42 Networking: A Beginner’s Guide other nodes must contend with that use for their own. In other words, if you’re using a network type with a capacity of 100 Mbps, that’s the total amount of bandwidth available to all of the nodes connected to the hub. NOTE Networks that are physically wired in a star topology are logically either a bus or a ring. This means that, despite what the network looks like, it still “behaves” as either a bus or a ring. Ethernet networks wired in a star fashion are logically a bus. Token Ring networks wired in a star fashion are logically a ring. Star topology networks can use one of several forms of Ethernet. The most common is 100Base-T Ethernet, which provides 100 Mbps of bandwidth. Quite a few older networks use 10Base-T Ethernet, which provides 10 Mbps of bandwidth. A newer standard called Gigabit Ethernet (1000Base-T) offers 1 Gbps of bandwidth. Most recently, a standard called 10 Gigabit Ethernet (or alternately 10GBase-X), which can run at 10 Gbps over fiber-optic cable, has been approved. 10Base-T requires a type of twisted-pair cable called Category 3 (Cat-3) cable. 100Base-T requires Category 5 (Cat-5) cable. 10Base-T can also use Cat-5, but 100Base-T cannot use Cat-3. These days, you should always use the most recent Cat-5 cable— called Cat-5E—even if it’s intended for only a 10Base-T network. (Cat-5 cable provides eight wires—four twisted pairs—and so can carry two connections in each cable if desired.) If cost is not an issue, consider even moving up to Cat-6. 10Base-T networks share the following wiring characteristics: N Require four actual wires (two twisted pairs in a single sheath); can be either unshielded twisted-pair or shielded twisted-pair N Can be run on either Cat-3 or Cat-5 cable N Are limited to a length of 100 meters (328 feet) for each node connection Figure 4-3. A star topology network 43 Chapter 4: Understanding Network Cabling N Are not limited in the number of nodes in a single logical segment N Use RJ-45 connectors for all connections (this type of connector is similar to a modular telephone connector, but the RJ-45 is larger) 100Base-T networks are similar to 10Base-T networks and have these characteristics: N Require four actual wires (two twisted pairs in a single sheath) N Must use Cat-5 cable or better N Are limited to a length of 100 meters (328 feet) for each node connection N Are not limited in the number of nodes in a single logical segment N Use RJ-45 connectors for all connections 1000Base-T networks are notable in that they can run over existing Cat-5 cable, but at ten times the speed of 100Base-T networks. Running over Cat-5 cable is a significant advantage for 1000Base-T, because over 75 percent of installed network cabling today 10Base-What? The various Ethernet standards referred to as, for instance, 10Base-2, 10Base-T, 100Base-T, and so on contain in their name all you need to know about what they do. The first portion—the number—can be 10, 100, or 1000, and this number indicates the data rate (in Mbps) that the standard carries. The word Base means the network is baseband rather than broadband. (A baseband connection carries only one signal at a given instant; a broadband connection carries multiple signals at any time.) The terminating letter or number indicates what sort of cable is used: T for twisted pair, 2 for thin coaxial, 5 for thick coaxial, and F or X usually indicating fiber-optic cable. Here’s a quick reference guide to the different standards commonly seen: 10Base-2 10 Mbps, coaxial (RG-58) cable 10Base-5 10 Mbps, coaxial (RG-8) cable 10Base-T 10 Mbps, twisted-pair (two pairs, Cat-3 or higher) cable 100Base-T 100 Mbps, twisted-pair (two pairs, Cat-5) cable; a variant called 100 Base-T4 designates four pairs 100Base-TX 100 Mbps, twisted-pair (two pairs, Cat-5) cable 100Base-FX 100 Mbps, fiber-optic cable 1000Base-T 1 Gbps, twisted-pair (four pairs, Cat-5) cable 10GBase-X 10 Gbps, fiber-optic cable 44 Networking: A Beginner’s Guide is Cat-5, and rewiring an entire building for a new networking standard is an extremely expensive proposition. 1000Base-T over Cat-5 networks have these characteristics: N Require eight actual wires (four twisted pairs in a single sheath) N Must use Cat-5 cable or better N Are limited to a length of 100 meters (328 feet) for each node connection N Are not limited in the number of nodes in a single logical segment N Use RJ-45 connectors for all connections Compared to bus networks, star topology networks are more expensive. Much more actual wire is required, the labor to install that wire is much greater, and an additional cost exists for the needed hubs. To offset these costs, however, star topologies are far more reliable than bus topologies. With a star topology, if any single network connection goes bad (is cut or damaged in some way), only that one connection is affected. While it is true that hubs echo all the network signals for the connected nodes to all other nodes on the hub, they also have the capability to partition, or cut off, any misbehaving node connections automatically—one bad apple won’t spoil the whole bunch. In addition, because each cable is run directly from the hub to the node, it is extremely easy to troubleshoot; you don’t need to go traipsing over an entire building trying to find the problem. Ring Topology A ring topology is actually not a physical arrangement of a network cable, as you might guess. Instead, rings are a logical arrangement; the actual cables are wired in a star, with each node connected on its own cable to the MAU. However, electrically, the network behaves like a ring, where the network signals travel around the ring to each node in turn. Figure 4-4 shows a sample ring topology network. Ring topology LANs are based on Token Ring instead of Ethernet. Some may also run Fiber Distributed Data Interface (FDDI)—a 100 Mbps fiber-optic network—instead of copper-based cable. Rings are also used for some larger telecommunications networks like Synchronous Optical Network (SONET), as well as in storage area networks and some other applications. DEFINE-IT! Physical Versus Logical You’ll often hear the terms physical and logical bandied about when discussing networks. These terms are used for quite a few different things. Physical, used in the context of networking, means the actual, physical thing—what you can see and feel. Logical means how something works, despite its appearance. For example, a Token Ring network is physically wired in a star; each cable radiates out from the MAU to each node. Logically, though, it’s a ring in which the signals travel from node to node in a circular fashion. The fact that the signals physically travel from the node to the MAU and back to the next node is usually unimportant when thinking about the logical circular arrangement of the Token Ring network. 45 Chapter 4: Understanding Network Cabling Figure 4-4. A sample ring topology network MAU Flow of tokens Ring topology, electrical representation Ring topology, physical representation 46 Networking: A Beginner’s Guide Comparing Rings to Stars and Buses To compare rings to stars and buses, you first need to understand the basic concept of how Ethernet networks work. Ethernet networks manage all the needed signals on the network using a technique called CSMA/CD, which stands for Carrier Sense Multiple Access with Collision Detection. CSMA/CD allows each node on a segment to transmit data whenever it likes. If two nodes try to transmit at the same time, they each detect this occurrence with their collision detection, and then both nodes wait a random amount of time (counted in milliseconds) to retry their transmissions. Considering how data packets flow on a network using CSMA/CD, you might think that it could quickly become a confusing mess, with data and collision retries causing more collisions. And you would probably think the potential exists for the network to reach a saturation point where virtually nothing gets transmitted because of excessive collisions. You would be right. For 10Base-T networks, this point occurs somewhere around 3.5 Mbps (about one-third of the 10 Mbps theoretical maximum that one node could achieve sending a stream of data to one other node). However, the reality is that excessive collisions don’t pose much of a problem on most networks these days for three reasons: N Most network traffic is bursty, and network nodes rarely consume all the bandwidth on a particular network for any significant length of time. N Even on a network where excessive collisions are hampering performance, breaking the network segment into smaller pieces and reducing the chances of collisions proportionately is relatively easy. N Currently, most networks use switches instead of hubs. Switches prevent data from colliding between their ports. Ultimately, CSMA/CD does the job, and Ethernet is the predominant network standard in the world because it works so well in practice and is so flexible. Token Ring networks operate on a different principle than CSMA/CD. Token Ring networks manage their bandwidth with a technique called token passing. Electrically, a data entity called a token circulates around the logical network ring. The token has two states: free and busy. When a node wants to transmit some data, it waits until the token coming into it is in a free state, and then the node marks the token as busy. Next, after adding to the token packet the data to be sent and the destination address, the node sends the packet on to the next node. The next node, finding the token set to its busy state, examines the destination address and passes the token on unchanged toward the destination. Once the destination node receives the token, it gets its data, marks the token as free, and sends it along to the next workstation. If the token somehow becomes “lost,” then a workstation generates a new, free token automatically after a set period of time passes. The beauty of Token Ring networks is that they behave predictably as the bandwidth needs of the nodes increase. Also, Token Ring networks are never bogged down by collisions, which are impossible in such a network. However, these benefits . data rate (in Mbps) that the standard carries. The word Base means the network is baseband rather than broadband. (A baseband connection carries only one signal at a given instant; a broadband. cable, has been approved. 10Base-T requires a type of twisted-pair cable called Category 3 (Cat-3) cable. 100Base-T requires Category 5 (Cat-5) cable. 10Base-T can also use Cat-5, but 100Base-T. over Cat-5 cable is a significant advantage for 1000Base-T, because over 75 percent of installed network cabling today 10Base-What? The various Ethernet standards referred to as, for instance,