Some terms are defined at the locations referenced in the Index.
| 0x | Indication that the following characters are a hexadecimal expression. |
| 5ESS | Class 5 Electronic Switching System, CO voice switch that serves end users. |
| AB | Two signaling bits in T-1 CAS when using a superframe. |
| ABCD | Four signaling bits in T-1 CAS when using the extended superframe. |
| ABNF | Augmented Backus—Naur Form. |
| AC | Area Code, in telephone number. |
| ACD | Automatic Call Distributor, PBX function or machine to spread calls among phones. |
| ACELP | Adaptive CELP. |
| ACF | Admission Confirm. |
| ACK | ACKnowledge, message type to confirm receipt of request or action taken. |
| ADC | Analog-to-Digital Converter, first step to encode voice as a digital stream. |
| address | IP address of a host; MAC, DLCI, or other L2 destination or source identifier; cf socket. |
| ADPCM | Adaptive Differential Pulse Code Modulation, form of compressed voice encoding. |
| AH | Authentication Header, form of IPsec that digitally signs but doesn't encrypt. |
| A-Law | Form of PCM companding used with E-1 links. |
| ALF | Application Level Framing. |
| ALG | Application Layer Gateway. |
| ALI | Automatic Location Information (Identifier), geographic indication associated with a telephone, landline or cellular, delivered to a PSAP with E911 call; kept in an ALI database. |
| AN | Acknowledgment Number, associated with an octet in a TCP stream or a packet. |
| analog | Can take on any value to represent sound; see digital. |
| ANI |
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