Network Topology
Introduction :
A topology defines the arrangement of nodes, cables and connecting devices that make up the
network. It determines the data paths that may be used between any pair of stations of the network. There
are two levels in a network topology and there are five kinds of network topologies. The networks can be
connected at two levels, the physical and the other is logical. A topology basically defines how the network
has to be designed and provides a structure to the network.
Network topology is the study of the arrangement or mapping of the elements (links, nodes, etc.) of a
network, especially the physical (real) and logical (virtual) interconnections between nodes. [1][2] A local area
network (LAN) is one example of a network that exhibits both a physical topology and a logical topology.
Any given node in the LAN will have one or more links to one or more other nodes in the network and the
mapping of these links and nodes onto a graph results in a geometrical shape that determines the physical
topology of the network. Likewise, the mapping of the flow of data between the nodes in the network
determines the logical topology of the network. The physical and logical topologies might be identical in
any particular network but they also may be different.
History :
Topological ideas are present in almost all areas of today's mathematics. The subject of topology
itself consists of several different branches, such as point set topology, algebraic topology and differential
topology, which have relatively little in common. We shall trace the rise of topological concepts in a number
of different situations.
Perhaps the first work which deserves to be considered as the beginnings of topology is due to Euler. In
1736 Euler published a paper on the solution of the Konigsberg bridge problem entitled Solutio problematis
ad geometrian situs pertinentis which translates into English as The solution of a problem relating to the
geometry of position. The title itself indicates that Euler was aware that he was dealing with a different type
of geometry where distance was not relevant.
The paper not only shows that the problem of crossing the seven bridges in a single journey is impossible,
but generalises the problem to show that, in today's notation, A graph has a path traversing each edge exactly
once if exactly two vertices have odd degree.
The next step in freeing mathematics from being a subject about measurement was also due to Euler. In
1750 he wrote a letter to Christian Goldbach which, as well as commenting on a dispute Goldbach was
having with a bookseller, gives Euler's famous formula for a polyhedron
v-e+f=2
where v is the number of vertices of the polyhedron, e is the number of edges and f is the number of faces. It
is interesting to realise that this, really rather simple, formula seems to have been missed by Archimedes and
Descartes although both wrote extensively on polyhedra. Again the reason must be that to everyone before
Euler, it had been impossible to think of geometrical properties without measurement being involved.
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Types of topologies :
The arrangement or mapping of the elements of a network gives rise to certain basic topologies
which may then be combined to form more complex topologies (hybrid topologies). The most common of
these basic types of topologies are (refer to the illustration at the top right of this page):
O
O
O
O
O
O
O
Bus
Star
Ring
Mesh
Tree
Hybrid
Point to Point
Classification of network topologies:
There are also three basic categories of network topologies:
1. Physical topologies
2. Signal topologies
3. Logical topologies
The terms signal topology and logical topology are often used interchangeably even though there is a
subtle difference between the two and the distinction is not often made between the two.
1) Physical topologies:
The mapping of the nodes of a network and the physical connections between them i.e. the layout of
wiring, cables, locations of nodes and the interconnections between the nodes and the cabling or wiring
system. Physical topologies can be further classified into :
O Point-to-point
O Bus
O Star
O Ring
O Mesh
O Tree
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 the value of guaranteed, or nearly so, communications between the two endpoints. The value of an ondemand point-to-point connection is proportional to the number of potential pairs of subscribers, and has
been expressed as Metcalfe's Law.
A. 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. Children's "tin-can
telephone" is one example, with a microphone to a single public address speaker is another. These are
examples of physical dedicated channels.
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B. 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.
Fig: Point to Point (switched) topology
Bus Topology :
In bus topology, each computer is connected to a single communication cable via the interface. In bus
topology, it is possible for every computer to communicate with every other computer in the network. Most
bus networks broadcast signals in both the directions, so it enables for all devices to receive the signal
directly. However, some buses are unidirectional when signals travel only in one direction. Bus topology can
be further divide into two categories :
A. 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 virtually
simultaneously (disregarding propagation delays).
B. 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).
Fig: Bus topology
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Advantages of Bus topology:
1. Short cable length : In bus topology, a single bus data path connects to all the nodes. So it
requires a very short cable length to connect all the nodes.
2. Easy Architecture : The bus architecture is very simple. It contains a single cable through which
all data passes and to which all the nodes are connected.
3. Easy to extend : It is possible to connect additional nodes easily in bus network.
Disadvantages of bus topology:
1. Difficult to diagnose a fault : In bus topology, all the terminals are connected to the common
bus. Also the control of the network is not centralized in any particular node. So it requires to
detect a fault from many points in the network.
2. Fault isolation is difficult : If the fault occurs in the network itself, then entire segment of the
bus must be disconnected.
Star Topology :
The type of network topology in which each of the nodes of the network is connected to a central
node with a point-to-point link in a 'hub' and 'spoke' fashion, the central node being the 'hub' and the nodes
that are attached to the central node being the 'spokes' (e.g., a collection of point-to-point links from the
peripheral nodes that converge at a central node) – all data that is transmitted between nodes in the network
is transmitted to this central node, which is usually some type of device that then retransmits the data to
some or all of the other nodes in the network, although the central node may also be a simple common
connection point (such as a 'punch-down' block) without any active device to repeat the signals.
Fig: Star topology
Star topology can be further divided into following categories :
1. 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
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node or beyond that which is supported by the standard upon which the physical layer of the physical star
network is based.
2. Distributed Star
A type of network topology that is composed of individual networks that are based upon the physical
star topology connected together 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').
Advantages of Star Topology:
1. Ease of service : Star topology has number of concentration points which are either at the central
node or at the intermediate node. This provides easy access for service of reconfiguration of the
network.
2. One device per connection : In the star topology, since all nodes are connected separately to the
server or hub, so if connection between a particular node and server fails, it will not affect the
functionality of other part of network.
3. Centralized Control : The central node (hub) is directly connected to other nodes in the network
so all the power is exercised by the hub.
4. Simple Access Protocols : Star Network Involves only the central node & one peripheral node.
So connection problem regarding the control of the medium for transmission purpose is easily
solved.
Disadvantages of Star topology:
1. Long cable length : In star topology, each node is directly connected to the server or central
computer. So it requires a large quantity of cable.
2. Difficult to expand : The addition of new node requires a connection to the central node.
Problem can arise as a longer cable is needed.
3. Central node dependency : If the central node in a star network fails, the entire network will
collapse.
Ring Topology :
The type of network topology in which each of the nodes of the network is connected to two other
nodes in the network and with the first and last nodes being connected to each other, forming a ring – all
data that is transmitted between nodes in the network travels from one node to the next node in a circular
manner and the data generally flows in a single direction only.
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Fig: Ring Topology
The type of network topology in which each of the nodes of the network is connected to two other
nodes in the network, with two connections to each of these nodes, and with the first and last nodes being
connected to each other with two connections, forming a double ring – the data flows in opposite directions
around the two rings, although, generally, only one of the rings carries data during normal operation, and the
two rings are independent unless there is a failure or break in one of the rings, at which time the two rings
are joined (by the stations on either side of the fault) to enable the flow of data to continue using a segment
of the second ring to bypass the fault in the primary ring.
Advantages of Ring Topology:
1. Short cable length : In ring topology, nodes are connected in a circular ring. Hence amount for
cabling required is less.
2. No wiring closet space is required : In ring topology, only onecable is connecting each node to
its immediate neighbors. So it does not require to allocate space for wiring closets.
3. Suitable for optical fibers : In ring topology, traffic travels in one direction. So it is easy to use
optical fiber as a medium of transmission.
Disadvantages of Ring Topology:
1. Node failure causes network failure : In ring topology, the data travels through all connected
nodes in the ring and then it will reach a required node. If one node fails to pass data through
itself, then entire network will fail.
2. Difficult to diagnose fault : Failure of one node will affect the entire network. To find a faulty
node, it requires to examine a series of adjacent nodes.
3. Network reconfiguration is difficult : In ring topology, to add a new node or to delete a node in a
network is difficult job. Also it is difficult to remove a particular node from the existing network.
Mesh Topology :
Mesh Network is a network where all the nodes are connected to each other and is a complete
network. In a Mesh Network every node is connected to other nodes on the network through hops. Some are
connected through single hops and some may be connected with more than one hope.While the data is
traveling on the Mesh Network it is automatically configured to reach the destination by taking the shortest
route which means the least number of hops. Data travels by hopping from one node to another and then
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reaches the destination node in a Mesh Topology Network. Mesh topology can be divided into further
categories
O Fully connected
The type of network topology in which each of the nodes of the network is connected to each of the
other nodes in the network with a point-to-point link – this makes it possible for data to be simultaneously
transmitted from any single node to all of the other nodes. The physical fully connected mesh topology is
generally too costly and complex for practical networks, although the topology is used when there are only a
small number of nodes to be interconnected.
O Partially connected
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.
Fig : Partially connected mesh topology
Advantages of mesh topology :
O
O
O
O
No traffic problem as there are dedicated links.
Robust as failure of one link does not affect the entire system.
Security as data travels along a dedicated line.
Points to point links make fault identification easy.
Disadvantages of mesh topology :
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O The hardware is expansive as there is dedicated link for any two nodes and each device should have
(n-1) I/O ports.
O There is mesh of wiring which can be difficult to manage.
O Installation is complex as each node is connected to every node.
Tree Topology :
The type of network topology in which a central 'root' node (the top level of the hierarchy) is
connected to one or more other nodes that are one level lower in the hierarchy (i.e., the second level) with a
point-to-point link between each of the second level nodes and the top level central 'root' node, while each of
the second level nodes that are connected to the top level central 'root' node will also have one or more other
nodes that are one level lower in the hierarchy (i.e., the third level) connected to it, also with a point-to-point
link, the top level central 'root' node being the only node that has no other node above it in the hierarchy
(The hierarchy of the tree is symmetrical.)
Fig : Tree topology
Advantages of a Tree Topology:
O Point-to-point wiring for individual segments.
O Supported by several hardware and software venders.
Disadvantages of a Tree Topology:
O Overall length of each segment is limited by the type of cabling used.
O if the backbone line breaks, the entire segment goes down.
O More difficult to configure and wire than other topologies.
2) Signal topology :
The mapping of the actual connections between the nodes of a network, as evidenced by the path that
the signals take when propagating between the nodes. The term 'signal topology' is often used synonymously
with the term 'logical topology', however, some confusion may result from this practice in certain situations
since, by definition, the term 'logical topology' refers to the apparent path that the data takes between nodes
in a network while the term 'signal topology' generally refers to the actual path that the signals (e.g., optical,
electrical, electromagnetic, etc.) take when propagating between nodes.
Example: In an 802.4 Token Bus network, the physical topology may be a physical bus, a physical star, or a
hybrid physical topology, while the signal topology is a bus (i.e., the electrical signal propagates to all nodes
simultaneously [ignoring propagation delays and network latency] ), and the logical topology is a ring (i.e.,
the data flows from one node to the next in a circular manner according to the protocol).
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3) Logical topology:
The mapping of the apparent connections between the nodes of a network, as evidenced by the path
that data appears to take when traveling between the nodes.
Classification of logical topologies
The logical classification of network topologies generally follows the same classifications as those in
the physical classifications of network topologies, the path that the data takes between nodes being used to
determine the topology as opposed to the actual physical connections being used to determine the topology.
O Daisy chains :
Except for star-based networks, the easiest way to add more computers into a network is by daisychaining, or connecting each computer in series to the next. If a message is intended for a computer partway
down the line, each system bounces it along in sequence until it reaches the destination. A daisy-chained
network can take two basic forms: linear and ring.
A linear topology puts a two-way link between one computer and the next. However, this was
expensive in the early days of computing, since each computer (except for the ones at each end) required
two receivers and two transmitters.
By connecting the computers at each end, a ring topology can be formed. An advantage of the ring is
that the number of transmitters and receivers can be cut in half, since a message will eventually loop all of
the way around. When a node sends a message, the message is processed by each computer in the ring. If a
computer is not the destination node, it will pass the message to the next node, until the message arrives at
its destination. If the message is not accepted by any node on the network, it will travel around the entire
ring and return to the sender. This potentially results in a doubling of travel time for data, but since it is
traveling at a fairly significant fraction of the speed of light, the loss is usually negligible.
O Centralization:
The star topology reduces the probability of a network failure by connecting all of the peripheral nodes
(computers, etc.) to a central node. When the physical star topology is applied to a logical bus network such
as Ethernet, this central node (traditionally a hub) rebroadcasts all transmissions received from any
peripheral node to all peripheral nodes on the network, sometimes including the originating node. All
peripheral nodes may thus communicate with all others by transmitting to, and receiving from, the central
node only. The failure of a transmission line linking any peripheral node to the central node will result in the
isolation of that peripheral node from all others, but the remaining peripheral nodes will be unaffected.
However, the disadvantage is that the failure of the central node will cause the failure of all of the peripheral
nodes also.
A tree topology (a.k.a. hierarchical topology) can be viewed as a collection of star networks arranged in
a hierarchy. This tree has individual peripheral nodes (e.g. leaves) which are required to transmit to and
receive from one other node only and are not required to act as repeaters or regenerators. Unlike the star
network, the functionality of the central node may be distributed.
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As in the conventional star network, individual nodes may thus still be isolated from the network by a
single-point failure of a transmission path to the node. If a link connecting a leaf fails, that leaf is isolated; if
a connection to a non-leaf node fails, an entire section of the network becomes isolated from the rest.
O Decentralization :
In a mesh topology (i.e., a partially connected mesh topology), there are at least two nodes with two or
more paths between them to provide redundant paths to be used in case the link providing one of the paths
fails. This decentralization is often used to advantage to compensate for the single-point-failure disadvantage
that is present when using a single device as a central node (e.g., in star and tree networks). A special kind of
mesh, limiting the number of hops between two nodes, is a hypercube. The number of arbitrary forks in
mesh networks makes them more difficult to design and implement, but their decentralized nature makes
them very useful. This is similar in some ways to a grid network, where a linear or ring topology is used to
connect systems in multiple directions. A multi-dimensional ring has a toroidal topology, for instance.
O Hybrids:
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, but two star networks connected together exhibit
a hybrid 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 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.
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