In some on premise IP-PBX VoIP installations there will be analog devices and/or non-ip phones that need to be connected to the IP-PBX. These endpoints could include phones where adding an Ethernet cable is not cost effective and other analog devices that are not to be switched to IP, such as fax machines. The best way to make the connection and add dial tone for these endpoints is by using a gateway device with enough ports for the number of devices, assigning each an extension in your IP-PBX. These FXS Gateway devices offer RJ-11 ports to connect up analog office phones, fax machines and other analog devices that then communicate with your VoIP PBX system. Depending on the number of analog devices, you will want to choose a device with a sufficient number of RJ-11 ports, but perhaps a best practice installation would allow for one or two additional ports when dealing with more than four endpoints.
When only one or two devices need to be connected the best choice should be a small ATA (Analog Telephone Adapter), like the Cisco SPA112 or SPA122. These ATAs are excellent for use as a fax interface or when you need to connect a few analog telephones or DECT phones. We have even completed installations using three of these devices in the same location and there are benefits in using several smaller devices, as one device can become damaged and only that one will need to be replaced, rather than having to replace one large more costly unit. ATAs typically come in various sizes based on the number of FXS ports, from one and two FXS port models to larger 4 port or 8 port gateways. When there are 6, 8, 12 or more analog phones/devices, then consider a larger multiport gateway as a good choice, like the new Grandstream HT818, a larger version of the well established HT814. Depending on the installation and other variables, cost and complexity should be weighed when making the decision for multiple smaller ATAs versus one or two larger ones. Typically a device that offers more ports comes in at a less cost per port cost. However, the trade off could be the possibility of losing a device from a storm hit. At times you'll find a single port can be become defective and an easy fix is to just just swap to a good port if one exists. If more damage is done, then the ATA will need to be replaced. Replacing one SPA112 is less costly than an entire HT414 or HT818. Another consideration is the ATA used for the fax machine, if needed. The Cisco SPA ATAs have a good track record being used for a fax interface when the proper settings are configured.
FXS (Foreign eXchange Subscriber) An RJ-11 port used to connect in premise to an analog office phone, fax machine or other analog device. FXS ports are the traditional telephone jack on the wall that provides telephony services and supplies battery power, provides dial tone and generates ringing voltage to the phone.
FXO (Foreign eXchange Office) designates a telephone signaling interface (port) that receives POTS, aka plain old telephone service. An IP-PBX appliance requires the use FXO ports to accept each active copper telephone line from the telco service provider. If you are keeping one, two or more traditional telephone lines (service) when installing a VoIP PBX system, then the best solution is an appliance with enough built-in FXO ports, like the Grandstream UCM6204. FXO ports are used to connect traditional POTS (Plain Old Telephone Service or analog telephony) telco lines and generates off-hook and on-hook.
The IP-PBX connects up to the VoIP carrier via SIP trunks or PRI (T1 telephony card required). Each analog device is assigned an extension in the IP-PBX and connected via the ATA gateway via RJ-11 then to Ethernet to the LAN. Existing RJ-11 connections are usually in the telco closet, keeping all equipment organized. In some mixed use buildings (warehouse-office) using analog DECT cordless phone(s), such as ones offered by Panasonic, give mobility, especially in a warehouse setting if conditions are not unfavorable; an added low cost solution.
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