Download ITT04103-LAN-Topologies-Lecture-2

LAN Topologies
Part 1
What is topology?
Topology is the physical or logical interconnection of communicating
devices
Physical Topology: The physical layout of devices on a network.
Every LAN has a topology, or the way that the devices on a
network are arranged and how they communicate with each other.
It can also be define as the way that the workstations (computers)
are connected to the network through the actual cables that
transmit data -- the physical structure of the network
Logical Topology: Also known as Signal Topology. The logical
topology 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.
Logical topology can also be defined as relationship between
nodes (workstations) in the network as seen by the software that
handles data delivery from station to station
Logical topologies are bound to the network protocols that direct
how the data moves across a network.
Note: Logical topology of a given network is not necessarily the
same as its physical topology.
Categories of LAN Topology
Framing
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To control the transmission of data, all topology carry data in
small units called frames
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Each Frame contains a part of data to be sent and a header or a
trailer (or both)
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The header normally contains the sender and receiver physical
address
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The header or trailer can also contain control information
such as a sequence number or redundant information to be
used in error control
Framing
Addressing
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In all topologies, an addressing mechanism ensure that a frame is
received by one or more destinations
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Each station (node) is assigned a unique address
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When a station (node ) sends a frame includes Source Address
and Destination Address
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When the frame is received by a station , it checks to see if the
destination address matches one of its three addresses: unicast,
multicast and broadcast
Unicast: Unicast addressing means one – to – one communication.
A station sends a frame that is received by only one station
Multicast : Multicast addressing means one –to- many
communication; a station sends a frame that can be received by a
selected number of stations
 A group of stations can have a common multicast address, which
is different from their unicast addressees
 When a station wants to send a frame to a group of stations, it
uses multicast address defining that group
Broadcasting: Broadcasting means one – to – all communication; a
station sends a frame that can be received by all other stations
• Broadcast can be considered a special case of multicast
communication ; the group includes all of the communication
Mesh Topology
Mesh Topology
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In a mesh topology, each of the node or station (computer) and
other devices, are interconnected with one another. That is every
node is connected to every other node in the network.
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This type of topology is very expensive as there are many
redundant connections, therefore it is not mostly used in
computer networks.
•
It is a preferred topology in wireless networks.
Number of links in mesh topology
Where x is the number of nodes in mesh network
Number of I/O interface per node
Where p is number of I/O ports a node must have
Advantages of Mesh Topology
• Even if one of the components fails there is always an alternative present. So
data transfer doesn’t get affected. That is it has multiple links, so if one route is
blocked then other routes can be used for data communication.
• Each connection can have its own data load, so the traffic problem is
eliminated.
• It is easy to troubleshoot mesh topology compared with other topologies.
• Its performance is not affected with heavy load of data transmission.
• The arrangement of the network nodes is such that it is possible to transmit
data from one node to many other nodes at the same time.
• Data can be transmitted from different devices simultaneously. This topology
can withstand high traffic.
• Expansion and modification in topology can be done without disrupting other
nodes.
Disadvantages of Mesh Topology
• There are high chances of redundancy in many of the network
connections.
• Overall cost of this network is way too high as compared to other
network topologies.
• Set-up and maintenance of this topology is very difficult. Even
administration of the network is tough.
Bus Topology
Bus Topology
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In Bus Topology all the nodes (computers or stations) are
connected to the single cable (called bus), by the help of
interface connectors (T-Connector).
Interface connector (T-Connector ) is also called medium
interface
This central cable is the backbone of the network
Every workstation communicates with the other device through
the Bus. The connection is multipoint. All stations share the
same medium
A signal from the source is broadcasted and it travels to all
workstations connected to bus cable. Although the message is
broadcasted but only the intended recipient, whose destination
address matches, accepts it.
If the destination address of machine doesn’t match with the
intended address, machine discards the signal
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A terminator is added at ends of the central cable (bus), to
prevent bouncing of signals.
A barrel connector can be used to extend the bus.
Bus Topology
Addressing in Bus Topology
•
Bus
topology
allows
unicast,
multicast
and
broadcast
addressing. Each station checks the destination address of the
frame received.
•
If the destination address is unicast, it must match the physical
address of the station; otherwise the frame is discarded.
•
If the destination address is multicast, the station checks to see if
the station belong to that group. If it does, the station keeps the
frame; otherwise the frame is discarded.
•
If the destination address is broadcast, the station keeps the
frame because the frame is destined for all stations
Bus Topology Operation
Bus Topology Operation
Bus Topology Operation -Explanation
 Bus topology Operation
figures shows
the mechanism of
sending a unicast frame from one station to another.
 The frame is received by all stations, but it is discarded by all
except the unique destination
 In the figure, a frame from station A is destined to station C is
received and discarded by station B
Advantages of Bus Topology
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Easy to connect a computer or peripheral to a linear bus.
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Requires less cable length than a star topology.
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It is easy to set-up and extend bus network.
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Bus topology is not expensive compared with other topologies.
Disadvantages of Bus Topology
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Entire network shuts down if there is a break in the main cable.
That is dependency on central cable in this topology has its
disadvantages. If the main cable (i.e. bus ) encounters some
problem, whole network breaks down
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Terminators are required at both ends of the backbone cable.
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Difficult to identify the problem if the entire network shuts down.
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Efficiency of Bus network decreases as the number of devices
connected to the bus increases. That is there is a limit on central
cable length and number of nodes that can be connected
It is not suitable for networks with heavy traffic.
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