k. Figure 2-2. Hub connecting multiple hosts to a network When designing Ethernet networks, repeaters and hubs get treated the same way. The 5-4-3 rule of Ethernet design states that between any two nodes on an Ethernet network, there can be only five segments, connected via four repeaters, and only three of the segments can be populated. This rule, which seems odd in the context of today's networks, was the source of much pain for those who didn't understand it. As hubs became less expensive, extra hubs were often used as repeaters in more com- plex networks. Figure 2-3 shows an example of how two remote groups of users could be connected using hubs on each end and a repeater in the middle. Repeater Figure 2-3. Repeater joining hubs Hubs are very simple devices. Any signal received on any port is repeated out every other port. Hubs are purely physical and electrical devices, and do not have a presence on the network (except possibly for management purposes). They do not alter frames or make decisions based on them in any way. Figure 2-4 illustrates how hubs operate. As you might imagine, this model can become problematic in larger networks. The traffic can become SO intensive that the network becomes saturated-if someone prints a large file, everyone on the network will suffer while the file is transferred to the printer over the network. Port 4 Port 5 Port 6 Port 7 Port 0 Port 1 Port 2 Port 3 Figure 2-4. Hubs repeat inbound signals to all ports, regardless of type or destination If another device is already using the wire, the sending device will wait a bit, and then try to transmit again. When two stations transmit at the same time, a collision occurs. Each station records the collision, backs off again, and then retransmits. On very busy networks, a lot of collisions will occur. With a hub, more stations are capable of using the network at any given time. Should all of the stations be active, the network will appear to be slow because of the excessive collisions. Collisions are limited to network segments. An