A network's topology is the configuration, or shape, of the wiring used in the network. Network architectures are standards for communication, such as Ethernet and Token Ring. This section describes the common technologies and architectures. The terms network architecture and network topology are often used interchangeably, even in some Microsoft documentation. For clarity throughout this book, topology refers to the configuration of network wiring and communication (star, bus, ring), and architecture refers to standards (Ethernet, Token Ring, ARCnet). Different types of LANs are wired in different ways. The nodes might be connected to each other, to a central hub, or to a continuous cable (bus). The four major network topologies are discussed below. Each type of network has a physical topology (the actual wiring) and a logical topology (the path the data follows). In some types of networks these are identical. However, some networks use different physical and logical topologies. For example, Token Ring networks use a physical star topology and a logical ring topology. Bus In a bus topology, a single cable supports an entire network segment. This cable is the bus, sometimes called a backbone. Nodes are attached at various points along the cable. Depending on the network architecture, nodes may be connected directly to the bus with T-connectors, or a cable called a drop cable can be connected between the bus and each node. Bus networks typically use coaxial cable. Devices called terminators are used at either end of the bus. These absorb the signal to prevent signals from reflecting back and forth on the bus, which creates extra traffic. The bus topology is usually inexpensive for smaller netwroks, since no devices are required aside from the cable and connectors, and a minimum length of cable is required. Ethernet 10Base2 and 10Base5 are common bus networks. The chief disadvantage of a bus topology is that a break in any point in the bus will bring the network down. Also, the coaxial cable used in these networks is generally harder to work with than twisted pair cable. Star In a star topology, each node is connected with its own cable to a central device node called a hub. The hub internally connects each node to the other nodes. Star-wired nodes typically use UTP cable. Ethernet 10BaseT is the most common network with a star topology. While a greater length of cable is required for this topology, it is more reliable than a bus because each node has its own cable. A problem in a cable will generally only affect a single node. Most hubs have visual indicators to make it easy to diagnose cable problems. Star networks using UTP cable are often less expensive than bus networks using coaxial cable because the ease of wiring and inexpensive wiring offsets the added expense of hubs. They are also easier to expand, since a new node can be wired to the hub without disconnecting other nodes. Ring In a ring topology, the nodes are connected to each other to form a circle. Each node receives signals from its upstream neighbour, and passes them on to its downstream neighbour. Ring networks often use token passing, as describe in 802.5, for media access. FDDI and Token Ring are two common networking systems that use ring topologies. Token Ring networks are actually physically wired as a star, but use a special hub that is wired internally as a ring, and function in a logical ring. Ring topologies offer the advantage of equal access to the network media through token passing, so they are often used in networks with many clients or with high-speed clients. The main disadvantage of a ring topology is the same as a bus: a single node's failure can disrupt the entire network. Ring networks can also be more difficult to troubleshoot and expand. Mesh A mesh topology provides fault tolerance through redundant links. In this system, each node is connected to every other node with seperate cables. Thus, a three-node network would use three cables; a four node network would use eight cables; and a 10-node network would require 45 cables. The main advantage of this system is a high degree of reliability. Any cable (or even several, depending on the size) could be severed without any nodes losing access to the network. The obvious disadvantage is that mesh topologies require large amounts of cable, making them very expensive to install and expand. A mesh topology can use routers (described below) to choose the best path for each network transmission. This allows redundant links to provide increased efficiency as well as relability. Hybrid A hybrid topolgy is any combination of the above topologies. One common hybrid technology is a star bus, in which several star-wired networks segments are interconnected with a bus. This topology is useful in networks where groups of workstations are close together, but several distant groups need to be connected. Another hybrid technology is a star ring, or star-wired ring. This is the topology used by Token Ring networks. The wiring forms a star topology, but hub is interally connected a ring. The mesh topology, while too expensive to be practical in itself, is useful in hybrid forms. For example, workstations might be connected by a star topology while three of four critical servers are wired in a mesh. This adds reliablity to complex networks.