ransmission speeds than LEDs. A laser diode is a pure light source that provides coherent energy with little distor- tion. Therefore, laser diodes are commonly used for long-haul and high-speed transmission. Laser diodes offer better performance than LEDs, and they are more expensive, although the cost of these components has been dropping about 40% per year. As the costs drop, performance is also improving; in the very near future, we should see the introduction of light sources that pulse one trillion bits per sec- ond and beyond, although this, of course, will require a lot more power. To carry traffic over the long haul, the best combination is single-mode fiber with laser diodes. For very short implementations, such as in a campus network Chapter 2 Traditional Transmission Media environment, the cost-efficiencies of multimode fiber and LEDs may make this combination a more appropriate solution. But in general, as we look forward to the new developments in optical equipment-such as wavelength division multiplex- ers, optical cross-connects, and optical switches-we will need higher-quality fiber to interface to. It appears that roughly 95% of the world's fiber plant is not prepared to operate at the high speed that we are evolving to with optical equipment. Even though we have been actively deploying fiber for years, it is not all compatible with the next generation of optical equipment. This means that we will see new compa- nies laying new highways and using the latest and greatest in fiber, as well as older companies having to upgrade their plants if they want to take advantage of what optical equipment has to offer. How Fiber-Optic Transmission Works As shown in Figure 2.9, in fiber-optic transmission, the digital bitstream enters the light source, in this case the laser diode. If a one bit is present, the light source pulses light in that time slot, but if there is a zero bit, there is no light pulse (or vice versa, depending on how it is set up). The absence or presence of light therefore represents the 