking as a technical one. The available FTTx test tools are deceptively simple in appearance but very advanced in their ease of operation and embedded metrology. Because many test tools will be required, they have to be low cost but just as accurate as a benchtop unit in the selected subset of measurements they make.
1. Active vs. Passive Access Networks, World Wide Packets, November 2004.
2. Gagon, N., An Innovative Solution for In-Service Troubleshooting on Live FTTH Networks,• Application Note 130, EXFO.
on the Agilent E6020B Spark FTTx
on the Agilent N2620A
on the EXFO Model FOT-930
MaxTester Multifunction Loss Tester
on the Acterna OLT-55 and OLP-57
SMART FTTx instruments
on the NetTest CMA 50 FTTx
on the NetTest CMA 5000