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Unformatted text preview: a common data-compression standard. Most modems can determine
and adjust to the data compression capabilities of another modem automatically.
Base Data Rates
9,600 bps Throughputs with'Data Compression
38,400 bps 14,400 bps 28,800 bps 57,600 bps Figure 17.10. Effective throughputs of base data rates with the two standard data
compression specifications used in modems.
6. Intelligence. Some of the more expensive modems contain microprocessors,
storage chips, and specialized communication chips that allow them to operate and
function under a large number of different circumstances. These modems can be
programmed to automatically perform dialing, answering, and disconnecting
functions. For instance, some modems can automatically dial-up a remote
computer (auto-dial), even answer incoming calls from other computers (autoanswer). Similarly, some modems have callback security feature that provides a
mechanism of protecting its computer from unauthorized access. These modems
include a list of valid phone number and password combinations with routines for
automatic passward check and callback. As soon as a remote computer tries to
establish a connection with a computer having this type of modem, the modem
calls back the remote computer immediately to ensure that the connection is
indeed established with the authorized remote user. Modems having such special
features are called smart modems or intelligent modems.
Analog versus Digital Transmission
If data is sent over long distances, the analog signal can become weak and
distorted. To avoid this, amplifiers are used. An afnplifier'used in a
communication system is similar to the amplifier used in a home stereo system - it
strengthens or amplifies a weak signal. Amplifiers are typically placed along
communication lines between modems.
Today, signals are also sent in digital mode. In this case modems are not needed.
When digital mode transmission is used over long distances, a repeater is used to
receive and then transmit the signal. Like amplifier, the...
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- Spring '14