Simplex Stop and Wait

1
Simplex Stop and Wait

• Sender
• Get data from network layer
• Send it to physical layer
• Wait for Ack
• Receiver
• Wait for frame arrival
• Get data from physical layer
• Send it to network layer
• Send Ack to the receiver

2
Normal operation
3
Analysis

• What if the channel is noisy.
• Data can be lost during the transmission
• Add a timer
• Resend data if receiver times out
• Sender starts timer when it sends a frame
• Receiver sends Ack only if data is correctly
  received

4
Stop-and-Wait ARQ, lost frame
5
Analysis

• What if the Ack is lost
• Receiver correctly receives the data and expect
  the next frame
• Sender times out because Ack is lost
• Sender resends the frame
• Receiver gets the duplicate data without noticing
  it

6
Stop-and-Wait ARQ, lost ACK frame
7
Positive Ack with Retransmission

• Sender
• Send next-frame-to-send to physical layer
• Start timer
• Wait for event (frame arrival, error, timeout)
• If(event frame arrival)
• Get Ack from physical layer
• If(Ack next-frame-to-send)
• Stop timer
• Get next frame to send from network layer
• Increment next-frame-to-send

• Receiver
• Wait for event(frame arrival, error)
• If(event frame arrival)
• Get data from physical layer
• If(frame expected)
• Pass data to network layer
• Increment frame expected
• Ack 1 frame expected
• Send Ack to physical layer

8
Piggybacking (delay Ack until next outgoing frame
- bidirectional)
9
Go-Back-N
10
Selective Repeat ARQ

• Go-Back-N ARQ inefficient because multiple frames
  are resent when errors or losses occur
• Selective Repeat retransmits only an individual
  frame
• Timeout causes individual corresponding frame to
  be resent
• NAK causes retransmission of oldest un-acked
  frame
• Receiver maintains a receive window of sequence
  numbers that can be accepted
• Error-free, but out-of-sequence frames with
  sequence numbers within the receive window are
  buffered
• Arrival of frame with Rnext causes window to
  slide forward by 1 or more

11
Selective Repeat ARQ