Notation for Digital-to-Analog Converter. D/A converts digital pulses to analog signal.

Delta or Data channel. An ISDN channel that carries signaling information at 16 kbps in BRI or 64 kbps in PRI. Also see B channel.

Layer 2 in the OSI model. The Data Link Layer defines procedures for data transmission, retransmission, error detection, and error correction. The Data Link Layer is divided into two sub-layers, namely LLC as the upper sublayer and MAC as the lower sublayer.
Ethernet, Token Ring, Frame Relay, ATM, and PPP are examples of Layer 2 protocols.
Also see OSI model.

Digital Audio Visual Council. A standard defined by ETSI for interactive multimedia communication over CATV network.

decibel. A relative measurement of power level difference.
dB = 10log(output power/input power)

A measurement of power level (dB) at one milliWatt (mW).
dBm = 10log(power/1mW)
0 dBm is equivalent with 1 mW.

Direct Broadcast Satellite. Broadcast technology using satellite that transmits signals directly to home users.

Direct Cable Connection. A direct connection between two I/O ports of two computers using a single cable or wireless link. DCC is one of connection types supported by Windows operating system. DCC can be implemented through serial port, parallel port and infrared port.

Data Communication Equipment. A device that functions to encode/decode signals and establish, maintain and terminate a connection. Example: modem.
Also see DTE.

The reverse process of encryption where the encrypted message is deciphered using an assigned key and converted to the original form.
Contrast with encryption.

Digital Enhanced Cordless Telecommunications.

A LAN access method that requires each node in a LAN to request permission (to transmit) to the hub before sending any data. When it actually transmits, data is not broadcast to all nodes but directed to the destination node only. Every node can transmit and receive at the same time. Demand priority supports traffic prioritization.
100VG-AnyLAN technology employs demand priority media access control method.
Compare with contention and token passing.

The opposite of modulation in which baseband signal (information) is taken out from a modulated carrier.
Contrast with modulation.

Data Encryption Standard. A legacy encryption standard using 56-bit encryption key. It is superseded by 3DES and AES.

Dynamic Host Configuration Protocol. A protocol that enables dynamic IP addresses assignment to client computers. Dynamic IP address is meant for ISPs to efficiently use the available IP addresses. Dynamic IP address is also a means of protection from harmful access from the Internet.
A residential gateway (broadband router) usually has built-in DHCP server to assign dynamic IP addresses to computers in a home network. DHCP server at an ISP assigns dynamic IP addresses to its subscribers therefore vacant IP addresses can be used to whoever requests for Internet connection.
Also see dynamic IP address.

Internet connection using voiceband modem. A voiceband modem at  a subscriber site dials a telephone number provided by the ISP to initiate communication with the corresponding modem at the ISP site. The connection takes place over public telephone network (PSTN). Therefore, the telephone line can't be used for telephone call during an Internet connection.
Dial-up connection is considered as narrowband service, but data rate has increased significantly since the beginning of the Internet from below 1 kbps to 56kbps downstream and 33 - 48 kbps upstream. Dial-up is still the most widely used, although its share is slowly bitten by broadband services, i.e. DSL and cable Internet access.
Compare with ADSL and cable modem.

Differentiated Service. A standard defined by the IETF that enables QoS support and traffic prioritization in IP network.

Digital signal refers to signal that discretely varies with time and has finite number of levels accordingly. It is represented by a pulse train. It is generated from the conversion of analog signal by an analog-to-digital converter.
Digital in general refers to technology that transmits/receives and processes digital signal. In digital system, information is encoded into a binary stream of digits 1 and 0.
See picture. Compare with analog.

Antenna that transmits and receives signals in a narrow angle.
Also see antenna. Contrast with omnidirectional antenna.

Digital Living Network Alliance. An industry association that was formed in June 2003 to promote the adoption of Interoperability Guidelines among consumer electronics, computing and mobile devices that will enable the development of consumer-friendly products.

Digital Media Adapter. A network-able device that functions to enable the streaming or playback of digital media content such as photo (jpeg), music (mp3), and movie (mpeg) stored in a computer (server) to television and stereo system.
A DMA box is usually equipped with a remote control.

Discrete Multi Tone. A technique to spread data transmission into parallel streams that is employed by most ADSL modems. Popularly known as OFDM.
Also see ADSL. Compare with CAP.

Demilitarized Zone. A part of an organization network that is exposed to the external network (e.g. the Internet). This sub network usually contains servers (e.g. Web, mail, FTP) accessible to the public.

Domain Name System. DNS server maps domain names with their corresponding IP addresses. Therefore, a user doesn't have to type an IP address in her browser address bar. What she types is an easy to remember name like http://www.conniq.com. Then the name is translated by DNS server to its IP address that is used by routers to route her browser request to the server that stores this Web site.

Data Over Cable Service Interface Specification. A standard that specifies how a cable modem at a cable TV subscriber site and the corresponding modem at a cable TV operator site (Head End) communicate to transfer data securely over a CATV network.
Also see cable modem and CATV.

A group of computers that are part of a network and share a common directory database.

The wireless link that is used by a wireless base station, access point, or satellite to transmit information (signal) to a user station.
Contrast with uplink.

The transfer of a content from a server to a client. For example, if you visit a Website, your browser downloads Web pages from the Website server.
Download (or downloadable) also refers to a file stored in a server that is available for users who have access right to transfer it to their computers. Software, e-books, photos, and music are examples of objects that are sold as downloads on the Internet.
Contrast with upload.

The stream of information from an ISP or the Internet to a client (user) computer. This term is often referred to in DSL or cable Internet access service.
Contrast with upstream.

Digital Subscriber Line. Also referred to as xDSL. A collection of copper enhancement technologies that expands the capacity of a copper wire by utilizing higher frequencies in copper wire spectrum to carry data signals. DSL is divided into two categories, i.e. symmetric and asymmetric DSL. Symmetric DSL has the same downstream and uptream rates. It is typically preferred by business users. Asymmetric DSL has different  downstream and upstream rates in which downstream rate is much higher. It is the choice of most home Internet surfers. For them, DSL refers specifically to ADSL.
Also see DSL flavors: ADSL, HDSL, SDSL, SHDSL, VDSL.

Digital Subscriber Line Access Multiplexer. A rack usually located at a DSL operator Central Office or in an access node that contains DSL modem modules (line cards). DSL modem modules in a DSLAM terminate DSL lines from the subscribers, take the data signals, multiplex them into aggregate connections for transporting them to their destinations on the Internet.
Also see modem and multiplexing.

Direct Sequence Spread Spectrum. DSSS is a spread spectrum technique in which the amplitude of an already modulated signal is amplitude modulated by a very high rate NRZ (Non-Return-to-Zero) binary stream.
DSSS is used in 802.11b wireless LAN (Wi-Fi).
Also see spread spectrum and 802.11b. Compare with FHSS.

Data Terminal Equipment. A device that acts as the data source, the data sink, or both. Examples: computer, printer, typewriter.
Also see DCE.

Dial-Up Networking profile in Bluetooth. With DUN, a Bluetooth device can use the modem of other Bluetooth device for connecting it to the Internet. A notebook, that is linked to a mobile phone via Bluetooth, can use the mobile phone as a GPRS modem to connect it to the Internet.
Also see Bluetooth.

Communication in both directions, transmit and receive.
Also see Full Duplex and Half Duplex.

Digital Video Broadcast.

Dense Wavelength Division Multiplexing. A WDM technology that multiplexes more wavelengths in a fiber optic strand thus the wavelengths are closely spaced. DWDM and WDM are often used interchangeably since today most WDM products gets more dense.
Also see WDM and fiber optic. Contrast with CWDM.

IP address that is assigned by a DHCP server in a residential gateway or an ISP's access server to a client (user) computer that changes every time the user logs on or at a predefined regular interval.
Also see DHCP. Contrast with static IP address.