Flow control ensures that a transmitting station does not overflow a receiving
station with data. ▫ We will discuss two protocols for flow control: ▫ Stop-and-
Wait.
Data Link Layer Flow Control
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Flow Control n
n
n
Flow Control is a technique for speed-matching of transmitter and receiver. Flow control ensures that a transmitting station does not overflow a receiving station with data We will discuss two protocols for flow control: n
Stop-and-Wait
n
Sliding Window
For the time being, we assume that we have a perfect channel (no errors)
Copyright by Jorg Liebeherr 98,99
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1
Stop-and-Wait Flow Control n n
n
Simplest form of flow control In Stop-and-Wait flow control, the receiver indicates its readiness to receive data for each frame Operations: 1. Sender: Transmit a single frame 2. Receiver: Transmit acknowledgment (ACK) 3. Goto 1.
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Copyright by Jorg Liebeherr 98,99
Analysis of Stop-and-Wait End of Transmit of ACK
Last bit received
Receiver propagation delay
Frame
Frame transmission delay Start Transmit
Copyright by Jorg Liebeherr 98,99
End Transmit
ACK
Efficiency = ?
ACK Sender Last bit of ACK received
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Sliding Window Flow Control n
n
Major Drawback of Stop-and-Wait Flow Control: n
Only one frame can be in transmission at a time
n
This leads to inefficiency if propagation delay is much longer than the transmission delay
Sliding Window Flow Control n n n
Copyright by Jorg Liebeherr 98,99
Allows transmission of multiple frames Assigns each frame a k-bit sequence number Range of sequence number is [0..2k-1], i.e., frames are counted modulo 2 k 5
Operation of Sliding Window n
Sending Window: n
At any instant, the sender is permitted to send frames with sequence numbers in a certain range (the sending window) Frames already transmitted
Window of frames that may be transmitted
0 1 2 3 4 5 6 7 0 1 2 3 4 5 6 Frame sequence number Copyright by Jorg Liebeherr 98,99
Last frame transmitted Window shrinks as frames are sent
Window expands as acknowledgements are received
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3
Operation of Sliding Window n
Receiving Window: n
The receiver maintains a receiving window corresponding to the sequence numbers of frames that are accepted Frames already received
Window of frames that are accepted by receiver
0 1 2 3 4 5 6 7 0 1 2 3 4 5 6 Last frame acknowledged Window shrinks as frames are received
Window expands as acknowledgements are sent
Copyright by Jorg Liebeherr 98,99
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Operation of Sliding Window n
How is “flow control” achieved? n
n
n
Receiver can control the size of the sending window By limiting the size of the sending window data flow from sender to receiver can be limited
Interpretation of ACK N message: n
Copyright by Jorg Liebeherr 98,99
Receiver acknowledges all packets until (but not including) sequence number N 8
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Example Transmitter
Receiver
0 1 2 3 4 5 6 7 0 1 2 3 F0 F1 F2
0 1 2 3 4 5 6 7 0 1 2 3
0 1 2 3 4 5 6 7 0 1 2 3
0 1 2 3 4 5 6 7 0 1 2 3 ACK4 3 0 1 2 3 4 5 6 7 0 1 2 3
0 1 2 3 4 5 6 7 0 1 2 3
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Copyright by Jorg Liebeherr 98,99
Example Continued Transmitter
Receiver
0 1 2 3 4 5 6 7 0 1 2 3 F3 0 1 2 3 4 5 6 7 0 1 2 3 F4 F5 F6 0 1 2 3 4 5 6 7 0 1 2 3
0 1 2 3 4 5 6 7 0 1 2 3
Copyright by Jorg Liebeherr 98,99
ACK3 4 0 1 2 3 4 5 6 7 0 1 2 3
0 1 2 3 4 5 6 7 0 1 2 3
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Analysis of Sliding Windows 0
1st frame received a+1
a
Wth frame received a+W
... 0
1
2
W
Receiver
Sender W
2a+1 1st ACK received 11
Copyright by Jorg Liebeherr 98,99
Analysis of Sliding Windows n
n
If the window size is sufficiently large the sender can continuously transmit packets: W ≥ 2a+1: Sender can transmit continuously normalized efficiency = 1
n
W < 2a+1: Sender can transmit W frames every 2a+1 time units normalized efficiency =
Copyright by Jorg Liebeherr 98,99
W 1 + 2a
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ARQ Error Control n n
n
Types of errors: Lost frames, damaged frames Most Error Control techniques are based on (1) Error Detection Scheme (e.g., Parity checks, CRC), and (2) Retransmission Scheme Error control schemes that involve error detection and retransmission of lost or corrupted frames are referred to as Automatic Repeat Request (ARQ) error control 13
Copyright by Jorg Liebeherr 98,99
ARQ Schemes n
n
The most schemes:
common
n
Stop-and-Wait ARQ
n
Go-Back-N ARQ
n
Selective Repeat ARQ
ARQ
retransmission
The protocol for sending ACKs in all ARQ protocols are based on the sliding window flow control scheme
Copyright by Jorg Liebeherr 98,99
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Stop-and-Wait ARQ n
n n
n
n
Stop-and-Wait ARQ is an addition to the Stop-andWait flow control protocol: Frames have 1-bit sequence numbers (SN = 0 or 1) Receiver sends an ACK (1-SN) if frame SN is correctly received Sender waits for an ACK (1-SN) before transmitting the next frame with sequence number 1-SN If sender does not receive anything before a timeout value expires, it retransmits frame SN 15
Copyright by Jorg Liebeherr 98,99
Stop-and-Wait ARQ n
Lost Frame
Timeout
ACK 0
1
ACK 1
Frame
1
0
1
Frame
Frame
Frame
ACK 0
A
B Copyright by Jorg Liebeherr 98,99
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8
Stop-and-Wait ARQ n
Lost ACK
Timeout
ACK 1
0
ACK 1
Frame
0
1
Frame
Frame
ACK 0
A
B Copyright by Jorg Liebeherr 98,99
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Go-Back-N ARQ n
Go-Back-N uses the sliding window flow control protocol. If no errors occur the operations are identical to Sliding Window
Copyright by Jorg Liebeherr 98,99
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Go-Back-N ARQ n
Operations: n
n
n
A station may send multiple frames as allowed by the window size Receiver sends a NAK i if frame i is in error. After that, the receiver discards all incoming frames until the frame in error was correctly retransmitted If sender receives a NAK i it will retransmit frame i and all packets i+1, i+2,... which have been sent, but not been acknowledged 19
Copyright by Jorg Liebeherr 98,99
Go-Back-N ARQ n
Frames 4,5,6 are retransmitted
Lost Frame
A Frame
Frame
Frame
Frame
Frame
Frame
Frame
1
2
3
4
5
6
4
5
6
ACK 6
Frame
0
NAK 4
Frame
ACK 3
Frame
B
Copyright by Jorg Liebeherr 98,99
Frames 5 and 6 are discarded
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10
Go-Back-N ARQ n
Frames 0-4 are retransmitted
Lost ACK Timeout
A Frame
Frame
Frame
Frame
Frame
Frame
Frame
Frame
1
2
3
4
0
1
2
3
4
5 ACK 5
Frame
0
ACK 3
Frame
ACK 3
Frame
B Frames 3 and 4 are discarded
Copyright by Jorg Liebeherr 98,99
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Selective-Repeat ARQ n
n
Similar to Go-Back-N ARQ. However, the sender only retransmits frames for which a NAK is received Advantage over Go-Back-N: n
n
Fewer Retransmissions.
Disadvantages: n n
n Copyright by Jorg Liebeherr 98,99
More complexity at sender and receiver Each frame must be acknowledged individually (no cumulative acknowledgements) Receiver may receive frames out of sequence
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Selective-Repeat ARQ n
only Frame 4 is retransmitted
Lost Frame
Frame
Frame
Frame
5
6
4
7
0
ACK 0
Frame
4
ACK 5
Frame
3
ACK 6, NAK 4 ACK 7
Frame
2
ACK 4
Frame
1
ACK 3
Frame
0
ACK 2
Frame ACK 1
Frame
ACK 1
A
B Frames 5 and 6 are buffered
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Copyright by Jorg Liebeherr 98,99
Example of Selective-Repeat ARQ Frames waiting for ACK/NAK
1 2
A
3 2
A
3 4 2 3 4 5
3
Frames received
2
B
1
ACK2 frame 1 is correct, send ACK 2
4
3 NAK2
B
1
frame 2 is in error, send NAK2
A
5
2
retransmit frame 2
Copyright by Jorg Liebeherr 98,99
B
Receiver must keep track of `holes’ in the sequence of delivered frames Sender must maintain one timer per outstanding packet
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