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A B C D E F G H I J K L M N O P Q R S T U V W X Y Z [NUMBER ENTRIES]

dB

"Decibel" - A decibel is a logarithmic unit representing a change ratio and, because it represents a ratio, the dB is, itself, a dimensionless unit (that is, it doesn't imply milliwatts, watts, or any other particular unit of measurement). The dB unit is 1/10th of a "bel", named after Alexander Grahm Bell. (Originally the ratio was called the TU "Transmission Unit" but was renamed in the 1920's by the engineers at Bell Telephone Laboratory) A bel is the exponent of 10 (hence, the logarithm) representing a ratio. An example will clarify this definition. Suppose that a signal is transmitted with a power level of 100 mW. At some distance from the transmitter the signal is measured and found to have attenuated to a level of 1 mW. The ratio of the measured signal to the original signal is 1:100. The measured signal is 100 times smaller than the original. The number 100 is equal to 10 raised to the second power (100=10^2). The exponent (2) is the "bel" unit. It could be said that the measured signal was 2-bels less than the transmitted signal. As it turns out, the bel ratio metric is too big for most measurements and so the deci-bel (1/10th of a bel) is the common value used. In the example, the transmitted signal has experienced a 20 dB loss at the measurement point. In practical terms, when a dB value goes up or down by 3 it means that the ratio between the two measured values is 2 (up) or 1/2 (down). If a signal experiences an attenuation of 3dB it is now half as strong as it was originally. A value of 3 dB implies .3 bels (since 1 bel = 10 decibels), and 10^.3=1.995 (essentially=2). To convert from dB to the ratio, divide the dB value by 10 (to convert to bels) and use the resultant number as an exponent of 10.

dBm

"dB milliwatts" - The dB ratio is used in conjunction with a predetermined denominator to represent actual units of measurement (as opposed to dimensionless ratios). When the denominator of the ratio is 1 milliwatt the resulting logarithmic representation of milliwatts is called a "dB milliwatt". Hence, 10 dBm = 10^1 milliwatts = 1 mW. Applying the rules for dB ratios, and using 1mW as the denominator in the calculations, the following conversion formulae may be applied:

Diffie-Hellman

A key exchange algorithm that was the first public-key algorithm ever invented. Diffie-Hellman can be used anonymously (without authentication). This means that two devices can exchange an encryption key between them without requiring an external authentication server.

DES

Data Encryption Standard. A federally approved symmetric encryption algorithm in use for many years and replaced by the Advanced Encryption Standard (AES). See also 3DES.

DiffServ

Differentiated services. An architecture for providing different types or levels of service for network traffic. Diffserv aggregates flows in the network so that routers and switches need to distinguish only a relatively small number of aggregated flows, even if those flows contain thousands or millions of individual flows.