V.24
V.24 is an ITU-T standard for a physical layer interface between DTE and DCE. V.24 is essentially the same as the EIA/TIA-232 standard.

V.25bis
V.25bis is an ITU-T specification describing procedures for call setup and tear-down over the DTE-DCE interface in a PSDN.

V.32
V.32 is an ITU-T standard serial line protocol for bidirectional data transmissions at speeds of 4.8 or 9.6 kbps.

V.32bis
V.32bis is an ITU-T standard that extends V.32 to speeds up to 14.4 kbps.

V.34
V.34 is an ITU-T standard that specifies a serial line protocol. V.34 offers improvements to the V.32 standard, including higher transmission rates (28.8 kbps) and enhanced data compression.

V.35
V.35 is an ITU-T standard describing a synchronous, physical layer protocol used for communications between a network access device and a packet network. V.35 is most commonly used in the United States and in Europe, and is recommended for speeds of up to 48 kbps.

V.42
V.42 is an ITU-T standard protocol for error correction using Link Access Procedure for Modems (LAPM).

V.xx
V.xx refers to various types of ITU-T communication standard. Some are for simple serial line communication (e.g. V.24, otherwise known as RS232), others are for modem communication (e.g. V.21, V.22, V.23, V.32), and others are for special uses (e.g. V.42, an error correction protocol). Sometimes, these standards are up-dated, and changed slightly, then they have bis added to the end (e.g. V.42bis a data compression protocol).

V5UA: V5.2-User Adaptation Layer
V5.2-User Adaptation Layer (V5UA) is a protocol in the SIGTRAN protocol stack for the backhauling of V5.2 messages over IP using the Stream Control Transmission Protocol (SCTP). This protocol may be used between a Signaling Gateway (SG) and a Media Gateway controller (MGC). It is assumed that the SG receives V5.2 signaling over a standard V5.2 interface.

Vacant Code Tone
Vacant Code Tone is used in crossbar systems to indicate that the dialed office code is unassigned. In step-by-step areas, this signal is called vacant level tone. For operator-originated calls, the verbal announcement is preceded by two flashes. In modern systems, recorded verbal announcements are used for this service. Vacant Code Tone is Low Tone for 0.5 seconds on, 0.5 seconds off, 0.5 seconds on and 1.5 seconds off.

Vacant Position Tone
Vacant Position Tone is a steady low tone. It is applied to all straightforward trunks terminating in a vacated position in manual offices.

VAD: Voice Activity Detection
Voice Activity Detection(VAD) is a function when enabled on a voice port or a dial peer, silence is not transmitted over the network, only audible speech. When VAD is enabled, the sound quality is slightly degraded but the connection monopolizes much less bandwidth.

VAD: Voice Activity Detector
Voice Activity Detector (VAD) is the device that detects voice activity and allows DTX to operate. In conjunction with DTX, VAD reduces power consumption in the mobile station and RF interference in the system by muting the transmitter when there is no voice to transmit.

Validity Check
Validity Check refers to any check that is designated to ensure the quality of a transmission in telecommunications system. A validity check compares a group of bits with the code being used to ensure that group of bits constitutes a valid character.

Variable Length Buffer
Variable length buffer is a buffer in telecommunication system. The characteristic of this buffer is that data may be entered at one rate and removed at another rate without the data sequence.

VAS: Value-Added Service
Value-Added Service (VAS), in telecommunications industry, refers to the non-core services or all services beyond standard voice calls. Conceptually, value-added services add value to the standard service offered by the telecommunications carrier, encouraging the user to use their phone more and allowing the carrier to drive up their ARPU (Average Revenue Per User).

VCDX: Very Compact Digital eXchange
Very Compact Digital eXchange (VCDX) refers to the smallest configuration of the Lucent 5ESS family which is a compact, packet-ready switch that delivers a full range of services from a single platform, using the same hardware and software as the full-size 5ESS switch. The VCDX can easily evolve into a full-size switch or become an element in a packet network within our Circuit-to-Packet Networks architecture, a convergent architecture that provides both cost-saving packet functionality and high-quality voice functionality. It is the product for bridging the transition of circuit networks to Asynchronous Transfer Mode (ATM)/Internet Protocol (IP) networks.

VCO: Voice Carry Over
Voice Carry Over (VCO) refers to a method for using TTYs (Teletypewriter). VCO allows people who are hard of hearing or oral deaf (deaf with intelligible speech) to use a TTY without typing. They only use the TTY for reading during the inbound direction of the call. They talk to their party by speaking into a microphone. VCO is often used in confunction with a Realy Service.

VCS: Virtual Circuit Service
Virtual Circuit Service (VCS) refers to a service made possible by virtual circuit technologies. Virtual Circuit is created within a shared telecommunication network system between two network devices. There are two types of Virtual Circuits: switched virtual circuits (SVCs) and permanent virtual circuits (PVCs).

VDSL: Very-High-Data-Rate Digital Subscriber Line
Virtual Circuit Service (VCS) refers to a service made possible by virtual circuit technologies. Virtual Circuit is created within a shared telecommunication network system between two network devices. There are two types of Virtual Circuits: switched virtual circuits (SVCs) and permanent virtual circuits (PVCs).

VDSL2: Second Generation VDSL
Virtual Circuit Service (VCS) refers to a service made possible by virtual circuit technologies. Virtual Circuit is created within a shared telecommunication network system between two network devices. There are two types of Virtual Circuits: switched virtual circuits (SVCs) and permanent virtual circuits (PVCs).

Vertical Redundancy Check
Vertical Redundancy Check is the scheme or process of checking errors in communication/modem transmissions by combining vertical error checking and longitudinal error checking.

VIC: Voice Interface Card
Voice Interface Card(VIC) connects the system to either the PSTN or to the PBX.

Virtual Call Capability
Virtual call capability, also called a virtual call facility, is a service feature in telecommunications. Virtual call capability has the following characteristics: (1) A call set-up procedure and a call disengagement procedure determine the period of communication between two data terminal equipments (DTEs) in which user data is transferred by the network in the packet mode of operation. (2) End-to-end transfer control of packets within the network is required. (3) Data may be delivered to the network by the call originator before the call access phase is completed, but the data is not delivered to the call receiver if the call attempt is unsuccessful. (4) The network delivers all the user data to the call receiver in the same sequence in which the data is received by the network. (5) Multi-access DTEs may have several virtual calls in progress at the same time.

Virtual Collocation
Virtual Collocation may means the following: 1) an interconnection agreement where one telephone company owns, installs and maintains equipment to connect networks at another telephone company's request. 2) a logic connection between end users that has a defined route and endpoints in ATM network.

Virtual Switching
The Virtual Switching is a technology allowing multiple switching functions to happen in one physical device or single functional switching to happen in multiple physical devices across network, while in the situation of a real switch, switching is conducted in one physical switch. Virtual switching technology enables a single switch to be used for many different applications. Each different function may have its own discrete performance and security controls. Using virtual switching technology, Service providers can create a dynamic service mix, as desired, without requiring new hardware and enabling the gradual evolution to include new services or a common control plane. Virtual switching is a core concept in the Multiservice network switching architecture.

VMB: Voice Mail Box
Voice Mail Box (VMB), in a telephony network, is a service that provides answering machine services for hundreds or thousands of users. Each user will have their own Voice Mail Box on the system. Each mail box will have a box number and a pass code.

Voice Frequency
Voice Frequency refers to the frequency range for the average human voice, which, when applied to station carrier, is 300 to 3400 Hz.

Voice Grade Channel
Voice Grade Channel is a channel suitable for the transmission of speech, digital or analog data, or facsimile, generally having a frequency range of about 300 to 3400 Hz.

Voice Mail
Voice Mail is a centralized telecommunication system to manage telephone messages for a large group of subscribers. It replaces the function of traditional telephone answering machine, uses a standard telephone handset for the user interface with indication of voice messages waiting and access method. Voice Mail is more sophisticated than traditional answering machines with much more functions such as remotely accessing the voice mail box from different phones in different locations.

Voice Port
Voice Port is an apparatus placed at the telephone subscribers premises that modulates and demodulates the DS0 voice upstream and downstream channels in cable-TV networks.

Voice Response Unit
Voice Response Unit refers to a technology that, by using a phone line, interfaces a person to information held in a PC.

VoIP: Voice over IP
Voice over IP (VOIP) refers to a group of technologies that use the Internet Protocol (IP) to transmit voice as packets over an IP network. Using VOIP protocols, voice communications can be achieved on any IP network regardless it is Internet, Intranets or Local Area Networks (LAN). In a VOIP enabled network, the voice signal is digitized, compressed and converted to IP packets and then transmitted over the IP network. VOIP signaling protocols are used to set up and tear down calls, carry information required to locate users and negotiate capabilities. The key benefits of Internet telephony (voice over IP) are the very low cost, the integration of data, voice and video on one network, the new services created on the converged network and simplified management of end user and terminals.

VT: Virtual Tributary
Virtual Tributary (VT) is a signal designed for transport and switching of sub-STS-1 payloads.

VT-n: Virtual Tributary Level n
Virtual Tributary Level n (VT-n) is a SONET format for mapping a lower-rate signal into a SONET payload. For example, VT-1.5 is used to transport a DS-1 signal.