P.O.T.S.

The buzz about communications today is about computer networks, notably the Internet. A forgotten stepchild of the communications infrastructure is the massive yet unnoticed system that provides the entire world with compatible communications—the telephone. The telephone is the most ubiquitous communications system and carries an astounding amount of traffic—voice and data to every corner of the world. Today it is supplanted by cellular systems, satellite phones and a plethora of other recent inventions, but the basic telephone system in many ways have remained the same for over 100 years.

A crude prototype of telephone was invented by Alexander Graham Bell (working with Thomas Edison) in 1876 and was put into commercial use in 1878. By 1885 long distance calls were made possible though switchboard operators connecting wires strung across large distances. The telephone today is essentially the same things as what existed in 1900 and is often referred to as POTS (or Plain Old Telephone System). While the public knows it as POTS, the telecom engineers prefer to call it PSTN (Public Switched Telephone Network).

Acronyms aside, the phone system consists of a telephone instrument located at the end-users premises connected by two wires to a “telephone exchange” located in a big windowless concrete building. The two wires are called the “tip” and the “ring”. When the telephone is idle, there is a 48-volt DC signal between tip and ring. When the telephone is “lifted” it puts a load on the circuit that is detected by the exchange. The exchange then drops the voltage to about 9-volts and sends an audible signal on the line called the dial tone. If you use a rotary dial, as the dial rotates back it “disconnects” the telephone momentarily—if you dial 1 there is 1 disconnect, if you dial 7 there are 7 disconnects. The exchange detects these disconnects and notes the digit you have dialed, which is of course followed by more digits. Current phone systems use tone dialing, which speeds things up by sending a short burst of a tone, different pitch for different numbers dialed, rather than the clickety-click of a series of disconnects for every digit.

After the dialing is over the central switch connects you to the other end and rings that telephone. The ringing is achieved by sending a 90-volt, AC wave at 20Hz to the telephone causing the bell in the phone to vibrate and ring. When the phone is picked up, a voice circuit is established and over the 2-wire connection voices can flow in both directions. A simple circuit consisting of a microphone, a headphone, a balancing transformer and a battery achieves this two-way communication. The techniques used are simple, effective and highly reliable. Remarkably not much has changed in the working of the POTS thus allowing a circa-1940 rotary telephone set to work quite fine on the most modern telephone system used today.

The exchange used in the early days of telephony, called a switchboard, comprised of wires terminating in plugs, one plug per telephone subscriber, on a large wooden board. Operators connected one subscriber to another by plugging two wires into a common patch panel. Connections that needed two exchanges needed two operators who simultaneously plugged the subscribers into the same inter-exchange wire, called a “trunk”.

The biggest change in telephone technology has happened at the exchange end. The first automated exchange was the Strowger exchange invented in 1892, but pressed into service in 1896 after the invention of the telephone dial. As the dial clicked out the numbers, large rotating arms at the exchange flipped around myriads of contacts, coming to rest on the selected destination thus connecting the call. Eventually in 1938 the crossbar switch was invented, and the “delightful clinking, whirring and squeak-squeak” was replaced by little relays that were activated by dialing pulses. Even though crossbars were vastly superior to Strowgers, they were costly, bulky and horrendously complicated. 

The mechanical switching of crossbars became history soon after the most radical invention of the century—the transistor. Of course, transistors replaced the relays and the electronic exchange was born. The electronic exchange was not the final answer—soon the digital exchange became the panacea and today, the world’s telephones run of off a scheme that has no resemblance to the POTS of yester-years.

Today, when you pick up the phone, the call does not go to the exchange, but it is detected by a little electronic gadget in a junction box located quite close to home called a concentrator. The concentrator converts the dialing and voices into digital pulses and collects all the digital pulses from all the phones in your neighborhood and sends them over a common wire to the exchange. At the exchange hundred of inputs from concentrators are mixed together and put on a trunk that connects all the exchanges in a city.

Suppose you are talking to a friend across town. As your voice impinges on the microphone, it gets converted to an electrical signal whose voltage varies with time and is called an analog signal. This analog signal travels out of your home, over the telephone wires leading to the nearest concentrator, which converts it into a series of digital bits. A digital voice channel is a bit-stream flowing at the rate of 64 thousand bits per second. Your bits then get intermixed with bits from the neighbors as it traverses over the link from the concentrator to the exchange.

At the exchange, your bits along with all the bits of all the people at all the concentrators get dumped into a trunk. The trunk is a wire that snakes its way to all the telephone exchanges in a city. This trunk is often not a real wire, but a glass fiber (also called fiber-optic cable). The bits over the glass fiber are no longer electrical but are pulses of light. Fiber optics allows millions of voices to be simultaneously transmitted over the same physical cable.

Suppose we color your bits red, and leave all the other uncolored. Then we can see your red bits flowing over the trunk going to every exchange connected to the trunk.  However, the destination exchange servicing your friend’s telephone recognizes the red bits and picks them up (the other exchanges ignore them). Then the exchange sends these bits to the concentrator near your friends home. This concentrator recognizes the red bits and plucks them and sends then to a converter where an analog signal emerges. The analog signal then is fed into the wires leading to your friends phone. That is how he or she can hear you, crisp and clear.

The POTS system of voltages, dial pulses, analog signals and such were invented around the technology of the Strowger exchanges of 1920. It is not only archaic, but also very inefficient. The phone system if designed today, would not use such beastly technology. Yet it has survived many a technological revolution.

There have been many challenges to POTS. The notable ones are digital phone systems with names such as Merlin, Rolm and a digital infrastructure called ISDN. All of these systems share two major qualities. They are much better than POTS and they are extinct.

POTS is here to stay, possibly forever. At the subscriber end, the telephones have changed. They are cheaper, lighter, better, smarter and significantly more fashionable. Cordless phones eliminate the wire from the handset to the base. Electronics in the base ensures better sound quality and adds features like memory, redial and so on. Yet the wire that connects the phone to the outside world is the same old POTS.

The antiquity of POTS was seen as a major hindrance to the modernization of the telephone system. Hence the pushes for significantly better solutions like ISDN (Integrated Services Data Network). ISDN provides multiple voice channels, fast data channels, better signaling and a whole slew of other obscure features, available to the end subscriber. ISDN is available from almost every phone company in every country (Calcutta Telephones has provided ISDN for over a decade). Yet, if you know of any person who has ISDN service at his/her home, you must know a real oddball (apologies to any reader with ISDN service).

One major reason for the failure of “better” phone systems such as ISDN is that innovations built around POTS can provide almost all the bells and whistles that ISDN provides. Need multiple voice circuits? Install extra phone lines, or install a PBX. Want data connectivity? Buy a modem (cheap). Want enhanced phone services? That is now available via POTS—for example is it possible to call your bank and get your account balance by authenticating yourself to a computer that reads back the information, all via the tone pad on your POTS phone.

There is another reason why POTS reigns supreme—plain, convoluted, circular economics. Arrogant, but true, the consumer product market (worldwide) is largely driven by the needs of American consumers. In the US, there is no charge for local POTS calls, due to an old quirk in telecommunication laws. These laws do not apply to ISDN calls, and hence the phone companies charge by the minute for ISDN calls. Hence hardly anyone subscribes to ISDN. Hence hardly anyone makes ISDN-based gadgets such as cordless phones, answering machines, fax machines and modems. Hence no one wants phone service, on which a cordless phone does not work. End of story.

Partha Dasgupta is on the faculty of the Computer Science and Engineering Department at Arizona State University in Tempe. His specializations are in the areas of Operating Systems, Cryptography and Networking. His homepage is at http://cactus.eas.asu.edu/partha