16-05-2013, 02:19 PM
Ethernet Over Plastic Optical Fibre
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INTRODUCTION
Plastic Optical Fibre, or POF as it is commonly referred to, has emerged as a low cost alternative to traditional copper cabling in office, home and automotive networks. Continued growth for POF is estimated to be worth in excess of $1 Billion/year by the end of 2008.
POF offers many benefits to the user. It is lightweight, robust, cheap and easy to install. A simple sharp blade can be used to cut POF, without the need for polishing or expensive termination equipment afterwards. The use of 650nm red light (LED) makes it completely safe. This safety feature brings additional benefits to the installer, as red light can be easily seen by the human eye to diagnose if the link is good. Traditional glass fibre 850nm laser solutions are not visible to the naked eye nor classified as ‘eye safe’. Perhaps though one of the biggest advantages in today’s complex and congested world of networking is the fact that being fibre, it is totally immune to electro-magnetic noise and just as important, itself emits no radiation. Just think of 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 simplified further as existing mains cable ducting can be used to route the fibre without any impact from noise interference or safety issues.
Industrial networks have been slower to adopt POF compared to automotive manufacturers who have been keen to exploit such technology to connect car infotainments systems and even some safety-critical applications like the airbag. 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 the most common. In fact next generation car specifications can be easily predicted by observing trends in home networking. For the ‘24/7/365’ world we live in the user demands the same services at the office, home and now the car.
A bus protocol called Media Oriented Systems Transport (MOST) led by car manufacturers Damlier Chrysler, BMW and Audi was devised in the late 1990’s to meet the rapidly increasing demands for in-car data bandwidth. MOST25 offers 25Mbps and more recently MOST50 up to 50Mbps data bandwidth using POF as the physical media.
Differences Between Plastic and Glass Fibre
POF differs from traditional optical fibre in material and the core / cladding dimensions. The core is the highly refractive centre of the fibre which acts as a ‘light guide’. For standard telecommunication Single-Mode Fibre (SMF), the core diameter is around 9um and the cladding diameter 125um. SMF is utilised in long haul applications with transmission distances of up to 100km, without the need for any repeater. Multi-Mode Fibre (MMF) uses a core / cladding diameter of typically 50um / 125um, providing less reach, up to approximately 2km, due to increased dispersion as a consequence of the larger diameter core. POF has a much larger core diameter compared to both SMF and MMF, commonly 980um / 1000um. Although this results in lower data rates (hundreds of Mbps) and reach, the big advantage is cost. The large core means the accuracy of alignment between the LED driver and fibre is less critical, to a point where even slightly damaged fibre is acceptable. Most of the expense in fibre systems today is not in the BOM (Bill of Materials) but production set up and alignment costs. The core / cladding diameters of Single-Mode, Multi-Mode and POF fibre are depicted in Figure 1 below.
Ethernet Over POF
Today POF has generally been utilised in more niche applications where the fore mentioned advantages have outweighed the need for high data bandwidth or extended reach of greater than 100m. Advances in LED and VCSEL (Vertical Cavity Surface Emitting Laser) technology have now enabled POF to support data rates of 1.25Gbps over 40m POF under laboratory conditions. Such bandwidth capability should open the doors to more mainstream applications.
For POF to become a real serious alternative to copper cable networks it must be able to support Ethernet. Why? Because Ethernet is almost exclusively used as the lower layers in any office network and is fast beginning to dominate 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 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 SetTop Box (STB) without the added expense and complication of wiring, it is not always feasible. Each home is totally unique in its structure and layout and already congested with wireless signals from mobiles, DECT phones, microwaves, wireless PCs, printers and gaming. Telcos predict that the necessary quality of service required for a wireless video link between router and STB/TV, cannot be delivered in possibly up to 30% of homes. In these cases, a wired alternative solution is required.
Conclusion
POF technology offers an attractive alternative to traditional glass optical fibre as well as copper for industrial, office, home and automotive networks. It is low cost, weighs little, 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. We have hopefully shown that despite a lack of standards, interfacing POF to Ethernet can be a straightforward task.