- Assumption:
- Sender and receiver are initialized with the same Frame#
- The Stop-and-Wait protocol:
- Sender:
Start: Transmit frame with send seq no = Frame#; Set a timeout; Wait: // Wait for an ACK frame to arrive before time out... while ( not timeout && ACK not received ) { Wait for ACK frame; } if ( timeout expired ) { go to Start; } // ACK was received... Check for correct ACK if ( ACK seq no ≠ Frame# ) { go to Wait; // Wrong ACK ! } Frame# = Frame# + 1; Done;
- Receiver:
// Receiver is ready to receive frame number R Receive frame; if ( CRC check is OK ) { if ( send seq no == R ) { Send ACK frame with recv seq no = R to sender; Deliver frame; R = R + 1; // Accept next new frame } else { // Received an OLD data frame Send ACK frame with recv seq no = (R-1) to sender; Discard frame; // because it's an old frame } } else { // Bit errors !!! Discard frame; }Special attention::
- When the receiver
receives an old frame:
- the receiver must send an ACK frame to recover the ACK frame loss
- To distinguish an
old ACK frame and
the current ACK frame (= R),
the receiver can use
the value:
(R-1)
- When the receiver
receives an old frame:
- Sender:
- After making these
2 modification
to the (my) basic protocol:
- Add a Send
Sequence Number to
the data frames (messages) to:
- Identify a data frame uniquely
- Add a
Receive Sequence Number to
the ACK frames to:
- Identify a ACK frame uniquely
the resulting protocol is the called:
- Stop-and-Wait protocol.
(Stop-and-Wait is one of the "classic" reliable communication protocols).
- Add a Send
Sequence Number to
the data frames (messages) to:
- Note:
- I will skip the
following scenarios:
- Error-free
- Data frame is lost
- ACK frame is lost
because: the operation of the Stop-and-Wait protocol is similar to that of the modified basic protocol
- Example:
when there are
no erros,
the
the sender and receiver will operate like this:
- I will skip the
following scenarios:
- How
the Stop-and-Wait
protocol solves the
delayed ACK problem:
Explanation:
- When the receiver receives the
2nd frame #4,
the receiver will
detect that:
- this
frame #4 is
a re-transmission
because:
- Send sequence no = 4
- R = 5 !!!!
- this
frame #4 is
a re-transmission
because:
- The receiver will
transmit:
ACK 4 // (R-1) = 5-1 = 4
- When the sender receives
the ACK 4, the
sender finds:
- ACK no = 4
- Frame# = 5
The sender can tell the the ACK 4 is not acknowlodging the data frame 5
- The sender will continue
waiting for
ACK 5
- In fact, the sender will timeout and retranmits the lost frame #5 again (and recovers correctly)
- When the receiver receives the
2nd frame #4,
the receiver will
detect that:
- The complete name of the
Stop-and-Wait protocol is:
- Stop-and-Wait
Automatic Repeat reQuest
protocol
Or: Stop-and-Wait ARQ protocol
Wikipedia page: click here
- Stop-and-Wait
Automatic Repeat reQuest
protocol
- ARQ protocols:
- ARQ protocol =
a
family of
error-control methods
for data transmission
- ARQ protocols use:
- ACK frames and
- timeouts
to achieve reliable data transmission over an unreliable transmission medium.
Wikipedia page: click here
- ARQ protocol =
a
family of
error-control methods
for data transmission
- Stop-and-Wait:
- The Stop-and-Wait ARQ protocol is the simplest member of the ARQ protocols
- We will soon
study a more
complex
(and
more efficient)
ARQ protocol:
- The Sliding Window ARQ protocol