As we explained in Lesson Four, wireless computer networks are becoming a major part of the networking market. This is true not only for residences, but also for commercial installations.
Almost all residential networking installations include the home's telephone system. However, it is critical to remember that under the residential networking standard, all telephone outlets must receive their own home run. They may not be run from one outlet to another (daisy-chaining), as they have been traditionally. This is illustrated in Fig. 1 on page 37.
In addition, it is recommended that the telephone cables you install in networked homes be Cat. 5 or better. Not only is it a good trade practice to install the most up-to-date materials, but most installers find it much easier to carry only one type of telephone cable from job to job, rather than several types.
THE OPERATION OF TELEPHONES
Modern telephones operate on essentially the same principles that were developed more than 100 years ago. They use a single pair of wires that connect the phones and a power source. When the phones are connected, the power source causes a current to flow in a loop, which is modulated by the voice signal from the microphone in one handset and excites the earphone in the opposite headset. Dialing was originally done by a rotary dial that simply switched the current on and off in a number of pulses corresponding to the number dialed. Dialing is now mostly (but not entirely) accomplished with tones.
By operating on a current loop, phones can be powered from a central source and extended simply by adding more wire and phones in parallel. Now, most phones are electronic (that is, they use semiconductors rather than electro-mechanical devices), but they use the same type of wiring, frequently called current-loop wiring.
Telephone wiring is simple because the bandwidth of telephone signals is low, generally around 3,000 hertz (cycles per second). Bandwidth is similar to speed, a low bandwidth requires lower frequencies, and a higher bandwidth requires higher frequencies.
Computer modems use sophisticated techniques to send much faster digital signals over low-bandwidth phone connections.
Because of the low bandwidth and current loop transmission, telephone wire is easy to install and test. It can be pulled without fear and if it is continuous, it should work.
The most common pieces of telecom equipment that you will be mounting are the following:
Telephones. The main concern here is that the proper phone is connected in the proper location.
Punch down blocks. It is important that punch-down blocks are mounted securely. If not, connections will be difficult to make, and the block will loosen during the termination process. Fig. 2 shows the wiring of the standard telephone punch-down block — the “66” block.
Wall jacks and connection modules. Here there are two concerns. First, that the conductors are terminated in the proper order, and second, that the jack or module is secure; it may suffer a fair amount of abuse during its useful life.
Equipment racks. Larger systems may require equipment racks. These must be not only secure, but must also be installed plumb and level.
Central consoles. Central consoles must be properly placed and installed near a source of electrical power.
Computer cards. Especially when modifying an existing system, you will have to install computer cards in existing personal computers. This requires some experience with computers, and just a bit of gentleness.
Although telephones and telephone company practices may vary dramatically from one locality to another, the basic principles underlying the way they work are the same everywhere.
Every telephone consists of three separate subassemblies, each capable of independent operation. These assemblies are the speech network, the dialing mechanism and the ringer. Together, these parts — as well as any additional devices such as modems, dialers and answering machines — are connected together, and transfer voice communications through telephone circuits.
A telephone is usually connected to the telephone exchange by an average of about three miles of #22 AWG or 0.5 mm copper wires. This wiring back to the phone company office is commonly called the loop, or the neighborhood loop. Although copper is a good conductor, it does have resistance, especially in small gauges. The resistance of #22 twisted-pair AWG wire is 16.46 Ohms per thousand feet at 77 degrees F (25 degrees C). Telephone companies describe loop length in kilofeet (thousands of feet).
Because the telephone apparatus is generally considered to be current driven, all phone measurements refer to current consumption, and not voltage. The length of the wire connecting the subscriber to the telephone exchange affects the total amount of current that can be drawn by anything attached at the subscriber's end of the line.
In the United States, the voltage applied to the line to drive the telephone is 48VDC. In other places, 50VDC is sometimes used. Note that telephones are peculiar in that the signal line is also the power supply line. The voltage is supplied by lead-acid cells, thus assuring a hum-free supply and complete independence from the electric company, which is why telephones work during power outages.
At the telephone exchange, the DC voltage and audio signal are separated by directing the audio signal through two 2 uF capacitors and blocking the audio from the power supply with a 5 Henry choke (inductive coil) in each line. Usually these two chokes are the windings of a relay that switches your phone line at the exchange; in the United States this relay is known as the “A” relay. The resistance of each of these chokes is 200 Ohms.
You can find out how well a phone line is operating by using Ohm's law and an ammeter. The DC resistance of any device attached to the phone line is often quoted in telephone company specifications as 200 Ohms; this will vary in practice from between 150 to 1,000 Ohms. You can measure the DC resistance of your phone with an Ohmmeter.
Using these figures, you can estimate the distance between your telephone and the telephone exchange. In the United States, the telephone company guarantees you no lower current than 20 mA — or what is known to your phone company as a long loop. A short loop will draw 50 to 70 mA, and an average loop, about 35mA. (Remember, in telephone work, current measurements are used, not voltage measurements.)
Telephone companies prefer the DC resistance of their lines to be about 10 megOhms when no apparatus is in use. This helps them avoid people using their 50V DC feed into the home or office for other uses. (The state of not being in use is called on-hook in telephone jargon.) A phone that is on-hook can draw no more than 5mA.
For older systems, the two phone wires connected to your telephone should be red and green. The red wire is negative and the green wire is positive. Your telephone company calls the green wire Tip and the red wire Ring. Most installations have another pair of wires — yellow and black — which can be used for many different purposes, if they are used at all. If two separate phone lines are installed in a home or office with older cables, you will find the yellow and black pair carrying the second telephone line. In this case, black is Tip, and yellow is Ring. The above description applies to a standard line with a DC connection between your end of the line and the telephone exchange. Most phone lines in the world are of this type, known as a metallic line.
Very long lines may have amplifiers, sometimes called loop extenders, on them. Some telephone companies use a system called a subscriber carrier, which is basically an RF (radio frequency) system in which your telephone signal is raised up to around 100 KHz and then sent along another subscriber's pair of conductors.
When a telephone is taken off the hook, the line voltage drops from 48V to between 9V and 3V, depending on the length of the loop. If another telephone in parallel is taken off the hook, the current consumption of the line will remain the same and the voltage across the terminals of both telephones will drop. Bell Telephone specifications state that three telephones should work in parallel on a 20 mA loop; transistorized phones tend not to pass this test, although some manufacturers use ICs (integrated circuits) that will pass.
While low levels of audio may be difficult to hear, overly loud audio can be painful. Consequently, a well designed telephone will automatically adjust its transmit and receive levels to allow for the attenuation — or lack of it — caused by the length of the loop. This adjustment is called loop compensation.
Because a telephone is a duplex device, both transmitting and receiving on the same pair of wires, the network must ensure that not too much of the caller's voice is fed back into his or her receiver. This function, called sidetone, is achieved by phasing the signal so some cancellation occurs in the speech network before the signal is fed to the receiver. Callers faced with no sidetone at all will consider the phone dead. Too much sidetone causes callers to lower their voices and not be heard well at the other end of the line. Too little sidetone will convince callers that they're not being heard.
Touchtone, the modern form of dialing, is faster and less prone to error than pulse dialing. Compared to pulse, its major advantage is that its audio band signals can travel down phone lines further than pulse, which can travel only as far as your local exchange. Touchtone can therefore send signals around the world via telephone lines, and can be used to control phone answering machines and computers.
Each transmitted digit consists of two separate audio tones that are mixed together. The four vertical columns on the keypad are known as the high group and the four horizontal rows as the low group; the digit 8 is composed of 1336 Hz and 852 Hz. The level of each tone is within 3 dB of the other, (the telephone company calls this Twist). A complete touchtone pad has 16 digits, as opposed to 10 on a pulse dial. Besides the numerals 0 to 9, a DTMF “dial” has *, #, A, B, C and D.
A large-scale view of a telephone network is shown in Fig. 3.
DATA OVER TELEPHONE LINES
Data transmission over the common telephone system was almost un-thought-of only 10 years ago. Now, it is a huge business, with tens of millions of people doing this every day. In fact, data transmission is becoming a big part of the telephone business, and will probably be more important than voice transmission in a few years. So, there is a world of opportunity open to those who understand this work, and can apply their knowledge intelligently.
But the telephone business is also full of change and controversy. Things are changing. Historically, telephone companies have been government-protected monopolies. In other words, you cannot just open up your own phone company; the federal government decides who can have a phone company, and which areas they can serve. In the early 1980s, the government allowed other companies to compete with AT&T in long-distance service. That opened the way for MCI, Sprint and much lower long-distance rates.
But local-area service is still a protected monopoly. Furthermore, the phone companies see data communication as a serious threat, and are trying to delay it as best they can. On the other hand, the computer industry is trying to get the phone companies to loosen-up and allow people to get high-bandwidth lines for the best computer features.
Sending data over phone lines comes in two basic forms:
Using the phone company's switched network for low-speed modem traffic.
Using dedicated circuits that can function at high speeds and more or less independently of the network switching system.