is, collision occurs if two or more computers anywhere in the network transmit SO that two or more frames overlap. Introduction to Telecommunications Network Engineering Twisted pair, 100m or fiber connection 10Base-F Switch up to 2 km collision collision domain domains separated by switch Figure 6.32 Network extension with and without a switch. Bandwidth utilization of the shared networks (i.e., inside one collision domain) is poor because one transmitting workstation seizes the whole network although only the segments to the source and destination comput- ers are needed for communication. As a rule of thumb, approximately 40 % of the network data capacity can be utilized on average in one collision domain; that is, all computers share the 4-Mbps transmission capacity of a 10-Mbps Ethernet network. If higher capacity is needed by worksta- tions, the number of collisions increases, delays increase, and the average capacity used by successful transmissions decreases because frequent colli- sions occupy the channel. The switched LAN that is discussed next divides a LAN into multiple collision domains and the bandwidth utilization is much improved. However, broadcast frames propagate to all segments in switched Ethernet and the router is needed at the border of the broadcast domain. 6.5.7 Switched Ethernet Switches and Bridges We can improve the performance of a CDMA/CD network by using switches (or switching hubs or bridges) instead of repeaters or hubs. Switches have replaced hubs in most CDMA/CD networks including coaxial ones where we called them bridges. We may connect many repeater hubs as shown in Figure 6.32 to each other by a switch or a switching hub. The switches create separate collision domains because they do not forward collision signals from one port to another. Switches do not transmit all received frames to all ports like repeaters. Instead they use MAC addresses and transmit frames to the direction where the destination is known to be located. Bridges are able to learn by listening Data Communicat