Using a Phone Line for Data


The telcos of the world were originally created to let you talk to someone else using a telephone. Almost every home in America has a local phone line installed in it. That phone line runs from your house, typically underground, to a local telco central office (CO). You can use your phone to make a call almost anywhere in the world, and the call goes over the phone line, called a local loop, to the telco CO, through its network, and on to the other phone.

When you make a call, the telco creates a new voice circuit on your behalf. A voice circuit is just telco lingo for the ability to send and receive voice between two phones. Another term you should know is Public Switched Telephone Network (PSTN), which refers to the combined telco networks in the world.

Although the local phone line was originally intended for supporting voice, it can also support the transmission of data. Next, you learn a little about how that works and then see how you can use a local phone line to send data to and from an ISP.

Making Data Sound Like Voice

First, let's cover a little bit about how voice works and how it works over a phone line. Then, I'll cover how to send data over that same phone line.

Sound waves travel through the air by vibrating the air. The human ear hears the sound because the ear vibrates as a result of the air inside the ear moving, which in turn causes the brain to process the sounds that the ear hears.

The telco, however, cannot forward vibrating air particles over its network. To send the voice over the phone line, something must convert the sound (the vibrating air) into an electrical signal, because the telco local loop (the phone line between your house and the CO) was designed to carry electrical signals. To convert the sounds into electricity, a telephone includes a microphone. In case you've never stopped to think about it, a microphone simply converts sound waves into an analog electrical signal. The telco can send the electrical signal between one phone and another. On the receiving side, the phone converts the electrical signal back to sound waves using a speaker that is inside the part of the phone that you put next to your ear.

Enough talk about this voice stuffour end goal is to be able to send data over the phone line. To understand how that happens, consider what occurs when a phone sends the electrical signal over a phone line, as shown in Figure 16-2.

Figure 16-2. Analog Electrical Signal: Frequency and Amplitude


The graph shows that the voltage level on the wire changes continuously over time. Because the voltage changes continuously, the signal is considered to be an analog electrical signal. You might recall that Chapter 4, "How to Build a Local (Network) Roadway," covered how Ethernet uses a digital electrical signal to transmit data, with the digital signal having a discrete value for a time period, and then changing to another discrete value so that the graph shows a bunch of right angles. Digital transmission is convenient for transmitting data because when you transmit data, it's already in digits0s and 1swhich are also discrete values.

The telcos created the local loop to support voice; that's why the loop uses analog electrical signals. The sounds that your voice makes happen to be a continuously changing sound wave, so analog electrical signals that continuously change work well for voice. In short, the word analog refers to a continuously changing signal, and the term digital refers to a signal that has discrete or exact states that imply a particular value, or digit.

The analog signal has a couple of characteristics that I'll define briefly, and then we'll talk about how to transmit data using such a signal:

  • Frequency Defined as how the number of times the signal repeats itself, from peak to peak, in one second (assuming that the sound the human makes doesn't change for a whole second). Figure 16-2 shows a frequency of 3 Hertz (Hz), or 3 per second. The greater the frequency of the electrical signal, the higher the pitch of the sound being represented.

  • Amplitude Represents how strong the signal is; a higher amplitude peak represents a louder sound.

The goal of the original telco was to create a circuit between any two phones. Each circuit consisted of an electrical path between two phones, which in turn supported the sending of an analog electrical signal in each direction, allowing the people on the circuit to have a conversation. Remember: The original telco predated the first vacuum tube computers, so the concept of support data communication between computers wasn't a consideration for the original telco. The original telco just wanted to get these analog electrical signals, which represented sounds, from one place to the other.

What Phones Do for Voice, Modems Do for Data

When you make a phone call and say something, the telco transmits the sounds you make to the telephone on the other end of the circuit. You and the other person, being humans, can speak to generate sound waves and listen to hear sound waves. The telephone converts what the human can do (sound waves) to what the telco network can do (analog electrical signals).

To use those same telephone lines to send data, you need a device that has some similarities to a phone. Like a phone, this device needs to know how to generate and receive the analog electrical signals. However, instead of converting sound waves to and from those analog electrical signals, the device needs to convert binary 0s and 1s on behalf of a computer. Figure 16-3 shows the general idea.

Figure 16-3. Comparing a Phone to a Modem


In the top half of the figure, the phones generate and receive analog electrical signals. The equipment in the telco CO is called a telephone switch because the device in the CO is built to support telephone traffic, and it thinks that there's a telephone on the other end of the local loop cable. To create a voice phone call, the telco causes the analog electrical signals to go from one side of the network to the other.

Modems allow two computers to send and receive a serial stream of bits over an analog phone circuit, with no physical changes required on the typical analog local loop between a residence and the telco CO. Because the telephone switch in the CO expects to send and receive analog voice signals over the local loop, modems simply send an analog signal to the PSTN and expect to receive an analog signal from the PSTN. However, instead of voice that a human speaker creates, the analog signal represents some bits that the computer needs to send to another computer. Similar in concept to a phone converting sound waves into a representative analog electrical signal, a modem converts a string of binary digits on a computer into a representative analog electrical signal. In fact, although most people simply use the term modem, you can also call these devices analog modems because they create and interpret analog electrical signals.

Modems encode a binary 0 or 1 onto the analog signal by varying the analog signal, for instance, by varying frequency or amplitude. Changing these characteristics of the analog signal is referred to as modulation. For instance, one of the earliest modem standards used an analog signal of 2250 Hertz for a binary 1 and 2100 Hz for a binary 0. (Remember: Frequency is measured in Hertz.) A modem could modulate, or change, between the two frequency levels to imply a binary 1 or 0. Figure 16-4 outlines the basics of how a modem can transmit data.

Figure 16-4. Basic Operation of Modems over PSTN


Modems agree to a particular encoding scheme to transmit and receive bits. In Figure 16-4, PC2 uses a higher frequency, shown as an analog waveform that moves up and down more quickly, to mean binary 0. PC2 uses a lower frequency (fewer movements up and down) to mean a binary 1. The receiverin this case a router with a modemuses those same rules to interpret the meaning of the signal. The telco passes the signal because the signal is just an analog electrical signal, and that's what the telco expects to happen on the local loop.

In the figure, the PC modulates (changes) between 30-Hz and 10-Hz signals to encode a binary 0 or 1, respectively. When the receiver interprets the analog signal, converting it back to binary digits using those same rules, the process is called demodulation. The term modem is a shortened version of the combination of the two words modulation and demodulation.

How Fast Can You Talk?

To achieve a particular transmission speed, the modems need to send and receive a signal that can change at that rate. For instance, Figure 16-4 implied that the frequency changed every .1 seconds, meaning that the PC could send 10 bits in every second, or for a rate of 10 bps. That's a ridiculously low speed, but showing faster speeds in the figure would have required a much larger piece of paper! Some early modems sent at 9600 bps, so the sending modem would change the signal (as necessary) every 1/9600 of a second. Similarly, the receiving modem would sample the incoming analog signal every 1/9600 of a second, interpreting the signal as a binary 1 or 0. Today, modems approach speeds around 56 Kbps.




Computer Networking first-step
Computer Networking First-Step
ISBN: 1587201011
EAN: 2147483647
Year: 2004
Pages: 173
Authors: Wendell Odom

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