Understanding Data Communications

Understanding Data Communications

Data communications is the last frontier of the computer world. It's an environment where everything is magic--and mainly black magic, at that. Data communications practitioners even speak an obscure language with confusing acronyms such as DTR/DSR, RTS/CTS, DTE/RTE, CCITT V.32bis, PBX, PPM, and PCM. But mere mortals can learn some of this data communications magic, and Understanding Data Communications, 4th Edition, by Gilbert Held, will initiate you into the mysteries.

Held's mission is to explain how data communications systems and the associated hardware and software work. His purpose is not to tell you how to build the equipment or code software for data communications. The book explains the basics of data communications systems in general and shows you how to use that knowledge in making connections.

To set the stage, the author provides a history of communications that goes back to the telegraph, which is a two-state (on or off) signaling system. By page 10, he introduces you to one of the first dual standards in the electronic communications age, with ASCII and Extended Binary Coded Decimal Interchange Code (EBCDIC).

The illustrations of data processing during the 1960s and 1970s will bring back memories for some of you and amuse the PC generation. The pace of the book accelerates rapidly, and the author introduces gateways, routers, and bridges. Soon, Held begins to remove some of the mystery around data communications acronyms and offers a general description of a data communications system.

The second chapter moves into the part of data communications most visible to you--the terminals. You see another illustration of progress, this time between an early teleprinter and a palmtop computer with a Personal Computer Memory Card International Association (PCMCIA) fax modem. Held covers various types of terminals, ranging from printers and Cathode Ray Tube (CRT) terminals to dumb terminals, and then to personal computers used as terminals. Whatever the terminal, digital data is transmitted by either serial or parallel methods. In either case, operators are interested in issues of timing and synchronous or asynchronous transmission. Held explains the Universal Asynchronous Receiver Transmitter (UART) and gives it credit for all the work it does every time you use a modem.

When you send a message, it travels between the terminals via a transmission channel. Held spends the third chapter discussing the various types of channels involved in data communications, including wire pairs, coaxial cable, waveguides, fiber-optic cables, radio waves, microwaves, satellite radio systems, and cellular connections. Each channel has a characteristic bandwidth, and therefore a theoretical information-handling capacity. You can send analog or digital signals using various carrier methods, and Held outlines those methods here.

Chapter 4 starts with an explanation of why modems are entrusted to carry so much information when digital computers are so prolific. Held claims that the answer comes from society's reliance on the telephone line. If you want to pass your information via telephone line, you need to use analog modulation. After Held details the different types of modulation you can use, he explains modems, beginning with low-speed modem operation and the old Bell 212A standard. He progresses to the V.22bis international standard, and then to the V.24 and RS-232 standard interface, which he covers pin by pin. Next, the author describes types of modems, duplex and half-duplex transmission and smart modems, and the Hayes command set. The Hayes command set is the de facto standard for modems today. Microcom Networking Protocol (MNP) and V.42 error correction are also covered in this chapter.

Chapter 5 covers synchronous modems, digital transmission, and Digital Service Units (DSUs). This chapter mentions vestigial sideband modulation and quadrature amplitude modulation. These might sound like gibberish to you now, but Held will tell you that the V.32 standard is based on a modified quadrature amplitude modulation technique. How many people know that? Just in case the V.32 standard comes up in casual conversation, you'll be able to impress people with your knowledge. If they press for more details, you'll have learned that the V.32bis standard uses echo cancellation to get full-duplex transmission, whereas the V.33 uses four wire-leased lines and does not need echo cancellation. The V.34, or V.fast, standard was still under development when the book was written, but it will surely receive more coverage in the next edition.

Unless you send a signal over a dedicated line, your signal will most likely be multiplexed with other signals as the communications carrier tries to maximize the use of its equipment. Among the many techniques for multiplexing, are PAM, PWM, PPM, and PCM. Held does a good job of sorting out this set of acronyms and explaining what each one means.

Chapter 7 begins with a discussion of how fiber-optic systems work. Held writes that most long-distance communication is now through optical systems. So, when you place a call to a bulletin board in another state, at some point your data is converted to pulses of light. Underwater optic cables offer capacity equal to that of satellite systems, but with lower cost, less transmission delay, lower noise, and far better security. This chapter also describes satellite systems.

All communicating parties must follow the same rules if they want to transmit data successfully. These rules, or protocols, are described in Chapter 8. You will find some of them, such as XMODEM, YMODEM, and ZMODEM, and KERMIT, familiar from PC communications packages. Many other terms you often see in PC modem software, including cyclic redundancy checks (CRC) and duplexing, are also clearly explained. Chapter 9 gives Procomm Plus as an example of a PC-based communications package. It is the DOS version, which many of us remember as the shareware program we used with our first modem.

Modems are not the only means of data communication: The LAN is in common use within many companies. Held presents the "ideal LAN" and then lists the major obstacles to the ideal LAN, just in case anyone thinks it can actually be built.

After describing the International Standards Organization (ISO) model for Open Systems Interconnection (OSI)--which shows up in almost every book on networks or operating systems these days--Chapter 10 goes into detail about the various layers of the model, and the various types of networks.

The final chapters cover packet networks, network design and management, and Integrated Services Digital Network (ISDN). Telecommunications books are out-of-date before they reach the printer, and the only obvious omission in this one is that it mentions Asynchronous Transfer Mode (ATM) only briefly. This omission will most likely be corrected in the next edition.

The format of the book is a little unusual: A summary is at the end of each chapter, followed by a multiple-choice quiz. This format suggests that the book has potential as a textbook, and instructors and students will want to note that the answers are provided at the back. Also provided is a thorough, cross-referenced index.

If you want to learn more about the mysteries of data communications, from Morse code to palmtop computers and PDAs, I recommend this book. It's interesting for the beginner to the intermediate-level reader.