While computer memory and disk space are addressed through a binary numbering system, speed is calculated by decimal measurement. Because all IT pros should have a solid foundation in the differences between these two types of measurements, I’m going to explain how speed measurements are calculated, and how those calculations differ from those used for memory and disk-storage space.
The importance of sine waves
Before you can understand how transmission speeds are measured, you need to understand the concept of a sine wave.
A sine wave looks similar to the graphic in Figure A. The horizontal line through the middle of the sine wave is a null zone. The peak above the null zone is an indication of positive voltage, while the valley below the null zone is an indication of negative voltage.
The portion of the sine wave that’s shown in Figure A represents a single cycle. If other cycles existed, they would appear as additional peaks and valleys.
Another important concept to understand about sine waves is timing. Not every sine wave will have identical timing, but all of the waves within an individual sine wave have identical timing.
For example, if the cycle that’s shown in Figure A took one second to complete, then all other cycles in the session would also take one second each to complete.
This is a one-cycle sine wave.
Sine of an electrical current
So how does a sine wave relate to transmitting data? As you may recall from any Introduction to Computers class, the smallest possible piece of data is a bit. When using binary expressions, either a one or a zero represents a bit. However, electronically, a bit is represented by either the presence or the absence of electrical current. This is where the sine wave comes in.
Note
Unlike memory and disk-storage space, which are measured in kilobytes, megabytes, and gigabytes, transmission speeds are measured in baud, kilobits, megabits, and gigabits.
As you saw earlier, a sine wave is a series of evenly spaced waves. There are lots of different types of sine waves, but the one thing that all sine waves have in common is that each wave has consistent timing. Now, imagine for a moment that you knew that a series of waves would be flowing down a wire at a rate of one wave per second. You could treat each wave as an individual bit, and base the bit’s content (0 or 1) on the shape of the wave.
The wave shown in Figure A is representative of an analog sine wave. Analog sine waves are used for analog communications, such as the type that occur between modems. As you probably know, an analog modem translates digital data into audible tones. The receiving modem then translates the tones back into digital data. The audible tones allow the data to flow over a phone line in analog format. The various pitches of tones have different meanings to the communications session. With this in mind, you can see how a sine wave is similar to a sound wave, and how the peaks in the wave could be used to represent a one, while valleys in the wave could represent a zero.
Digital and analog sine wave differences
In digital communications, the same principle of evenly spaced waves is used, but the waves look different. In digital communications, the waves are square shaped, as shown in Figure B. As you look at a cycle, if the wave looks like a square, the square represents a binary one. If no square is present during the cycle, the cycle is interpreted as binary zero.
This is what a digital wave looks like.
From baud to bits
Whether analog or digital, communications streams are composed of evenly spaced waves. The shape of the wave determines the value of a bit, and each wave cycle represents a single bit. The term for a bit cycle is a baud. Therefore, one cycle equals one baud.
If you used modems prior to the early 1990s, you may remember the now antiquated term "baud rate." A modem’s baud rate was a direct reflection of how many cycles were generated per second and thus how many bits per second were transmitted. Therefore, a 300-baud modem used 300 cycles per second to move 300 bits of data.
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