Simultaneous voice and data, or SVD, is a telecommunications term that refers to transceiving

Transceiver

A transceiver is a device comprising both a transmitter and a receiver which are combined and share common circuitry or a single housing. When no circuitry is common between transmit and receive functions, the device is a transmitter-receiver. The term originated in the early 1920s...

 both voice and primary data

Data

The term data refers to qualitative or quantitative attributes of a variable or set of variables. Data are typically the results of measurements and can be the basis of graphs, images, or observations of a set of variables. Data are often viewed as the lowest level of abstraction from which...

 concurrently over one PSTN modem

Modem

A modem is a device that modulates an analog carrier signal to encode digital information, and also demodulates such a carrier signal to decode the transmitted information. The goal is to produce a signal that can be transmitted easily and decoded to reproduce the original digital data...

.

Background

Before telecommunication networks began to support data related services, the bearer channel was dedicated solely to voice information. When support for data related services was originally released, the bearer channel

Bearer channel

A bearer channel is a DS-0 that carries call content i.e. one that does not carry signaling.In the Common Channel Signaling scheme for telecommunications, signaling is sent out-of-band, while all other traffic rides bearer channels...

 was switched between being dedicated to voice information or dedicated to primary data information, depending on the subscriber's needs at any given time. Eventually, support for simultaneous voice and data transmission was added to modems

Modem

A modem is a device that modulates an analog carrier signal to encode digital information, and also demodulates such a carrier signal to decode the transmitted information. The goal is to produce a signal that can be transmitted easily and decoded to reproduce the original digital data...

 and mobile devices, to enable subscribers to use voice and data services at the same time.

Types

• Digital Simultaneous Voice and Data, or DSVD, defined by the ITU
  Itu
  Itu is an old and historic municipality in the state of São Paulo in Brazil. The population in 2009 was 157,384 and the area is 641.68 km². The elevation is 583 m. This place name comes from the Tupi language, meaning big waterfall. Itu is linked with the highway numbered the SP-75 and are flowed...
  
   V.70 standard.
• Analog Simultaneous Voice and Data
  V.61
  ITU V.61 is a standard for Analog Simultaneous Voice and Data , a MODEM related standard for use over a POTS line or leased line.The can be downloaded free of charge in several languages.-See also:*V.70 Digital Simultaneous Voice and Data...
  
  , or ASVD, defined by the ITU
  Itu
  Itu is an old and historic municipality in the state of São Paulo in Brazil. The population in 2009 was 157,384 and the area is 641.68 km². The elevation is 583 m. This place name comes from the Tupi language, meaning big waterfall. Itu is linked with the highway numbered the SP-75 and are flowed...
  
   V.61
  V.61
  ITU V.61 is a standard for Analog Simultaneous Voice and Data , a MODEM related standard for use over a POTS line or leased line.The can be downloaded free of charge in several languages.-See also:*V.70 Digital Simultaneous Voice and Data...
  
   standard.
• Data over signalling
  Data over signalling
  Data over signalling, or DoS, is a telecommunications term that refers to sending primary data over a signalling channel, instead of the subscriber's bearer channel.-Background:...
  
  , or DoS, wherein data is multiplexed onto the signalling channel and voice is dedicated to the bearer channel.
• Combined voice and data, wherein the data is multiplexed with voice onto the bearer channel.