| ||ACK (Acknowledge Packet)|| |
| ||In the Transmission Control Protocol, ACK packets are used to acknowledge receipt of a packet. They are used by both ends of the connection to move in between states, and are the basis of TCP's reliability. |
| ||ADPCM (Adaptive Differential Pulse-Code Modulation)|| |
| ||ADPCM is a technique for converting sound or analog information to binary information (a string of 0's and 1's) by taking frequent samples of it and expressing the value of the sampled sound modulation in binary terms.|
It is a variant of DPCM that varies the size of the quantization step, to allow further reduction of the required bandwidth for a given signal-to-noise ratio. Where circuit costs are high and loss of voice quality is acceptable, it sometimes makes sense to compress the voice signal even further. An ADPCM algorithm is used to map a series of 8 bit PCM samples into a series of 4 bit ADPCM samples. In this way, the capacity of the line is doubled. The technique is detailed in the G.726 standard. Some ADPCM techniques are used in Voice over IP communications.
| ||ADSL (Asymmetric Digital Subscriber Line)|| |
| ||ADSL is a form of DSL, a data communications technology that enables faster data transmission over copper telephone lines than a conventional modem can provide. ADSL has the characteristic that the data can flow faster in one direction than the other, i.e., asymmetrically. |
For conventional ADSL, downstream rates start at 256 kbit/s up to 9 Mbit/s (about 300 m from the central office) but can go as high as 52 Mbit/s over short ranges of within 100 metres (so-called VDSL). Upstream rates start at 64 kbit/s and typically reach 256 kbit/s but can go as high as 768 kbit/s. The name ADSL Lite is sometimes used for the slower versions.
A newer variant called ADSL2 provides higher downstream rates (up to 12 Mbit/s for spans of less than 2.5 km). Higher symbol rates and more advanced noise-shaping are responsible for these increased speeds. ADSL2+ boosts these rates to up to 25 Mbit/s for spans of less than 1.5 km.
Because of the relatively low data-rate (compared to optical backbone networks) ATM is an appropriate technology for multiplexing time-critical data such as digital voice with less time-critical data such as Web traffic; ATM runs widely over ADSL technology to ensure that this remains a possibility.
| ||AM (Amplitude Modulation)|| |
| ||Amplitude modulation (AM) is a method used to modulate a signal, typically using radio. In the case of an analog signal to be sent, the amplitude of the radio wave is modulated to be directly proportional to the value of the analog signal at the time. This should be compared to frequency modulation (FM), which modulates the frequency rather than the amplitude. |
Amplitude modulation typically produces a modulated output signal that has twice the bandwidth of the modulating signal, with a significant power component at the original carrier frequency.
| ||AMI (Alternate Mark Inversion)|| |
| ||AMI is a line encoding technique (line code) for T1s which uses bipolar pulses to represent logical 1 values. It is therefore a three-level system. A logical 0 is represented by no symbol, and a logical 1 by pulses of alternating polarity. The alternating coding prevents the build-up of a DC voltage in the cable. This is considered an advantage since the cable may be used to carry a small DC current to power intermediate equipment such as line repeaters. |
AMI suffers the drawback that a long run of 0's produces no transitions in the data stream (not containing sufficient transitions to guarantee lock of a DPLL). Therefore this type of encoding is not transparent to the sequence of bits being sent. For voice traffic the least significant bit of each frame (every eighth bit) is always set to '1', generating enough transitions on the line. This is acceptable for voice, but not for data.
Example of AMI encoding: The pattern of bits "1 0 0 0 0 1 1 0" encodes to "+ 0 0 0 0 - + 0".
| ||Amplitude Distortion|| |
| ||Distortion occurring in a system, subsystem, or device when the output amplitude is not a linear function of the input amplitude under specified conditions. |
Amplitude distortion is measured with the system operating under steady-state conditions with a sinusoidal input signal. When other frequencies are present, the term amplitude refers to that of the fundamental frecuency only.
| ||Analog Signal|| |
| ||An analog (analogue in British English spelling) signal is any continuously variable signal. It differs from a digital signal in that small fluctuations in the signal are meaningful. Analog is usually thought of in an electrical context, however mechanical, pneumatic, hydraulic, and other systems may also use analog signals. |
The primary disadvantage of analog signalling is that any system has noise -that is, random variations- in it. As the signal is copied and re-copied, or transmitted over long distances, these random variations become dominant. Electrically these losses are lessened by shielding, good connections, and several cable types such as coax and twisted pair.
| ||Angle Modulation|| |
| ||Modulation in which the phase or frequency of a sinusoidal carrier is varied with the information contained in the modulating waveform. Angle modulation has two forms: phase and frequency modulation.|
| ||Application Layer|| |
| ||The application layer is the 7th level of the seven-layer OSI model. This layer interfaces directly to and performs common application services for the application processes; it also issues requests to the Presentation Layer. |
The common application services provide semantic conversion between associated application processes. Examples of common application services of general interest include the virtual file, virtual terminal, and job transfer and manipulation protocols.
| ||ARP (Address Resolution Protocol)|| |
| ||A method for finding a host's Ethernet address from its Internet address. The sender broadcasts an ARP packet containing the Internet address of another host and waits for it (or some other host) to send back its Ethernet address. Each host maintains a cache of address translations to reduce delay and loading. ARP allows the Internet address to be independent of the Ethernet address but it only works if all hosts support it.|
| ||ARQ (Automatic Repeat-reQuest)|| |
| ||Automatic Repeat-reQuest is an error control technique for data transmission in which the receiver detects transmission errors in a message and automatically requests a retransmission from the transmitter. Few types of ARQ protocols are Stop-and-wait ARQ, Go-Back-N ARQ and Selective Repeat ARQ.|
| ||ATDM (Asynchronous Time-Division Multiplexing)|| |
| ||Asynchronous Time-Division Multiplexing (ATDM) is a method of Time Division Multiplexing (TDM) where the time slots are allocated as needed rather than preassigned.|
| ||ATM (Asynchronous Transfer Mode)|| |
| ||ATM is a cell relay network protocol which encodes data traffic into small fixed sized (53 byte) cells instead of variable sized packets as in packet-switched networks (such as the Internet Protocol or Ethernet). |
ATM was intended to provide a single unified networking standard that could support both synchronous channel networking (PDH, SDH) and packet-based networking (IP, Frame relay, etc), whilst supporting multiple levels of quality of service for packet traffic.