Modem over VoIP (sometimes abbreviated as ''MoIP'') is the use of
voice over IP
Voice over Internet Protocol (VoIP), also known as IP telephony, is a set of technologies used primarily for voice communication sessions over Internet Protocol (IP) networks, such as the Internet. VoIP enables voice calls to be transmitted as ...
(VoIP) to carry analog
modem
The Democratic Movement (, ; MoDem ) is a centre to centre-right political party in France, whose main ideological trends are liberalism and Christian democracy, and that is characterised by a strong pro-Europeanist stance. MoDem was establis ...
signals, which are then in turn used to carry a two-way digital data link.
G.711 passthrough
Since most VoIP codecs are designed only to carry speech signals, successful modem-over-VoIP operation generally requires the use of
G.711 coding on the VoIP audio link, emulating the channel capabilities of a digital
PSTN
The public switched telephone network (PSTN) is the aggregate of the world's telephone networks that are operated by national, regional, or local telephony operators. It provides infrastructure and services for public telephony. The PSTN consists ...
connection. ITU-T
V.152 describes one way of implementing G.711 passthrough.
G.711 uses 8 bits per sample, either μ-law and A-law encoded, at a sample rate of 8 kHz, and signals carried over G.711 have historically been associated with a maximum practical modem speed of 56 kbps, based on the ITU-T
V.92 standard. This use of G.711 coding is sometimes called 'fax pass-through' as it enables analog
fax
Fax (short for facsimile), sometimes called telecopying or telefax (short for telefacsimile), is the telephonic transmission of scanned printed material (both text and images), normally to a telephone number connected to a printer or other out ...
transmission (which also uses a modem connection) over VoIP.
G.711 modem-over-VoIP communication can be useful for connecting an Internet-connected computer to a dial-up system that only has modem connectivity over the PSTN. It can also be performed end-to-end by hobbyists for the purposes of amusement.
V.150.1
An alternative to G.711 passthrough is the use of ITU-T
V.150.1, also known as Modem over IP, which demodulates and remodulates modem data at gateways, forwarding the data over IP between them. This has the advantage that it can also handle some of the signalling aspects of modem connections.
References
See also
* ITU-T
T.38, also known as Fax over IP, which encapsulates
T.30 fax data directly over IP
Voice over IP
Modems
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