mirror of https://github.com/tLDP/LDP
175 lines
6.1 KiB
XML
175 lines
6.1 KiB
XML
<sect1 id="Topologies-and-Architectures">
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<title>Topologies-and-Architectures</title>
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<para>
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A network's topology is the configuration, or shape, of the wiring used in
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the network. Network architectures are standards for communication, such
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as Ethernet and Token Ring. This section describes the common technologies
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and architectures.
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</para>
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<para>
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The terms network architecture and network topology are often used
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interchangeably, even in some Microsoft documentation. For clarity throughout
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this book, topology refers to the configuration of network wiring and
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communication (star, bus, ring), and architecture refers to standards
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(Ethernet, Token Ring, ARCnet).
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</para>
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<para>
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Different types of LANs are wired in different ways. The nodes might be
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connected to each other, to a central hub, or to a continuous cable (bus).
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The four major network topologies are discussed below.
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</para>
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<para>
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Each type of network has a physical topology (the actual wiring) and a logical
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topology (the path the data follows). In some types of networks these are
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identical. However, some networks use different physical and logical
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topologies. For example, Token Ring networks use a physical star topology and
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a logical ring topology.
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</para>
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<para><variablelist>
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<varlistentry><term>Bus</term>
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<listitem><para>
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<para>
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In a bus topology, a single cable supports an entire network segment. This
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cable is the bus, sometimes called a backbone. Nodes are attached at various
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points along the cable. Depending on the network architecture, nodes may be
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connected directly to the bus with T-connectors, or a cable called a drop
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cable can be connected between the bus and each node.
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</para>
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<para>
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Bus networks typically use coaxial cable. Devices called terminators are used
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at either end of the bus. These absorb the signal to prevent signals from
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reflecting back and forth on the bus, which creates extra traffic.
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</para>
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<para>
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The bus topology is usually inexpensive for smaller netwroks, since no devices
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are required aside from the cable and connectors, and a minimum length of cable
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is required. Ethernet 10Base2 and 10Base5 are common bus networks.
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</para>
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<para>
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The chief disadvantage of a bus topology is that a break in any point in the
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bus will bring the network down. Also, the coaxial cable used in these networks
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is generally harder to work with than twisted pair cable.
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</para>
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</para></listitem></varlistentry>
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<varlistentry><term>Star</term>
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<listitem><para>
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<para>
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In a star topology, each node is connected with its own cable to a central
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device node called a hub. The hub internally connects each node to the
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other nodes. Star-wired nodes typically use UTP cable. Ethernet 10BaseT
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is the most common network with a star topology.
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</para>
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<para>
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While a greater length of cable is required for this topology, it is more
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reliable than a bus because each node has its own cable. A problem in a cable
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will generally only affect a single node. Most hubs have visual indicators to
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make it easy to diagnose cable problems.
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</para>
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<para>
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Star networks using UTP cable are often less expensive than bus networks
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using coaxial cable because the ease of wiring and inexpensive wiring offsets
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the added expense of hubs. They are also easier to expand, since a new node can
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be wired to the hub without disconnecting other nodes.
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</para>
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</para></listitem></varlistentry>
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<varlistentry><term>Ring</term>
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<listitem><para>
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<para>
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In a ring topology, the nodes are connected to each other to form a circle.
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Each node receives signals from its upstream neighbour, and passes them on to
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its downstream neighbour. Ring networks often use token passing, as describe in
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802.5, for media access.
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</para>
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<para>
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FDDI and Token Ring are two common networking systems that use ring topologies.
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Token Ring networks are actually physically wired as a star, but use a special
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hub that is wired internally as a ring, and function in a logical ring.
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</para>
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<para>
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Ring topologies offer the advantage of equal access to the network media
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through token passing, so they are often used in networks with many
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clients or with high-speed clients. The main disadvantage of a ring
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topology is the same as a bus: a single node's failure can disrupt the
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entire network. Ring networks can also be more difficult to troubleshoot
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and expand.
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</para>
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</para></listitem></varlistentry>
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<varlistentry><term>Mesh</term>
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<listitem><para>
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<para>
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A mesh topology provides fault tolerance through redundant links. In this
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system, each node is connected to every other node with seperate cables. Thus,
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a three-node network would use three cables; a four node network would use
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eight cables; and a 10-node network would require 45 cables.
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</para>
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<para>
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The main advantage of this system is a high degree of reliability. Any cable
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(or even several, depending on the size) could be severed without any nodes
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losing access to the network. The obvious disadvantage is that mesh
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topologies require large amounts of cable, making them very expensive
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to install and expand.
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</para>
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<para>
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A mesh topology can use routers (described below) to choose the best path for
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each network transmission. This allows redundant links to provide increased
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efficiency as well as relability.
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</para>
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</para></listitem></varlistentry>
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<varlistentry><term>Hybrid</term>
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<listitem><para>
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<para>
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A hybrid topolgy is any combination of the above topologies. One common hybrid
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technology is a star bus, in which several star-wired networks segments are
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interconnected with a bus. This topology is useful in networks where groups of
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workstations are close together, but several distant groups need to be
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connected.
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</para>
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<para>
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Another hybrid technology is a star ring, or star-wired ring. This is the
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topology used by Token Ring networks. The wiring forms a star topology,
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but hub is interally connected a ring.
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</para>
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<para>
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The mesh topology, while too expensive to be practical in itself, is useful in
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hybrid forms. For example, workstations might be connected by a star topology
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while three of four critical servers are wired in a mesh. This adds reliablity
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to complex networks.
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</para>
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</para></listitem></varlistentry>
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</variablelist></para>
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</sect1>
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