The role of the ATA or Analog Telephone Adaptor is to turn regular telephones into VoIP telephones. To perform this magic, the ATA has to fool the telephone set into thinking it is plugged into a standard telephone line. It also has to fool the broadband Internet connection into thinking it is plugged into a SIP (Session Initiation Protocol) telephone compatible with the VoIP provider's service.
The electrical interface is perhaps the easiest to achieve. On the analog side, the phone plugs into a standard RJ-11 jack like you would find on a wall socket. This delivers the standard -48 V DC to power the handset along with the dial tone, AC ringing, and Caller ID signals when needed. It has to detect standard DTMF (Dual Tone Multi-Frequency) touch-tone waveforms from the keypad and convert them to digital format for IP signaling using Session Initiation Protocol, the standard for VoIP.
How does the analog voice become digital and back to analog again? That's the function of the Codec or Coder/Decoder. The standard in use since digital telephony was invented is G.711. But there are other codecs that must also supported. These may include G.723, G.729, G.728 and G.726.
On the network side, The ATA has to terminate a standard Ethernet connection, usually 10/100 Mbps through a RJ-45 connector labeled as WAN. Many newer ATAs take this a step further by including a second Ethernet port labeled LAN. This is for connection to a PC in lieu of connecting the PC directly to DSL, Cable Modem or T1 WAN. The ATA performs the function of an Ethernet switch or router. This is not just to save the cost of a separate device so that PC and telephone can share the same broadband connection. When a router is included in the ATA, it then has the ability to ensure quality of service by giving voice packets priority over computer data packets so that using the PC doesn't interrupt telephone calls.
Maintaining voice quality while minimizing bandwidth is also the function of signal processing within the ATA. Some common processes include line echo cancellation and dynamic jitter buffer that compensates for variations in Internet line speed. PLC or Packet Loss Concealment deals with missing or out of sequence packets. Either the packet is replaced with a zero packet, a copy of an already received packet, or an interpolation between the preceding and following packets. VAD or Voice Activity Detection senses when you are talking and stops generating packets while you are listening to reduce bandwidth needs. Because digital lines have no traditional analog "hiss", people sometimes equate silence with a lost connection. CNG or Comfort Noise Generation recreates that low level background noise to indicate that the connection between phones is still in place.
All of these functions have been integrated into a package you can hold in your hand, in the HandyTone 502 ATA that is provided at no cost when you order PhonePower VoIP telephone service for residential and small business use. You can use the analog phone you have now, including cordless phones with multiple handsets.
Other VoIP service providers have also gone the route of providing their customers with complimentary ATAs to make the transition from traditional analog to digital VoIP as easy as possible.