Networking All-in-One For Dummies. Doug Lowe

different network protocols and standards can be used in various layers of the OSI model, the most common standard found at layers 1 and 2 is Ethernet. Similarly, the most common standard at layer 3 is IP. I cover more about Ethernet and IP in Chapters 2 and 3 of Book 2, but keep in mind that most of what follows in this chapter is related to Ethernet and IP.

Recognizing Network Topology

      The term network topology refers to the shape of how the computers and other network components are connected to each other. Several different types of network topologies exist, each with advantages and disadvantages.

      

In the following discussion of network topologies, I use two important terms:

       Node: A node is a device that’s connected to the network. For your purposes here, a node is the same as a computer. Network topology deals with how the nodes of a network are connected to each other.

       Packet: A packet is a message that’s sent over the network from one node to another node. The packet includes the address of the node that sent the packet, the address of the node the packet is being sent to, and data.

      Bus topology

The first type of network topology is called a bus, in which nodes are strung together in a line, as shown in Figure 2-1. The key to understanding how a bus topology works is to think of the entire network as a single cable, with each node “tapping” into the cable so it can listen in on the packets being sent over that cable. If you’re old enough to remember party lines, you get the idea.

      In a bus topology, every node on the network can see every packet that’s sent on the cable. Each node looks at each packet to determine whether the packet is intended for it. If so, the node claims the packet. If not, the node ignores the packet. This way, each computer can respond to data sent to it and ignore data sent to other computers on the network.

      FIGURE 2-1: Bus topology.

If the cable in a bus network breaks, the entire network is effectively disabled. Obviously, the nodes on opposite sides of the break can continue to communicate with each other, because data can’t span the gap created by the break. But even those nodes that are on the same side of the break may not be able to communicate with each other, because the open end of the cable left by the break disrupts the proper transmission of electrical signals.

      In the early days of Ethernet networking, bus topology was commonplace. Although, for most networks, bus topology has given way to star topology (see the next section), many networks today still have elements that rely on bus topology.

      Star topology

In a star topology, each network node is connected to a central device called a hub or a switch, as shown in Figure 2-2. Star topologies are commonly used with LANs.

      FIGURE 2-2: Star topology.

If a cable in a star network breaks, only the node connected to that cable is isolated from the network. The other nodes can continue to operate without interruption — unless, of course, the node that’s isolated because of the break happens to be the file server.

      

You should be aware of the somewhat technical distinction between a hub and a switch. Simply put, a hub doesn’t know anything about the computers that are connected to each of its ports. So, when a computer connected to the hub sends a packet to a computer that’s connected to another port, the hub sends a duplicate copy of the packet to all its ports. In contrast, a switch knows which computer is connected to each of its ports. As a result, when a switch receives a packet intended for a particular computer, it sends the packet only to the port that the recipient is connected to.

      Strictly speaking, only networks that use switches have a true star topology. If the network uses a hub, the network topology has the physical appearance of a star, but it’s actually a bus. That’s because when a hub is used, each computer on the network sees all the packets sent over the network, just as in a bus topology. In a true star topology, as when a switch is used, each computer sees only those packets that were sent specifically to it, as well as packets that were specifically sent to all computers on the network (those types of packets are called broadcast packets).

      Expanding stars

      Physicists say that the universe is expanding, and network administrators know they’re right. A simple bus or star topology is suitable only for small networks, with a dozen or so computers. But small networks inevitably become large networks as more computers are added. For larger networks, it’s common to create more complicated topologies that combine stars and buses.

      For example, a bus can be used to connect several stars. In this case, two or more hubs or switches are connected to each other using a bus. Each of these hubs or switches is then the center of a star that connects two or more computers to the network. This type of arrangement is commonly used in buildings that have two or more distinct workgroups. The bus that connects the switches is sometimes called a backbone.

      Another way to expand a star topology is to use a technique called daisy-chaining. When you use daisy-chaining, a switch is connected to another switch as if it were one of the nodes on the star. Then this second switch serves as the center of a second star.

      Ring topology

A third type of network topology is called a ring (see Figure 2-3). In a ring topology, packets are sent around the circle from computer to computer. Each computer looks at each packet to decide whether the packet was intended for it. If not, the packet is passed on to the next computer in the ring.

      FIGURE 2-3: Ring topology.

      Years ago, ring topologies were common in LANs, as two popular networking technologies used rings: ARCNET and token ring. ARCNET is still used for certain applications such as factory automation, but it’s rarely used in business networks. Token ring is still a popular network technology for IBM midrange computers. Although plenty of token ring networks are still in existence, not many new networks use token ring any more.

      Ring topology was also used by FDDI, one of the first types of fiber-optic network connections. FDDI has given way to more efficient fiber-optic techniques, however. So, ring networks have all but vanished from business networks.

      Mesh topology

A fourth type of network topology, known as mesh, has multiple connections between each of the nodes on the network, as shown in Figure 2-4. The advantage of a mesh topology is that if one cable breaks, the network can use an alternative route to deliver its packets.