Running Ethernet over plastic optical fiber

POF offers many benefits to the user. It is lightweight, robust, cheap, and easy to install. Even a simple pair of scissors can be used to cut POF and the use of a 650nm red light (LED) makes it completely safe for the user and installer's vision. This safety feature brings additional benefits to the installer, since red can be easily seen by the human eye to diagnose if the link is good.

One of the biggest advantages in today's complex and congested world of networking is that fiber is totally immune to electromagnetic interference (EMI) and noise. Just as important, it emits no radiation, a major benefit given all the wireless networks that are present today in the home and office. This is extremely important for video and voice streaming, where such noise can affect picture or service quality. Installation is further simplified as existing main cable ducting can be used to route the fiber without noise interference.

Industrial networks have been slower to adopt POF, in contrast to automotive manufacturers who have been as keen to exploit such technology for connecting car infotainments systems and even some safety-critical applications such as airbags. Today's high-end cars are processor intensive, supporting devices such as radio, CD, DVD, navigation systems, Bluetooth, telephones, TV tuners and gaming, to name some common examples. In fact, next generation car specifications can be easily predicted by observing trends in home networking. For the "24/7/365" world we now live in, consumers expect and demand the same services in the office, home and now, even the car.

A bus protocol called Media Oriented Systems Transport (MOST), (promoted and organized by the MOST Cooperation, see Reference 1) led by car manufacturers Damlier Chrysler, BMW, and Audi, was devised in the late 1990s to meet the rapidly increasing in-car data bandwidth. MOST25 initially offered 25 Mbps and more recently, MOST50 offers up to 50 Mbps data bandwidth using POF as the physical media.

Differences between plastic and glass fiber
POF differs from traditional optical fiber in material and the core/cladding dimensions. The core is the highly refractive centre of the fiber which acts as a "light guide". For standard-telecommunication Single-Mode Fiber (SMF), the core diameter is around 9 μm and cladding diameter 125 μm. SMF is used in long-haul applications with transmission distances of up to 100 km and does not need a repeater. Multimode Fiber (MMF) uses a core/cladding diameter of typically 50 μm/125 μm, providing less reach, up to approximately 2 km, due to increased dispersion as a result of the larger diameter core.

Conversely, POF has a much larger core diameter compared to both SMF and MMF, commonly 980 μm/1000 μm. Although this results in lower data rates (hundreds of Mbps) and reach, the most compelling advantage is cost. The large core means the accuracy of alignment between the LED driver and fiber is less critical, to a point that even a slightly damaged fiber is acceptable. Most of the expense in fiber systems today is not in the BOM (Bill of Materials). Rather, the bulk of the cost lies in the production set up and alignment costs. The core/cladding diameters of Single-Mode, Multi-Mode and POF fiber are shown in Figure 1.

Figure 1: Single-mode, multimode, and POF fiber core/cladding diameters
(Click to Enlarge Image)

Standard fiber-optic cables use a glass-quartz core and cladding, where impurities are added to the core to produce the desired refractive index to act a guide for the light. Glass fiber-optic cable offers lower attenuation than its plastic counter part. POF typically consisting of a polymethylmethacrylate (PMMA) core and a fluoropolymer cladding. The plastic nature of POF provides a more rugged cable, capable of withstanding tighter bend radius than glass fiber-optic cable.

POF over Ethernet
Today, POF has generally been used mostly in niche applications where the cited advantages have outweighed the need for high data bandwidth. Advances in LED and VCSEL (Vertical Cavity Surface Emitting Laser) technology have now enabled POF to support data rates of 3 Gbps and above. Such bandwidth capability should open the door to more mainstream applications.

For POF to become a serious alternative to copper-cable networks, it must be able to support Ethernet. This is because Ethernet is almost exclusively used as the lower layers in any office network, and is quickly dominating factory and home networks. So why "reinvent the wheel" by devising unique application-specific protocols, when you can benefit from today's mass market deployment of Ethernet? Ethernet is a low-cost, proven open standard, providing the bandwidth capability and Quality of Service (QoS) to support today's most demanding triple play (voice, video and data) services.

It is naive to assume that wireless will always be the answer for home triple-play networking. Although highly desirable to connect a broadband modem or router to IPTV or IP Set-top Box (STB) without the added expense and complication of wiring, it is not always feasible. Each home is unique in structure and layout, and already congested with wireless signals from mobiles, DECT phones, microwaves, wireless PCs, printers and gaming. Telephone companies are now predicting that the necessary quality-of-service (QoS) required for a wireless video link between router and STB/TV cannot be delivered in up as many as 30 percent of homes. In these cases, a wired alternative solution will be required.

There are some obvious candidates for this wired link. Coaxial cable may be common in U.S. households but not in Europe. Laying relatively thick coax or CAT5 cable in the home is awkward at best, and ugly at worst. HomePlug offers a solution using existing powerline cabling in the home, but this, too, has disadvantages of security and doubts over the quality of delivering video and voice.

POF's home-friendly characteristics provide a more promising solution. Thin, flexible plastic optical fiber can be inconspicuously laid along skirting boards (baseboards) and door frames, cut to the desired length, and safely self-terminated. It is immune to electromechanical noise and this allows the provider to offer a guaranteed QoS for video and voice services.

Designers who search the market will find a number of different manufacturers for Fiber-Optic Transceivers (FOTs) designed to provide Fast Ethernet (100 Mbps) over POF. However, unlike traditional FOTs, such transceivers lack a common interface standard; both electrically and physically. So, will this hold back the introduction of Ethernet over POF? Not necessarily, as it depends upon the flexibility of existing Ethernet device specifications. Micrel Semiconductor, for example, offers an Ethernet family of multi-port switches and PHYs well-suited for POF transceivers. With devices such as these, nearly any differential input can be interfaced to the receiver due to the wide-range common-mode and voltage-swing input specification (50mV pk-pk to V_dd).

Figure 2 shows an example for interfacing one of Micrel's Ethernet devices to the Firecomms EDL300E POF transmitter and EDL300D receiver.

Figure 2: And example of an Ethernet-to-POF interface circuit
(Click to Enlarge Image)

This implementation provides a Fast Ethernet link over POF with a typical reach of 100 meters, comparable to a standard CAT5 solution. Any differences in the signals' common-mode voltage are removed by ac coupling of the interfaces. An adjustable voltage reference (LM4041) is used to ensure that the FXSD input is in the correct FX mode when a signal is detected (greater than 2.2V) and loss of signal (between 1 V and 1.8 V). The 10 kΩ and 6 kΩ resistors are then used to adjust the reference voltage across the LM4041 to around 2 V. This acts to clamp the FXSD input at 1.3 V (V_cc - V_ref) when the EDL300D Signal Detect Pin (SD) is low (LVCMOS < 0.4V). When a signal is present, the SD pin is high (>2.4 V), thus no current will flow through the reference voltage diode and FXSD will follow the SD pin voltage (>2.4 V).

Conclusion
POF technology offers an attractive alternative to traditional glass optical fiber, as well as copper wire for industrial, office, home and automotive networks. It is low cost, lightweight, easy and safe to install and EMC immune. However, to become a mainstream alternative to copper cabling in such networks, POF needs to support Ethernet. This article has hopefully shown that despite a lack of standards, interfacing POF to Ethernet can be a straightforward task.

Reference
MOST Cooperation, www.mostcooperation.com

About the author
Mike Jones is a senior field application engineer with Micrel, Inc., San Jose, CA.