Notation of Analog-to-Digital converter (ADC). ADC converts analog signal to digital pulses.

Authentication, Authorization, and Accounting. AAA (Triple A) is the function of RADIUS server to control clients (users) access to the Internet.

Available Bit Rate. A service class in ATM (Asynchronous Transfer Mode) network that adjusts bandwidth according to congestion level.
Also see ATM.

The last mile in a telecommunication network. Refers to the segment from the Local Exchange (LE) or the Central Office (CO) down to the subscribers' homes. Very often mentioned as Local Loop.
In Internet access, access network refers to the segment between a subscriber terminal up to her ISP point of presence (PoP). 

The transport network termination usually located in a curbside cabinet or a building telecom room that functions as service distribution point.

Refers to wireless LAN (Wi-Fi) access point, that is a device that connects computers and devices together in a Wi-Fi network. An access point also bridges the Wi-Fi network and a wired network such as Ethernet and provides interface to a broadband modem or a router.
See picture.

A process to record network usage of a user account, i.e. her connection time or number of bytes transferred.
Accounting is part of AAA function that is handled by RADIUS server in ISP. The accounting data is then fed to a billing server for billing calculation of each subscriber account.
Also see AAA.

Asynchronous Connectionless (ACL) link. A point-to-multipoint link between the master and all the slaves participating on the piconet. In the slots not reserved for the synchronous connection-oriented (SCO) links, the master can establish an ACL link on a per-slot basis to any slave, including the slaves already engaged in an SCO link.
Also see Bluetooth and SCO link.

A temporary network of computers, peripherals, and mobile devices that is typically created in a spontaneous manner. In most cases, it uses wireless technology such as Wi-Fi, Bluetooth and IrDA.
Ad-hoc network is often used for information exchange during a meeting or a conference.
Also see Bluetooth and IrDA.

A person who is responsible for overseeing a network and has full access right to the network. Often called admin for short. 

Asymmetric Digital Subscriber Line. A popular DSL technology that provides asymmetric downstream and upstream data rate over a telephone line by spreading data to higher frequencies above voice frequencies.

To have ADSL service, an ADSL modem must be installed at a subscriber home and her ADSL operator (in DSLAM rack). However, not all telephone lines within a neighborhood are qualified for ADSL. ADSL availability depends on the condition of each telephone line and the distance from a subscriber to the Central Office or Access Node where a DSLAM is located. ADSL is distance sensitive, meaning the closer a subscriber to the DSLAM the higher the data rate, the farther the lower the data rate.

There are two types of ADSL, i.e. Full Rate ADSL and ADSL Lite (a.k.a. Universal ADSL, splitterless ADSL, or G.Lite). Full Rate ADSL installation requires an external splitter (low pass filter) to split telephone signal from broadband data. In ADSL Lite, the filter function is integrated in the modem. ADSL maximum downstream data rate is 8 Mbps (Full Rate) or 1.5 Mbps (G.Lite) and upstream data rate is 1 Mbps (Full Rate) or 512 kbps (G.Lite). ADSL Full Rate is defined in ITU-T G.992.1 (G.dmt) while ADSL Lite in ITU-T G.992.2 (G.lite).

Also see DSL.

A new member in xDSL family. ADSL2 introduces several benefits to ADSL, i.e. real-time connection diagnostic, real-time rate adaptation, fast start-up, flexible power management, improved data rate, all digital mode (ADSL without POTS), enlarged service coverage, and data rate bonding.
To the end users, this means having a more responsive technical support, faster Internet connection, and service availability beyond ADSL traditional coverage.
ADSL2 is defined in ITU-T G.992.3 (G.dmt.bis) and ITU-T G.992.4 (G.lite.bis). Maximum downstream rate is 12 Mbps and coverage up to 18,600 feet (5.67 km) from the Central Office.

ADSL2plus is an improvement to ADSL2 that enables the doubling of potential bandwidth and increases the reach of ADSL2 by up to about 40%. This will expand the broadband coverage to a greater area.
ADSL2+ is defined in ITU-T G.992.5. Maximum downstream rate is 24 Mbps and coverage up to 22,000 feet (6.7 km) from the Central Office.

Advanced Encryption Standard. An encryption standard that uses a key size of 128, 192, or 256 bits. AES was designed to replace DES.
Also see DES.

Audio Frequency. Refers to sound frequencies that are audible to human ears. It ranges from about 20 Hz to 20 kHz.

Adaptive Frequency Hopping. Refers to a technique introduced in Bluetooth 1.2 Specification that enables a Bluetooth transmitter to adapt its frequency hopping sequence to skip those frequencies occupied by other ISM band users.
AFH is used to allow coexistence of Bluetooth and Wi-Fi with very minimum interference.

Amplitude Modulation. A modulation technique where the carrier amplitude varies in accordance with the information (baseband signal) amplitude variations.
The most familiar application is in AM radio broadcast. AM radio stations broadcast at frequency range from about 540 kHz to 1800 kHz.
Also see modulation.

Analog signal refers to signal that continuously varies with time and has infinite number of levels accordingly. It is represented by a sinusoidal wave. It is generated from the conversion of natural event like human speech into electrical signal by a component called transducer.
Analog in general refers to technology that transmits/receives and processes analog signal.
See picture. Also see digital.

American National Standards Institute. The US standard body who coordinates the development of national standards and influences the development of international standards.

A device that radiates and captures radio frequency (RF) signals. Antenna is always present in radio communications. Antenna characteristics (e.g. aperture, gain, radiation pattern, polarization) are calculated in the design of a radio communication system.

Automatic Private IP Addressing. The feature of TCP/IP in Windows XP and some previous versions of Windows (Windows 98, 98 SE, ME, 2000, Server 2003) that automatically assigns itself a Class B IP address within this range 169.254.0.1 - 169.254.255.254 and a subnet mask of 255.255.0.0 in the absence of a DHCP server, when the TCP/IP of a network adapter is configured for dynamic addressing.

Asynchronous Transfer Mode PON. The first PON (Passive Optical Network) specification that was defined by the FSAN (Full Service Access Network) group of service providers in 1998. APON uses ATM encapsulation for transporting data.
Also see PON.

A computer network technology that was developed by Apple Computer. AppleTalk consists of protocols that strictly follows the protocol stack in the OSI Model. It supports proprietary access method (i.e. LocalTalk), Ethernet, Token Ring, and FDDI.
Also see OSI model.

Layer 7 in the OSI model. Application Layer interfaces user applications with network services. Protocols that operate at this layer are: HTTP, FTP, SMTP, SNMP, POP3, IMAP4, Telnet.
Also see OSI model.

Attached Resource Computing Network. A LAN architecture that employs token passing media access control, transmits at 2.5 Mbps, and uses bus or star topology.

Address Resolution Protocol. A protocol that is used when a client computer wants to know the destination node's physical (MAC) address by broadcasting the IP address of the destination node.
ARP matches an IP address to a MAC address.

Advanced Research Projects Agency NETwork. An experimental packet-switched network developed and funded by the US Department of Defense (DoD) in 1969. ARPANET is often mentioned in literatures as the beginning of the Internet.

Automatic Retransmission (or Repeat) reQuest. A request sent by a receiver to a transmitter to retransmit a packet when it detects an error.
Also see FEC.

Average Revenue Per User.

A transmission system in which each character is preceded by a start bit and terminated with one or two stop bits. The function of the stop bit is to assure the detection of the next start bit.
Contrast with synchronous.

Asynchronous Transfer Mode. A Layer 2 (in the OSI model) technology that divides information into fixed length cells of 53 bytes. Each cell consists of a 48-byte payload and a 5-byte header. The use of very small and fixed length cells makes ATM switch very fast and suitable for real-time transmission.
ATM network consists of ATM switches that establish logical circuits end-to-end when a communication occurs thus supporting QoS. ATM network usually runs over SDH/SONET fiber optic transport network.
ATM is the choice of multiservice telecom operators, large enterprises, and ISPs for their backbone networks.
Also see ATM QoS levels: ABR, CBR, UBR, VBR.

ADSL Transceiver Unit. A terminology that is used by DSL Forum for ADSL Transmitter/Receiver device or ADSL modem. The unit that is placed at the Central Office is called ATU-C and the other unit at Remote location (customer premises) is ATU-R.
Also see ADSL.

Attachment Unit Interface. In Thicknet (10Base5), an AUI connector that connects to a computer's DB-15 connector is used to attach an NIC to a transceiver unit.

A process to identify and verify a client (user) using password, PIN, biometric devices, or digital signature (certificate) before allowing her to access a network.
Authentication is part of AAA function that is handled by RADIUS server in ISP. An ISP subscriber must be authenticated before being granted access to the Internet.
Also see AAA.

A process to determine what a client (user) is permitted to do on a network. Network users have different access levels based on their functions or profiles.
Authorization is part of AAA function that is handled by RADIUS server in ISP. An ISP subscriber must be authorized before being able to use resources and services provided by the ISP.
Also see AAA.

The capability of a hub or a switch to sense the traffic condition and automatically change the transmission rate from 10Mbps to 100 Mbps or vice versa.

American Wire Gauge. A US measurement standard for non-ferrous wire diameter. Non-ferrous or non-iron conductor includes copper wire used in telecommunication and networking. The larger the diameter the lower the AWG.
Most copper wires used in telephone and computer networks have AWG value between 22 and 26.