Reducing the Effects of Large Propagation Delays on High Speed IEEE 802.3 CSMA/CD Networks using Collision Truncation

A half-duplex repeater is the least-cost method for connecting multiple hosts to an IEEE 802.3 Ethernet LAN. However, if we increase the data rate from 10 Mb/s Ethernet to 100 Mb/s Fast Ethernet, shared access via CSMA/CD becomes less efficient because of the resulting increase in size of the collision fragments. For example, in a typical star-wired single-repeater network with a diameter of 200 meters, upgrading from 10BASE-T to 100BASE-T increases the round-trip delay (including bit budget for the network cards and repeater) from about 100 bit times to almost 500 bit times. Moreover, a further upgrade to 1000 Mb/s Gigabit Ethernet would increase the round-trip delay to about 4000 bit times— which is eight times larger than the minimum frame size. In this paper, we introduce a simple change to the handling of incoming signals at the repeater ports known as collision truncation, and show that it reduces the average duration of a collision by more than 50 percent for networks with large round-trip delays. For the standard BEB Ethernet protocol operating at moderate load, collision truncation reduces the mean endto-end network delay by more than 25% for 100 Mb/s Fast Ethernet and by more than 50% for 1000 Mb/s Gigabit Ethernet. For BLAM, collision truncation increases the maximum throughput by more than 10%, and reduces the mean end-to-end delay substantially under high load. Collision truncation requires no changes to the host network adapters, and its use is completely transparent to the hosts. The only behavioral change that is visible to the attached hosts is the illusion that the network diameter appears to have been reduced. The relative cost of adding collision truncation to a repeater is very small compared to upgrading to a frame-level device, such as a bridge.