CSCI 465 D ata Communications and Networks Lecture 10 Martin van Bommel CSCI 465 Data Communications & Networks 1 Data Link Control Protocols when sending data, to achieve control, a layer of logic is added above the Physical layer data link control or a data link control protocol to manage exchange of data over a link: frame synchronization, flow control, error control Addressing control and data on same link link management CSCI 465 Data Communications & Networks 2 Flow Control Necessary when data is being sent faster than it can be processed by receiver Computer to printer is typical setting Can be from computer to computer when a processing program is limited in capacity Influenced by: transmission time propagation time a bit to traverse the link Usually a buffer is filled, and transfer is stopped until buffer is emptied CSCI 465 Data Communications & Networks 3 X-On / X-Off Byte-oriented flow control When all is well, X-On signal active When buffers near full, X-Off signal to tell sender to wait When buffers near empty, X-On again Pause on screen CSCI 465 Data Communications & Networks 4 Model of Frame Transmission CSCI 465 Data Communications & Networks 5 Stop-and-Wait Simplest form of Frame-Oriented Control Source may not send new frame until receiver acknowledges previous one Very inefficient, especially when a single message is broken into many small frames buffer size of receiver is limited No one station can dominate medium if error, detected sooner and less data need be retransmitted CSCI 465 Data Communications & Networks 6 Bit Length of Link B = R d/V B = length of link in bits R = data rate of link, in bps d = length, or distance, of link in meters V = velocity of propagation in m/s a = B / L a = propagation time (normalized) L = length of frame in bits CSCI 465 Data Communications & Networks 7 Stop-and-Wait Link Utilization 8 Stop-and-Wait Example 40 km link, 10 Mbps rate, 1000 bit frame Signal in copper propagates at 2 x 10 8 m/sec Takes 0.1 msec to transmit frame 0.2 msec delay to begin arriving Total 0.3 msec to receive, 0.2 msec to ACK Line busy for 0.5 msec for 0.1 msec frame 20% efficiency CSCI 465 Data Communications & Networks 9 Sliding-Window Flow Control allows multiple numbered frames to be in transit receiver has buffer W frames 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 CSCI 465 Data Communications & Networks 10 Sliding Window Diagram 11 Sliding Window Example 12