### Units of Digital Data

You learned previous sections. Let’s examine them again in more detail, with emphasis on their relevance to computers. Recall that one bit (1 b) is a single binary digit, and one byte (1 B) is a unit of digital data consisting of a string of eight contiguous bits (8 b) in most systems. One byte constitutes roughly the same amount of data as one character, such as a letter, numeral, punctuation mark, or space.

#### Memory and Storage Capacity

Computer memory and storage involves files that are huge in terms of bytes. Therefore, kilobytes (units of 210 = 1024 bytes), megabytes (units of 220 = 1,048,576 bytes), and gigabytes (units of 230 = 1,073,741,824 bytes) are used. The abbreviations for these units are KB, MB, and GB, respectively. Alternatively you might see them abbreviated as K, M, and G.

As computer technology advances, you’ll hear more and more about a unit of data called a terabyte (TB or T). This is equivalent to 240 bytes, or 1,048,576 MB. Someday we will commonly use the terms petabyte (PB or P), which refers to 250 bytes or 1,048,576 GB, and exabyte (EB or E), which refers to 260 bytes or 1,048,576 TB. Here are all these data units listed as a hierarchy:
1 KB = 1024 B
1 MB = 1024 KB
1 GB = 1024 MB
1 TB = 1024 GB
1 PB = 1024 TB
1 EB = 1024 PB

Computer memory is usually specified in megabytes or gigabytes. The same holds true for removable data storage media. The hard drive in a computer generally has capacity measured in gigabytes, although a few get into the terabyte range. Some external storage media, used for data archiving (saving it for long-term reference), have capacity measured in terabytes and petabytes.

#### Data Speed

The speed at which computers send digital data to and from each other is almost always expressed in bits per second (bps) and large multiples thereof. Multiples of bits per second involve the same pre fixes as multiples for bytes, but when talking or writing about bits per second, these prefixes refer to powers of 10 rather than powers of 2.

A kilobit per second (1 kbps) is equal to 103 = 1000 bps, a megabit per second (1 Mbps) is equal to 106 = 1,000,000 bps, and a gigabit per second (1 Gbps) is equal to 109 = 1,000,000,000 bps. (You won’t often hear of data speed units larger than the gigabit per second.) Hierarchically:
1 kbps = 1000 bps
1 Mbps = 1000 kbps
1 Gbps = 1000 Mbps

Note the lowercase “k” in reference to “kilo-” meaning 103 or 1000, and the uppercase “K” in reference to “kilo-” meaning 210 or 1024. Note also that for the other prefix multipliers, uppercase letters are always used. These are not typos! The “k versus K” distinction is a notational peculiarity, and is often ignored or overlooked. You will sometimes see “Kbps” rather than “kbps,” or “kB” rather than “KB,” in technical papers and other documents. As long as you know whether the author is writing about data speed (bits per second) or memory/storage (bytes), it should not be a problem. But if you want to be rigorous, this peculiarity is worth remembering.