How Analog Telephony Adapters (ATA) Connect IP PBX to FXO (Analog Trunks) and FXS (Analog Stations)

In this article, let us look at what Analog Telephony Adapters are, why ATA’s are required, generic connectivity scenario/ architecture diagram of ATA’s, if ATA’s can work without an IP PBX, salient/ important points about ATA’s.

What is an Analog Telephony Adapter (ATA)?

An IP Telephony solution is never complete without an ATA – Analog Telephony Adapter. These devices are very useful for connecting an IP PBX (IP Telephony Softswitch, IP PBX Appliance, VOIP Server, or any Call Control application in IP environment), Analog Trunks (PSTN Analog Lines from a service provider for making outgoing voice calls – denoted by FXO) and Analog Stations (Analog devices like Analog Phones, Analog Fax etc, that are normally useful in a telephony environment – denoted by FSX).

Why are Analog Telephony Adapter’s (ATA’s) required?

If a company is implementing IP PBX/ VOIP (click here to read the various advantages of IP Telephony), most of the components involved in the solution would be IP (Internet Protocol) based. But invariably, some analog connections also need to be integrated with IP PBX like Analog Trunk Lines (To make Local/ STD outgoing calls) and Analog Phones / Fax Machines. In order to connect them in the IP environment, we need an adapter that can convert the analog signals generated by them into IP signals so that they can be interpreted correctly by the IP PBX . So, an Analog Telephony Adapter is a device that enables both IP and Analog Communication devices/ lines to co-exist and  interact with each other. Among other things, they enable an analog phone to directly make a call over the Internet(via the IP PBX)!

Generic Connectivity Scenario and Architecture Diagram of Analog Telephony Adapter’s (ATA’s):

Analog Telephony Adapter Connectivity Scenario and Architecture Diagram - ATA

Analog Telephony Adapter (ATA) Connectivity Scenario and Architecture Diagram

In the above architecture diagram of ATA, there are two locations – Head Office and a Small Branch Office. There is an IP PBX in the head office, that connects to PSTN trunk lines (telephone lines) through FXO Gateway H (ATA). Actually, it connects to the FXO gateway H through the LAN. There are a lot of IP Phones and Soft-phones that connect to the IP PBX through the IP Network (LAN). All the Analog Telephones in the head office connect to the IP PBX (Via LAN) through the FXS Gateway H. There is also a Router which connects to the LAN on one side and the Internet/WAN at the other.

We also have the remote branch office where there is a smaller router/ modem that connects to the Internet at one end and the FXS gateway R (through LAN) at the other. The FXS gateway directly connects to a couple of Analog Phones. There is also a local FXO gateway R at the branch that connects to the LAN at one end and the PSTN/ Telephone Lines (Analog Trunks) at the other end.

All the IP Phones and the Gateway devices (ATA’s) are registered to the IP PBX/ VOIP Server using IP addresses over the network (each of them have their own IP address – Local IP address). All the Analog Phones connect to the IP PBX in the HO through the FXS Gateway H and the Analog Trunk Lines (PSTN telephone network) connects to the IP PBX through the FXO Gateway H. In the branch, both the FXO Gateway R and FXS Gateway R are registered to the IP PBX (Over the WAN/Internet via VPN – bringing them on to the same network) . In the branch, the analog telephones connect to the FXS Gateway R and hence get an extension number through the IP PBX. The FXO Gateway R connects to the PSTN lines which can be accessed by the IP PBX (VOIP Server) in the Head Office.

Any IP Phone/ Analog Phone (via FXS Gateway H) in the Head Office can now dial to the PSTN network using both FXO Gateway H and FXO Gateway R (Through the IP PBX) and the Analog Phones in the remote branch office can also dial to the PSTN network with both FXO Gateway H and FXO Gateway R. All the phones at either locations can also dial to outside numbers using an ITSP (Internet Telephony Service Provider) as ITSP accounts can be registered in both the IP PBX (VOIP Server) in the head office as well as the ATA Gateway appliance in the branch office. The fax machine in the Head Office also connects to the IP Telephony Server (VOIP Server) through the FXS Gateway H and hence can send and receive faxes through the PSTN network. In certain countries (like India), it is not allowed to bypass the Toll for making Local and STD calls over the IP network (but international calls are allowed over the IP Network) as shown in the above diagram – So it is better to check with your local regulations.

Can Analog Telephony Adapter’s (ATA’s) work without IP PBX?

Yes, they can and a couple of scenarios where such ATA’s can be used as stand-alone devices (without IP PBX/ VOIP Server) is given below:

FXS (ATA) to ITSP Connectivity Architecture Diagram

FXS (ATA) to ITSP Connectivity Architecture Diagram

This is a very simple scenario and can be used in SOHO (Small Office and Home Office) scenario. Here, up to a couple of analog telephones can directly connect to the FXS Gateway and make outgoing calls through an ITSP (Internet Telephony Service Provider) over the Internet. There is no IP PBX required in this scenario. But in scenarios like this, its better to check if the ITSP can provide servcie all the regions that you want to dial out (to any cell phones/ land-lines in those regions). In some countries (Like India), ITSP cannot yet provide Local and STD call service over the Internet due to regulations. But international calls can be made using the above set up.

Analog phones connected to FXS Gateway using FXO gateway to make outgoing calls

Analog phones connected to FXS Gateway using FXO gateway to make outgoing calls

In the above scenario, analog phones are connected to the FXS ATA gateway in one location. In-turn, this FXS gateway is connected to the FXO ATA Gateway (through VPN – Internet network) in another location. The FXO gateway is connected to the analog trunk lines in that location. However, the analog phones (from one location) can access the analog trunks (in another location) through the gateways and dial out to any phone number using the analog trunks. So, someone sitting in one location makes a call to any phone number in another location and still pays only local call charges! Again, this is not allowed within certain countries (like India), and hence its better to check with your local regulations.

In both the above scenarios, the gateway devices can provide basic PBX functions like call hold, call waiting, call transfer, 3-way conference call etc, to the analog phones that are connected to them.

Salient / Important Points about ATA – Analog Telephony Adapter:

  • Both the FXO and FXS ATA gateway devices come with either 1 or 2 LAN/WAN ports and have 1-8 FXO ports/ 1-24 FXS ports.
  • Some ATA devices have integrated router capabilities.
  • Some of them can connect to analog CCD Surveillance cameras and enable to see their output at the remote end.
  • The FXS ATA’s can connect to analog phones as well as analog fax machines.
  • The ATA devices have web based interface that enable the administrators to configure them even from a remote location.
  • Some ATA devices allow multiple SIP accounts to be registered to them directly.
  • Each ATA device has its own IP address and some of them have built-in DHCP servers that can assign IP addresses to devices connected to it.
  • A traditional Analog/Digital EPABX can be connected (integrated) with an IP PBX with limited number of channels (of communication between them) using the Analog Telephony Adapters.
  • Some FXS ATA devices have a PSTN fail-over port that lets the analog phones connected to it make outgoing calls using the PSTN line in case the connection to the Internet (and hence to the ITSP/ SIP Server) fails. The FXS ATA device can also fail over to an FXO gateway instead of a single PSTN line, if appropriately configured.

This article was written based on the Grandstream FXO & FXS Analog Telephony Adapter (ATA). If you live in the United States, you can have a look at the best price for Grandstream Analog Telephony Adapters in US from Amazon.

You might also be interested to read: A Comprehensive introduction and Architecture Diagram for VOIP (IP Telephony) Systems

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