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First Things First Telecommunications, like all highly visible and interesting fields, is full of apocryphal stories, technical myths, and fascinating legends. Everyone in the field seems to know someone who knows the outside plant repair person who found the poisonous snake in the equipment box in the manhole', the person who was on the cable-laying ship when they pulled up the cable that had been bitten through by some species of deep water shark, some collection of seriously evil hackers, or the backhoe driver who cut the cable that put Los Angeles off the air for 12 hours.
There is also a collection of techno jargon that pervades the telecommnications industry and often gets in the way of the relatively straightforward task of learning how all this stuff actually works. To ensure that such things don't get in the way of absorbing what's in this book, I'd like to begin with a discussion of some of them.
This is a book about telecommunications, which is the science of communicating over distance (tale-,from the Greek tale, "far off'). It is, however, fundamentally dependent upon data communications, the science of moving traffic between computing devices so that the traffic can be manipulated in some way to make it useful. Data, in and of itself, is not particularly useful, consisting as it does of a stream of ones and zeroes that is only meaningful to the computing device that will receive and manipulate those ones and zeroes. The data does not really become useful until it is converted by some application into information, because a human can generally understand information. The human then acts upon the information using a series of intuitive processes that further convert the information into knowledge, at which point it becomes truly useful. Here's an example: A computer generates a steady stream of ones and zeroes in response to a series of business activities involving the computer that generates the ones and zeroes. Those ones and zeroes are fed into another computer, where an application converts them into a spreadsheet of sales figures (information) for the store from which they originated. A financial analyst studies the spreadsheet, calculates a few ratios, examines some historical data (including not only sales numbers but demographics, weather patterns, and political trends), and makes an informed prediction about future stocking requirements and advertising focal points for the store based on the knowledge that the analyst was able to create from the distilled information.
Data communications rely on a carefully designed set of rules that governs the manner in which computers exchange data. These rules are called protocols, and they are centrally important to the study of data communications. Dictionaries define protocol as "a code of correct conduct." From the perspective of data communications, they define it as "a standard procedure for regulating the transmission of data between computers," which is itself "a code of correct conduct." These protocols, which will be discussed in detail later in this book, provide a widely accepted methodology for everything from the pin assignments on physical connectors to the sublime encoding techniques used in secure transmission systems. Simply put, they represent the many rule sets that govern the game. Many countries play football, for example, but the rules are all slightly different. In the United States, players are required to weigh more than a car, yet be able to run faster than one. In Australian Rules football, the game is declared forfeit if it fails to produce at least one body part amputation on the field or if at least one player doesn't eat another. They are both football, however. In data communications, the problem is similar; there are many protocols out there that accomplish the same thing. Data, for example, can be transmitted from one side of the world to the other in a variety of ways including Tl, E1, microwave, optical fiber, satellite, coaxial cable, and even through the water. The end result is identical: the data arrives at its intended destination. Different protocols, however, govern the process in each case.
A discussion of protocols would be incomplete without a simultaneous discussion of standards. If protocols are the various sets of rules by which the game is played, standards govern which set of rules will be applied for a particular game. For example, let's assume that we need to move traffic between a PC and a printer. We agree that in order for the PC to be able to transmit a printable file to the printer, both sides must agree on a common representation for the zeroes and ones that make up the transmitted data. They agree, for example (and this is only an example) that they will both rely on a protocol that represents a zero as the absence of voltage and a one as the presence of a three-volt pulse on the line, as shown in Figure 1-1. Because they agree on the representation, the printer knows when the PC is sending a one and when the PC is sending a zero. Imagine what would happen if they failed to agree on such a simple thing beforehand. If the transmitting PC decides to represent a one as a 300-volt pulse and the printer is expecting a three-volt pulse, the two devices will have a brief (but inspired) conversation, the ultimate result of which will be the release of a small puff of silicon smoke from the printer.
Now they have to decide on a standard that they will use for actually originating and terminating the data that they will exchange. They are connected by a cable (see Figure 1-2) that has nine pins on one end and nine jacks on the other. Logically, the internal wiring of the cable would look like Figure 1-3. However, when we stop to think about it, this oneto-one correspondence of pin-to-socket will not work. If the PC transmits on pin 2, which in our example is identified as the send data lead, it will arrive at the printer on pin 2-the send data lead. This would be analogous to holding two telephone handsets together so that two communicating parties can talk. It won't work without a great deal of hollering. Instead, some agreement has to be forged to ensure that the traffic placed on the send-data lead somehow arrives on the receive data lead and vice versa. Similarly, the other leads must be able to convey information to the other end so that normal transmission can be started and stopped. For example, if the printer is ready to receive the print file, it might put voltage on the data terminal ready (DTR) lead, which signals to the PC that it is ready to receive traffic. The PC might respond by setting its own DTR lead high as a form of acknowledgment, followed by transmission of the file that is to be printed. The printer will keep its DTR lead high until it wants the PC to stop sending. For example, if the printer senses that it is running out of buffer space because the PC is transmitting faster than the slower printer can print, it will drop the DTR lead, causing the PC to temporarily halt its transmission of the print file. As soon as the printer is ready to receive again, it sets the DTR lead high once again, and printing resumes. As long as both the transmitter and the receiver abide by this standard set of rules, data communications will work properly This process of swapping the data on the various leads of a cable, incidentally, is done by the modem-or by a null modem cable that makes the communicating devices think they are talking to a modem. The null modem cable is wired so that the send-data lead on one end is connected to the receive data lead on the other end and vice-versa; similarly, a number of control leads such as the carrier detect lead, the DTR lead, and the data set ready (DSR) leads are wired together so that they give false indications to each other to indicate that they are ready to proceed with the transmission, when in fact no response from the far end modem has been received.
Of course, the UN and its sub-organizations cannot perform this task alone, nor should they. Instead, they rely upon the input of hundreds of industry-specific organizations as well as local, regional, national, and international standards bodies that feed information, perspectives, observations, and technical direction to the ITU, which serves as the coordination entity for the overall international standards creation process. These include the American National Standards Institute (ANSI), the European Telecommunications Standards Institute (ETSI, formerly the Conference on European Post and Telegraph, CEPT), Telcordia (formerly Bellcore, now part of SAIC), the International Electrotechnical Commission (IEC), the European Computer Manufacturers Association (ECMA), and a host of others.
It is worthwhile to mention a bit about the ITU as a representative standards body. Founded in 1947 as part of the United Nations, it descended from a much older body called the Union Telegraphique, founded in 1865 and chartered to develop standards for the emerging telegraph industry. Over the years since its creation, the ITU and its three principal member bodies have developed three principal goals:
|Ch. 1||First things first||1|
|Ch. 4||The Byzantine world of telecom regulation||225|
|Ch. 5||Premises technologies||243|
|Ch. 6||Access technologies||281|
|Ch. 7||Transport technologies||369|
|Ch. 8||Telecom market segments||453|
|Ch. 9||Final summary||481|