Modem Terminology Overview
Modem Terminology Overview - Page
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SPOT 14.4
SPOT 14.4 User's Guide
User's Guide
38GDE00001SCF - Issue 4 (11/96)
38GDE00001SCF - Issue 4 (11/96)
Modem Terminology Overview
Modem Terminology Overview
What Does a Modem Do?
What Does a Modem Do?
Your SPOT provides a link between the digital world inside your
computer and the analogue world of the telephone network. Before we
look at how your SPOT works, lets explain some of the terms that you
may meet.
Bits and Bytes
Bits and Bytes
Inside your computer, data and commands pass between components in
digital form. That is, as electrical ON or OFF pulses. You may also hear
these pulses called HIGH and LOW; ONE and ZERO or MARK and
SPACE. Each pulse is called a BIT (a contraction of Binary Digit).
A single BIT does not convey much information as it only has two values,
ON or OFF. These are equal to a value of 1 or 0. Computers normally act
on groups of 8 bits at a time. With 8 bits you can represent 256 values (
from 00000000 to 11111111 in binary). A group of 8 bits is called a
BYTE.
ASCII Code
ASCII Code
To be useful, the combination of bits in a byte needs to represent
something. Various coding methods have been devised over the years.
The code most widely used for text is ASCII (American Standard Code
for Information Interchange). For example, in ASCII, K is represented by
a value of 75, which is 01001011 in 8 bit binary.
The ASCII code only contains 128 characters. Thus it could be
represented using just 7 bits. Because computers handle internal data on
an 8 bit basis, the eighth bit is still sent even though it may be surplus
(always 0). Some programs do use the 8th bit, extending the ASCII
characters table to include foreign characters and symbols but this is not
standard. For this reason, when you come to use your modem you are
sometimes given an option of using 7 bit or 8 bit ASCII.