Analogue Telephony

So what is FXS,FXO,E&M and all that?

• FXS - Foreign eXchange Subscriber (Device)
• FXO - Foreign eXchange Office (Line)
• E&M - Ear & Mouth
• PSTN - Public Switched Telephone Network
• POTS - Plain Old Telephone Network
• PBX - Private Branch eXchange
• ISDN - Integrated Services Digital Network
• DTMF - Discrete Tone Multiple Frequency
• VoIP - Voice over Internet Protocol

These terminologies relate to Analogue telephony and the characteristics of their signalling on the PSTN.

An FXO device will connect

The line from your telco to the PSTN is typically FXS and your phone or fax machine is an FXO device.

E&M describes a line connected between two locations. If you have ever watched an old black and white movie where the user is frantically tapping away on the hook of a phone with no dial to gain the attention of the operator? This user is sending a "wink" which is a request for service and is the most basic use for a telephone service and is used on an E&M curcuit. This is still used to provide a leased-line or tie-line between remote PBX systems where use is not charged for but rental for thie line is higher. However, any modern system would almost always use a digital (ISDN) line.

The FXO line signals:

• Dial tone after a Loop closure (if there is no ringing voltage)
• Operational power
• Ring voltage to alert the FXS device that a call is on the line
• Call waiting (Option)
• CallerID (Option)
• Message Waiting (Option)

FXS device signals:

• Hookflash
• Sending DTMF to the exchange to request an outbound call
• Loop closure to alert the FXS that it hasaken the call
• Hangup at the end of a call

So why all the information on this old technology now in the age of VoIP?

VoIP technology is powerful and provides a vast array of features and signalling options. Having said this, most home telephone users just want to make a call. They would probably want to reuse their existing telephone as they know how to use it and may have phone books stored.

Business users may have invested in a PBX and handsets with time spent on user training and system familiarity. This is where VoIP becomes really useful.

Integration is the key, providing the cost and feature benefits of VoIP whilst reusing existing investments. This also enables a migration where a company or individual could use legacy POTS at the same time as trialing VoIP services. The PBX may use some trunks connected to the PSTN and some VoIP trunks.

There are many VoIP interfaces available to provide PSTN/POTS type signalling to enable integration of the old and new techniologies. An FXS line can be picked up by an FXO device enabling a line to be rerouted to a new location. This may aid an office move or allow a home line to be answered remotely.

The opportunities are endless and can provide strong economic benefits to a company as long as the right technology is used in the right place. If you need to discuss any solution based on this article, please contact OpusVL.