Working with Hubs and Switches
Despite the advantages of switches over hubs, hubs are still widely used in older networks. Whether working with hubs or switches, it is important to be aware of some of their characteristics to troubleshoot a network. For instance, if performance-monitoring tools show network bottlenecks or a congested network, the hubs may need to be replaced with switches for increased performance. This is especially important when working with both hubs and switches in a production environment.
Hub and Switch Ports
Hubs and switches have two types of ports: medium dependent interface (MDI) and medium dependent interface crossed (MDI-X). The two types of ports differ in their wiring. As the X implies, an MDI-X port’s wiring is crossed; this is because the transmit wire from the connected device must be wired to the receive line on the other. Rather than use a crossover cable (which is discussed in the next section, "Cables Connecting Hubs and Switches"), you can use the more simple straight-through cable (also discussed in the next section) to connect systems to the switch or hub.
On most modern hubs and switches, a special port called the uplink port allows you to connect two hubs and switches to create larger networks. Because the aim of this type of network connection is to make each hub or switch think that it is simply part of a larger network, the connection for the port is not crossed; a straight-through network cable is used to connect the two hubs or switches together. Figure 3.6 shows the uplink port on an Ethernet switch.
Figure 3.6 The uplink port on an Ethernet switch.
In the absence of an uplink port, you can connect two hubs or switches together by using MDI-X ports, but you must use a crossover cable to do so.
Cables Connecting Hubs and Switches
Two types of cables are used to connect devices to hubs and switches: crossover cables and straight-through cables. The difference between the two types is that in a crossover cable, two of the wires are crossed; in a straight-through cable, all the wires run straight through.
Specifically, in a crossover cable, Wires 1 and 3 and Wires 2 and 6 are crossed: Wire 1 at one end becomes Wire 3 at the other end, Wire 2 at one end becomes Wire 6 at the other end, and vice versa in both cases. You can see the differences between the two cables in Figures 3.7 and 3.8. Figure 3.7 shows the pinouts for a straight-through cable, and Figure 3.8 shows the pinouts for a crossover cable.
Figure 3.7 Pinouts for a straight-through twisted-pair cable.
Figure 3.8 Pinouts for a crossover twisted-pair cable.
Hubs and switches are sometimes equipped with a network connection for another cable type, such as coaxial. Such switches that accommodate different media types such as fiber-optic cable and UTP, are referred to as hybrid switches. Other higher-end devices simply have empty sockets into which you can plug connectivity modules of choice. This approach lets you create very fast networks. For example, three 24-port 10/100 Ethernet switches could be connected to each other by a Gigabit Ethernet fiber-optic connection. This would create a very fast network structure in which switch-to-system communication can occur at 200Mbps (in full-duplex mode) and switch-to-switch communication can occur at Gigabit Ethernet speeds. The result is a very fast local area network (LAN).
Hub and Switch Indicator Lights
Both hubs and switches use light-emitting diodes (LEDs) to indicate certain connection conditions. At the very least, a link light on the hub will indicate the existence of a live connection. On higher-end devices, additional lights might indicate activity, the speed of the connection, whether the connection is at half- or full-duplex, and sometimes errors or collisions. The LEDs provide an immediate visual indicator about the status of the device, so familiarizing yourself with their function is a worthwhile exercise. A further discussion of hub and switch LEDs is provided in Chapter 13, "Troubleshooting Tools and Utilities."
Rack-Mount, Stackable, and Freestanding Devices
Some hubs and switches, as well as many other networking devices, are designed to be placed in a rack, whereas others are labeled as stackable or freestanding. Rack-mount devices are designed for placement into equipment racks, which are a common sight in computer rooms. The racks are approximately 19 inches wide; devices designed to be rack-mounted are slightly smaller than freestanding devices, so they can fit in the racks. Small metal brackets are screwed to the sides of the devices to allow them to be fitted into the racks.
If you don’t have racks, you need to use stackable or freestanding devices. These devices can literally be placed on top of one another. Many network equipment manufacturers realize that not everyone has racks, and so they make their equipment usable in either a rack or a freestanding configuration.
Managed Hubs and Switches
Both hubs and switches come in managed and unmanaged versions. A managed device has an interface through which it can be configured to perform certain special functions. For example, it may allow for port mirroring, which can be useful for network monitoring, or allow ports to be specified to operate at a certain speed. Because of the extra functionality of a managed device, and because of the additional components required to achieve it, managed devices are considerably more expensive than unmanaged devices. When you’re specifying switches or hubs, consider the need for manageability carefully. If a switch will be used to connect servers to the network, a managed device might make the most sense—the extra functionality might come in handy. On parts of the network that accommodate client computers, unmanaged devices generally suffice.
At the time of this writing, switches are still quite a bit more expensive than hubs with equivalent capacity, but the gap is narrowing quickly. Some manufacturers have stopped producing hubs and instead are putting all their efforts into developing switches. This would seem to be a sound strategy. In all but the smallest networks or companies with the most restrictive budgets, hubs are rapidly being replaced by switches. In new implementations, hubs are unlikely to be specified and installed.