Digital Communication System:
Digital Communication System has several distinct advantages over analog transmission systems, but there are definite disadvantages to it as well. Because of the heavy investment in analog facilities, it will be many years before digital becomes the major technique in communication system. In fact, if it were not for the many analog earth stations already in existence, satellite communications would probably have been almost wholly digital by now. The following lists provide a good summary of the advantages and disadvantages of digital communication.
- Transmission quality is almost independent of the distance between The error rate is virtually unaffected by distance due to the regeneration, or cleaning up, of the signal at the regenerators.
- A mixture of traffic, ranging from telephony and telegraphy to data and video information can be easily carried.
- The capacity of certain existing transmission systems can be increased. For instance, the capacity of a single channel over a cable pair can be increased by employing digital multiplexing equipment, whereby several signals are combined into a higher rate signal without any significant impairment. Similarly, the capacity of a satellite communication system can be increased by going to TDM access.
- Digital cable systems are more economical for distances of 15-40 km, particularly if cable capacity is nearly exhausted. It is more economical to convert to digital on the old cable rather than by laying new cables or provide new ductways.
- Digital Communication System lends itself to such novel facilities as cryptography, storage and other forms of digital processing.
- Digital Communication System is more suitable for the newer types of transmission media, such as light beams in optical fibers and circular or helical wave guides operating around 30 to 100 GHz.
- Digital signal characteristics are convenient for electronic switching, in which groups of digits are selected to be switched in turn onto various This is called packed switching.
- One of the major disadvantages of a digital system is its large band width requirement. An audio channel, which may normally require an analog band width of 4 kHz, will require 64 kHz when transmitted by a PCM (digital) system. This can be reduced by signal processing and the employment of special modulation techniques, but at the expense of increased costs and complexity.
- Time division digital transmission is not compatible with the frequency division analog transmission in current use. Both cannot be carried simultaneously, as can be done, for instance, with mono and stereo FM.
Because of the basic incompatibility, however, it is very difficult to gradually change over from analog to digital transmission. Unless digital systems become highly economical or demand becomes high, it will be some time before an international satellite digital system becomes viable.