Modem Standards
o Bell modems
-First
commercial modems by Bell Telephone Co.
-Developed in
early 1970s
o ITU-T modem standards
-V-series:
Today's most popular modem standards
-Bell modem
compatible: V.21/22/23/26/27/29
o Intelligent modems
-Hayes (or
Hayes-compatible) modems
-Modem is
controlled by instructions (AT commands)
-Automatic
answering, dialing, etc.
BELL Modems
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™ Traditional Modems
o Traditional modems are limited to a data
rate of 33.6 Kbps as determined by
Shannon's
formula Data rate ∝
Signal-to-noise ratio
o New modems with bit rated of 56 Kbps are
wide spread now
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