A brief refresher in Traditional Analog and Digital Voice Telephony

Bell System Logo

Bell System Logo

If you have a telephone company provided wall jack for each individual phone line in your home or office, you probably have POTS – Plain Old Telephone Service – an affectionate name for traditional Analog copper phone line service.    This is a physical pair of copper wires that are directly connected between your home or office and the Telephone Company Central Office.  Each pair of copper wires from the Telephone Company provides an individual dial tone and phone number for your Telephone.

Traditionally, it is the responsibility of the Telephone Company Central Office to provide dial tone, line voltage, and ring voltage to that copper pair – the power that makes the phone work.  As many of us remember in the days before the Internet, even if the Utility Power was out, we could still make and receive phone calls.  Telephone sets had mechanical or electronic bells completely powered by the electricity provided from the Telephone Company Central Office.

If you have a larger organization with dozens of telephones and each person has their own direct dial telephone number and/or dedicated extension, you probably have a PBX (Private Branch Exchange – On-Premise Telephone Switch or Phone System) in a closet with one or more Primary Rate Interface (PRI) Digital Circuits from the Telephone Company.   Each PRI is capable of providing up to 23 simultaneous voice conversations.  A PRI is a special type of copper wire circuit, again between the Telephone Company Central Office and your Office.

Unlike the Analog POTS circuit described above, where there is a one to one relationship between the pair of copper and a telephone number, with a PRI, a virtually unlimited quantity of telephone numbers can be supported but only 23 simultaneous voice conversations can occur at one time per PRI circuit.

In either case, the similarities are that there is very long copper wire between the Telephone Company and your home or office.

If you have Telephone Service from a Cable company, you have a hybrid service that is a combination of both Digital and Analog services.  The “long copper wire” described above is replaced by a digital VoIP (Voice Over IP) service provided through a Cable Modem.

VoIP is a fancy term for delivering traditional telephone services over an IP Data Network, in this context, the Internet.

The VoIP service connects back to the Cable Company Telephone Central Office over the same Coaxial Cable that carries your television service.  Your existing telephone plugs in to an ATA (Analog Telephone Adapter – usually built in to the Cable Modem) that converts the digital VoIP data in to the traditional Analog, two wire pair that your telephone can use.

If you have Telephone Service from Verizon FiOS, you have a hybrid service that is also a combination of both Digital and Analog services similar to that which is provided by the Cable Company.  The difference is that the digital VoIP (Voice Over IP) service travels through a Fiber Optic Cable as laser pulsed light, instead of as electrical signals through Coaxial Cable, back to the Verizon Telephone Central Office.  Your existing telephone plugs in to an ATA (Analog Telephone Adapter) that is built in to an ONT – Optical Network Transmission unit.  The ONT is a specialized piece of equipment that converts the laser pulsed light in the Fiber Optic cable to an electrical signal that the ATA can use to provide dial-tone and a telephone number to your single line telephone.

The Telephony infrastructure is changing at a rapid pace.   It is now extremely rare to have an actual single pair of copper wire connected between your home or office and a Telephone Central Office five or twenty five miles away.  From the beginning of telephone service as we know it dating back over a century, this was common practice and exactly how the original AT&T Telephone Network was built.

In the examples above, Verizon FiOS is converting the Analog electrical signal that your telephone needs to a Digital Light Pulse inside the ONT installed in your home or office.   In essence, Verizon has replaced the miles of traditional copper wire with a Fiber Optic Cable directly connecting your home or office to the Verizon Telephone Central Office.

In areas where Verizon cannot bring Fiber Cable directly to your home or office, they replace the hundreds of pairs and miles of copper wire with Fiber Optic Cable to Junction Boxes in each neighborhood.  (One Fiber Optic Cable can replace hundreds or even thousands of copper pairs of wires.)  In each Junction Box is a monster size ONT that does exactly the same thing as the one used in the Verizon FiOS installation in your home or office above:  It converts the Analog Electrical Signals for the hundreds of pairs of copper wires that run from the Junction Box to your home or office in to pulses of light that travel over the Fiber Optic Cable back to the Verizon Central Telephone Office.


  1. noblewolf says:


    I have a question that might not be that important to the regular person, but very important to me.

    As a medical device patient that relies on analog telephony to transmit critical device monitoring data back to the device manufacturer/doctor, it is critical for me, and the other 1.5 million Americans that use this type of remote monitoring, to have a traditional analog land line for the monitors to function properly. They transfer the data by dialing to the manufacturer and sending data via the phone line, just like a fax machine.

    Would you happen to know when all of the telecom companies will end analog transmission and transfer completely to digital? Every day, more and more people using these monitors are having connectivity issues that do not allow them to continue remote monitoring.

    I hear verizon and at&t are already communicating that they are shutting down traditional analog land lines in some markets.

    Thank you for any info you can provide.


    You can PM me if you want.

    • The issue is not the conversion from analog to digital but perhaps to a Session Internet Protocol (SIP) or Voice-over-Internet-Protocol (VoIP) that is the potential issue.

      As long as you are with your incumbent local telephone carrier, like Verizon with FiOS or a well know Cable Provider, like Optimum, Cox, Comcast, AT&T, etc. you should not have any issue. The major carriers mostly properly support analog modem/fax data without issue.

      If you move to a Hosted VoIP provider – one where you access their Central Office by plugging in a VoIP phone or an Analog Gateway Adapter to your existing Internet connection (Rotuer, network, etc.), then that is where analog modem/fax transmissions fail. Since these types of phone providers rely on “where-is / as-is” Internet connectivity, they cannot control the quality of service required to support analog data transmission.

      You usually need to work with a telephone provider (local utility like an actual phone company or cable company) that controls their own infrastructure to support analog modem and fax data over digital lines.

      Keep in mind that many of the medical device monitoring companies are upgrading and/or offering solutions that use the Internet either as a primary monitoring method or as a backup.

      Thank you for your question,

      Jason Palmer.

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