- Fact:
- Previously,
we have described the
Aloha network protocol
informally
- We will now give the formal description of the Aloha network protocol
- Previously,
we have described the
Aloha network protocol
informally
- The Aloha protocol:
Explanation:
- If a node has a
message to transmit:
- transmit the message (immediately)
- Then:
- Wait a time out period for the ACK frame
- If ACK frame is received within
time out:
- done
Otherwise: (the data frame was probably lost due to collision !!)
- Wait a random interval (to avoid another collision )
- Retransmit the message
- If a node has a
message to transmit:
- Note:
- The most likely
cause that
there was no ACK reply
is:
- The transmission was corrupted due to collision (simultaneous transmission)
- The random delay in included
into the Aloha protocol to:
-
reduce
the likelihood
of a
repeated
collision
(Different nodes will pick a different delay before they transmit again !!!)
When different nodes pick different random delays, simultaneous reception is less likely
-
reduce
the likelihood
of a
repeated
collision
- The most likely
cause that
there was no ACK reply
is:
- Sensing:
- Sensing = listening for transmissions on the broadcast channel
-
Important fact:
- When multiple nodes
transmits
at the same time,
the simultaneous transmissions
will cause a
collision
- All frames in a collision will be lost !!!!
- Broadcast network
often use
sensing to
minimize the
likelihood that
mmultiple nodes
will transmit at the
same time
- When multiple nodes
transmits
at the same time,
the simultaneous transmissions
will cause a
collision
- How sensing can
help
avoid collisions:
- Other terminals (nodes)
can "heard"
the transmission of
a terminal:
-
When a node
detects a transmission, it can
avoid a collision by:
- delaying its own transmission until the channel is clear
- Other terminals (nodes)
can "heard"
the transmission of
a terminal:
- The Aloha protocol:
does not use channel sensing !!!
- $64,000 question:
- Why does a node
not use
sensing
to avoid a
potential collission ????
Answer:
- the messages were
short
(nodes send text
in those days, not graphics)
and
- the distance
between the islands were large.
Result:
- Sensing will not help a node to avoid a collision
- Why does a node
not use
sensing
to avoid a
potential collission ????
- Important fact:
- A
collision
of multiple transmissions
take place at the
receiver (= main campus)
(Not at the sender !!!)
Graphically:
- A
collision
of multiple transmissions
take place at the
receiver (= main campus)
- Why
does sensing
not help to
avoid collision in
Hawaii ???
Because:
- If a node
senses an
idle transmission medium and
transmits:
- The transmission may result in a collision
Example:
- Suppose
node A and
B wants to
transmit a
data frame to the
main campus:
- A and
B
will sense that
the channel is
idle:
Both nodes will start their transmission:
And their transmission will collide at the receiver:
- If a node
senses an
idle transmission medium and
transmits:
- Furthermore:
- Sensing a busy channel is not an indication for a collision !!!
Example:
- Supose A
sends a message to the
main campus:
- A's messages
reaches the node
B:
- If node
B start
tranmitting
NOW:
The transmission will not collide at the reciver:
Therefore:
- Sensing that the
channel is idle
can not help you
do the right thing
- Sensing that the channel is busy can not help you do the right thing
So: why bother sensing the channel at all ???