An analog telephone adapter (ATA) is a device for connecting traditional analog telephones, fax machines, and similar customer-premises devices to a digital telephone system or a voice over IP telephony network.
An ATA is often built into a small enclosure with an internal or external power adapter, an Ethernet port, one or more foreign exchange station (FXS) telephone ports. Such devices may also have a foreign exchange office (FXO) interface for providing alternative access to traditional land line telephone service.
The ATA provides dial tone, Ringing Generator, DC power, caller ID data and other standard telephone line signaling (known collectively as BORSCHT) to the telephone connected to a modular jack.
The digital interface of the ATA typically consists of an Ethernet port to connect to an Internet Protocol (IP) network, but may also be a USB port for connecting the device to a personal computer.
Using such an ATA, it is possible to connect a conventional telephone to a remote VoIP server. The ATA communicates with the server using a protocol such as H.323, SIP, MGCP, SCCP or IAX, and encodes and decodes the voice signal using a voice codec such as G.711, G.729, GSM, iLBC or others. Since the ATA communicates directly with the VoIP server, it does not require a personal computer or any software such as a softphone. It uses approximately 3 to 5 watts of electricity, depending on model and brand.
An ATA is connected between an IP network (such as a broadband connection) and an existing telephone jack in order to provide service nearly indistinguishable from public switched telephone network (PSTN) providers on all the other telephone jacks in the residence. ATAs are used by many VoIP companies selling a telco-alternative VoIP service, where the device is used to replace a user's connection to a traditional telephone company. When sold in connection with a VoIP service, the ATA is often locked so it cannot be used with a competing service, and the user can only partly change its configuration. Some providers do sell devices that are not locked and can be used with any compatible provider. This type of service, which is fixed to one location, is generally offered by broadband Internet providers such as cable companies and telephone companies as a cheaper flat-rate traditional phone service.