Networking Components (part 2)

Hubs, MAU s, and Repeaters Once you have the network card inserted into the computer system or network device, you will next need to connect each of the systems together using devices such as hubs, repeaters, or MAUs. Hubs are one of the most important components of a network because they act as a central point for all network devices to connect to. You can easily remember the layout of a hub if you think of a wheel and picture how the spokes radiate out from the hub of the wheel. In a network, each spoke is a connection, and the hub of the wheel is the hub of the network where all of the cables come together.

The Role of Hubs in Networking The hub, also known as a concentrator, is responsible for allowing all systems a central point of connection, so that when a computer sends a piece of data to another computer, the electrical signal leaves the network card of the sending system and reaches the hub, and the hub sends the signal to all ports on the hub so that all systems can check to see whether the data is destined for them.

The Role of Hubs in Networking

The Role of Hubs in Networking When looking at Figure 3-12, you will notice that the hub displays link lights for each port on the hub that is active. This can be used in troubleshooting, just like the link light on the back of a network card. You also will notice that there is a collision indicator on the hub, which indicates whether or not your network currently is having a lot of collisions. Once again, this could be very useful when troubleshooting network problems—excessive collisions could lead to systems dropping off the network.

Cascading Hubs

Cascading Hubs Looking at Figure 3-13, you will notice that there is a switch on the hub to indicate that you wish to use the fifth port to cascade, or connect, to another hub. If you were to try to chain two hubs together with the switch set to normal, it would be similar to trying to connect two computers together with a straightthrough cable; it can’t be done because you need to switch the send and receive wires on one end of the cable.

Cascading Hubs We have discussed using a crossover cable, but manufacturers of hubs have given us a solution that eliminates the need for crossover cables by creating a port on the hub that is already crossed over; you simply need to switch the setting to use the fifth port as the cascade port. Also known as the uplink port, the cascade port connects to a normal port on the next hub.

Cascading Hubs If you wanted to connect three 24-port hubs together, you would need to uplink from port 24 on the first hub to any port on the second hub (I usually uplink to the first port on the second hub), then use port 24 on the second hub to uplink to the first port on the third hub, as shown in Figure 3-14. Before we get into the different types of hubs, a point that I want to stress is that a hub sends the signal to all ports on the hub. This means that, if you have a 24-port hub linked to another 24-port hub and a workstation sends data to another workstation, the data will be sent to all 48 ports on the network. This leads to a lot of unneeded traffic and contention across the entire network that will slow network performance.

Cascading Hubs

Passive Hubs The function of a passive hub is simply to receive data from one port of the hub and send it out to the other ports. For example, an eight-port hub receives data from port 3 and then resends that data to ports 1, 2, 4, 5, 6, 7, and 8. A passive hub contains no power source or electrical components, there is no signal processing (such as when the hub receives the electrical signal), and there is no regenerating of the signal to ensure that it is readable at the destination. A passive hub simply attaches the ports internally and enables communication to flow through the network.

Active Hubs An active hub provides the same functionality as a passive hub with an additional feature. Active hubs rebuild (regenerate) the data before sending it to all of the destination ports on the hub. Using active hubs, you can increase the length of your network, because although the signal weakens with distance, when the active hub receives the signal, it rebuilds the data, allowing it once more to go a greater distance. It is important to remember that UTP cabling can be run a maximum of 100 meters.

Active Hubs With an active hub, you can run this type of cable 100 meters on each side of the hub. An active hub has a power source and built-in repeaters to boost the signal. Extra electronics built into an active hub allow for signal regeneration. Remember that a passive hub does not regenerate the signal as the active hub does; therefore, the cable distance between two PCs is the total cable length and not PC-to-hub length, as with active hubs. Remember the difference between an active hub and a passive hub: An active hub can boost the signal, whereas a passive hub cannot.

Hybrid Hubs A hybrid hub is a hub that can use many different types of cables in addition to UTP cabling. A hybrid hub usually is cabled using thinnet or thicknet Ethernet along with popular cable types such as twisted-pair cabling. A few years ago, hybrid hubs were fairly popular. UTP seems to be the popular cable type today, so you may not see the thinnet or thicknet connector on the hub as well.

Hybrid Hubs

Multistation Access Units A multistation access unit (MAU) is a device to which multiple workstations are connected in order to communicate on a Token Ring network. A MAU is a hub-type device for Token Ring networks with some features that make it a little bit different from a hub—for example, a MAU regenerates the signal when it reaches the MAU. Because Token Ring networks use token passing instead of CSMA/CD, there is no chance for collisions on a Token Ring network. The first difference you will notice with MAUs over hubs is that a MAU does not have collision indicators on it because you can’t have collisions on a Token Ring network.

Multistation Access Units

Multistation Access Units Another major difference with a MAU is that MAUs don’t actually use an uplink port. With Token Ring, there is a logical ring within the MAU, and when you connect to another MAU, you must complete a full ring structure again. Therefore, you will notice on the Token Ring MAU that it has a ring-in port and a ring-out port. When you wish to connect two MAUs together, you must ring out of the first MAU and, with that cable, ring in to the second MAU. Then you must ring out of the second MAU and ring in to the first MAU.

Multistation Access Units

Repeaters One of the pitfalls of networking environments is that the electrical signal that is traveling the wire is weakened over distance as a result of outside interference. Eventually, if two systems are too far from one another, the signal is so weak that by the time it reaches the other side it is unreadable. This is where repeaters come in.

Repeaters If your network layout exceeds the normal specifications of the cable, you can use repeaters to allow the signal to travel the distance by placing the repeaters at different points in the network. For example, if you are using thinnet cabling, you know that thinnet is limited to 185 m. But what if you want to connect two systems together that are 235 m apart? You would place a repeater somewhere before the 185 m mark so that the repeater will regenerate or rebuild the signal, allowing it to travel the extra difference.

Repeaters

Bridges A bridge is a network connectivity device that is used to break the network down into multiple network segments. A bridge runs at layer 2, the data link layer, and is used to filter traffic by only forwarding traffic to the destination network segment.

Bridges

Bridges Let’s look at an example of how a bridge filters network traffic. Assume that you have just completed connecting the bridge to the network segments shown in Figure 3-19. When Workstation A sends data to Workstation F, the data will go out the network card of Workstation A and will travel the full length of segment 3 in both directions. The signal will reach the bridge, and the bridge will look at the destination MAC address of the packet. Once the bridge looks at the destination MAC address, it will compare that MAC address with the MAC addresses in its bridging table.

Bridges The bridging table is a table in memory that lists all known MAC addresses and which network segment that MAC address lives on. This table is critical to the bridge’s filtering features. Since this is the first piece of data sent on the network, the MAC address for Workstation F is not in the bridging table, so the bridge will need to forward the data to both segment 1 and segment 2. It will not forward the information to network segment 3 because that is where the data came from, and if Workstation F existed on that network, it would already have the data.

Bridges When the bridge received the initial data from Workstation A, it recorded the MAC address of Workstation A and the network segment that Workstation A resides on in the bridging table. This way if anyone sends data to Workstation A, the bridge will have an entry for workstation A in the bridging table, and the bridge will forward the data only to network segment 3 and not to the other segments.

Bridges Also note that when Workstation F replies to Workstation A, the data will need to pass through the bridge, so the bridge will know what network segment Workstation F resides on and will record that MAC address in the bridging table. Over time, the bridging table will be filled with MAC addresses and their associated network segments. In our example, after the bridging table has been constructed, if Workstation A sends data to Workstation C, the data will reach the bridge and the bridge will forward the data only to network segment 1.

Bridges