tands for collision detection. These schemes have given way to forms of controlled access. Economy and efficiency had more to do with this changeover than anything else. We will describe token ring, which is one of the protocols that uses a token to control access. Most new LANs and older CSMA/CD LANs have been upgraded using the electrical interface of CSMA and CSMA/CD and now incorporate hubs, switching hubs, switches, and/or routers. This eliminated any possibility of collisions and mitigated the very limited traffic capacity typical of Ethernet (CSMA/CD). ENTERPRISE NETWORKS I: LOCAL AREA NETWORKS 11.6.2 CSMA and CSMA/CD Access Techniques Carrier sense multiple access (CSMA) is an early LAN access technical, which some simplistically call "listen before transmit." This "listen before transmit" idea gives insight into the control mechanism. Essentially, if user No. 2 is transmitting, user No. 1 and all others hear that the medium is occupied and refrain from using it. The technique was prone to collisions due to propagation and processing delays to detect that the "line is busy." In that open period before carrier detection, user No. 1, for example, could start transmitting where that transmission would then be corrupted. User No. 2's transmission would also be corrupted. Every station must wait until the two corrupting transmission finish before starting a transmission of its own. CSMA/CD (collision detection) reduced the time to detect another user on the medium and thus did not transmit. They would backoff based on a special algorithm and wait a specified time before trying again. Both systems were inefficient and only permitted a traffic load up to some 35% of capacity. More and more LAN operators have turned to some sort of switching technique where each required connectivity was on a point-to-point basis. Just two stations would be involved, the sender and the receiver, or the sender and a switch or router port. Collisions and backoff time were completely eliminated and loading could be up to nearly 100%