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Problem:
the vulnerable period (length of time
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in
which a frame can be destroyed) for pure
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ALOHA
is 2t, where t is the time required to
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broadcast
a frame
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Slotted
ALOHA: devised by Roberts in 1972 as
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a
means for doubling channel utilization:
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divide
time into discrete intervals or slots, each slot
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corresponds
to a frame. Stations must wait for the
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beginning
of a slot before broadcasting
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Under
light load, average waiting time is
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increased
from zero to one-half the slot length
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Under
heavy load, the utilization is doubled, to
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about
37% (because vulnerability period
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dropped
from 2t to t)
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