Flow Control

• flow control is a technique for assuring that a transmitting station does not overwhelm a receiving station with data
• two flow control mechanisms
  • stop-and-wait
    • also referred as "alternating bit" or "send and wait"
  • sliding window

Stop and Wait

• source transmits frame
• destination receives frame and replies with acknowledgement (ACK)
• source waits for ACK before sending next
• destination can stop flow by not send ACK
• works well for a few large frames
• stop and wait becomes inadequate if large block of data is split into small frames

Pros and Cons

• simple
• inefficient use of resources (low utilization, especially for systems with long propagation delays)

Sliding window

• allows multiple numbered frames to be in transit
• receiver has buffer W long
• transmitter sends up to W frames without ACK
• ACK includes number of next frame expected
• sequence number is bounded by size of field (k)
  • frames are numbered modulo $2^k$
  • giving max window size of up to $2^k - 1$
• receiver can ACK frames without permitting further transmission (Receive Not Ready)
• must send a normal acknowledge to resume
• if have full-duplex link, can piggyback ACKs

Go back N

• based on sliding window
• if no error, ACK as usual, indicating next frame expected
• use window to control number of outstanding frames
• if error, reply with rejection
  • discard that frame and all future frames until error frame received correctly
  • transmitter must go back and retransmitted that frame and all subsequent frames

• Receiver detects error in frame $i$
• Receiver sens rejection$-i (RRi)$
• Transmitter gets rejection-$i$
• Transmitter transmits frame $i$ and all subsequent frames

• Frame $i$ lost and no additional frame sent
• Receiver gets nothing and returns neither acknowledgement nor rejection
• Transmitter times out and sends acknowledgement frame with $P$ bit set to 1
• Receiver interprets this as command which it acknowledgement with the number of the next frame it expects (frame $i$)
• Transmitter then retransmits frame $i$

• Receiver gets frame $i$ and sends acknowledgement ($i + 1$) which is lost
• Acknowledgements are cumulative, so next acknowledgement ($i+n$) may arrive before transmitter times out on frame $i$
• if transmitter times out, it sends acknowledgement with $P$ bit set as before
• this can be repeated a number of times before a reset procedure is initiated
• NOTE: either damaged or lost, for sender is the same since it can't reconstruct the acknowledgement frame to be able to "read" it

Error Control

• error control techniques use some or all of these mechanisms
  • Automatic repeat request
    • error detection
    • positive acknowledge
    • retransmission after timeout
    • negative acknowledgement and retransmission
• Some versions of ARQ
  • Stop-and-Wait ARQ
  • Go-back-N ARQ
  • Selective-reject ARQ

Selective Reject

• also called selective retransmission
• only rejected frames are retransmitted
• subsequent frames are accepted by the receiver and buffered
• minimizes retransmission
• receiver must maintain large enough buffer
• more complex logic in transmitter
• hence less widely used
• saves bandwidth
• useful for satellite links with long propagation delays