A hub or concentrator is a device used to connect all of the computers on a star or ring network. From the outside, a hub, shown in Figure 2.14, is nothing more than a box with a series of cable connectors in it. Hubs are available in a variety of sizes, from four- and five-port devices designed for home and small business networks to large rack-mounted units with 24 ports or more. Installing a single hub is simply a matter of connecting it to a power source and plugging in cables connected to the network interface adapters in your computers. However, it'simportant for a network technician to understand what goes on inside a hub.
Figure 2.14 Hubs have ports into which you plug the cables connected to your computers' network interface adapters
Like network interface adapters, hubs are associated with specific data-link layer protocols. Ethernet hubs are the most common because Ethernet is the most popular data-link layer protocol, but Token Ring MAUs are hubs also, and other protocols, such as FDDI, can also use hubs.
An Ethernet hub is also called a multiport repeater. A repeater is a device that amplifies a signal as it passes through it to counteract the effects of attenuation. If, for example, you have a thin Ethernet network with a cable segment longer than the prescribed maximum of 185 meters, you can install a repeater at some point in the segment to strengthen the signal and extend the maximum segment length. This type of repeater, with only two BNC connectors, is rarely seen these days. The hubs used on UTP Ethernet networks are repeaters as well, but they have many RJ45 ports instead of just two BNC connectors.
When data enters the hub through any of its ports, the hub amplifies the signal and transmits it out through all of the other ports. This enables a star network to share a single medium, even though each computer has its own separate cable. The hubrelays every packet transmitted by any computer on the network to all of the other computers while amplifying the signals. The maximum segment length for a UTP cable on an Ethernet network is 100 meters. A segment is defined as the distance between two communicating computers. However, because the hub also functions as a repeater, each of the cables connecting a computer to a hub port can be up to 100 meters long, allowing a segment length of up to 200 meters on a one-hub network.
The hubs used on most Ethernet networks are purely physical-layer devices. This means that the hub works with the signals native to the network medium, such as electrical voltages, but does not interpret the signals, read the data inside packets, or even recognize that there is data there. This type of hub is relatively inexpensive. However, there are Ethernet hubs with more intelligence that can process the data they receive in more elaborate ways.
Some hubs with greater data processing capabilities provide a service called store and forward, which means that the hub contains buffers in which it can retain packets to retransmit them out through specific ports as needed. This is one step short of a switch, which reads the destination address from each incoming packet and transmits it only to the system for which it is intended.
Some intelligent hubs also include management features that enable them to monitor the operation of each of the hub's ports. In most cases, an intelligent hub uses the Simple Network Management Protocol (SNMP) to transmit periodic reports to a centralized network management console. This type of manageability isn't necessary on a small LAN, especially because it significantly increases the price of the hardware, but for a large enterprise network that has dozens of hubs, it can be a boon to the network administrator.
You can build a simple Ethernet LAN by plugging a number of computers into a single hub, but what happens when your network outgrows your hub? The solution is to get another hub and connect it to the first one. Large networks can have many interconnected hubs forming large LANs, which are in turn connected by routers. Almost every Ethernet hub on the market has an extra port called anuplink port, which is used to connect to another hub instead of to a computer. The uplink port is wired differently from the other ports in the hub.
As explained in Lesson 1, UTP cables contain eight wires in four pairs, and each pair consists of a signal wire and a ground. Computers transmit data over one wire pair and receive data over another. In most cases, the other two pairs of wires are left unused. For two computers to communicate, the transmit contacts on each system must be connected to the receive contacts on the other system. In all but exceptional cases, UTP cables are wired straight through, meaning that each of the eight pins in the connector at one end of the cable is wired to the corresponding pin in the connector at the other end. If you were to use a cable like this to connect two computers, you would have the transmit pins connected tothe transmit pins and the receive pins to the receive pins, making communication impossible.
Another function of a hub is to provide the crossover circuit that connects the transmit pins to the receive pins for each connection between two computers. The uplink port is the one port in the hub that does not have the crossover circuit. When you connect the uplink port in one hub to a regular port in another, you enable the computers on one hub to connect to those on the other, with only a single crossover between them. Without the uplink port, connecting one hub to another would cause a connection between computers on different hubs to go through two crossover circuits, canceling each other out. To avoid such a connection failure, you shouldn't plug an uplink port on one hub into the uplink port on another.
Run the CrossoverCabling video located in the Demos folder on the CD-ROM accompanying this book for a demonstration of an Ethernet hub's crossover circuit.
On some hubs, the uplink port is switched, meaning that you can choose whether that port uses the crossover circuit or not. This is an important factor to consider when evaluating hubs, because the switched port might count toward the total number of usable ports in the hub. In other words, a hub advertised as having eight ports might have one that is switchable, and an eight-port hub with a dedicated uplink port might have eight regular ports and one uplink port, for a total of nine. Be sure you know what you're getting before making a purchase.
It is possible to create a simple two-node Ethernet network without using a hub by connecting the network interface adapters of two computers directly, using a crossover cable. A crossover cable is a UTP cable that has the transmit pins on one end of the cable wired to the receive pins on the other end, thus eliminating the need for the crossover circuit in the hub. You can create this sort of network with two computers, and because you're eliminating the repeater from the network, the crossover cable can be no longer than 100 meters.
The MAUs used on Token Ring networks may look similar to Ethernet hubs, but they could not be more different. Unlike Ethernet hubs, Token Ring MAUs are passive devices, meaning that they are not repeaters. They perform certain data-link layer functions that are crucial to network operation. The primary difference in the operation of an MAU is that it does not retransmit all incoming traffic out through the other ports simultaneously. Instead, the MAU transmits a packet arriving through port 5, for example, out through port 6, and then waits for the packet to return to the MAU through port 6, after which it transmits it through port 7, waits for it to return, and so on. This pattern is shown in Figure 2.15. After the MAU has transmitted the packet to each of the computers on the network and has received it back, it sends the packet to the system that originated it, and that system removes it from the network. This process enables the computers in a physical star topology to communicate as though they are cabled in a ring topology.
Figure 2.15 Token Ring MAUs relay packets serially, whereas Ethernet hubs transmit them in parallel
When forwarding packets in this way, the role of the computer is at least asimportant as that of the MAU. If a computer is not there to return each packet that was sent to it back to the MAU, that packet can never be forwarded out through the next port. To prevent packets from being lost in this way, Token Ring computers perform an initialization process at startup that informs the MAU of their presence. Once the MAU receives the proper signals from the NIC in the computer, it figuratively adds the system to the logical ring and begins forwarding packets to it. Ports to which no computer is connected are never added to the ring, and the MAU skips them when forwarding packets. These unused ports are said to be in the loopback state. Token Ring MAUs also do not have an uplink port like an Ethernet hub, but they do have dedicated Ring In and Ring Out ports that are used to connect one MAU to another.
Match the concept in the left column with the definition that best describes it in the right column.
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