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A Simple Telephone

Surprisingly, a telephone is one of the simplest devices you have in your house. It is so simple because the telephone connection to your house has not changed in nearly a century. If you have an antique phone from the 1920s, you could connect it to the wall jack in your house and it would work fine! The very simplest working telephone would look like this inside:

As you can see, it only contains three parts and they are all simple: • A switch to connect and disconnect the phone from the network - This switch is generally called the hook switch. It connects when you lift the handset. • A speaker - This is generally a little 50-cent, 8-ohm speaker of some sort. • A microphone - In the past, telephone microphones have been as simple as carbon granules compressed between two thin metal plates. Sound waves from your voice compress and decompress the granules, changing the resistance of the granules and modulating the current flowing through the microphone. That's it! You can dial this simple phone by rapidly tapping the hook switch -- all telephone switches still recognize "pulse dialing." If you pick the phone up and rapidly tap the switch hook four times, the phone company's switch will understand that you have dialed a "4."

A Real Telephone

The only problem with the phone shown on the previous page is that when you talk, you will hear your voice through the speaker. Most people find that annoying, so any "real" phone contains a device called a duplex coil or something functionally equivalent to block the sound of your own voice from reaching your ear. A modern telephone also includes a bell so it can ring and a touch-tone keypad and frequency generator. A "real" phone looks like this:

Still, it's pretty simple. In a modern phone there is an electronic microphone, amplifier and circuit to replace the carbon granules and loading coil. The mechanical bell is often replaced by a speaker and a circuit to generate a pleasant ringing tone. But a regular $6.95 telephone remains one of the simplest devices ever. Telephone Network: Wires and Cables

The telephone network starts in your house. A pair of copper wires runs from a box at the road to a box (often called an entrance bridge) at your house. From there, the pair of wires is connected to each phone jack in your house (usually using red and green wires). If your house has two phone lines, then two separate pairs of copper wires run from the road to your house. The second pair is usually colored

M.C. Edith García Cárdenas 1 yellow and black inside your house. (See What do the little boxes that the phone company has around our neighborhood do? for a description of the telephone boxes and wires that you see by the road.)

A typical phone company box that you see by the side of the road. Click here to learn more. Along the road runs a thick cable packed with 100 or more copper pairs. Depending on where you are located, this thick cable will run directly to the phone company's switch in your area or it will run to a box about the size of a refrigerator that acts as a digital concentrator. http://electronics.howstuffworks.com/telephone-image.htm

The Telephone Network: Digitizing and Delivering

The concentrator digitizes your voice at a sample rate of 8,000 samples per second and 8-bit resolution (see How Analog and Digital Recording Works for information on digitizing sounds). It then combines your voice with dozens of others and sends them all down a single wire (usually a coax cable or a fiber- optic cable) to the phone company office. Either way, your line connects into a line card at the switch so you can hear the dial tone when you pick up your phone. If you are calling someone connected to the same office, then the switch simply creates a loop between your phone and the phone of the person you called. If it's a long-distance call, then your voice is digitized and combined with millions of other voices on the long-distance network. Your voice normally travels over a fiber-optic line to the office of the receiving party, but it may also be transmitted by satellite or by microwave towers. (See How does a long-distance call work? for a more detailed description.)

Creating Your Own Telephone Network

Not only is a telephone a simple device, but the connection between you and the phone company is even simpler. In fact, you can easily create your own intercom system using two telephones, a 9-volt battery (or some other simple power supply) and a 300-ohm resistor that you can get for a dollar at Radio Shack. You can wire it up like this:

Your connection to the phone company consists of two copper wires. Usually they are red and green. The green wire is common, and the red wire supplies your phone with 6 to 12 volts DC at about 30 milliamps. If you think about a simple carbon granule microphone, all it is doing is modulating that current (letting more or less current through depending on how the sound waves compress and relax the granules), and the speaker at the other end "plays" that modulated signal. That's all there is to it! The easiest way to wire up a private intercom like this is to go to a Hand Generated! You know the hand crank on those old-fashioned telephones? M.C. Edith García Cárdenas It was used to generate the ring-2 signal AC wave and sound the bell at the other end! hardware or discount store and buy a 100-foot phone cord. Cut it, strip the wires and hook in the battery and resistor as shown. (Most cheap phone cords contain only two wires, but if the one you buy happens to have four, then use the center two.) When two people pick up the phones together, they can talk to each other just fine. This sort of arrangement will work at distances of up to several miles apart. The only thing your little intercom cannot do is ring the phone to tell the person at the other end to pick up. The "ring" signal is a 90-volt AC wave at 20 hertz (Hz). Ne Calling Someone

If you go back to the days of the manual switchboard, it is easy to understand how the larger phone system works. In the days of the manual switchboard, there was a pair of copper wires running from every house to a central office in the middle of town. The switchboard operator sat in front of a board with one jack for every pair of wires entering the office. Above each jack was a small light. A large battery supplied current through a resistor to each wire pair (in the same way you saw in the previous section). When someone picked up the handset on his or her telephone, the hook switch would complete the circuit and let current flow through wires between the house and the office. This would light the light bulb above that person's jack on the switchboard. The operator would connect his/her headset into that jack and ask who the person would like to talk to. The operator would then send a ring signal to the receiving party and wait for the party to pick up the phone. Once the receiving party picked up, the operator would connect the two people together in exactly the same way the simple intercom on the previous page was connected! It is that simple!

Tones

In a modern phone system, the operator has been replaced by an electronic switch. When you pick up the phone, the switch senses the completion of your loop and it plays a dial tone sound so you know that the switch and your phone are working. The dial tone sound is simply a combination of 350-hertz tone and a 440-hertz tone, and it sounds like this: • Click here to hear a dial tone. (For more information on tones, see How Guitars Work.) You then dial the number using a touch-tone keypad. The different dialing sounds are made of pairs of tones, as shown here: 1,209 Hz 1,336 Hz 1,477 Hz 697 Hz 1 2 3 770 Hz 4 5 6 852 Hz 7 8 9 941 Hz * 0 #

M.C. Edith García Cárdenas 3 A typical number that you dial sounds like this:

If the number is busy, you hear a busy signal that is made up of a 480-hertz and a 620-hertz tone, with a cycle of one-half second on and one-half second off, like this:

Telephone Bandwidth

In order to allow more long-distance calls to be transmitted, the frequencies transmitted are limited to a bandwidth of about 3,000 hertz. All of the frequencies in your voice below 400 hertz and above 3,400 hertz are eliminated. That's why someone's voice on a phone has a distinctive sound. Compare these two voices:

You can prove that this sort of filtering actually happens by using the following sound files: • 1,000-hertz tone • 2,000-hertz tone • 3,000-hertz tone • 4,000-hertz tone • 5,000-hertz tone • 6,000-hertz tone Call up someone you know and play the 1,000-hertz sound file on your computer. The person will be able to hear the tone clearly. The person will also be able to hear the 2,000- and 3,000-hertz tones. However, the person will have trouble hearing the 4,000-hertz tone, and will not hear the 5,000- or 6,000-hertz tones at all! That's because the phone company clips them off completely. For lots more information on telephones, telephone networks and related technologies, check out the links on the next page.

Cordless telephones are one of those minor miracles of modern life -- with a cordless phone, you can talk on the phone while moving freely about your house or in your yard. Long before cell phones became so cheap that anyone could afford one, cordless phones gave everyone the freedom to walk and talk within the privacy of their own homes. Cordless phones have many of the same features as standard telephones, and there are many models available. In this article, we will examine how these cordless telephones work and see why there are so many different types on the market today.

The Basics

A cordless telephone is basically a combination telephone and radio transmitter/receiver (see How Telephones Work and How Radio Works for details on these two technologies). A cordless phone has

M.C. Edith García Cárdenas 4 two major parts: base and handset. • The base is attached to the phone jack through a standard phone wire connection, and as far as the phone system is concerned it looks just like a normal phone. The base receives the incoming call (as an electrical signal) through the phone line, converts it to an FM radio signal and then broadcasts that signal. • The handset receives the radio signal from the base, converts it to an electrical signal and sends that signal to the speaker, where it is converted into the sound you hear. When you talk, the handset broadcasts your voice through a second FM radio signal back to the base. The base receives your voice signal, converts it to an electrical signal and sends that signal through the phone line to the other party. The base and handset operate on a frequency pair that allows you to talk and listen at the same time, called duplex frequency.

A Brief History

Cordless phones first appeared around 1980. The earliest cordless phones operated at a frequency of 27 MHz. They had the following problems: • limited range • poor sound quality - noisy and ridden with static because walls and appliances interfered with the signals • poor security - people could easily intercept signals from another cordless phone because of the limited number of channels In 1986, the Federal Communications Commission (FCC) granted the frequency range of 47-49 MHz for cordless phones, which improved their interference problem and reduced the power needed to run them. However, the phones still had a limited range and poor sound quality. Because the 43-50 MHz cordless phone frequency was becoming increasingly crowded, the FCC granted the frequency range of 900 MHz in 1990. This higher frequency allowed cordless phones to be clearer, broadcast a longer distance and choose from more channels. However, cordless phones were still quite expensive, about $400. In 1994, digital cordless phones in the 900 MHz frequency range were introduced. Digital signals allowed the phones to be more secure and decreased eavesdropping -- it was pretty easy to eavesdrop on analog cordless phone conversations. In 1995, digital spread spectrum (DSS) was introduced for cordless phones. This technology enabled the digital information to spread in pieces over several frequencies between the receiver and the base, thereby making it almost impossible to eavesdrop on the cordless conversations. In 1998, the FCC opened up the 2.4 GHz range for cordless phone use. This frequency has increased the distance over which a cordless phone can operate, and brought it out of the frequency range of most radio scanners, thereby further increasing security.

\Anatomy of a Cordless Telephone To illustrate the parts of a cordless telephone, we will show you the inside of this one from General

M.C. Edith García Cárdenas 5 Electric (GE). It was made in 1993 and operated in the 43-50 MHz range.

GE cordless phone, including handset and base unit As mentioned above, all cordless phones have a base and a handset. Let's look at these parts individually. Base The base unit of the cordless phone is plugged into the telephone jack on your wall.

Base unit components If you open up the base and expose the circuit board, you see several components that carry out the functions of the base: • phone line interface - receives and sends telephone signals through the phone line • radio  amplifies signals to and from phone-line interface, user controls and speaker phone (if present)  broadcasts and receives radio signals to and from the handset • power - supplies low voltage power to the circuits and recharges the battery of the handset

Circuit board in the base of the GE cordless phone Phone Line Interface Phone line interface components do two things. First, they send the ringer signal to the bell (if it's on the base) or to the radio components for broadcast to the handset. This lets you know that you have an incoming call. Second, they receive and send small changes in the phone line's electrical current to and from the radio components of the base. When you talk, you cause small changes in the electrical current of the phone line, and these changes get sent to your caller. The same happens when the caller talks to you. Radio Components The radio components receive the electrical signals from the phone line interface and user controls (keypads, buttons). The radio components convert the signals to radio waves and broadcast them via the antenna. Radio components use quartz crystals to set the radio frequencies for sending and receiving. There are two quartz crystals, one for sending signals and one for receiving signals. Remember that the base and handset operate on a frequency pair that allows you to talk and listen at the same time (duplex). The radio components include an audio amplifier that increases the strength of the incoming electrical signals. Power Components A DC power cube transformer supplies the low voltage required by the electrical components on the circuit board. The power components on the circuit board work with the power cube to supply electrical current to re-charge the battery of the handset. In addition to the above components, some bases also have audio amplifiers to drive speakers for speaker phone features, keypads for dialing, liquid crystal displays (LCDs) for caller ID, light-emitting diodes (LEDs) for power/charging indicators, and solid state memory for answering machine or call- back features. Handset You can carry the handset with you throughout the house or outside within the range of the base transmitter. The handset has all of the equipment of a standard telephone (speaker, microphone, dialing

M.C. Edith García Cárdenas 6 keypad), plus the equipment of an FM radio transmitter/receiver.

Block diagram of handset components When you open up the handset, you see these components: • speaker - converts electrical signals into the sound that you hear • microphone - picks up your voice and changes it to electrical signals • keypad - input for dialing • buzzer or ringer - lets you know that you have an incoming call • radio components  amplify electrical signals to and from microphone and speakers  send and receive FM radio frequencies • LCD or LED displays - indicator lights • re-chargeable battery - supplies electrical power to handset

Parts of the GE cordless phone's handset, showing the fronts of the circuit boards Parts of the GE cordless phone's handset, showing the backs of the circuit boards, the speaker, microphone, ringer and battery Speaker The speaker receives the electrical signals from the audio amplifier in the radio components and converts them into sound. When you remove the cover from the speaker, you see a large round permanent magnet with a hole in the middle and a deep groove surrounding the hole. Within this deep groove is a coil of fine copper wire that is attached to a thin plastic membrane. The plastic membrane covers the magnet and coil.

Close-up view of the speaker in the GE cordless telephone handset Close-up of the speaker with the top removed Close-up of the speaker with the plastic membrane and attached coil lifted out. The large metal disc is the magnet. Close-up of the speaker's plastic membrane with attached wire coil To hear sounds, the following events happen: 1. Electrical signals come from the radio components. 2. The electrical signals travel in the coil of copper wire. 3. The electrical signals induce magnetic currents in the coil of wire, thereby making it an electromagnet. 4. The electromagnetic coil moves in and out of the groove within the permanent magnet. 5. The coil moves the attached plastic membrane in and out at the same frequencies as the changes in electric currents. 6. The movements of the membrane move air at the same frequencies, thereby creating sound waves that you can hear. Microphone The microphone changes the sound waves from your voice into electrical signals that are sent to the audio amplifier of the radio components. A microphone is essentially a speaker that works in reverse. When sound waves from your voice move the membrane, they make tiny electric currents either by moving a coil of wire within a magnet or by compressing the membrane against carbon dust (see How do microphones work? for details).

M.C. Edith García Cárdenas 7 Close-up of handset's keypad circuit board with attached microphone and buzzer Keypad The keypad allows you to dial a number. It transfers the pressure from your fingertip on the appropriate key into an electrical signal that it sends to the radio components. Below the rubber keypad is a circuit board with black conductive material under each button (shown above). The keypad works like a remote control. When you press a button, it makes a contact with the black Duplex Example material and changes its electrical conductance. The conductance sends an Base: electrical signal to the radio components indicating that you have selected that number. • 44.32 MHz transmitter • 49.28 MHz receiver Buzzer or Ringer When the radio components of the handset receive the ringer signal from Handset: the base, they send electrical signals to the buzzer. The buzzer changes • 49.28 MHz transmitter those electrical signals into sound much like the speaker does. You hear • 44.32 MHz receiver the buzzer sound and know that someone is calling you. In some phones, the speaker is used to make the ringer sound and there is no need for a separate ringer. Radio Components The radio components of the handset are like those of the base -- they convert electrical signals from the microphone into FM radio signals and broadcast them at the same frequency as the receiving crystal of the base unit. The radio components also receive radio signals at the same frequency as the broadcasting crystal from the base, convert them to electrical signals and send them to the speaker and/ or buzzer (ringer). Remember that the base and handset operate on a duplex frequency pair that allows you to talk and listen at the same time. LCD or LED Displays Most handsets have one or more light-emitting diodes (LED) that indicate various things, such as when the phone has an open line or when the battery is low.

LED indicator light on the handset of the GE cordless phone Some handsets have an LCD that can display numbers for caller ID features, similar to a cell phone. The LCD may be reflective or backlit so that you can see it when the room light is low. Battery The handset's battery supplies the power for all of the electrical components in the handset. All cordless phone handsets have a rechargeable battery (nickel-cadmium, nickel-metal hydride or lithium). When the battery runs low, an indicator light on the handset usually lights up or flashes. In some phones, a "beeping" sound may also indicate a low battery. You then recharge the battery on the base of the cordless phone. The GE cordless phone that we dissected was from 1993. Modern cordless phones have the same functions and much of the same hardware. However, many of the electronic circuits that were once achieved with transistors, resistors and capacitors have been replaced with integrated circuits. This advancement allows the handset to be either smaller with the same functions or the same size with more functions. In summary, a cordless phone is basically a combination of a telephone and an FM radio transmitter/receiver. Because it is a radio transmitter, it broadcasts signals over the open airways rather than specifically between the base and handset.

M.C. Edith García Cárdenas 8 Many cordless phone conversations can be easily picked up by radio scanners. Because of this open broadcast, It is possible for other people to listen to your phone conversation by using a radio scanner. So an important issue and feature to look for in a cordless phone is security -- DSS offers the best protection against eavesdropping Features

Cordless phones have many of the same features as standard telephones, and there are many models, offering lots of different features. Major Features Remember that a cordless telephone is a combination of a telephone and a radio transmitter/receiver. Because it is a radio transmitter/receiver, you have the following issues that you do not have on a standard cord phone: • range • sound quality • security The range is the distance that the handset can be from the base. The sound quality can be affected by the distance, the way the information in the radio signal is transmitted, and interfering structures such as walls and appliances. Security is an issue because the radio signals from both handset and receiver go over the open airways, where they can be picked up by other devices (other cordless phones, baby monitors, radio scanners). The above issues relate to the following features of your cordless phone: • radio frequency • analog vs. digital technology • number of channels Frequency Because your cordless phone is a radio transmitter/receiver, it operates on various radio frequencies, which are set by the Federal Communications Commission (FCC) as with any other radio. Cordless phones operate over three major frequency bands (base and receiver use two closely related but separate frequencies within the band so that you can talk and listen at the same time): • 43-50 MHz • 900 MHz • 2.4 GHz • 5.8 GHz The 43-50 MHz band was common to early cordless telephones and is still available in low-cost models. Because of the low frequency, these phones have short ranges (about 1,000 ft / 330 m) and poorer sound quality (due to interference from structures and appliances). The 43-50 MHz phone signals can also be picked up easily on radio scanners and nearby baby monitors. The 900 MHz band (actually 900-928 MHz) is the most common frequency for cordless phones today. The higher frequency gives it a greater range (5,000 to 7,000 ft / 1,500 to 2,100 m) and better sound quality. However, 900 MHz signals can be picked up easily by most commercially available radio scanners.

M.C. Edith García Cárdenas 9 In 1998, the FCC opened up the 2.4 GHz range for cordless phone use. A 2.4 GHz or 5.8 GHz cordless phone can operate over a greater distance and is above the frequencies that can be picked up by most commercially available radio scanners; therefore, it is more secure than lower frequency models. Analog vs. Digital Analog technology is common in cordless telephones, especially in inexpensive models. Analog signals tend to be more noisy, or prone to interference with respect to sound quality. In addition, analog signals are easily picked up and interpreted by radio scanners. In contrast, digital technology, like that found in a CD, allows the phone signals to sound clearer. Furthermore, digital signals are more secure. In 1995, digital spread spectrum (DSS) was introduced for cordless phones. DSS spread the digital information in pieces over several frequencies between the receiver and the base, thereby making it almost impossible to eavesdrop on cordless phone conversations. Channels Each frequency band (43-50 MHz, 900 MHz, 2.4 GHz or 5.8 GHz) can be subdivided into different increments or channels. For example, on some models, when you're talking on your 900 MHz phone, the base searches for a pair of frequencies (channels) within that range, that is not already in use, in order to talk to the handset. So, if the base is capable of searching more increments, it can more easily find a frequency pair that is clear from interference, providing better sound quality. The number of cordless phone channels can vary as follows: • 10 to 25 channels - 43-50 MHz phones, some inexpensive 900 MHz phones • 20 to 60 channels - most 900 MHz phones • 50 to 100 channels - high-end 900 MHz and 2.4/5.8 GHz phones voz/ip

Protocols

As we've seen, on each end of a VoIP call we can have any combination of an analog, soft or IP phone as acting as a user interface, ATAs or client software working with a codec to handle the digital-to- analog conversion, and soft switches mapping the calls. So how do you get all of these completely different pieces of hardware and software to communicate efficiently to pull all of this off? The answer is protocols. There are several protocols currently used for VoIP. These protocols define ways in which devices like codecs connect to each other and to the network using VoIP. They also include specifications for audio codecs. The most widely used protocol is H.323, a standard created by the International Telecommunication Union (ITU). H.323 is a comprehensive and very complex protocol that was originally designed for video conferencing. It provides specifications for real-time, interactive videoconferencing, data sharing and audio applications such as VoIP. Actually a suite of protocols, H.323 incorporates many individual protocols that have been developed for specific applications.

M.C. Edith García Cárdenas 10 As you can see, H.323 is quite a large collection of protocols and specifications. That's what allows it to be used for so many applications. The problem with H.323 is that it is not specifically tailored to VoIP. An alternative to H.323 emerged with the development of Session Initiation Protocol (SIP). SIP is a much more streamlined protocol, developed specifically for VoIP applications. Smaller and more efficient than H.323, SIP takes advantage of existing protocols to handle certain parts of the process. Media Gateway Control Protocol (MGCP) is a third commonly used VoIP protocol that focuses on endpoint control. MGCP is geared toward features like call waiting. You can learn more about the architecture of these protocols at Protocols.com: Voice Over IP. One of the challenges facing the worldwide use of VoIP is that these three protocols are not always compatible. VoIP calls going between several networks may run into a snag if they hit conflicting protocols. Since VoIP is a relatively new technology, this compatibility issue will continue to be a problem until a governing body creates a standard universal protocol for VoIP. The overall hurdle facing VoIP is that there are currently no overriding standards. This includes hardware, protocols and virtually every aspect of the system. In the end, VoIP is a vast improvement over the current phone system in terms of efficiency, cost and flexibility. Like any emerging technology, VoIP has some challenges to overcome, but it is clear that developers will keep refining this technology until it eventually replaces the current phone system.

M.C. Edith García Cárdenas 11