1. EE 4272Spring, 2003 Chapter 7 Data Link Control Objectives: Effective & reliable data communication between two directly connected transmitting-receiving stations Requirements:  Frame Synchronization: Data are sent in blocks called frames. The beginning & end of each frame must be recognizable.  Flow Control: The sending station must not send frames at a rate faster the receiving station can absorb them.  Error Detection: There is no 100% reliable data transmission existing, bit errors should at least be detected.  Error Control: Bit errors introduced by the transmission system should be corrected.

2. EE 4272Spring, 2003 Flow Control Ensuring the sending entity does not overwhelm the receiving entity  Preventing buffer overflow Transmission time  Time taken to emit all bits into medium Propagation time  Time for a bit to traverse the link

3. EE 4272Spring, 2003 Model of Frame Transmission

4. EE 4272Spring, 2003 Stop and Wait Source transmits frame Destination receives frame and replies with acknowledgement Source waits for ACK before sending next frame Destination can stop flow by not send ACK Works well for when the frame size is large SenderReceiver

5. EE 4272Spring, 2003 Stop and Wait Link Utilization Large block of data may be split into small frames  Limited buffer size  Errors detected sooner (when whole frame received)  On error, retransmission of smaller frames is needed Prevents one station occupying medium for long periods Stop and wait becomes inadequate

6. EE 4272Spring, 2003 Stop and Wait Link Utilization

7. EE 4272Spring, 2003 Sliding Windows Flow Control Allow multiple frames to be in transit Receiver has buffer W long Transmitter can send up to W frames without ACK Each frame is numbered ACK includes the number of next frame expected Sequence number bounded by size of field (k)  Frames are numbered modulo 2 k SenderReceiver T ime … … 1 2 3 … W

8. EE 4272Spring, 2003 Sliding Window Diagram

9. EE 4272Spring, 2003 Example Sliding Window

10. EE 4272Spring, 2003 Sliding Window Enhancements Receiver can acknowledge frames without permitting further transmission (Receive Not Ready) Must send a normal acknowledge to resume If duplex, use piggybacking  If no data to send, use acknowledgement frame  If data but no acknowledgement to send, send last acknowledgement number again, or have ACK valid flag (TCP)