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D

dB

decibel. A logarithmic representation of magnitude relationships commonly used in radio and sound measurement. A ratio of decibels to an isotropic antenna that is commonly used to measure antenna gain. As a rule, the greater the dBi value, the higher the antenna gain value. This results in a more acute angle of coverage.

dBd

dB dipole. The gain an antenna has over a dipole antenna at the same frequency. A dipole antenna is the smallest, least-gain practical antenna that can be made. To convert from dBd to dBi, just add 2.14 to the number (0 dBd = 2.14 dBi).

dBi

dB isotropic. The gain a given antenna has over a theoretical isotropic (point-source) antenna. Unfortunately, an isotropic antenna cannot be made in the real world, but it is useful for calculating theoretical fade and system operating margins.

dBm

dB milliwatt. A signal strength or power level. 0 dBm is defined as 1 mW (milliwatt) of power into a terminating load such as an antenna or power meter. Smaller signals are expressed as negative numbers (for example, 83 dBm).

DBPSK

Differential Binary Phase Shift Keying. Modulation technique used by IEEE 802.11-compliant WLANs for transmission at 1 Mbps.

dBW

dB relative to 1 watt. See also dB.

DDP

Datagram Delivery Protocol. The protocol used to facilitate communication among wireless devices such as the AP to client communication.

demodulate

To convert a modulated signal back to the original signal.

DES

Data Encryption Standard. An encryption standard issued by the National Bureau of Standards. The 3DES protocol uses a version of this encryption that carries three times the encryption protection.

de-spreading

The process used by a correlator to recover the information from a spread-spectrum signal.

dipole antenna

Antenna with the gain, pattern, and impedance defined at and near resonance of one-half wavelength. The antenna is split at its electrical center for connection to a transmission line. The radiation pattern is maximum at right angles to the axis of the antenna. Cisco dipole antennas are made of rubber; therefore, this antenna is sometimes referred to as a rubber duck. This started a long time ago when a very popular dipole manufacturer once referred to them in this manner and to this day still uses the duck logo in advertisements.

direct sequence (DS)

A spread-spectrum modulation technique where a pseudo-random code directly phase modulates a carrier, increasing the bandwidth of the transmission. The resulting signal has a noiselike spectrum. The signal is de-spread by correlating with a pseudo-random code identical to and in synchronization with the code used to spread the carrier at the transmitter. Sometimes referred to as DSSS.

directional antenna

An antenna that concentrates transmission power into a direction thereby increasing coverage distance at the expense of coverage angle. Directional antenna types include Yagi, patch, and parabolic dish.

diversity

The intelligent system of two antennas continually senses incoming radio signals and automatically selects the antenna best positioned to receive it. This feature proves useful for operating in areas susceptible to the affects of multipath interference.

down tilt

The angle difference between perfectly horizontal (compared to the earth) and the center of the vertical beam width of an antenna.

DQPSK

Differential Quadrature Phase Shift Keying. Modulation technique used by IEEE 802.11-compliant WLANs for transmission at 2 Mbps.

DSSS

direct sequence spread spectrum. This transmission technology spreads a signal across a wide range of frequency spectrum. The data signal is broken up into several bits (known as a chipping sequence) and sent across the medium. The receiving station can reassemble the bits into the original information, even if some of the bits are missing due to interference.

dual band

Referred to when a WLAN device has more than one radio or a radio capable of operating on more than radio band (set of channels). The Cisco 1200 series AP is a dual-band unit.