Networking: A Beginner’s Guide Fifth Edition- P18 ppsx

67 Chapter 6: Understanding Network Hardware Putting together all the necessary pieces in the proper way is the art of network design. Chapter 15 discusses important aspects of assembling these devices so they work together optimally, but first you need to know what they are and what they can do. The following sections discuss these essential network devices. Repeaters A repeater is a device that extends the distance of a particular network run. It takes a weak network signal in on one side, boosts the signal, and then sends it out its other side. You most often see repeaters on Thin Ethernet networks, but they are available for virtually any network connection. For instance, if you need to run a 100Base-T Cat-5 cable longer than 100 meters (328 feet), a repeater enables you to double that distance. Repeaters operate at the physical layer of the OSI networking model. They do not have the intelligence to understand the signals they are transmitting. Repeaters merely amplify the signal coming in either side and repeat it through their other side. (Remember that they also amplify any noise on the cable!) Repeaters are used to connect only the same type of media, such as 10Base-2 Thin Ethernet to 10Base-2 Thin Ethernet, or Token Ring twisted-pair to Token Ring twisted-pair. In practice, repeaters are usually used with 10Base-2 networks (Thin Ethernet), which are discussed in Chapter 4. Repeaters do have a small amount of intelligence that can be useful. They can separate one of their connections from the other when there is a problem. For example, consider two segments of Thin Ethernet that are connected using a repeater. If one of those segments is broken, the repeater allows the good segment to continue working within itself. Users on the good segment will be unable to connect to resources on the broken segment, but they can still use the good segment without trouble. (But remember that this capability does you little good if your servers are on the broken segment and your workstations are on the good segment!) Figure 6–1 shows a network extension using repeaters. Figure 6-1. Using repeaters to extend network length (10Base-2 Thin Ethernet shown) 68 Networking: A Beginner’s Guide Hubs and Concentrators Intelligent LAN concentrators—usually just called concentrators or, even more simply, hubs—are used to connect network nodes to network backbones. Nodes are connected to hubs in a physical star fashion (cables fan out from the hub to each node), whether they are used for a star topology or a ring topology network (these topologies are discussed in Chapter 4). A simple network might consist of just a hub or two; smaller networks usually don’t require a network backbone. Hubs are available for virtually any network media type, with the higher-end units using replaceable modules to support multiple media types. For example, you can purchase a high-end hub chassis that can house both Ethernet and Token Ring modules. You can purchase hubs in a variety of sizes, ranging from those that support only 2 workstations to those that support more than 100 workstations. Many network designers use stackable hubs, which usually support 24 node connections each. These hubs are often used in concert with switches, which are discussed in the next section. Hubs have two important properties: N Hubs echo all data from each port to all the other ports on the hub. Although hubs are wired in a star fashion, they actually perform electrically (logically) more like a bus topology segment in this respect. Because of this echoing, no filtering or logic occurs to prevent collisions between packets being transmitted by any of the connected nodes. N Hubs can automatically partition (in this context, cut off) a problematic node from the other nodes—in effect, shutting down that node. Such partitioning occurs if a cable short is detected, if the hub port is receiving excessive packets that are flooding the network, or if some other serious problem is detected for a given port on the hub. Automatic partitioning keeps one malfunctioning connection from causing problems for all of the other connections. Hubs are becoming much more sophisticated. They often have a number of advanced built-in features, including the following: N Built-in management, where the hub can be centrally managed over the network, using SNMP or other network management protocols and software. N Autosensing of different connection speeds. For example, Ethernet hubs that can automatically detect and run each node at either 10 Mbps (10Base-T) or 100 Mbps (100Base-T) are common. N High-speed uplinks that connect the hub to a backbone. These usually operate at ten times the basic speed of the hub. (For example, for a 100 Mbps hub, the uplink ports might run at 1 Gbps.) N Built-in bridging and routing functions, which make it unnecessary to use separate devices to perform bridging and routing. N Built-in switching, where nodes on the hub can be switched instead of shared. 69 Chapter 6: Understanding Network Hardware When ordering a hub, it’s important to know how many nodes you want to connect, how much bandwidth each requires, and what type of network backbone is being used. Backbones can be anything from shared 10 Mbps Thin Ethernet, to 100 Mbps 100Base-TX, to higher-speed backbones. Your choice of a backbone technology depends on the total amount of bandwidth that you need and the various other network design criteria that you must meet. Each hub is a separate collision domain, or an area of the network in which collisions can occur. Connecting all hubs together in some fashion generally results in a larger collision domain, encompassing all the hubs. The exception to this rule is a configuration where all the separate hubs are connected to a switch (see the next section), which keeps each hub in its own collision domain. Figure 6–2 shows an example of a network using hubs. Switches Switches, as their name implies, can switch connections from one port to another, and they can do so rapidly. They are connection-oriented and dynamically switch among their various ports to create these connections. Think of a train yard, with many trains coming in on some tracks and leaving on other tracks. The yard manager orders the Figure 6-2. A typical hub arrangement Common backbone cable 70 Networking: A Beginner’s Guide track “switches” to take place so the trains can get to their destination. A network switch is much like the yard manager, except that the switch directs packets rather than trains and uses Ethernet cabling rather than train tracks to transport its cargo. NOTE Switches are a lot like bridges, except that they have many ports and otherwise look like hubs. You might think of a switch as a bridge with multiple ports. Because switches form one-to-one connections between any two ports, all the ports coming into a switch are not part of a single collision domain. In this sense, the switch acts as a sort of super bridge (bridges are discussed in the next section). Switches are often used to connect a number of hubs to a much faster backbone. For example, suppose that you have 10 hubs, each with 24 workstation nodes connected. If you simply connect all the hubs together on a common backbone, all 240 workstations would share a single collision domain, which could hurt performance quite a bit. Instead, a much better approach is to install a 12-port switch and connect each hub to one of the ports on the switch. For instance, it is common to use 100Base-T Ethernet for workstation connections, but 1000Base-T (or some other faster network connection) for the backbone. This allows all the traffic being generated by each of the 10 hubs to continue to run at about a 100 Mbps connection speed to the servers, even though all the hubs are sharing the backbone. Figure 6–3 illustrates this approach. NOTE Switches often are used simply to connect two given ports (such as traffic from port 5 to port 21, for instance), but they are also intelligent enough to echo certain types of broadcast packets to all ports simultaneously. Figure 6-3. A network built using hubs and switches Switch Hubs Server 71 Chapter 6: Understanding Network Hardware Switches have become inexpensive and are blazingly fast. For local area network (LAN) connections, switches make more sense than hubs, partly because of their cost and their relative simplicity. In fact, purchasing bridges has become difficult, as switches now dominate the market. Additionally, most new networks eschew hubs in favor of a 100 percent–switched approach. In fact, it’s virtually impossible to purchase hubs any longer, because manufacturers typically offer only switches. (You may still be able to purchase very small hubs, with four to eight ports, but even in these small applications, switches are preferable and not much more expensive.) It’s important that you understand the difference between hubs and switches, because you may still encounter hubs installed in existing networks. For new networks, you will use switches exclusively. Doing so dramatically reduces the opportunity for network packet collisions, which are more likely in a hub arrangement. Bridges Bridges are, in a nutshell, more intelligent versions of repeaters. Bridges can connect two network segments together, but they have the intelligence to pass traffic from one segment to another only when that traffic is destined for the other segment. Bridges are used to segment networks into smaller pieces. Some bridges can span different networking systems and media, such as from coaxial Thin Ethernet to twisted-pair Token Ring. As you might recall, repeaters operate at the physical layer (layer 1) of the OSI networking model. Bridges operate one layer higher, at the data-link layer (layer 2). Bridges examine the media access control (MAC) address of each packet they encounter to determine whether they should forward the packet to the other network. Bridges contain address information about all the parts of your network, through either a static routing table that you program or a dynamic, learning-tree system that discovers all the devices and addresses on the network automatically. Is It Better to Use Fewer Large Switches or More Small Switches? Larger switches that can host hundreds of connections within a single chassis are generally more powerful than their smaller 24-port siblings, and they tend to have more built-in redundancy, such as redundant power supplies in the unit and so forth. However, sometimes it’s easier and less expensive to build a network using smaller 24-port switches. You can simply purchase an extra 24-port unit as a hot-swap backup (a backup unit that can be quickly swapped in to take the place of a failed unit) that you can manually implement at a moment’s notice. The only real disadvantage to this approach is that the redundancy is not automatic. If one 24-port switch fails, you’ll need to move its connections to the backup switch. In contrast, a larger unit can switch to redundant features automatically. As always, consider such trade-offs carefully for your particular company and its needs. . swapped in to take the place of a failed unit) that you can manually implement at a moment’s notice. The only real disadvantage to this approach is that the redundancy is not automatic. If one. much like the yard manager, except that the switch directs packets rather than trains and uses Ethernet cabling rather than train tracks to transport its cargo. NOTE Switches are a lot like bridges,. connection-oriented and dynamically switch among their various ports to create these connections. Think of a train yard, with many trains coming in on some tracks and leaving on other tracks. The yard manager