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 refers to the
placement of the network's various components, including device location and
cable installation, while logical topology
shows how data flows within a network, regardless of its physical design.
Distances between nodes, physical interconnections, transmission rates, and/or
signal types may differ between two networks, yet their topologies may be
identical.
A good example is a local area network (LAN): Any given node in the LAN
has one or more physical links to other devices in the network; graphically
mapping these links results in a geometric shape that can be used to describe
the physical topology of the network. Conversely, mapping the data flow between
the components determines the logical topology of the network.
Topology
There are two basic categories of
network topologies:
- Physical topologies
- 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 logical classification of
network topologies generally follows the same classifications as those in the
physical classifications of network topologies but describes the path that the data
takes between nodes being used as opposed to the actual physical
connections between nodes. The logical topologies are generally determined by
network protocols as opposed to being determined by the physical layout of
cables, wires, and network devices or by the flow of the electrical signals,
although in many cases the paths that the electrical signals take between nodes
may closely match the logical flow of data, hence the convention of using the
terms logical topology and signal topology interchangeably.
Logical topologies are often closely
associated with Media Access Control methods and protocols. Logical
topologies are able to be dynamically reconfigured by special types of
equipment such as routers and switches.
Diagram of different network
topologies.
The study of network topology
recognizes eight basic topologies:
- Point-to-point
- Bus
- Star
- Ring or circular
- Mesh
- Tree
- Hybrid
- Daisy chain
Point-to-point
The simplest topology is 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.
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 Topology
Bus network topology
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.[1]
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 Topology
Star network topology
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 Topology
Ring network topology
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.
Mesh Topology
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 quadratic ally with the number of nodes,
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 Topology
Tree network topology
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This particular type of network
topology is based on a hierarchy of nodes. The highest level of any tree
network consists of a single, 'root' node, this node connected either a single
(or, more commonly, multiple) node(s) in the level below by (a) point-to-point
link(s). These lower level nodes are also connected to a single or multiple
nodes in the next level down. Tree networks are not constrained to any number
of levels, but as tree networks are a variant of the bus network topology, they
are prone to crippling network failures should a connection in a higher level
of nodes fail/suffer damage. Each node in the network has a specific, fixed
number of nodes connected to it at the next lower level in the hierarchy, this
number referred to as the 'branching factor' of the tree. This tree has
individual peripheral nodes.
1.
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.
2.
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.
3.
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.
4.
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.
5.
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.
Advantages
- It is scalable. Secondary nodes allow more
devices to be connected to a central node.
- Point to point connection of
devices.
- Having different levels of the
network makes it more manageable hence easier fault identification and
isolation.
Disadvantages
- Maintenance of the network may
be an issue when the network spans a great area.
- Since it is a variation of bus
topology, if the backbone fails, the entire network is crippled.
definition: Tree topology is a combination of
Bus and Star topology.
An example of this network could be
cable TV technology. Other examples are in dynamic tree based wireless networks
for military, mining and otherwise mobile applications. The Naval
Postgraduate School,
Monterey CA, demonstrated such tree based wireless networks for border
security. In a pilot system, aerial cameras kept aloft by balloons relayed real
time high resolution video to ground personnel via a dynamic self healing tree
based network.
Hybrid
Hybrid networks use a combination of
any two or more topologies in such a way that the resulting network does not
exhibit one of the standard topologies (e.g., bus, star, ring, etc.). For
example a tree network connected to a tree network is still a tree network
topology. A hybrid topology is always produced when two different basic network
topologies are connected. Two common examples for Hybrid network are: star
ring network and star bus network
- A Star ring network consists of
two or more star topologies connected using a multistation access
unit
(MAU) as a centralized hub.
- A Star Bus network consists of
two or more star topologies connected using a bus trunk (the bus trunk
serves as the network's backbone).
While grid and torus networks have
found popularity in high-performance
computing
applications, some systems have used genetic algorithms to design custom networks that have
the fewest possible hops in between different nodes. Some of the resulting layouts
are nearly incomprehensible, although they function quite well.
A Snowflake topology is really a
"Star of Stars" network, so it exhibits characteristics of a hybrid
network topology but is not composed of two different basic network topologies
being connected.
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