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Lecture 6 – LAN Topology
Objective
After attending this lecture, students should have a brief understanding of:
 three topology of Local Area Network: bus topology, star, and extended star
topology – network layout, and signal transmission method
 Ethernet – architecture, access method
 Choice of local area network topology
Terminology
Terms
Topology
Network Architecture
Access Method
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Meaning
The physical layout of a Local Area Network.
Describes how devices are connected together, what
media is supported and its limitation, and how
electrical signal is transmitted over the media
Defines the rules and layout of a network
Defines the components
Defines communication protocol
A regulatory mechanism that allows devices on a
network to share the networking media equitably and
in some cases cost effectively as well.
It is a set of predefined rules or protocols that
determine how a device gains access or entry to the
networking media.
1. Bus topology
1.1
General Characteristics of Bus topology
 All devices on a LAN are attached independently to a linear networking media
known as bus, trunk line, or highway.
 The bus wire must end in a terminating resistance, or terminator.
 Example of bus topology – Ethernet
W
T
C
W
Symbol :
C–
TW–
1.2.
C
W
Connector
Terminator
Workstation (including server, workstation, etc.)
Signal transmission over the media
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A transmitted signal travels in both directions;
Each device can check the signal on the media through its network card;
Each signal contains the source MAC address, source IP address, destination
MAC address, and destination IP address
Only the device with matching destination IP address, and destination MAC
address can copy the data and pass it to higher layer;
Other devices with non-matching IP address, or non-matching destination address
ignore the data;
A signal, at reaching the end of a bus, is absorbed by the terminator. This prevent
the signal bouncing back to the media.
Remark:
A broadcast signal means every device on a LAN has a matching destination
address.
1.3
Only one workstation at a time transmits data
 Avoidance of signal collision (Carrier Sense)
Before a workstation transmits its data to the media, the transceiver (a feature
inside a network card, or a physical component) checks whether the media is
being used by other devices.
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Collision impact
- When a collision occurs, the data from each device impact - that is, the
voltage pulses from each device are both present on the common bus wire at
the same time - and thus the data from both devices is damaged.
- The network area within which data packets originate and collide is called a
collision domain
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Collision Recovery
- On detection of collision, the network card of a network device issues a
backoff; enforcing different retransmission delay for each device on the network.
1.4
Advantage & Disadvantages
Advantage
Simple wiring layout
Low installation cost
Network still function in case of a node
failure.
2. Bus topology
Disadvantage
High management cost;
Long fault diagnosis time;
The bus itself is a single point of failure.
E.g. When adding new device to a bus
topology, the network is required to shut
down.
2.1
General Characteristics of Bus topology
 All devices on a LAN are attached independently to a central point (usually
known as HUB).
 An active hub connects the networking media, and regenerates the electrical
signal, thus supporting longer distance connection.
 A passive hub is simply a device used to connect networking media.
 E.g. Horizontal cabling
W
HUB
W
Symbol :
W–
W
Workstation (including server, workstation, etc.)
2.2. Signal transmission over the network
 A transmitted signal travels to the central point (HUB) along its own connection
line.
 Each signal contains the source MAC address, source IP address, destination
MAC address, and destination IP address
 Only the device with matching destination IP address, and destination MAC
address can copy the data and pass it to higher layer;
 Address recognition is performed by the hub.
Remark:
A broadcast signal means every device on a LAN has a matching destination
address.
2.3 Advantage & Disadvantages
Advantage
Easier to design
Easy for maintenance – diagnosis is
simpler;
Greater reliability – any node failure does
not affect the rest of a network
Disadvantage
Higher media installation cost
A hub itself is the single point of failure
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Limited distance, support up to 100
meter between the HUB and a
workstation due to the use of UTP
cable.
Maximum distance between two
workstations are 200 meter apart
(with HUB sitting in the middle)
Maximum coverage area is 200m *
200m
3. Extended Star Topology
3.1
Signal Loss
 When transmitting over a media, electrical signal could be weakened due to heat
loss, radiation, or leakage.
 Each media has its own limited distance to support data transmission before signal
loss.
3.2
Extending a star topology
 By using repeater, a weakened signal can be amplified.
 A star topology can provide a larger area coverage by using repeaters.
 A star topology can be extended by using internetworking devices that do not
result in attenuation of the signal. When this occurs, the resulting topology is
referred to as an extended star topology.
Review Questions
1. Explain why star topology does not require external terminator whilst bus topology
needs such components at both ends?
2. Identify in what situation that a bus net breaks down while a star network does not.
3. Which topology would you prefer?
4.
4.1
Ethernet
Criteria for a network architecture
Criteria
How to achieve
reliability
 Provide error detection mechanism.
 Provide error recovery facility.
modularity
 Relatively small variety mass-produced blocks in a broad
assortment of network devices.
connectivity
 Incorporate a variety of hardware and software products.
 Devices and products can function in an interoperable and
unified manner.
Ease of use
 Users need have no concern for or knowledge of the
network's structure or implementation
modification
 Evolve and adapt as needs change or expand, or as new
technologies emerge.
implementation
 Follow industry networking standards.
 Allow for the installation of a variety of configurations
4.2.
History of Ethernet
Time & Place
1960s at the University of
Hawaii.
In the 1970s the Xerox
1980s IEEE
4.3
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Event
The access method used by Ethernet was
developed.
To advance and popularize Ethernet
Standardization of Ethernet architecture– 802.3
Ethernet access method
CSMA/CD - carrier sense multiple access with collision detection
Functions:
 Transmitting and receiving data packets
 Decoding data packets and checking them for valid addresses before passing them
to the upper layers of the OSI model.
 Detecting errors within data packets or on the network.
Function to be
performed at a device
Wants to xmit data
(Carrier Sense)
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Media Access
Collision detection
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Action taken after
collision
Action taken by the device
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Receiving data by a
receiver
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Data verification at
receiver
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Perform listen before transmit mode.
Listen to the media and check whether it is in use (by
electrical voltage / pulse).
Transmit data if not busy.
Sender keeps on listen to the media to ensure no
device is using the media at the same time.
A collision is detected if signal strength is doubled.
Sending devices continue transmission to ensure the
collision is detected by all workstations.
Each device invokes a backoff algorithm, resulting in
different delays at devices attempting to perform
transmission.
No priority for collided senders when attempting for
re-transmission.
Only the device whose MAC address and IP address
matches the destination MAC address and
destination IP address carried by inside Ethernet data
frame will copy the data.
Validate data packet using frame check sequence (a
parity check field inside Ethernet data packet).
Ignore data if error found. Pass data to upper layer
for further checking/process if no error.
No acknowledgement back to sender.
Characteristics of Ethernet
 Ethernet is a broadcast transmission medium
 All devices on a network can see all data that passes along the networking media,
but not processing data.
 A connectionless network architecture.
 Also known as a best-effort delivery system.
4.4
Ethernet vs Token Ring
Item
Access Method
Physical layout
Data delivery
Ethernet
CSMA / CD
 Linear Bus (10Base2
or 10Base5)
 Star (10BaseT)
Opportunistic
Token Ring
Token Passing
Circular wiring
Assurance
Review Questions
1. Can collisions occur in a Token Ring Network architecture? Why
or Why not?
2. Which of the two network architectures listed above could be considered to be
deterministic?
3. In which of the two network architectures listed above is a broadcast access
method used?
4. What is token passing?
5. Why and how can network architecture determine the type of topology used?