Attenuation is a measure of the decrease in signal strength as is propagates along a medium. Attenuation occurs in any transmission environment, including optical, electrical, electromagnetic. We also experience it in acoustical (i.e., sound) energy. Because we are talking about changes in energy, attenuation is expressed in decibels (dB). Most cables are rated for attenuation in decibels per unit of distance.
Sources of Attenuation
Attenuation is generally caused by one of two phenomena.
- Absorption: Some of the energy of the transmitted signal is literally absorbed by the medium or surrounding elements. For example, radio signals (electromagnetic) transmitted at particular frequencies can be trapped by water molecules. The energy is converted to heat. This is actually the principle on which the microwave oven works, and is one of the reasons some people still worry about the effects of microwave radio on the human person (the power levels, however, are significantly smaller).
- Scattering: Some of the energy of the transmitted signal is redirected or scattered. On an optical facility, for example, some of the light energy can be scattered by minute defects in the glass. If there is a splice in the cable, some of the light energy can be deflected by minute imperfections at the splice point.
In the radio space, a third element is present, known as the inverse square law. This is best understood in the context of omnidirectional antennae. The signal originates at the transmitter and spreads out in all directions. The leading edge of the signal forms a ring around the transmitter in much the same way a wave of water moves away from a dropped stone in a pond. The total amount of energy in that ring was determined at the transmitter, but as the ring expands, the amount of energy at any one point on the ring drops because the ring itself is larger. The amount of energy lost is a function of the relationship between the radius of a circle and its circumference.
Attenuation and Transmission Systems
In general, we can rank media by their susceptibility to attenuation phenomena.
- Optical fiber is the least susceptible transmission system. Single mode fiber can support transmission distances measured in tens of kilometers. Multimode fiber can support distances measured in single kilometers, however the limitation is more due to dispersion effects than to attenuation.
- Coaxial cable transmission systems are next in terms of attenuation characteristics. They can support transmission distances measured in hundreds of meters, depending on the bandwidth required. They have less attenuation than twisted pair because they have less signal leakage.
- Twisted pair is next on the list, but this is a fairly broad statement. There are many types of twisted pair that range from the low quality cable used by [LEC]]s in the outside plant to the high-quality unshielded twisted pair (e.g., Category 7 UTP) used for extremely high rate on-premises cabling. These cables have a range of attenuation characteristics as well as a range of noise immunity characteristics. The latter is predominantly what makes the high speeds possible. For high rate transmissions, this medium is typically limited to distances measured in tens of meters, up to about 100 meters.
- Radio frequency (RF) transmissions tend to be the most susceptible to attenuation, largely due to their existence in free space and the wide variety of objects that can be present in that space to attenuate the signal, and to the inverse square law.
Attenuation and Frequencies
Attenuation does not impact all frequencies equally. In fact, higher frequencies are more significantly impacted than lower frequencies. You have seen this effect if you have ever heard a loud rock music concert from a distance. The base (low frequency) tones of the drums and base guitar survive the trip, but the higher frequency tones of the lead guitar are more significantly impacted.
In a transmission system, this means the further you extend the transmission the fewer working frequencies you can use. Because there is a relationship between the range of frequencies available for transmission (i.e., passband) and the transmission rate of the channel, as a general rule, the further you go (without repetition or amplification), the slower you have to go.
This is why it is difficult to pin down how far you can extend a particular cable plant (or separate transmitters and receivers in a wireless environment. To truly make a definitive statement about distance limitations you have to know the attenuation characteristics of the medium, the power of the transmitter, the sensitivity of the receiver, the desired number of frequencies. For digital transmission systems, the latter can be determined by understanding the desired bit rate, and the specific encoding scheme being used.