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 | William Stallings Data and Computer Communications Chapter 7 Data Link Control 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 * Model of Frame Transmission * 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 a few large frames * Fragmentation 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 * Stop and Wait Link Utilization * Sliding Windows Flow Control Allow multiple frames to be in transit Receiver has buffer W long Transmitter can send up to W frames . | William Stallings Data and Computer Communications Chapter 7 Data Link Control 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 * Model of Frame Transmission * 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 a few large frames * Fragmentation 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 * Stop and Wait Link Utilization * 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 number of next frame expected Sequence number bounded by size of field (k) Frames are numbered modulo 2k * Sliding Window Diagram * Example Sliding Window * 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) * Error Detection Additional bits added by transmitter for error detection code Parity Value of parity bit is such that character has even (even parity) or odd (odd parity) number of ones Even number of bit errors goes undetected * Cyclic Redundancy Check For a block of k bits transmitter generates n bit sequence Transmit k+n bits which is exactly divisible by some number Receive divides frame by that number If no remainder, assume no error For math,
