an insulating layer, a shielding layer, and a weather proof outer jacket, as shown in figure 1-10. It is almost as easy to install as twisted-pair, and is the preferred medium for many of the major local-area networks. Coaxial cable is used extensively in local-area networks whenever the distance involved is relatively short, generally less than 2 miles for baseband LANs and 10 miles for broadband LANs. It is used in both  baseband and broadband  networks. Wait a minute! You say you want to know what the terms baseband and broadband mean and how they relate to networks? Not to worry; we explain them to you a little later in the text, but for now, all you need to know is that they both deal with the way data is transmitted (in the form of electrical signals) through some type of medium. Fiber Optic Cable Fiber optic cable is to coaxial cable is to twisted- pair as the F-18 Hornet is to the Corvette is to the model T. It is the newest of the communication mediums, one that   was   spurred   by   the   development   of   laser technology. Fiber optic cable (shown in fig. 1-11) consists of thousands of clear glass fiber strands, each approximately   the   thickness   of   a   human   hair. Transmission  is  made  possible  by  the  transformation  of digital data into  modulated  light beams, which are sent through  the  cable  by  a  laser  light-emitting  diode  (LED) type  device  at  incredibly  fast  speeds.  Transmission rates available (as of 1990) range up to approximately 1 billion (or giga) bits per second (Gbps), with speeds over 2 Gbps possible. When thinking in terms of frequencies,  light  frequencies  are  extremely  high.  They are approximately 600,000 times that of the highest television  channel.  In  terms  of  data  communications, the  higher  the  frequency  of  the  signal,  the  more information it can carry. Put simply, every hairlike fiber within a fiberoptic cable has the capacity to carry many hundreds    of    local-area    network    channels simultaneously. When dealing with fiber optic cable, you will hear such terms as: Figure  1-10.—Coaxial  cable, l l * Figure 1-11.—Fiber optic cable. Monomode—  Single  fiber  cable Multimode—  Several  fibers  within  a  cable Graded index—  A  variation  of  multimode Some of the major advantages of fiber optics over wire media include speed, size, weight, longevity, and resistance to tapping without being noticed. Since it carries no electrical current, it is immune to electrical interference of any kind, and there is no worry of it being  a  shock  hazard. One big disadvantage of fiber optic is the tighter restrictions on how much the cable can be bent. Other disadvantages include higher cost, and the inability to add on new workstations while other stations are active. Although it is relatively easy to splice the fiber optic cable and add new stations, the network or a portion of the network must be down while preparing the splice. On  the  other  hand,  if  your  activity  has  serious interference problems, or has a need for absolute network security, or the need to send signals several miles, then fiber optics might be the only solution. Cable  Selection About now, you may be asking yourself, why all the fuss over transmission speeds?  Why not just simply choose  the  cheapest  transmission  medium  available  and use it? It may not be the ideal situation, but it would get the job done, right? This is true; and with that in mind, we ask you this question. Would you put regular unleaded gasoline in your brand new car that happens to have a high-performance engine? The engine may not run as well as you would like, but it would get the job done, right? The same is true of transmission speeds and the different levels of speed within a computer system. To put it another way, the speed of transmission is very much related to the type of transmission medium used between stations in a network. 1-17