Wireless Community NetworksA Guide for Library Boards, Educators, and Community LeadersVersion 1.0Chapter 5. Data Transfer Rates: A PrimerYou've probably seen them. Ks, Ms, and Gs. They're always used in computer ads: 56K, 6.4GB, 32MB. Sometimes they have other letters after them, like Kbps or MBps. But what do they mean? Discussing network connectivity requires one to establish a basic vocabulary of data transfer speeds. Data transfer is the process of moving computer data from one point to another. The speed, or rate, of that movement is called the data transfer rate. There is a more common word used to describe this measurement also: bandwidth. BandwidthBandwidth is a term borrowed from the world of wireless transmission, such as TV or radio broadcasting or ham radio. Bandwidth refers to the frequency width of a particular band of radio transmission. For example, the VHF television band extends from the 54 megahertz frequency to the 216 megahertz frequency. The width of that band is 21654, or 162 megahertz. Electrical signals are carried over network cables at different speeds depending on the frequencies used on specific wire types. So the term bandwidth is used in the computer networking world as well. In networking it is used to describe the maximum amount of data that can be transmitted across a medium in one second. Other terms used commonly to describe this same measurement are line speed, throughput, and capacity. In this chapter I've used data transfer rate as a more descriptive phrase. These terms are used interchangeably in many networking discussions although they have slightly different technical meanings. The basic unit of storage in a computer system is the bit. A secondary unit is the byte. Bandwidth is normally measured in some form of bitspersecond (bps). Since the number of bits that can be transmitted through most media can be quite large, researchers and technicians use common abbreviations for large numbers. These are the Ks, Ms, and Gs we mentioned up front. Before we get to them, though, let's examine those two words I just threw at you: bits and bytes. Bits and BytesWhile computers can do some amazing things, they are fundamentally simple machines. In fact, they do just a few simple things: they recognize two states of electrical current (high and low); they recognize magnetic polarity (North and South); they "open" and "close" gates; they shift bits left and right. Out of these four basic operations come all the number crunching and visual operations we've come to expect and demand from them. Since computers only recognize distinct states of electrical current and magnetic polarity, all data used by computers—letters, numbers, punctuation marks, pictures, and sounds—must be represented by using a series of two symbols. This type of system, where only two single values are possible, is known as a binary system. In mathematics, the binary (base 2) number system describes a binary system using two digits, 0 and 1. All counting is accomplished using just these two symbols. (Obviously, this is not the counting system people use! We have ten fingers and ten toes, so our number system is based on having ten different counting symbols, or digits. This number system is called the decimal [base 10] number system.) These symbols are called binary digits, or bits for short. They represent the smallest unit of storage on a computer. In order to program a computer to operate on textual data (letters, numerals, punctuation marks, and other special symbols), a binary code was necessary. Because storage space was very expensive in the early years of computers, the code needed to provide enough symbols but not waste storage space. Eventually, a coding system using a sequence of eight bits was designed (eight bits provides up to 256 different text symbols). Because most computer data is textual in nature, the eightbit sequence became a very common unit of storage. It is called a byte. Each letter in a document required one byte of storage. Oddly enough, because the byte became so prevalent in storage measurements, the sizes of even nontext objects like graphics and sounds are now expressed in bytes. So, when measuring data transfer rates and storage sizes, different values are used. Bits are typically used in data transfer rates, and bytes are used to indicate storage space. A lowercase b is used to signify bits. An uppercase B is used to signify bytes. (In a few data transfer rate specifications—mainly for hard drives and tape drives—a byte value is used instead of bits. One can always see the difference in the symbol used, Bps instead of bps.) If you'd like to see an example of how textual data is stored on microcomputers, read the sidebar "Encoding Text." Otherwise, let's get started on those Ks, Ms, and Gs.
Large UnitsA 20page wordprocessed document might take up 60,000 bytes or more of storage space. A small photograph on a web page might take up as many as 100,000 bytes. In most cases, measuring storage space or transfer rates requires large numbers. To make the notation of these measures easier, a series of unit abbreviations have been commonly used. The letter K (for "kilo") is commonly used to indicate 1,000 units (e.g., kilograms). However, with computers such a number is not really a "round" number. The closest "round" number in the binary number system is 1,024. So, computer scientists have adopted the K designation as a substitute for 1,024 units. This symbol is combined with the proper computer unit used to create a complete measure: Kb for kilobits and KB for kilobytes. Likewise, the letter M (for "mega") represents a million units. Once again, one million is not a "round" number in the binary number system, so the exact measure is a little more: 1,048,576 (1,024 x 1,024). Mb stands for megabits, and MB stands for megabytes. In general discussions of storage requirements, we tend to use the round number we're familiar with: 1,000 and 1,000,000. For example, when an advertisement says a Zip disk holds 100MB of data, it really means about 100 million bytes, not 104,857,600 bytes (the technical value represented by 100MB). Data transfer rates are quoted using the same rounded numbers. Over the past three years, even larger quantities of hard drive storage products have become common. And a new, very fast networking technology has emerged into highend applications. These products have gigabyte and gigabit specifications, respectively. "Giga" indicates approximately a billion units. We use the letter G to represent this unit. For purists, a gigabit equals 1,024 x 1,024 x 1,024 = 1,073,741,824 bits. Wow! Gigabit Ethernet networking is being used primarily where highend graphics applications and huge data sets are used. Schools and libraries probably will not use this level of networking until realtime, highquality desktop video is common. Such use would include distance learning, conferences, and others. Now you know the basics for decoding all the data transfer rates you'll run across in reading computer magazines or books. But we have talked about these transfer rates in numbers that are basically meaningless. The addition of some perspective might be in order. What does 1Mbps really mean? How much stuff is it? A Picturesque PerspectiveIt's time to put data throughput in terms we're familiar with. Let's start with some basic, common transfer rates and build mental pictures.
Most home computer users have a modem providing a data transfer rate of at least 28.8Kbps. Some have newer 56K modems. Some still have older 14.4Kbps modems. It turns out that the latter is a very convenient and meaningful number. Remember when we used to type all our documents on a typewriter? An 8 1/2 x 11 sheet of paper had a oneinch margin on the left and right sides, top and bottom. An extra halfinch at the top of the page was used for a page number and heading. Most typewriters we used had a Pica typeface, which put ten characters per inch across the page. Usually the lines were doublespaced. This provided about 26 typed lines per page, with about 65 characters per line—a total (26 x 65) of 1,690 characters per page. Do you remember how many bits it takes to represent a character? (If you said eight, give yourself a reward!) 1,690 x 8 equals 13,520 bits. Add a few hundred bits for the packaging required to send data across a network and you end up with a number very close to the persecond speed of a 14.4Kbps modem. This gives us a nice mental picture. One normal, doublespaced page of text takes about one second to transfer with a 14.4Kbps modem. A 28.8Kbps modem can transfer about two pages per second. And the very latest technology, at 56Kbps, can transfer about four pages per second. Let's extend this picture to something else we can visualize. I read Tom Clancy novels. What about one of those twoinchthick, hardback titles? Just for fun I estimated the number of characters contained in Without Remorse, one of Clancy's books that appears to be about "average" length. It has about 1,484,000 characters in it. Converting that to bits, we get 11,872,000 bits. So, how long will it take to transfer one Tom Clancy novel at 56Kbps? 11,872,000 bits / 56,000 bps = 212 seconds (or, 3.53 minutes). In contrast, let's look at just one photograph the size of a common computer screen and stored in "real color" format. Just one requires 640 dots x 480 dots x 24 bits per dot = 7,372,800 bits to represent. Wow again! This photograph will take 132 seconds (2.2 minutes) to download at 56Kbps. So how long would the digital equivalent of a coffee table book of medieval art, with a hundred pages of photographs, take to download? Even with current technology that compresses photographs into smaller files, this would be a very intensive download. This shows why Gigabit Ethernet has become popular in certain fields relying heavily on graphics and motion video technologies. Table 9 below shows a comparison of the common data communication technologies. The "X" column shows how many times faster a specific technology is compared to a 28.8Kbps modem (considered the benchmark for our purposes). Or, you can look at it as a statement of how many 28.8K modems are required to equal the performance of the specified technology. The download column indicates how long, in seconds, it takes to transfer our reference document, a Tom Clancy novel, using each technology.

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