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Datagram

See Packet.

dB

See Decibel (dB).

dBi (dB isotropic)

The gain a given antenna has over a theoretical isotropic (point source) antenna. (An isotropic antenna is an imaginary, ideal transmitting antenna that radiates a signal equally well in all directions—theoretically a sphere.) Although isotropic antennas are only theory, they provide a useful tool for calculating theoretical fade and System Operating Margins. For instance, a dipole antenna has 2.14 dB gain over a 0 dBi isotropic antenna.

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. Small signals are negative numbers (e.g. -83 dBm). For example, typical 802.11b WLAN cards have +15 dBm (32mW) of output power. They also spec a -83 dBm Rx sensitivity (minimum Rx signal level required for 11Mbps reception). Additionally, 125 mW is 21 dBm, and 250 mW is 24 dBm.

Decibel (dB)

The difference (or ratio) between two signal levels; used to describe the effect of system devices on signal strength, which varies logarithmically, not linearly. Since the dB scale is a logarithmic measure, it produces simple numbers for large-scale variations in signals, allowing the use of whole numbers to calculate system gains and losses. Every time you double (or halve) the power level, you add (or subtract) 3 dB to the power level. This corresponds to a 50 percent gain or reduction. A 10 dB gain/loss corresponds to a tenfold increase/decrease in signal level. A 20 dB gain/loss corresponds to a hundredfold increase/decrease in signal level. Thus the decibel scale allows big variations in signal levels to be handled easily with simple digits.

DHCP

See Dynamic Host Configuration Protocol (DHCP).

DiffServ

Short for "differentiated services," this IETF standard protocol is a small, well-defined set of per-packet building blocks from which a variety of services may be built, thereby providing a framework for delivering Quality of Service (QoS) in networks. The protocol relies on traffic conditioners sitting at the edge of the network to indicate each packet's requirements. As such DiffServ can specify and control network traffic by class, which allows designated types of traffic to get precedence in the network flow. DiffServ might be used within enterprise networks, for example, to give preferential treatment to mission-critical data. DiffServ-capable routers need only track a small number of per-hop behaviors, and they service packets based on a single byte.

See also IntServ.

See also Quality of Service (QoS).

Digital

A description of data, which is stored or transmitted as a sequence of discrete symbols from a finite set, most commonly this means binary data represented using electronic or electromagnetic signals. In this book, "digital" most commonly refers to the use of a binary code (bits) to represent information. The main benefit of transmitting information digitally is that the signal can be produced precisely and although it will pick up interference or "garbage" along the way, the telecom industry has found ways to regenerate the signal back to crystal clarity. The signal is put through a Yes-No exercise—is this part of the signal a "one" or a "zero"? Then the signal is reconstructed to what it was at the beginning of the transmission, amplified and sent on its merry way. This all means that digital-based transmission is "cleaner" than analog transmission.

Digital Signal Processor (DSP)

This term refers to a specialized type of processor that is optimized for performing detailed algorithmic operations on analog signals after they have been digitized. For examples DSPs handle line signaling in modems.

Digital Subscriber Line (DSL)

Point-to-point public network access technologies that allow multiple forms of data, voice, and video to be carried over twisted-pair copper wire on the telco local loop between a network service provider's central office and the customer site.

Direct Sequence Spread Spectrum (DSSS)

A wide bandwidth signal with low amplitude, which allows it to appear to be "noise" when received on a non-spread spectrum receiver. DSSS uses a radio transmitter to continuously spread its signal containing data packets over a fixed range of a wide frequency band. DSSS is also a transmission technology used in wireless network transmissions where a data signal at the sending station is combined with a higher data rate bit sequence, or chipping code, that divides the user data according to a spreading ratio. Compare with Frequency Hopping Spread Spectrum.

Distributed Coordination Function (DCF)

A form of carrier sense multiple access with collision avoidance (CSMA/CA). A class of coordination functions where the same coordination function logic is active in every station in the basic service set whenever the network is in operation.

DNS

See Domain Name System/Service (DNS).

Domain Name System/Service (DNS)

Since IP addresses, which are numeric, are difficult for people to remember, host names or domain names such as cmp.com are generally used to identify the address of computers that are connected to the Internet. Because computers on the Internet only understand numeric IP addresses, not domain names, every webserver requires a DNS server to translate domain names into IP addresses. Note, however, that a domain name may identify one or more IP addresses.

DSP

See Digital Signal Processor (DSP).

DSSS

See Direct Sequence Spread Spectrum (DSSS).

Due Diligence

A comprehensive investigation and assessment of all attributes. Issues. and variables inherent in a target entity/person/product/service, which will impact upon the target's ability to achieve its strategic objectives.

Dynamic Channel Selection

See Dynamic Frequency Selection (DFS).

Dynamic Frequency Selection (DFS)

This technique, which is also known as dynamic channel selection (DCS), allows client devices to detect the clearest channels within the radio waveband.

Dynamic Host Configuration Protocol (DHCP)

A protocol in the TCP/IP suite that allocates IP addresses automatically to any DHCP client (any device attached to your network, such as your computer) so that addresses can be reused when the client no longer needs them.

Dynamic Rate Shifting

Refers to the ability to adjust connection speeds for more reliable connections. The IEEE 802.11 series of standards defines dynamic rate shifting, allowing data rates to be automatically adjusted for noisy conditions. This means Wi-Fi devices will transmit at lower speeds (e.g. 5.5 Mbps, 2 Mbps, and 1 Mps in the case of 802.11b networks) under noisy conditions. When the devices move back within the range of a higher-speed transmission, the connection will automatically speed up again.



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Going Wi-Fi. A Practical Guide to Planning and Building an 802.11 Network
Going Wi-Fi: A Practical Guide to Planning and Building an 802.11 Network
ISBN: 1578203015
EAN: 2147483647
Year: 2003
Pages: 273

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