Download The mesh network topology employs either of two

Network topology
In communication networks, a topology is a usually schematic description of the arrangement of a
network, including its nodes and connecting lines. There are two ways of defining network
geometry: the physical topology and the logical (or signal) topology.
The physical topology of a network is the actual geometric layout of workstations. There are several
common physical topologies, as described below and as shown in the illustration.
In the bus network topology, every workstation is connected to a main cable called the bus.
Therefore, in effect, each workstation is directly connected to every other workstation in the
network.
In the star network topology, there is a central computer or server to which all the workstations are
directly connected. Every workstation is indirectly connected to every other through the central
computer.
In the ring network topology, the workstations are connected in a closed loop configuration.
Adjacent pairs of workstations are directly connected. Other pairs of workstations are indirectly
connected, the data passing through one or more intermediate nodes.
If a Token Ring protocol is used in a star or ring topology, the signal travels in only one direction,
carried by a so-called token from node to node.
The mesh network topology employs either of two schemes, called full mesh and partial mesh. In
the full mesh topology, each workstation is connected directly to each of the others. In the partial
mesh topology, some workstations are connected to all the others, and some are connected only to
those other nodes with which they exchange the most data.
The tree network topology uses two or more star networks connected together. The central
computers of the star networks are connected to a main bus. Thus, a tree network is a bus network
of star networks.
Logical (or signal) topology refers to the nature of the paths the signals follow from node to node.
In many instances, the logical topology is the same as the physical topology. But this is not always
the case. For example, some networks are physically laid out in a star configuration, but they
operate logically as bus or ring networks.
A bus network is an arrangement in a local area network (LAN) in which each
node(workstation or other device) is connected to a main cable or link called the bus. The
illustration shows a bus network with five nodes. Each node is shown as a sphere, the bus appears
as a heavy horizontal line, and connections to the bus appear as vertical lines.A bus network is
simple and reliable. If one node fails to operate, all the rest can still communicate with each other.
For a major disruption to take place, the bus itself must be broken somewhere. Bus networks are
easy to expand. Additional nodes can be added anywhere along the bus. There are several
limitations to the bus network topology. The length of the bus is limited by cable loss. A bus
network may not work well if the nodes are located at scattered points that do not lie near a
common line. In situations like this, a ring network, mesh network, or star network may prove more
flexible and more cost effective.
A star network is a local area network (LAN) in which all nodes (workstations or other
devices) are directly connected to a common central computer. Every workstation is indirectly
connected to every other through the central computer. In some star networks, the central computer
can also operate as a workstation. The connections can be wired or wireless links. The star network
topology works well when workstations are at scattered points. It is easy to add or remove
workstations. If the workstations are reasonably close to the vertices of a convex polygon and the
system requirements are modest, the ring network topology may serve the intended purpose at
lower cost than the star network topology. If the workstations lie nearly along a straight line, the bus
network topology may be best. In a star network, a cable failure will isolate the workstation that it
links to the central computer, but only that workstation will be isolated. All the other workstations
will continue to function normally, except that they will not be able to communicate with the
isolated workstation. If any workstation goes down, none of the other workstations will be affected.
But if the central computer goes down, the entire network will suffer degraded performance or
complete failure. If redundancy is required, the mesh network topology may be preferable.
A ring network is a local area network
in which the nodes (workstations or other devices) are
connected in a closed loop configuration. Adjacent pairs of nodes are directly connected. Other
pairs of nodes are indirectly connected, the data passing through one or more intermediate nodes.
The connections can consist of wired or wireless links. The ring topology may prove optimum when
system requirements are modest and workstations are at scattered points. If the workstations are
reasonably close to the vertices of a convex polygon (such as the pentagon shown in the
illustration), the cost can be lower than that of any other topology when cable routes are chosen to
minimize the total length of cable needed. A break in the cable of a ring network may result in
degraded data speed between pairs of workstations for which the data path is increased as a result of
the break. If two breaks occur and they are not both in the same section of cable, some workstations
will be cut off from some of the others. When system reliability is a critical concern, a bus network
or star network may prove superior to a ring network. If redundancy is required, the mesh network
topology may be preferable. A token ring is a widely-implemented kind of ring network.
A Token Ring network is a local area network (LAN) in which all computers are connected in
a ring or star topology and a bit - or token-passing scheme is used in order to prevent the collision
of data between two computers that want to send messages at the same time. It works in this way:
1. Empty information frames are continuously circulated on the ring.
2. When a computer has a message to send, it inserts a token in an empty frame (this may
consist of simply changing a 0 to a 1 in the token bit part of the frame) and inserts a message
and a destination identifier in the frame.
3. The frame is then examined by each successive workstation. If the workstation sees that it is
the destination for the message, it copies the message from the frame and changes the token
back to 0.
4. When the frame gets back to the originator, it sees that the token has been changed to 0 and
that the message has been copied and received. It removes the message from the frame.
5. The frame continues to circulate as an "empty" frame, ready to be taken by a workstation
when it has a message to send.