Over the past few years, computer peripherals have been moving away from parallel or serial connection and to a new type of bus. That bus is the Universal Serial Bus (USB). The build-in serial bus of most motherboards generally offers a maximum of 2 external interfaces for connectivity to a PC, although add-on adapters can take that count up to as many as 16 serial interfaces. USB, on the other hand, can connect a maximum of 127 external devices. Also, USB is a much more flexible peripheral bus than either serial or parallel. USB supports connections to printers, scanners, and many other input devices (such as keyboards, joysticks, and mice).
When connecting USB peripherals, you must connect them either directly to one of the USB ports on the PC or to a USB hub hub that is connected to one of those USB ports. Hubs can be chained together to provide multiple USB connections. Although you can connect up to 127 devices, it is impractical in reality. Most computers with USB interfaces will support around 12 USB devices.
Because fiber-optic cable transmits digital signals using light impulses rather than electricity, it is immune to Electromagnetic Interference (EMI) and Radio Frequency Interference (RFI).
The cable itself comes in two different styles: single-mode fiber (SMF) and multimode fiber (MMF). The difference between single-mode fibers and multimode fibers is in the number of light rays (and thus the number of signals )they can carry. Generally speaking, multimode fiber is used for shorter-distance applications and single-mode fiber for longer distances.
If you happen to come across a strand of fiber in the field and want to know if it’s single mode or multimode, here are some general guidelines. First of all, it’s got a yellow jacket, it’s probabley single mode. If it’s got an orange jacket, it’s most likely multimode. Also, check the writing on the cable itself. you’ll find a number like 62.5/125. These are the outside diameters of the core and the cladding (respectively). If the first number is a 8,9 or 10 it is most likely a single mode. On the other hand, if the numbers read as before (62.5/125), it’s most likely a multimode strand of fiber. Use these two tips to help you identify that errant strand of fiber.
Although fiber-optic cable may sound like the solution to many problems, it has pros and cons just as the other cable types. Here are the Pros:
- It completely immune to EMI or RFI
- Can transmit up to 40 kilometers (about 25 miles)
- It difficult to install
- Requires a bigger investment in installation and meterials)
Fiber-optic cable can use a myriad different connectors, but the two most popular and recognizable are the straight tip (ST) subscriber (or square) cannector (SC) connectors. The ST fiber-optic connector, developed by AT&T, was one the most widely used fiber-optic connectors. It uses a BNC attachment mechanism similar to the Thinnet connection mecfhanism, which makes connections and disconnections relatively easy. Its ease of use is one of the attributes that makes this connector so popular. Figure 1.18 shows an example of an ST connector. Notice the BNC attachment mechanism.
The SC connector is another type of fiber-optic connector. As you can see in Figure 1.19, SC connectors are latched connectors. This latching mechanism holds the connector in securely while in use and prefents it from just falling out. SC connectors work with either single-mode or multimode optical fibers, and they will last for around 1000 matings. They are seeing increased use but aren’t as popular as ST connectors for LAN connections.