| ANI Automatic Number Identification A telephone function that transmits the billing number of the incoming call (Caller ID, for example). | MGCP Media Gateway Control Protocol A protocol for IP telephony that enables a caller with a PSTN phone number to locate the destination device and establish a session. |
| BLI Busy Lamp Indicator A light or LED on a telephone that shows which line is in use. | PBX Private Branch eXchange An in-house telephone switching system that interconnects telephone extensions to each other as well as to the outside telephone network. |
| DID Direct Inward Dialing The ability to make a telephone call directly into an internal extension without having to go through the operator. | PRI Primary Rate Interface An ISDN service that provides 23 64-Kbps B (Bearer) channels and one 64-Kbps D (Data) channel (23 B and D). |
| Diff-Serv Differentiated Services The Diff-Serv model divides traffic into a small number of classes to provide quality of service (QoS). | PSTN Public Switched Telephone Network The worldwide voice telephone network. |
| DNIS Dialed Number Identification Service A telephone function that sends the dialed telephone number to the answering service. | SIP Session Initiation Protocol A protocol that provides telephony services similar to H.323, but is less complex and uses less resources. |
| DTMF Dual-Tone Multifrequency The type of audio signals generated when you press the buttons on a touch-tone telephone | T1 A 1.544-Mbps point-to-point dedicated digital circuit provided by the telephone companies consisting of 24 channels. |
| E1 The European counterpart to T1 which transmits at 2.048 Mbps | TAPI Telephony API A programming interface that allows Windows client applications to access voice services on a server. |
| 802.1p An IEEE standard for providing QoS using three bits (defined in 802.1q) to allow switches to reorder packets based on priority level. | TOS Type of Service A method of setting precedence for a particular type of traffic for QoS. |
| 802.1q An IEEE standard for providing virtual LAN (VLAN) identification and QoS levels. Three bits are used to allow eight priority levels, and 12 bits are used to identify up to 4,096 VLANs. | Trunk A communications channel between two points, typically referring to large-bandwidth telephone channels between switching centers that handle many simultaneous voice and data signals. |
| H.323 An ITU standard for real-time interactive voice and videoconferencing over LANs and the Internet | VoIP Voice over (TCP/) IP A method of sending voice information over a packet-switched network, such as the Internet, using TCP/IP. |
