Data Communications Networking Devices - 4th Ed.pdf

614 ________________________________________________ LAN INTERNETWORKING DEVICESFigure 5.51Using a high-speed switch as a network backboneHandling speed incompatibilitiesOne of the main problems associated with the use of LAN switches occurs when adevice connected on a high-speed port requires communications with a deviceconnected on a lower operating rate port. For example, assume a server is connectedto a switch at 100 Mbps while a workstation is connected via a 10 Mbpsoperating rate. Since buffer storage on each port is ®nite, it becomes possible for aclient query followed by a long server response to result in the loss of frames once abuffer is ®lled. Asimilar problem can occur in the reverse direction when thecumulative operating rate of many clients attempting to access a common deviceexceeds the operating rate of the connection between the switch and that device.Recognizing this problem, vendors developed several methods to regulate thetransmission of information, a process referred to as ¯ow control. Flow control isprimarily applicable to Ethernet switches, since the rotating token can be used todelay transmission on Token-Ring switches. Two popular methods used by switchvendors to regulate the transmission of information include backpressure andserver software modules.BackpressureBackpressure is a term used to represent the generation of a false collision signal.Because a collision signal causes an Ethernet workstation or server to delay furthertransmission based on an exponential backoff algorithm, it provides a mechanismfor implementing ¯ow control. That is, once buffer storage in the switch hasreached a prede®ned level of occupancy, the switch will generate a false collision