 and inform the fish about its immediate environment. In the event that two Gymnarchus are in the same area, their emissions interfere with one another (intersymbol interference?), rendering their detection mechanisms ineffective. But, being the clever creatures that they are, Gymnarchus has designed a technique to deal with this problem. When the two fish "hear" each other's transmissions, they both immedi- ately stop pulsing. Each fish waits a measurably random period of time while continuing to swim, after which they begin to transmit again, but this time at slightly different frequencies to avoid interference. We hope that you understand that under the circumstances we cannot in good conscience grant this patent. Sincerely yours, U.S. Patent Office I don't think a fish can hold a patent, but if it could, Niloticus would hold the patent for a widely used technique called "Carrier Sense, Multi- ple Access with Collision Detection." Please read on. Perhaps the best-known, contention-based medium access scheme is Ethernet, a product developed by 3Com founder and Xerox PARC vet- eran Bob Metcalfe. In contention-based LANs, devices attached to the network vie for access using the technological equivalent of gladiatorial combat. "If it feels good, do it" is a good way to describe the manner in which they share access (hence the Berkeley Method). If a station wants to transmit, it simply does so, knowing that the possibility exists that the transmitted signal may collide with the signal generated by another sta- tion that transmits at the same time. Even though the transmissions are electrical and are occurring on a LAN, there is still some delay between the time that both stations transmit and the time that they both realize that someone else has transmitted. This realization is called a collision, and it results in the destruction of both transmitted messages as shown in Figure 5-19. In the event that a collision occurs as the result of simul- taneous transmission, both stations back off by immediately stopping the