What does an Analog Telephone Adapter do?

A short guide to using VoIP ATAs.

Introduction

A VoIP Analog Telephone Adapter is a device that takes one or more separate analog telephone lines and connects them to a LAN or the Internet. These analog lines can either be telephone lines connected to the telco's central office or station lines connected to analog desk phones.

Summary

ATAs take analog voice signals and put them onto the 'net. An ATA connected to an analog phone converts the analog phone into an IP phone, able to send voice communications anywhere the network permits. An ATA connected to a telco trunk can take incoming calls and send them over the 'net -- or it can enable other IP phones to place calls on that telco trunk.

There are many many uses for ATAs: this guide outlines some of the more popular and useful.

Connecting desk phones to IP PBXes

If one is building a PBX using Asterisk and want to connect (say) two dozen analog phones, there are two obvious ways of proceeding:

1. use a channel bank and T100P card to provide 24 FXS ports for the analog phones to plug into; or
2. use 12 Sipura SPA-2000 two-port FXS ATAs.

In smaller installations one can use Grandstream or Sipura ATAs in place of Digium four-port FXS cards.

One can also use ATAs to connect wireless phones and fax machines to Asterisk servers.

Connecting Branch Offices Together

In organisations with more than one location it is not uncommon for many if not most of the telephone calls to be between offices. This means that many of your telephone lines are being used to handle calls between your own offices. If the offices are distant then this can also mean hefty long distance bills for intra-company calls.

Both of these costs can be eliminated by connecting remote offices using VoIP. With ATA devices such as the Grandstream HandyTone or Sipura SPA one can connect PBXes in each office to the Internet and route calls between offices over the Internet, bypassing the telephone company and eliminating long distance charges on intra-company voice traffic.

Given the modest cost of the ATAs (staring at $260 a pair), it does not take long for this investment to be repaid. Even if you have only ten people in your office calling the other branch ten minutes a day, the pay back period will be less than three months!

Providing "Off-Premise Extensions"

Frequently it is desirable to install PBX extensions in offices that are separate from the main facility. Traditionally these "off-premises extensions" required expensive special trunks from the telephone company. No longer. By using pairs of ATAs, one in the remote office and the other at the PBX, one can put extensions anywhere in the world where Internet access is available. Calls routed over the ATAs travel free -- no long distance charges!

Integrating Remote Telephone Trunks

Imagine a scenario with a remote branch office with one or two phones lines. Instead of just having staff at that remote office using phones connected directly to those lines imaging the flexibility and efficiency of having those telephone lines brought back to the main office's PBX and having those remote staffers with PBX extensions. Calls that arrive on the branch office lines are routed into the PBX to be handled by anyone connected to the PBX -- whether they are at the remote office or headquarters. Calls can be transferred between offices seamlessly. And staffers at either location can place local calls to either location at no charge.

This scenario uses one or more SPA-3000 ATAs in the remote branch office to connect to the telephone lines and staff telephones; along with one or more SPA-2000 ATAs at headquarters to connect to the PBX. If HQ is running Asterisk then there is no need for the SPA-2000s -- Asterisk can talk to the SPA-3000s directly!

Technical Details

When selecting an ATA one must consider the type of analog telephone line(s) you need to connect to your ATA. There are three types you are most likely to encounter:

Office Trunk (FXO) - a telephone line that connects the ATA to the telephone company. Your home phone jack is such a type of telephone line.

Station Line (FXS) - a telephone line that connects the ATA to an analog desk telephone.

E & M Trunk - a telephone line that connects the ATA to a special port on some types of PBXs.

Each type of line must be plugged into a port of the same type on the ATA. Note that the ATAs listed below only support FXO and FXS ports. NetVOICE communications also carries ATAs that can handle E&M trunks for specialised PBX applications.

Recommended ATA Products and Comparison Table

* The SPA-2000 & SPA-3000 can support one G.729 session.

Please note that the features have been taken from manufacturers' literature and do not represent a warranty or commitment by netVOICE communications (a division of Silicon-by-the-Sea, Ltd.). Customers are urged to confirm that critical features/functionality are supported by their chosen ATA prior to purchase.