Star Topology
In a star topology, all devices are
connected to a common central location, typically a hub or a switch.
When a node sends data to the central location, the central device
retransmits the information and sends it to the destination. Because
all cabling is connected to a central device, if one link fails, only
that portion of the network will fail. The rest of the network will
not be affected. However, if the central device fails, the entire
network will also fail.
A star topology can have a maximum of
1,024 nodes on a LAN and is commonly used for 10BASE-T (IEEE 802.3) and 100BASE-TX
(IEEE 802.12) Ethernet.
Advantages of star topologies include reliability along with the ease of maintenance and installation.
Monitoring and troubleshooting can be maintained at the central
device, providing easier maintenance. Star topologies allow for
greater reliability because each node is connected to the central device by a
segment. If one segment breaks, only that node loses access to the
network, hence the rest of the network is not affected. Because each node is
connected to the central device, star topologies also allow for an
easy network layout, providing the network administrator easier
installation over the other topologies.
A disadvantage of this topology is
cost. With each device being connected to the central location, more cabling is required than other topologies. In
addition there is the cost of the central device.

Bus Topology
A bus topology connects multiple
devices onto one main cable and is sometimes referred to as a
backbone, trunk, or segment. Terminators must be connected at each end
of the topology to absorb any reflected signals. If coaxial cable is
used, without terminators, reflected signals will echo across the network,
causing the entire network to be unusable.
An advantage of a bus topology is cost
and ease of installation. Because this topology uses a simple cable
layout, it costs less and is easier to implement than the other
topologies.
A disadvantage is that if a cable segment or the
backbone breaks or fails, the network will fail. Another
disadvantage is only one node can transmit data onto the network at a
time. If two or more nodes attempt to send data at the same time, a
collision will occur; which will require a recovery procedure thereby slowing down the network. Once the
collision has occurred, all the data must be re-sent. A process
called Carrier Sense Multiple Access/Collision Detection (CSMA/CD)
prevents the occurrence of another collision. CSMA/CD is a process
where each node waits its "turn" to retransmit data. 
Mesh Topology
Normally used for WANs, a mesh topology
connects every device on the network together and provides a path to
and from each device. An advantage is that because all the devices are
connected to each other, the network has a higher fault tolerance and
reliability. If a cable segment breaks along the network, the devices
will find the quickest way to reroute the packet to its destination.
Therefore, the data will most often always reach its destination.
Disadvantages of this topology are cost
and difficulty in management. Because there are numerous connections,
to and from each device, there is a large number of cabling
requirements, causing a mesh topology to be somewhat expensive. If a
segment breaks on the network, with the complex design of the mesh
topology, finding the exact problem location can be very difficult.
Therefore, maintaining the network can be very complex. 
Ring Topology
Ring topologies consist of each device
on the network being connected with two other devices. There is no
beginning or end of the cable. This particular topology forms a
complete ring. The devices on this network use a transceiver to
communicate with their neighbors. Transceivers also act like repeaters to
regenerate each signal as it is passed through the device.
Advantages include better performance
because each device receives a "turn" to transmit signals
and has equal access to the network. An additional advantage is
that the signal is regenerated by each device it passes through,
thereby preventing the signal from degrading.
A disadvantage of using this topology
is that if one device on the cable fails, the entire network will also fail.
Locating the failure can sometimes be difficult. Another
disadvantage is that if any changes are made to the network, including adding to or
moving devices, the disruption will cause the network will fail. 
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