Transcript File

LAN
ARCHITECTURE
By
Dr. Khalid Hussain
1
Introduction
• Physical and Logical Topologies
• Topologies
– Bus
– Ring
– Star
– Extended Star
– Mesh
– Hybrid
2
Physical vs. Logical Topology
• The actual layout of a network and its media is
its Physical Topology
• The way in which the data access the medium
and transmits packets is the Logical Topology
• A glance at a network is not always revealing.
Cables emerging from a Hub does not make it
necessarily a Star Topology – it may actually be
a bus or a ring
3
Physical vs. Logical Topology (2)
• Your choice of Logical Topology will affect the
Physical Topology – and vice versa
• Design carefully – it may be difficult to change
part way through the installation
• Your choice will determine cable installation,
network devices, network connections,
protocols (and where you will drill holes in the
building !)
4
Factors
•
•
•
•
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Cost
Scalability
Bandwidth Capacity
Ease of Installation
Ease of fault finding and maintenance
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Bus Topology
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Bus Topology (2)
• Network maintained by a single cable
• Cable segment must end with a terminator
• Uses thin coaxial cable (backbones will be
thick coaxial cable)
• Extra stations can be added in a daisy
chain manner
7
Bus Topology (3)
• Standard is IEEE 802.3
• Thin Ethernet (10Base2) has a maximum
segment length of 200m
• Max no. of connections is 30 devices
• Four repeaters may be used to a total
cable length of 1000m
• Max no. of nodes is 150
8
Bus Topology (4)
• Thick Ethernet (10Base5) used for
backbones
• Limited to 500m
• Max of 100 nodes per segment
• Total of four repeaters , 2500m, with
a total of 488 nodes
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Bus Topology (5)
Advantages
• Inexpensive to install
• Easy to add stations
• Use less cable than
other topologies
• Works well for small
networks
Disadvantages
• No longer recommended
• Backbone breaks, whole
network down
• Limited no of devices can
be attached
• Difficult to isolate
problems
• Sharing same cable
slows response rates 10
Ring Topology
11
Ring Topology (2)
• No beginning or end (a ring in fact !!)
• All devices of equality of access to media
• Single ring – data travels in one direction only, guess what a
double ring allows !?
• Each device has to wait its turn to transmit
• Most common type is Token Ring (IEEE 802.5)
• A token contains the data, reaches the destination, data
extracted, acknowledgement of receipt sent back to transmitting
device, removed, empty token passed on for another device to
use
12
Ring Topology (3)
Advantages
• Data packets travel at
great speed
• No collisions
• Easier to fault find
• No terminators
required
Disadvantages
• Requires more cable
than a bus
• A break in the ring will
bring it down
• Not as common as
the bus – less devices
available
13
Star Topology
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Star Topology (2)
• Like the spokes of a wheel (without the
symmetry)
• Centre point is a Hub
• Segments meet at the Hub
• Each device needs its own cable to the Hub
• Predominant type of topology
• Easy to maintain and expand
15
Star Topology (3)
• Advantages
• Easy to add devices as the
network expands
• One cable failure does not bring
down the entire network
(resilience/flexible)
• Hub provides centralised
management
• Easy to find device and cable
problems
• Can be upgraded to faster
speeds
• Lots of support as it is the most
used
• Disadvantages
• A star network requires more
cable than a ring or bus
network
• Failure of the central hub can
bring down the entire network
• Costs are higher (installation
and equipment) than for most
bus networks
16
Extended Star Topology
A Star
Network
which has
been
expanded
to include
an
additional
hub or
hubs.
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Mesh Topology (Web)
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Mesh Topology (2)
•
•
•
•
Not common on LANs
Most often used in WANs to interconnect LANS
Each node is connected to every other node
Allows communication to continue in the event of
a break in any one connection
• It is “Fault Tolerant”
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Mesh Topology (3)
Advantages
• Improves Fault
Tolerance
Disadvantages
• Expensive
• Difficult to install
• Difficult to manage
• Difficult to
troubleshoot
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Hybrid Topology
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Hybrid Topology (2)
• Old networks are updated and replaced,
leaving older segments (legacy)
• Hybrid Topology – combines two or more
different physical topologies
• Commonly Star-Bus or Star-Ring
• Star-Ring uses a MAU (Multistation
Access Unit (see later slide)
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Types of Logical Topology
•
•
•
•
Previous slides showed Physical Topologies
Only two Logical Topologies (Bus or Ring)
Physical Bus or Ring easy to conceptualise
Physical Star could be either a Bus or Ring in
logical terms
• Confused ? See next slides
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Logical Bus
•Modern Ethernet networks are Star Topologies
(physically)
•The Hub is at the centre, and defines a Star Topology
•The Hub itself uses a Logical Bus Topology internally,
to transmit data to all segments
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Logical Bus
Advantages
• A single node failure
does not bring the
network down
• Most widely
implemented topology
• Network can be added
to or changed without
affecting other stations
Disadvantages
• Collisions can occur
easily
• Only one device can
access the network
media at a time
25
Logical Ring
• Data in a Star Topology can transmit data in
a Ring
• The MAU (Multistation Access Unit) looks like
an ordinary Hub, but data is passed internally
using a logical ring
• It is superior to a Logical Bus Hub – see later
slide
26
Logical Ring (2)
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Logical Ring (3)
Advantages
• The amount of data
that can be carried in Disadvantages
a single message is • A broken ring will
greater than on a
stop all
logical bus
transmissions
• There are no
• A device must wait
collisions
for an empty token
to be able to
transmit
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Summary
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•
•
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Bus Topology
Ring Topology
Star Topology
Other Topologies
Logical Topologies
Questions and Answers
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