There are also three basic categories of network
topologies:physical topologiessignal topologieslogical topologiesThe 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.
Tuesday, September 16, 2008
Physical topologies
The mapping of the nodes of a network and the physical
connections between them – i.e., the layout of wiring,
cables, the locations of nodes, and the interconnections
between the nodes and the cabling or wiring system.
connections between them – i.e., the layout of wiring,
cables, the locations of nodes, and the interconnections
between the nodes and the cabling or wiring system.
Classification of Physical Topologies
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 on-demand point-to-point
connection is proportional to the number of potential
pairs of subscribers, and has been expressed as
Metcalfe's Law.
Bus
Linear busThe 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)
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
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.
Ring
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.
Mesh
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.Tree (also known as hierarchical):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, each node in the
network having a specific fixed number, f, of nodes
connected to it at the next lower level in the
hierarchy, the number, f, being referred to as the
'branching factor' of the hierarchical tree.
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 on-demand point-to-point
connection is proportional to the number of potential
pairs of subscribers, and has been expressed as
Metcalfe's Law.
Bus
Linear busThe 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)
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
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.
Ring
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.
Mesh
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.Tree (also known as hierarchical):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, each node in the
network having a specific fixed number, f, of nodes
connected to it at the next lower level in the
hierarchy, the number, f, being referred to as the
'branching factor' of the hierarchical tree.
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.Note: 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.ExampleIn 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).
of a network, as evidenced by the path that the signals
take when propagating between the nodes.Note: 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.ExampleIn 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).
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.
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.
Daisy chains
Except for star-based networks, the easiest way to add
more computers into a network is by daisy-chaining, 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 insignificant multiple of the
speed of light, the loss is usually negligible.
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.If the central node is passive, the originating node
must be able to tolerate the reception of an echo of its
own transmission, delayed by the two-way round trip
transmission time (i.e. to and from the central node)
plus any delay generated in the central node. An active
star network has an active central node that usually has
the means to prevent echo-related problems.
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.
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 networkA 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 algorithms0 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.
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.
Daisy chains
Except for star-based networks, the easiest way to add
more computers into a network is by daisy-chaining, 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 insignificant multiple of the
speed of light, the loss is usually negligible.
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.If the central node is passive, the originating node
must be able to tolerate the reception of an echo of its
own transmission, delayed by the two-way round trip
transmission time (i.e. to and from the central node)
plus any delay generated in the central node. An active
star network has an active central node that usually has
the means to prevent echo-related problems.
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.
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 networkA 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 algorithms0 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.
Category 5 Cable
Category 5 cable, commonly known as Cat 5 or "Cable and
Telephone", is a twisted pair cable type designed for
high signal integrity. Many such cables are unshielded
but some are shielded. Category 5 has been superseded by
the Category 5e specification. This type of cable is
often used in structured cabling for computer networks
such as Ethernet, and is also used to carry many other
signals such as basic voice services, token ring, and
ATM (at up to 155 Mbit/s, over short distances).The specification for category 5 cable was defined in
ANSI/TIA/EIA-568-A, with clarification in TSB-95. These
documents specified performance characteristics and test
requirements for frequencies of up to 100 MHz.Category 5 cable includes four twisted pairs in a single
cable jacket. This use of balanced lines helps preserve
a high signal-to-noise ratio despite interference from
both external sources and other pairs (this latter form
of interference is called crosstalk). It is most
commonly used for 100 Mbit/s networks, such as
100BASE-TX Ethernet, although IEEE 802.3ab defines
standards for 1000BASE-T - Gigabit Ethernet over
category 5 cable. Cat 5 cable typically has three twists
per inch of each twisted pair of 24 gauge copper wires
within the cable.
Telephone", is a twisted pair cable type designed for
high signal integrity. Many such cables are unshielded
but some are shielded. Category 5 has been superseded by
the Category 5e specification. This type of cable is
often used in structured cabling for computer networks
such as Ethernet, and is also used to carry many other
signals such as basic voice services, token ring, and
ATM (at up to 155 Mbit/s, over short distances).The specification for category 5 cable was defined in
ANSI/TIA/EIA-568-A, with clarification in TSB-95. These
documents specified performance characteristics and test
requirements for frequencies of up to 100 MHz.Category 5 cable includes four twisted pairs in a single
cable jacket. This use of balanced lines helps preserve
a high signal-to-noise ratio despite interference from
both external sources and other pairs (this latter form
of interference is called crosstalk). It is most
commonly used for 100 Mbit/s networks, such as
100BASE-TX Ethernet, although IEEE 802.3ab defines
standards for 1000BASE-T - Gigabit Ethernet over
category 5 cable. Cat 5 cable typically has three twists
per inch of each twisted pair of 24 gauge copper wires
within the cable.
Category 5e
Cat 5e cable is an enhanced version of Cat 5 that adds
specifications for far end crosstalk. It was formally
defined in 2001 in the TIA/EIA-568-B standard, which no
longer recognizes the original Cat 5 specification.
Although 1000BASE-T was designed for use with Cat 5
cable, the tighter specifications associated with Cat 5e
cable and connectors make it an excellent choice for use
with 1000BASE-T. Despite the stricter performance
specifications, Cat 5e cable does not enable longer
cable distances for Ethernet networks: cables are still
limited to a maximum of 100 m (328 ft) in length (normal
practice is to limit fixed ("horizontal") cables to 90 m
to allow for up to 5 m of patch cable at each end). Cat
5e cable performance characteristics and test methods
are defined in TIA/EIA-568-B.2-2001.
specifications for far end crosstalk. It was formally
defined in 2001 in the TIA/EIA-568-B standard, which no
longer recognizes the original Cat 5 specification.
Although 1000BASE-T was designed for use with Cat 5
cable, the tighter specifications associated with Cat 5e
cable and connectors make it an excellent choice for use
with 1000BASE-T. Despite the stricter performance
specifications, Cat 5e cable does not enable longer
cable distances for Ethernet networks: cables are still
limited to a maximum of 100 m (328 ft) in length (normal
practice is to limit fixed ("horizontal") cables to 90 m
to allow for up to 5 m of patch cable at each end). Cat
5e cable performance characteristics and test methods
are defined in TIA/EIA-568-B.2-2001.
Connectors and Other Information
The cable exists in both stranded and solid conductor
forms. The stranded form is more flexible and withstands
more bending without breaking and is suited for reliable
connections with insulation piercing connectors, but
makes unreliable connections in insulation-displacement
connectors. The solid form is less expensive and makes
reliable connections into insulation displacement
connectors, but makes unreliable connections in
insulation piercing connectors. Taking these things into
account, building wiring (for example, the wiring inside
the wall that connects a wall socket to a central patch
panel) is solid core, while patch cables (for example,
the movable cable that plugs into the wall socket on one
end and a computer on the other) are stranded. Outer
insulation is typically PVC or LSOH.Cable types, connector types and cabling topologies are
defined by TIA/EIA-568-B. Nearly always, 8P8C modular
connectors, often incorrectly referred to as "RJ-45",
are used for connecting category 5 cable.The cable is terminated in either the T568A scheme or
the T568B scheme. It doesn't make any difference which
is used as they are both straight through (pin 1 to 1,
pin 2 to 2, etc); however mixed cable types should not
be connected in series as the impedance per pair differs
slightly and could cause signal degradation. The article
Ethernet over twisted pair describes how the cable is
used for Ethernet, including special "cross over"
cables.
forms. The stranded form is more flexible and withstands
more bending without breaking and is suited for reliable
connections with insulation piercing connectors, but
makes unreliable connections in insulation-displacement
connectors. The solid form is less expensive and makes
reliable connections into insulation displacement
connectors, but makes unreliable connections in
insulation piercing connectors. Taking these things into
account, building wiring (for example, the wiring inside
the wall that connects a wall socket to a central patch
panel) is solid core, while patch cables (for example,
the movable cable that plugs into the wall socket on one
end and a computer on the other) are stranded. Outer
insulation is typically PVC or LSOH.Cable types, connector types and cabling topologies are
defined by TIA/EIA-568-B. Nearly always, 8P8C modular
connectors, often incorrectly referred to as "RJ-45",
are used for connecting category 5 cable.The cable is terminated in either the T568A scheme or
the T568B scheme. It doesn't make any difference which
is used as they are both straight through (pin 1 to 1,
pin 2 to 2, etc); however mixed cable types should not
be connected in series as the impedance per pair differs
slightly and could cause signal degradation. The article
Ethernet over twisted pair describes how the cable is
used for Ethernet, including special "cross over"
cables.
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