- What is Aloha:
- The very first midium access protocol developed at the University of Hawaii around 1970.
- Their problem:
- The Uuniversity of Hawaii
had campuses on
different islands
- Computers (a.k.a. "main frames")
were only located
on the main campus
(computers were very exensive in those days):
- In those days,
we use terminals
connected (through wires) to the
main frame to
access the
main frame computer:
- The problem facing
the Univ. of Hawaii:
- It's too expensive to
run wires between
the main campus and the
different
satelite campussess
- How do you connect the terminals on campusses on other islands to gain access the computer (main frame) on the main campus ????
- It's too expensive to
run wires between
the main campus and the
different
satelite campussess
- The Uuniversity of Hawaii
had campuses on
different islands
- Their solution:
- Use radio communication !!!
- Property of
radio transmissions
(on the same frequency):
- Simultaneous reception
of multiple transmissions
will cause
both transmissions
to be received
in error:
- Simultaneous reception
of multiple transmissions
will cause
both transmissions
to be received
in error:
- Network protocol:
- Network protocol = a procedure that must be followed by every nodes on a network to regulate the access to the communication network
- Operation of
an Aloha terminal:
- When the user on the
terminal is
idle:
- The terminal is
idle
I.e.: does nothing
- The terminal is
idle
- When the user on the
terminal
hits/presses a
key:
- The terminal will transmit a message containing the ASCII code for the key (letter) pressed using the Aloha network protocol
- When the user on the
terminal is
idle:
- The Aloha
network protocol:
- The (radio) terminal
on a satelite campus
sends
the message (= frame) to
the main frame on
the main campus
and then
wait for
an ACK frame:
Note:
- The (radio) transmission can be "heard" (received) by all nodes (terminals/main frame) in the Aloha network !!!
- If the message was
received correctly,
the main frame transmits
an ACK message (frame)
back to acknowledge the
reception
- If the terminal did
not receive an
ACK message
(from the main frame) after
a time out period,
the termimal will:
-
Wait a
random period
- Then
re-tramsmit the
message
-
Wait a
random period
Note:
- The most likely reason
that a message was
lost is:
- There was a
collision
due to
simultaneous message arrival/reception
at the
main campus site:
- There was a
collision
due to
simultaneous message arrival/reception
at the
main campus site:
-
Waiting
a random time period
before
re-transmitting the
messages will:
-
Decrease the
likelihood of
another collision
due to
simultaneous transmissions
(it's not likely that will wait the same random period !!!)
-
Decrease the
likelihood of
another collision
due to
simultaneous transmissions
- The (radio) terminal
on a satelite campus
sends
the message (= frame) to
the main frame on
the main campus
and then
wait for
an ACK frame:
- Note:
- If the
main frame
wants to
transmit a message to
some terminal, the
main frame will
use the
same network protocol:
- Main frame sends the
data frame and then
waits for an
ACK frame from the
terminal
- If no ACK message is
received before the
times out,
the main frame will:
- Wait a
random time period
and then
- Re-transmit the message
- Wait a
random time period
and then
- Main frame sends the
data frame and then
waits for an
ACK frame from the
terminal
- If the
main frame
wants to
transmit a message to
some terminal, the
main frame will
use the
same network protocol:
- Some background information on
the operational environment in
Hawaii:
- The distance between
island is
very large (7-63 miles apart)
- Messages between
terminals and main computer
are very short
(We only had character-based terminals --- no graphics in 1970 !!!)
Result:
- The terminal
(on a remote island) will
have
finished transmitting
the message before
the radio signal has
reached the
main campus !!!
- The distance between
island is
very large (7-63 miles apart)