Below are some questions and answers related to FTTH and optical technology. These were taken form an issue of FTTH Council, Mar-Apr 2011.
Q: What is bandwidth?
A: In a network, bandwidth (what engineers call bitrate) is the ability to carry information. The more bandwidth a network has, the more information it can carry in a given amount of time. Networks
with high bandwidth also tend to be more reliable because fewer bottlenecks disturb the flow of information.
Q: How much bandwidth, or information delivered by bandwidth, do we need?
A: A standard-definition television signal requires a bandwidth of about 2 Mbps – two million bits (zeros and ones) per second. HDTV requires as little as 2.5 Mbps if the image is rather static – a person being interviewed, for instance. But fast action, such as in some sporting events, requires more – as much as 8 Mbps, even with new compression technology such as MPEG-4. Now, full-frame 3D is hot. While it can be delivered at 2.5 to 3 Mbps, a high-quality experience requires closer to 5 Mbps. 3D immersive HDTV – a technology already being used in some academic and industrial settings – will require between 50 and 300 Mbps.
Q: What about data?
A: Bandwidth requirements for many kinds of data are exploding. For example, the digital cameras that consumers buy can create larger and larger images. In health care, the medical images produced by equipment such as CT scanners are orders of magnitude larger than camera images – a hundred times larger, and more. The biggest growth in data transmission has been for video, and this trend is expected to continue at least for the rest of this decade. Video requires not only extra bandwidth but also extra reliability. The smallest delay in data transmission can result in distorted views.
Q: Can’t copper carry high bandwidth?
A: Yes, copper can support high bandwidth, but only for a few hundred yards. The longer the signal travels on copper, the lower the bandwidth. Optical fiber is unique in that it can carry high-bandwidth signals over enormous distances. Fiber uses laser light to carry signals. Under some circumstances, a signal can travel 40 miles (60 kilometers) without degrading enough to keep it from being received. Another difference is that fiber is better able to support upstream bandwidth – that is, from the user out to the network. High upstream bandwidth is important for video communication and for many business applications.
Q: What exactly makes fiber “future proof”?
A: The equipment necessary to send light signals keeps getting better. So equipping an existing fiber network with newer electronics and with lasers that pulse light faster, or lasers using different wavelengths of light, can vastly increase available bandwidth without changing the fiber itself. The new electronics are very cheap compared with the original cost of laying the fiber. Therefore, once fiber has been deployed, network operators can
keep increasing bandwidth at very little cost.
Q: How long has fiber optic technology been in use?
A: Fiber optic cable has actually been used in communications networks for more than 30 years. Fiber first delivered a signal directly to an American home (in Hunter’s Creek, Fla.) more than 20 years ago. Before that, fiber was – and still is – relied upon to carry communications traffic from city to city or country to country. The first trans-Atlantic fiber cable was laid in 1988. Almost every country on earth has some fiber, delivering services reliably and
Q: All providers seem to claim they have fiber networks. What’s different about fiber to the home?
A: Don’t be fooled! It is true that most cable and DSL networks use fiber. In these networks, the fiber carries the signal close enough to a home so that copper can carry it the rest of the way. However, this approach requires xpensive, difficult to maintain electronics at the point where the fiber meets the copper. The available bandwidth is far less than an all-fiber network. And these halfway approaches do not allow symmetrical bandwidth – existing cable and DSL systems can download much faster than they can upload information.
Q: Isn’t that good enough?
A: That depends on what you want to use your bandwidth for. If all you want is to send emails, download songs or share family photos, the bandwidth provided by today’s cable modems and DSL lines may be good enough. If you want to log on to the corporate LAN from home and work effectively, you’ll need more. And what about uploading a high-def video of the school play, or sitting down to dinner with family members a thousand miles away?
Q: Why does it matter how close to the home fiber comes in DSL and cable systems?
A: With copper cable, there is a marked relationship between distance and available bandwidth. For example, the latest commercial versions of DSL can carry a signal of more than 200 Mbps for about 750 feet. Over a distance of a mile, DSL can deliver only about 30 Mbps. That’s the theoretical limit. In practice, the real bandwidth is less.
Q: With cable and DSL, there’s often a difference between advertised and actual bandwidth. Is that
true for fiber?
A: Cable, DSL and even wireless networks are usually more heavily oversubscribed than fiber – that is, providers promise users more than the total amount of available bandwidth because they know all users aren’t going full throttle most of the time. As a result, copper-based networks slow down during periods of heavy use – such as when teenagers come home from school. Fiber has enough bandwidth that providers can guarantee high speeds with little or no oversubscription. If a fiber network is designed properly, users will always get the speeds that are advertised. The Federal Communications Commission now insists that carriers deliver the bandwidth they advertise.
Q: Is FTTH technology expensive?
A: In new construction, fiber costs about the same as copper to build, and it costs much less to operate and maintain. Building fiber to the home is expensive only when compared with not building anything – that is, with adding new electronics to an existing copper network or building fiber only part of the way to the home. The problem is that these less-expensive solutions don’t always meet users’ needs. In the last few years, the flood of video content has outrun the ability of older technologies to handle bandwidth demands. Providers are shutting off or slowing down service or imposing prohibitive fees for customers who exceed monthly bandwidth caps. Customers don’t like these restrictions, and they don’t appreciate being called “bandwidth hogs” for using services they have paid for. In addition, it’s not clear that providers save money by failing to meet users’ needs, because limiting bandwidth means limiting revenue potential as well.