Vocademy

The Telephone System

Parts of this page is in outline/bullet point from and will be fleshed out later

Audio Transducers

Understanding audio transducers is essential to understanding communications circuits. You are expected to review Audio Transduces in Analog circuits (particularly microphones and speakers) as part of your study of Communications Circuits. The invention of the telephone was essentially the invention of audio transducers.

The original telephone system

The telephone system that has been in use for over 100 years is often called:

PSTN - Public Switched Telephone Network
POTS - Plain Old Telephone System

This system emerged for the original telephone developed by Alexander Bell and Thomas Edison.

Bell had developed what we now call a dynamic speaker. This was essentially a paper diaphragm suspended such that it was able to vibrate. This diaphragm was connected to a coil of wire such that the coil was suspended in the field of a permanent magnet. When an electric current is passed through the coil the resulting magnetic field interacts with the field of the permanent magnet, causing the coil and paper diaphragm to move. When the current varied, the coil and diaphragm vibrate as the current varies. If this vibration is fast enough the diaphragm produces audible sound waves.

To produce a working telephone, Bell needed a way to convert sound waves into a varying electrical current. Thomas Watson, Bell's assistant discovered that the speaker they had already invented also worked in reverse; it also worked as a microphone. The dynamic speaker and the dynamic microphone are essentially the same device, but optimized to work in "opposite directions."

An actor portraying Bell speaking into his telephone

Bell and Watson made a working telephone by simply wiring two of the speakers together.

Schematic of Bell's Original Telephone

On February 14, 1876, the same day Bell's attorney filed a patent application for Bell's telephone, Elisha Gray filed a patent caveat (essentially an incomplete patent application) for a device that varied its electrical resistance in response to sound waves. It used a metal diaphragm connected to a needle. This was suspended such that the needle dipped into a metal container of water mixed with acid. There is strong evidence that Bell's attorney persuaded a clerk at the U. S. Patent Office to copy details of Gray's caveat to Bell's application. Despite the resulting lawsuits and conspiracy theories, Gray's device was not used in Bell's telephone (although Bell did test it as a proof of concept).

Bell's telephone was no more than two speaker/microphone transducers connected together. The system was self-powered, meaning there was no external power source. When speaking into one end it generated a signal that could be heard in the other end. You can make a modern version by wiring two small speakers or dynamic microphones together. Simply connect two speakers together with a length of double wire (such as speaker wire or lamp cord) to make a loop.

A modern version of Bell's telephone you can make yourself

Have a friend take one of the speakers into another room and close the door. Talk into one speaker and your friend will faintly hear you in the other. Sound-powered telephones work the same but with improved transducers optimized for the purpose.

Bell's telephone was almost unusable. The dynamic speaker/microphone was not sensitive enough as a microphone. The person talking had to yell into the transducer to be heard at the other end. Edison added a battery and carbon microphone to make the system that was in use for over 100 years. A person speaking into either microphone could be heard clearly in both speakers.[1]

Until the late 1960s the basic telephone system remained unchanged. A basic telephone circult (a connection between two customers) consists of two dynamic speakers, two carbon microphones (one speaker and microphone per telephone) and a battery to power the system. Early systems contained batteries in the phones themselves but these were replaced by batteries in the central telephone switch buildings. Telephones also has a hand-cranked magneto to produce the apporximately 90 volts AC required to ring a bell. Early telephones had a separate earpiece and hand-held microphone, requiring two hands to operate. When carbon microphones were developed that were insensitive to gravity they were placed in a handset with the speaker.

The Bell/Edison Telephone minus the bell-ringing magneto.

To make a call the call the customer turned the crank on the magneto to ring a bell or clicker at the central switchboard. This got the attention of the operator who used a plugboard to complete a circuit between one customer and another.

The early system required operators to connect customers.

the switchboard used 1/4-inch three-conductor plugs and jacks

Parts of the plug were called the tip, ring and sleeve.

  1. Sleeve: ground
  2. Ring: negative, red wire (primary line) or black wire (secondary line)
  3. Tip: positive, green wire (primary line) or yellow wire (secondary line)
  4. Insulating rings

The terms "tip" and "ring" are still used to denote the positive and negative connections to the POTS network.

The Strowger switch system

The Strowger switch served to route calls until electronic switches were invented in the late 1950s. A Strowger switch had ten levels and ten rotary positions. To place a call, the customer lifter the receiver. This closes the hook switch (the phone is now off-hook) causing the voltage on that line to drop from 48 volts to between six and 10 volts. This triggered the system to connect the phone to a Strowger switch and to an AC voltage producing the dial tone.

Rotating the telephone dial opened and closed a switch on the phone. This caused the system to disconnect the dial tone and to operate the Strowger switch. For example, if you dialed with the number five hole, the switch would open and close five times. This opened and closed the circuit, causing a solenoid to step the switch through the five levels. Each level had 10 contacts that connected to other Strowger switches. When the switch stopped at a particular level it automatically rotated to connect to the next switch that was not in use. Dialing the next digit caused the second switch to operate like the first. This continued until the last switch made a connection to the receiving customer. The final switch connected to up to 100 customers. The next to last number dialed took the switch to the corresponding level. That level connects to 10 customers. The final digit dialed caused the switch to rotate to the corresponding customer. Once a connection was made from one customer to another, a "line finder" connected the customers directly, freeing the Strowger switches to be used by other customers. If the final switch could not make the connection, because the receiving customer was already using his/her phone, the system directed the caller to a "busy signal".

A Strowger switch

As the telephone network developed, an amplifying unit was invented using a speaker physically coupled to a carbon microphone, allowing long distance calls. These were later replaced with vacuume tube and then transistor amplifiers. Radio techniques were developed to send multiple calls over single coaxial cables. Microwave links were used to send multiple calls over radio links. More recently the system has been augmented with fiber optic cables.

Early television networks used specially optimized telephone lines to distribute signals across the country.

Modern system

The switch is the building where telephone calls are routed. This is often called the exchange by people outside the industry.

A trunk is multiple lines between switches. These may be 900-pair wire bundles, coaxial cables, microwave links or fiber optics

The Local Loop or Subscriber Loop is the connection between the switch and the customer. Today, many customers are connected by fiber optics instead of copper wire. Customers connected by fiber optics have a device (an OLT) that converts the fiber optic signal into a POTS signal for the home telephones.

Local loop connections start as 900-pair cables. 900-pair cables are divided into 25-pair cables in a B-box.

A telephone B-box

Commercial customers are served by 25-pair cables. Residential customers are usually served by a 2-pair cable where only one pair is used unless the customer orders a second line.

25-pair cables terminate at the commercial customer at a 66-block. Not all commercial customers get all 25 pair. These are often shared by several customers (each with a partially filled 66-block). The wires are color coded where each wire has a colored vinyl coating with a painted stripe of a different color. The color of the vinyl coating is the major color and the stripe is the minor color.

                      

A 66-block and color code

The proper way to wire a 66-block is to use a punch-down tool to wire the 25 wires to the left side of the block. Individual telephones are wired to the opposite side of the block. Jumper bars are used to connect the sides. The jumper bars can be removed to isolate telephones from the telephone system. 66-blocks are often wired whatever way the technician finds convenient.

A 110-block is an alternative to the 66-block that is compatible with high-speed networking.

Part of two 110-blocks

A punch-down tool with blades for both the 66-block and the 110-block.

The POTS system continues to work even if there is a power failure since it is powered by 48-volt batteries at the telephone switch. However, wireless telephones need the mains power to work and still won't operate during a power failure.

Residential customers of all ages are largely abandoning "land lines" in favor of cell phones. Commercial customers are switching to Voice Over Internet Protocol (VOIP). VOIP telephones are essentially computers connected to an Ethernet network. Some VOIP systems use a POTS system throughout the office suite connected to a central VOIP switch in the telecom closet. This switch connects to the Internet. It is now popular for VOIP phones to connect individually to a VOIP provider via the Internet. With such systems, a phone call to the next cubical goes out to the Internet, through the VOIP provider's system and back in to the neighboring phone.

Most cable TV providers also provide telephone service. This is usually a type of VOIP or similar technology that is routed through the cable providers system rather than the Internet (the Internet is inherently unreliable so this makes cable TV phone systems independent of Internet interruptions). Telephone companies are also bundling television and Internet service with their telephone service, often via fiber optics. One system provided by telephone companies connects via fiber optic cable to an intermediate box. From there, a DSL signal (Digital Subscriber Line, a high-speed digital signal sent over a POTS line) is sent to the customer (this is called Fiber To The Curb or FTTC). The DSL signal is compatible with the POTS system. Customers get television and internet through their traditional telephone connection using DSL.

Some residential customers may use VOIP services such as Vonage, Ooma or MagicJack. For these systems a customer may connect a single phone to a VOIP router. Alternatively, the residence's wiring my be disconnected form the telephone company where the local loop enters the residence. A VOIP router connects to the customers telephone wiring from there.

Cell Phones

Cell phone technology is not related to traditional telephone technology (POTS/PSTN). Cell phones are compact computer-controlled radio transceivers. They connect by radio to nearby towers. Calls are then routed to the POTS network and then to traditional telephones or through the cellular tower network to other cell phones.



The Secret Life of Machines: The Telephone


AT&T Archives: The Step-By-Step Switch


—————————
1It is important for the talking person to hear him/herself in his/her own speaker. Otherwise they tend to yell into the microphone.
Vocademy