Network topology is the arrangement of the various elements (links, nodes, etc.) of a computer network.
Essentially, it is the topological
structure of a network and may be depicted physically or logically.
Physical topology is the placement of the various components of a network, including device location and cable installation, while
logical topology
illustrates how data flows within a network, regardless of its physical
design. Distances between nodes, physical interconnections,
transmission rates, or signal types may differ between two networks, yet
their topologies may be identical.
Contents
- 1 Topology
- 1.1 Point-to-point
- 1.2 Bus
- 1.3 Star
- 1.4 Ring
- 1.5 Mesh
- 1.6 Tree
- 1.6.1 Advantages
- 1.6.2 Disadvantages
- 1.7 Hybrid
- 1.8 Daisy chain
- 2 Centralization
- 3 Decentralization
Topology
There are two basic categories of network topologies: physical topologies and logical topologies.
The shape of the cabling layout used to link devices is called the
physical topology of the network. This refers to the layout of cabling, the locations of nodes, and the interconnections between the nodes and the cabling.
The physical topology of a network is determined by the capabilities of
the network access devices and media, the level of control or fault
tolerance desired, and the cost associated with cabling or
telecommunications circuits.
The logical topology in contrast, is the way that the signals act on
the network media, or the way that the data passes through the network
from one device to the next without regard to the physical
interconnection of the devices. A network's logical topology is not
necessarily the same as its physical topology. For example, the original
twisted pair Ethernet using repeater hubs was a logical bus topology with a physical star topology layout. Token Ring is a logical ring topology, but is wired a physical star from the Media Access Unit.
The study of network topology recognizes eight basic topologies: point-to-point, bus, star, ring or circular, mesh, tree, hybrid, or daisy chain.
Point-to-point
The simplest topology with a permanent link between two endpoints. Switched point-to-point topologies are the basic model of conventional telephony.
The value of a permanent point-to-point network is unimpeded
communications between the two endpoints. The value of an on-demand
point-to-point connection is proportional to the number of potential
pairs of subscribers and has been expressed as Metcalfe's Law.
- Permanent (dedicated)
- Easiest to understand, of the variations of point-to-point topology, is a point-to-point communications channel that appears, to the user, to be permanently associated with the two endpoints. A children's tin can telephone is one example of a physical dedicated channel.
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- Within many switched telecommunications systems,
it is possible to establish a permanent circuit. One example might be a
telephone in the lobby of a public building, which is programmed to
ring only the number of a telephone dispatcher. "Nailing down" a
switched connection saves the cost of running a physical circuit between
the two points. The resources in such a connection can be released when
no longer needed, for example, a television circuit from a parade route
back to the studio.
- Switched:
- Using circuit-switching or packet-switching
technologies, a point-to-point circuit can be set up dynamically and
dropped when no longer needed. This is the basic mode of conventional
telephony.
Bus
Main article: Bus network
- In local area networks where bus topology is used, each node is
connected to a single cable. Each computer or server is connected to the
single bus cable. A signal from the source travels in both directions
to all machines connected on the bus cable until it finds the intended
recipient. If the machine address does not match the intended address
for the data, the machine ignores the data. Alternatively, if the data
matches the machine address, the data is accepted. Since the bus
topology consists of only one wire, it is rather inexpensive to
implement when compared to other topologies. However, the low cost of
implementing the technology is offset by the high cost of managing the
network. Additionally, since only one cable is utilized, it can be the single point of failure. If the network cable is terminated on both ends and when without termination data transfer stop and when cable breaks, the entire network will be down.
- Linear bus
-
- The type of network topology in which all of the nodes of the
network are connected to a common transmission medium which has exactly
two endpoints (this is the 'bus', which is also commonly referred to as
the backbone, or trunk) – all data that is transmitted between nodes in the network is transmitted over this common transmission medium and is able to be received by all nodes in the network simultaneously.
-
- Note: When the electrical
signal reaches the end of the bus, the signal "echoes" back down the
line, causing unwanted interference. As a solution, the two endpoints of
the bus are normally terminated with a device called a terminator that prevents this echo.
- Distributed bus
-
- The type of network topology in which all of the nodes of the
network are connected to a common transmission medium which has more
than two endpoints that are created by adding branches to the main
section of the transmission medium – the physical distributed bus
topology functions in exactly the same fashion as the physical linear
bus topology (i.e., all nodes share a common transmission medium).
Star
Main article: Star network
- In local area networks with a star topology, each network host is
connected to a central hub with a point-to-point connection. In Star
topology every node (computer workstation or any other peripheral) is
connected to central node called hub or switch. The switch is the server
and the peripherals are the clients. The network does not necessarily
have to resemble a star to be classified as a star network, but all of
the nodes on the network must be connected to one central device. All
traffic that traverses the network passes through the central hub. The
hub acts as a signal repeater.
The star topology is considered the easiest topology to design and
implement. An advantage of the star topology is the simplicity of adding
additional nodes. The primary disadvantage of the star topology is that
the hub represents a single point of failure.
- Extended star
- A type of network topology in which a network that is based upon the
physical star topology has one or more repeaters between the central
node (the 'hub' of the star) and the peripheral or 'spoke' nodes, the
repeaters being used to extend the maximum transmission distance of the
point-to-point links between the central node and the peripheral nodes
beyond that which is supported by the transmitter power of the central
node or beyond that which is supported by the standard upon which the
physical layer of the physical star network is based.
-
- If the repeaters in a network that is based upon the physical
extended star topology are replaced with hubs or switches, then a hybrid
network topology is created that is referred to as a physical
hierarchical star topology, although some texts make no distinction
between the two topologies.
- Distributed Star
- A type of network topology that is composed of individual networks
that are based upon the physical star topology connected in a linear
fashion – i.e., 'daisy-chained' – with no central or top level
connection point (e.g., two or more 'stacked' hubs, along with their
associated star connected nodes or 'spokes').
Ring
Main article: Ring network
- A network topology that is set up in a circular fashion in which
data travels around the ring in one direction and each device on the
ring acts as a repeater to keep the signal strong as it travels. Each
device incorporates a receiver for the incoming signal and a transmitter
to send the data on to the next device in the ring. The network is
dependent on the ability of the signal to travel around the ring. When a
device sends data, it must travel through each device on the ring until
it reaches its destination. Every node is a critical link
In a ring topology, there is no server computer present; all nodes work
as a server and repeat the signal. The disadvantage of this topology is
that if one node stops working, the entire network is affected or stops
working.
Mesh
Main article: Mesh networking
The value of fully meshed networks is proportional to the exponent of
the number of subscribers, assuming that communicating groups of any
two endpoints, up to and including all the endpoints, is approximated by
Reed's Law.
- Fully connected network
Fully connected mesh topology
-
- A fully connected network is a communication network in which each of the nodes is connected to each other. In graph theory it known as a complete graph. A fully connected network doesn't need to use switching nor broadcasting.
However, its major disadvantage is that the number of connections grows
quadratically with the number of nodes,as per the formula
-
-
- and so it is extremely impractical for large networks. A two-node network is technically a fully connected network.
- Partially connected
Partially connected mesh topology
-
- The type of network topology in which some of the nodes of the
network are connected to more than one other node in the network with a
point-to-point link – this makes it possible to take advantage of some
of the redundancy that is provided by a physical fully connected mesh
topology without the expense and complexity required for a connection
between every node in the network.
Tree
Tree topology is structured like a tree in real world. Tree structure
has a root node, intermediate nodes and leaves. Root node is the main
or head node of the structure, and the leaves are the last nodes, which
has no further child nodes. This structure is arranged in a hierarchical
form, any nodes can have any number of the child nodes. But the tree
topology is practically impossible to construct, because the node in the
network is nothing, but the computing device can have maximum one or
two connections, so we cannot attach more than 2 child nodes to the
computing device (or parent node). There are many sub structures under
tree topology, but the most convenient is B-tree topology whereby
finding errors is relatively easy.
- A network that is based upon the physical hierarchical topology must
have at least three levels in the hierarchy of the tree, since a
network with a central 'root' node and only one hierarchical level below
it would exhibit the physical topology of a star.
- A network that is based upon the physical hierarchical topology and
with a branching factor of 1 would be classified as a physical linear
topology.
- The branching factor, f, is independent of the total number of nodes
in the network and, therefore, if the nodes in the network require
ports for connection to other nodes the total number of ports per node
may be kept low even though the total number of nodes is large; – this
makes the effect of the cost of adding ports to each node totally
dependent upon the branching factor and may therefore be kept as low as
required without any effect upon the total number of nodes that are
possible.
- The total number of point-to-point links in a network that is based
upon the physical hierarchical topology will be one less than the total
number of nodes in the network.
- If the nodes in a network that is based upon the physical
hierarchical topology are required to perform any processing upon the
data that is transmitted between nodes in the network, the nodes that
are at higher levels in the hierarchy will be required to perform more
processing operations on behalf of other nodes than the nodes that are
lower in the hierarchy. Such a type of network topology is very useful
and highly recommended.
