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A voice-and-data modem supports communication over a telephone connection for both speech and digital data. The phrase can describe a device that offers the two functions separately, or, more specifically, a modem that carries voice and data at the same time. Those meanings are not interchangeable.
What is a voice-and-data modem?
A modem converts serial digital data into a signal suitable for transmission over a telephone channel, then converts the received signal back into digital data. That is the general definition in the NIST glossary; it does not mean every modem also supports speech.
Adding “voice” indicates that the device has voice functionality as well as data capability. Depending on the design, it may switch between voice and data modes, or support both simultaneously. Check the device or standard’s specifications to determine which meaning applies.
Does it carry voice and data at the same time?
Not necessarily. “Voice and data” can refer to two available functions without implying that they operate simultaneously. A specific example of simultaneous operation is ITU-T Recommendation V.61, a historical standard for the general switched telephone network and leased point-to-point two-wire telephone circuits.
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V.61 specifies voice-plus-data signaling at 4,800 bit/s, with optional automatic switching to data-only signaling rates up to 14,400 bit/s. These are figures for that 1996 telephone-line standard, not broadband modem speeds or a description of current internet service. See the ITU-T V.61 recommendation.
How are voice calls, data calls, and voice-band data different?
Voice calls and modem data calls
Telephone voice calls and modem data calls are distinct call types. RFC 2806 defines separate URI schemes for voice, fax, and modem data calls; its modem scheme describes a terminal that can handle incoming data calls. A modem intended for data calls should not therefore be assumed to carry ordinary human speech. Read RFC 2806.
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Voice-band data
Voice-band data is a way to transport modem, fax, and text-telephony signals through a packet network’s voice channel using a suitable codec. It describes a transport method, not a separate modem product category. RFC 4734 distinguishes signaling events from the payload and discusses approaches including modem relay, voice-band data, clear-channel data, T.38 fax, and character-mode text. See RFC 4734.
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What does V.21 tell you about modem data rates?
V.21 is an example of a standardized modem protocol, not a definition of every voice-and-data modem. RFC 4734 gives V.21 a maximum rate of 300 bits/s and describes two channels that can support full-duplex data transmission. V.21 is also used to carry control data in other protocols. Those details concern this protocol and should not be generalized to other modems; see RFC 4734.
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What should you check when identifying one?
- Simultaneous or switched operation: Does the specification say voice and data can run together, or that the device switches between modes?
- Connection type: Is it designed for a telephone line, a particular telephone network, or another transport?
- Supported functions: Does it handle speech, fax, incoming modem data calls, or only data?
- Applicable standard and rate: Match any stated rate to the named standard and connection. For example, V.61’s telephone-circuit rates do not establish compatibility with broadband or VoIP.
For older telephone-line equipment, standard compliance alone does not establish that it will work with a current broadband or VoIP service. Compatibility depends on the specific device and network; the cited V.61 standard covers specified telephone circuits, not modern broadband service.
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