Previous | Table of Contents | Next |
Frequency and wavelength are inversely relatedas the frequency of the signal (number of cycles per second) increases, the wavelength (length of the electromagnetic waveform) of the signal decreases. In other words, the more waveforms per second, the shorter the length, or cycle, of each individual wave. Table 3.1 defines the frequency and wavelength of various types of radio and light-based communications systems as they relate to the electromagnetic spectrum [3-2], [3-3], and [3-4]. While higher frequency signals offer greater bandwidth, they also generally suffer to a greater extent from signal attenuation.
Band Designation | Frequency Range | Wavelength | Usage |
---|---|---|---|
Audible | 20 Hz20 kHz | >100 Km | Acoustics |
Extremely/Very Low Frequency (ELF/VLF) Radio | 3 kHz30 kHz | 100 Km10 Km | Navigation, Weather, Submarine Communications |
Low Frequency (LF) Radio | 30 kHz300 kHz | 10 Km1 Km | Navigation, Maritime Communications |
Medium Frequency (MF) Radio | 300 kHz3 MHz | 1 Km100 m | Navigation, AM Radio |
High Frequency (HF) Radio | 3 MHz30 MHz | 100 m10 m | Citizens Band (CB) Radio |
Very High Frequency (VHF) Radio | 30 MHz300 MHz | 10 m1 m | Amateur (HAM) Radio, VHF TV, FM Radio |
Ultra High Frequency (UHF) Radio | 300 MHz3 GHz | 1 m10 cm | Microwave, Satellite, UHF TV |
Super High Frequency (SHF) Ratio | 3 GHz30 GHz | 10 cm1 cm | Microwave, Satellite |
Extremely High Frequency (EHF) Radio | 30 GHz300 GHz | 1 cm.1 mm | Microwave, Satellite |
Infrared Light | 103105 GHz | 300µ3µ | Infrared |
Visible Light | 10131015 GHz | 1µ.3µ | Fiber Optics |
X-Rays | 10151018 GHz | 103µ107 µ | N/A |
Gamma and Cosmic Rays | >1018 GHz | <017 µ | N/A |
k = Kilo = 1,000 (1 thousand) | |||
M = Mega = 1,000,000 (1 million) | |||
G = Giga = 1,000,000 (1 billion) | |||
T = Tera = 1,000,000,000 (1 trillion) | |||
cm = centimeter (1/100 meter) | |||
mm = millimeter (1/1,000 meter) | |||
µ = micron (1/1,000,000 meter) |
The selection of the most effective transmission system for a given application must be made in the context of a number of key design considerations. Such considerations include general transmission characteristics such as bandwidth and error performance, both of which affect throughput. Additionally, one must consider the allowable distance between devices, as well as issues of propagation delay, security, mechanical strength, and physical dimensions. Finally, and perhaps most important of all, are issues of local availability and cost, including cost of acquisition, deployment, operation and maintenance, and upgrade or replacement.
The basic transmission characteristics of a given medium are of primary importance. Those characteristics include bandwidth, or capacity, error performance, and distance between network elements. These three dimensions of a transmission system, in combination, dictate effective throughput, the amount of information that can be put through the system.
Bandwidth, in this context, refers to the raw amount of bandwidth the medium supports. Error performance refers to the number or percentage of errors which are introduced in the process of transmission. Distance refers to the minimum and maximum spatial separation between devices over a link, in the context of a complete, end-to-end circuit. Clearly, any given transmission system increases in attractiveness to the extent that available bandwidth is greater, introduced errors are fewer, and the maximum distance between various network elements (e.g., amplifiers, repeaters, and antennae) is greater.
It should be noted that bandwidth, error performance, and distance are tightly interrelated. In a twisted pair network, for example, more raw bandwidth requires higher transmission frequencies. Higher frequencies attenuate (lose power) more rapidly than do lower frequencies. This fact results in more errors in transmission, unless the amplifiers/repeaters are spaced more closely together. For instance, a physical four-wire, digital T1 circuit typically is provisioned over twisted pair, providing bandwidth of 1.544 Mbps with excellent error performance through the placement of regenerative repeaters spaced approximately every 6,000 ft. A somewhat better grade of twisted pair can be deployed in a Local Area Network (LAN) to support transmission rates of 100 Mbps between a workstation and a LAN hub or switch, with excellent error performance as long as the device separation is [le]100 meters (some manufacturers back away from support at 20 meters). In either case, certain measures must be taken to avoid high levels of ambient noise, the gauge of the conductors must be considered, etc. While this comparison is simplified, it clearly demonstrates the close and direct relationship between bandwidth, distance, and error performance.
Previous | Table of Contents | Next |