PC Hardware in a Nutshell, 3rd Edition

Serial Data Transmission

A serial bit stream is actually placed on the wire as a rapidly changing series of voltages, with positive voltage used to represent logical zero, and negative voltage used to represent logical one. This serial bit stream is characterized by the nominal speed at which the bits are being sent, measured in bits per second, abbreviated bps and pronounced “bips.” At higher data rates, the bit stream is specified in kilobits per second (Kbps or Kb/s) or megabits per second (Mbps or Mb/s).

Tip

Capitalization is significant when referring to data rates. A lowercase “b” refers to bits, while an uppercase “B” refers to bytes. A lowercase “k” refers to decimal thousands (10³ or 1,000), and an uppercase “K” to binary thousands (2¹⁰ or 1,024). Similarly, lowercase letters refer to decimal millions (“m”) or billions (“g”), while uppercase letters refer to binary millions (“M”) or billions (“G”).

Serial data rates between devices cannot be set at just any arbitrary value. Instead, they are incremented into standard fixed rates that you select from. The lowest rate used is 50 bps. For historical reasons, the upward progression in data rates is somewhat odd. The next standard rate is 75 bps, followed by 110 bps and 150 bps. From there, the standard data rates double for a while, to 300 bps, 600 bps, 1,200 bps, 2,400 bps, 4,800 bps, 9,600 bps, 19,200 bps, and 38,400 bps. Beyond 38,400, speeds are less standardized. On a de facto basis, the next step up is usually 57,200 bps ...