<sect1 id="Topologies-and-Architectures">

<title>Topologies-and-Architectures</title>

<para>
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.
</para>

<para>
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).
</para>

<para>
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.
</para>

<para>
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.
</para>

<para><variablelist>

<varlistentry><term>Bus</term>
<listitem><para>
		
<para>
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.
</para>

<para>
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.
</para>

<para>
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.
</para>

<para>
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.
</para>

</para></listitem></varlistentry>

<varlistentry><term>Star</term>
<listitem><para>
				
<para>
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.
</para>

<para>
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.
</para>

<para>
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.
</para>

</para></listitem></varlistentry>

<varlistentry><term>Ring</term>
<listitem><para>
				
<para>
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.
</para>

<para>
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.
</para>

<para>
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.
</para>

</para></listitem></varlistentry>

<varlistentry><term>Mesh</term>
<listitem><para>
				
<para>
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.
</para>

<para>
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.
</para>

<para>
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.
</para>

</para></listitem></varlistentry>

<varlistentry><term>Hybrid</term>
<listitem><para>
				
<para>
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.
</para>

<para>
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.
</para>

<para>
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.
</para>

</para></listitem></varlistentry>

</variablelist></para>

</sect1>
