Data Link Layer. Flow Control. Flow Control

Transcription

1 Data Link Layer Flow Control 1 Flow Control 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: Stop-and-Wait Sliding Window For the time being, we assume that we have a perfect channel (no errors) 2 1

2 Stop-and-Wait Flow Control 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 (CK). Goto 1. nalysis of Stop-and-Wait propagation delay Last bit received Frame CK End of Transmit of CK Receiver Efficiency =? Frame transmission delay CK Sender Start Transmit End Transmit Last bit of CK received 4 2

3 Sliding Window Flow Control Major Drawback of Stop-and-Wait Flow Control: Only one frame can be in transmission at a time This leads to inefficiency if propagation delay is much longer than the transmission delay Sliding Window Flow Control llows transmission of multiple frames ssigns each frame a k-bit sequence number Range of sequence number is [0..2 k -1], i.e., frames are counted modulo 2 k 5 Operation of Sliding Window Sending Window: t 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 Frame sequence number Last frame transmitted Window shrinks as frames are sent Window expands as acknowledgements are received 6

4 Operation of Sliding Window Receiving Window: 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 Last frame acknowledged Window shrinks as frames are received Window expands as acknowledgements are sent 7 Operation of Sliding Window How is flow control achieved? Receiver can control the size of the sending window y limiting the size of the sending window data flow from sender to receiver can be limited Interpretation of CK N message: Receiver acknowledges all packets until (but not including) sequence number N 8 4

5 Example Transmitter Receiver F0 F1 F2 CK4 9 Example Continued Transmitter F F4 F5 F6 Receiver CK

6 nalysis of Sliding Windows 0 a 1st frame received a+1 Wth frame received a+w Receiver W W 2a+1 1st CK received Sender 11 nalysis of Sliding Windows If the window size is sufficiently large the sender can continuously transmit packets: W 2a+1: Sender can transmit continuously normalized efficiency = 1 W < 2a+1: Sender can transmit W frames every 2a+1 time units W normalized efficiency = 1 + 2a 12 6

7 RQ Error Control 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 utomatic Repeat Request (RQ) error control 1 RQ Schemes The most common RQ retransmission schemes: Stop-and-Wait RQ Go-ack-N RQ Selective Repeat RQ The protocol for sending CKs in all RQ protocols are based on the sliding window flow control scheme 14 7

8 Stop-and-Wait RQ Stop-and-Wait RQ is an addition to the Stop-and- Wait flow control protocol: Frames have 1-bit sequence numbers (SN = 0 or 1) Receiver sends an CK (1-SN) if frame SN is correctly received Sender waits for an CK (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 Stop-and-Wait RQ Lost Frame Timeout CK 0 CK 1 CK

9 Stop-and-Wait RQ Lost CK Timeout CK 0 CK 1 CK 1 17 Go-ack-N RQ Go-ack-N uses the sliding window flow control protocol. If no errors occur the operations are identical to Sliding Window 18 9

10 Go-ack-N RQ Operations: station may send multiple frames as allowed by the window size Receiver sends a NK i if frame i is in error. fter that, the receiver discards all incoming frames until the frame in error was correctly retransmitted If sender receives a NK i it will retransmit frame i and all packets i+1, i+2,... which have been sent, but not been acknowledged 19 Go-ack-N RQ Lost Frame Frames 4,5,6 are retransmitted Frame 5 Frame 4 Frame 6 Frame 5 Frame 4 Frame Frame 2 Frame 6 CK NK 4 CK 6 Frames 5 and 6 are discarded 20 10

11 Go-ack-N RQ Lost CK Timeout Frames 0-4 are retransmitted Frame Frame 2 Frame Frame 2 Frame 4 Frame 5 Frame 4 CK CK CK 5 Frames and 4 are discarded 21 Selective-Repeat RQ Similar to Go-ack-N RQ. However, the sender only retransmits frames for which a NK is received dvantage over Go-ack-N: Fewer Retransmissions. Disadvantages: More complexity at sender and receiver Each frame must be acknowledged individually (no cumulative acknowledgements) Receiver may receive frames out of sequence 22 11

12 Selective-Repeat RQ Lost Frame only Frame 4 is retransmitted Frame Frame 2 Frame 7 Frame 4 Frame 6 Frame 5 Frame 4 CK 1 CK 2 CK CK 4 CK 6, NK 4 CK 7 CK 5 CK 0 CK 1 Frames 5 and 6 are buffered 2 Example of Selective-Repeat RQ Frames waiting for CK/NK Frames received 2 1 CK2 frame 1 is correct, send CK 2 4 NK2 frame 2 is in error, send NK retransmit frame Receiver must keep track of `holes in the sequence of delivered frames Sender must maintain one timer per outstanding packet 24 12