Mesh networks are often used to link switches in a LAN. In Figure 2-4, the mesh has a total of seven connections. If any of these seven connections goes bad, any switch can still reach any other switch while traveling through at most one intermediate switch.
Mesh networks are also very common for metropolitan or wide area networks (WANs). These networks use routers to route packets from network to network. For reliability and performance reasons, routers are usually arranged in a way that provides multiple paths between any two nodes on the network in a mesh-like arrangement.
Considering Cable
You can find much more about the details of working with network cable in Book 3, Chapter 1, as well as Book 4, Chapter 1. But before we get too far, I want to give you an overview of what’s involved with cabling together a network.
For starters, network cable and all the bits and pieces that go along with it are the most important components of layer 1 of the OSI Reference Model. The following sections describe the most important layer 1 and cabling details you need to know.
Twisted-pair cable
There are several varieties of cable you can choose from, but the most common is called twisted-pair. It’s called that because inside the outer sheath of the cable are four pairs of small insulated wire. The wires are 24 gauge, which means they’re about half a millimeter in diameter. These pairs are color coded: blue, green, orange, and brown. For each pair, there is one solid color wire and one striped wire — so, the blue pair consists of a solid blue wire and a blue-and-white striped wire.
The two wires that make up each pair are twisted together in a way that prevents the electrical signals within each pair from interfering with the other pairs. To accomplish this, each pair is twisted at a different rate.
The maximum length of a single run of Cat-5e cable is 100 meters.
Cat-5e cable is able to carry network data at speeds of up to 1 gigabit per second (Gbps). The newer and somewhat more expensive Cat-6 cable can carry data at up to 10 Gbps but can sustain that speed for only 55 meters.
RJ45 connectors
Twisted-pair cable is attached to network devices using a special type of connector called an RJ45, which is a small block of plastic with eight metal contacts. RJ45 connectors resemble a telephone connector but are larger (telephone connectors have just four electrical contacts). For the cable to meet Cat-5e standards, the twists of the individual pairs must be maintained all the way up to the RJ45 connector.
RJ45 connectors come in both male (plug) and female (receptacle) varieties. Typically, the male connector is installed on the cables and the female connectors are installed in equipment. Thus, to connect a cable to a computer, you plug the male RJ45 plug on the cable into the female RJ45 receptacle on the computer.
Patch panels and patch cables
A patch panel is a group of RJ45 receptacles on a single metal plate, usually attached to a 19-inch equipment rack. Patch panels are used to bring cables run from individual computer locations to a single location where they can then be patched to other equipment using patch cables. A patch cable is simply a short length of twisted-pair cable with an RJ45 plug on both ends. Patch cables are usually 3 to 10 feet in length, but longer lengths are occasionally used.
Patch panels typically have either 24 or 48 ports. Depending on the size of your network, you may have more than one patch panel at a single location. For example, a large network may have four 48-port patch panels to support a total of 192 computers.
Repeaters and hubs
A repeater is a layer-1 device that is designed to circumvent the maximum length limitation of twisted-pair network cables. A repeater contains two RJ45 ports, which are connected internally by an amplifier. Electrical signals received on either of the two ports are boosted by the amplifier and sent through the other port. Thus, the cables on both ends of the repeater can be up to 100 meters. The repeater effectively doubles the reach of the cable.
A hub is a repeater with more than two ports. For example, a hub may have four or eight ports. These ports can each connect to another device on the network such as a client computer, a server, or a printer. A port on a hub can also connect to another hub, so that (for example) an eight-port hub can connect to seven computers and another eight-port hub, which can connect to seven more computers. In this way, two eight-port hubs can connect 14 computers to each other.
There are two very important things to know about hubs.
The second most important thing to know about hubs is that an electrical signal received on any of the hub’s ports is amplified and repeated on all the other ports in the hub. So, in an eight-port hub, any electrical signals received on port 1 are amplified and then sent out on ports 2 through 8. Any devices that are connected to ports 2 through 8 see the signals that were received on port 1. The same is true for signals received on any of the other ports; for example, any signals received on port 4 will be amplified and repeated on ports 1 through 3 as well as ports 5 through 8.
That’s the second most important thing to know. The first most important thing to know about hubs is that they’re almost never used anymore. That’s because simply repeating all incoming signals on all ports is an incredibly bad idea, for reasons that will become apparent later in this chapter and in Chapter 3 of this minibook. If your network still has hubs, you should seriously consider replacing them with switches, which are described in the next section and further explained in the next chapter.
Switches
A switch is a layer-2 device that is similar to a hub in that it allows you to connect more than one device, and packets received on one port are relayed to other ports. The difference, however, is that a switch is able to examine the actual contents of the data that it receives. As I explain in the “Pondering Packets” section, later in this chapter, data is sent in units called packets that contain a destination address. A switch looks at this destination address and repeats the incoming packet only on the port that can deliver the packet to the intended destination.
For example, suppose Computer A is connected to switch port 1, and Computer D is connected to switch port 4. If Computer A sends a packet to Computer D, that packet is received on switch port 1. The switch knows that Computer D is connected to switch port 4, so the switch sends the packet out on switch port 4. In this way, Computer D receives the packet. The computers or devices that are connected to the other ports on the switch are not bothered with the packet intended for Computer D.
If that doesn’t make a lot of sense, don’t worry: It will. The next two sections in this chapter explain the concept of MAC addresses, which are how networks identify the intended recipients of data packets, as well as how data packets work. Then, in Chapter 3 of this minibook, I dive deeper into how switches do their magic.
Perusing Ports,