Frame bursting: a technique for scaling CSMA/CD to gigabit speeds

Abstract

Gigabit Ethernet supports the transmission of ordinary Ethernet frames at a data rate of 1000 Mb/s. Both flow-controlled full-duplex point-to-point links and half-duplex shared collision domains are included in the IEEE 802.3z draft standard. The parameters for half-duplex operation were chosen to align with the requirements of current generic building cabling standards, rather than to match the natural way that network size scales inversely with speed, so a star-wired single repeater topology with a maximum diameter of 200 m is permitted. Thus, gigabit Ethernet is the first time that the CSMA/CD medium access control algorithm has been applied to networks in which the round-trip propagation delay can be much greater than the transmission time for a minimum length frame. In this article, we describe the changes made to CSMA/CD that allow it to support large propagation delays without increasing the minimum frame length or changing its existing one-frame-at-a-time service interface. First, carrier extension is used to decouple the slot time from the minimum frame length, so the slot time can be increased without changing the Ethernet frame format. Second, frame bursting is used to reduce the overhead for transmitting small frames by allowing a host to transmit more than one frame without ever releasing control of the channel. Using simulation, we show that CSMA/CD with carrier extension and frame bursting operating on 1000 Mb/s links provides a significant performance increase over 100 Mb/s Fast Ethernet.