 VSAT terminal. One application is to deliver, in near real time, point-of-sale (POS) information, forwarding it to headquarters where the VSAT hub is located. Efficiency of bandwidth use is not a primary motivating factor in system design. Thus for the interactive VSAT data network environment, low delay, simplicity of implementation, and robust operation are generally of importance than the bandwidth efficiency achieved. Message access on any shared system can be of three types: (1) fixed assigned, (2) contention (random access), or (3) reservation (controlled access). There are hybrid schemes between contention and reservation. In the fixed assigned multiple access, VSAT protocols are SCPC/FDMA, CDMA spread spectrum technique), and TDMA. 17 All three are comparatively inefficient in the bursty environment with hundreds of thousands of potential users. 9.4 FIBER-OPTIC COMMUNICATION LINKS 9.4.1 Applications Fiber optics as a transmission medium has a comparatively unlimited bandwidth. It has excellent attenuation properties, as low as 0.25 dB/km. A major advantage fiber has when compared with coaxial cable is that no equalization is necessary. Also, repeater separation is on the order of 10-100 times that of coaxial cable for equal transmission bandwidths. Other advantages are: Electromagnetic immunity Ground loop elimination Security Small size and lightweight Expansion capabilities requiring change out of electronics only, in most cases No licensing required Fiber has analog transmission application, particularly for video/TV. However, for this discussion we will be considering only digital applications, principally as a PCM highway or "bearer." Fiber-optic transmission is used for links under 1 ft in length all the way up to and including transoceanic undersea cable. In fact, all transoceanic cables currently being installed and planned for the future are based on fiber optics. Fiber-optic technology was developed by physicists and, following the convention of optics, wavelength rather than frequency is used 