Types of Network Topologies and

Networking Devices Functions

Dr.S.Sivakumar, Associate
Professor, SENSE,VIT
 Multiplexing
• Multiplexing is a way of sending multiple
signals or streams of information over a shared
link.
• Types
– Frequency Division Multiplexing
– Time Division Multiplexing
– Code Division Multiplexing
– Wavelength Division Multiplexing
– Frequency Division Multiplexing
• it divides the spectrum or carrier bandwidth in logical
channels and allocates one user to each channel.
– Time Division Multiplexing
• The shared channel is divided among its user by means
of time slot.
• Each user can transmit data within the provided time
slot only.
• It works on synchronized mode in both ends.
– Code Division Multiplexing
• Multiple data signals can be transmitted over a single
frequency by using Code Division Multiplexing.
• Each station is assigned with a unique code.
– Wavelength Division Multiplexing
• In fiber optic mode, multiple optical carrier signals are
multiplexed into an optical fiber by using different
wavelengths
 Switching Techniques
• The data transmission from source to
destination is routed through various
intermediate nodes.
• Types of switching
– Circuit switching
– Message switching
– Packet switching
 Circuit switching
• Dedicated physical link between the source
and destination.
• Example: Telephone switching devices creates
a path that connects the dialer and receiver by
making a physical connection.
• Drawbacks
• Expensive
• High bandwidth
• Takes long time to establish the connection
 Message Switching
• There is no need of dedicated path between the
source and destination.
• When source want to send the message, it
appends the destination address to the message.
• In this method, each node uses a store and
forward techniques.
• Drawbacks
– Large storing capacity
– Not suitable for interactive applications.
 Packet switching
• Messages are divided and grouped into the
small units called packets.
• It connectionless network switching
techniques.
• The packets are individually routed from
source to destination and there is no need of
dedicated link for communication.
• Advantages
– Reduced delay
 Physical Topology
• Term physical topology refers to the way in which a network is
laid out physically
• Two or more devices connect to a link. Two or more links form
a topology
• “Topology of a network is the geometric representation of
the relationship of all the links and linking devices (usually
called nodes) to one another”.
• Four basic topologies possible: mesh, star, bus and ring
• Types of connections  1)pt-to-pt connections
• 2) Multi-point connections (multidrop connection)
Spatially shared or time-shared connection
Physical Topology
 MESH Topology
• Fully connected Mesh , n=5 devices, 10 links

Station A

Station C Station B

Station D Station E
 MESH Topology
• Every device has a dedicated point-to-point link to every other device.
• The term dedicated means that the link carries traffic only between the two
devices it connects.
• To find the no. of physical links in a fully connected mesh n/w with ‘n’
nodes, we consider that each node must be connected to every other node.
• Node 1 must be connected to n – 1 nodes, Node 2 must be connected to n –
1 nodes and finally Node n must be connected to n – 1 nodes
• We thus need n(n - 1) physical links.
• However, if each physical link allows communication in both directions
(duplex mode), mesh topology, we can divide the no. of links by 2.
• Now we need n(n -1) / 2 duplex-mode links. To accommodate that many
links, every device must have n-1 I/O ports to be connected to other n-1
stations.
 Advantages of Mesh Topology
1. The use of dedicated links guarantees that each connection can
carry its own data load, thus eliminating traffic problems that can
occur when links must be shared by multiple devices
2. A mesh topology is robust; That is, if one link becomes unusable,
it does not incapacitate the entire system
3. The 3rd advantage is privacy or security. When every message
travels along a dedicated line, only the intended recipient sees it.
Physical boundaries prevent other users from gaining access to
messages
4. Finally, the pt-to-pt links make fault identification and fault
isolation easy. Traffic can be routed to avoid links with suspected
problems. This enables the n/w manager to discover the location
of fault and aids in finding its cause & solution
 Disadvantages of Mesh Topology

• The major disadvantages of a mesh are related to the amount of

cabling and the no. of I/O ports required
1. Because each node must be connected to every other node,
installation and reconnection are difficult
2. The sheer bulk of the wiring can be greater than the available space
(in walls, ceilings, or floors) can accommodate
3. Finally, the H/W required to connect each link (I/O ports and
cable) can be prohibitively expensive
• For these reasons, a mesh topology is implemented in a limited
fashion, for example, as a backbone connecting the main
computers of a hybrid n/w that can include several other
topologies.
• A practical example is the connection of telephone regional office
to be connected to every other regional office
 Star Topology

HUB

Station A Station B Station C Station D

 Star Topology
• Each device has a dedicated point-to-point link only to a
central controller, usually called a hub.
• The controller acts as an exchange: If one device wants to
send data to another, it sends the data to the controller,
which then relays the data to the other connected device.
• Advantages:
– less expensive
– Easy installation and reconfiguration.
– Robustness
– fault isolation
• Drawbacks
– If the hub goes down, the whole system is dead
 Advantages of Star Topology

1. A star topology is less expensive than a mesh topology

2. In a star, each device needs only one link and one I/O port to
connect it to any no. of others. This factor makes it easy to
install and reconfigure
3. Less cabling required, as additions, moves and deletions
involve only one connection between that device & hub
4. A star topology is also robust; That is, if one link fails, only
that link is affected. All other links remain active
5. The factor mentioned above allows fault identification and
fault isolation can be done easier as long as the hub is
working
 Disadvantages of Star Topology
1. One big disadvantage of a star topology is the dependency
of the whole topology on one single point, the hub.
2. If the hub goes down, the whole system is dead
3. Although a star requires far less cable than a mesh, each
node must be linked to a central hub. Hence due to this,
often more cabling is required in a star than in some other
topologies (such as ring or bus)

Practically, the star topology is used in LANs. High-speed LANs

often use a star topology with a central hub
 Bus Topology

Station A Station B
Drop line Drop line

Tap Tap
Station C
 Bus Topology
• Multi-point connections.
• One long cable acts as a backbone to link all the devices in
a network.
• Nodes are connected to the bus cable by drop lines and taps.
• Advantages:
– ease of installation
– a bus uses less cabling than mesh or star topologies
• Drawbacks
– Difficulties in reconnection and fault isolation.
– a fault or break in the bus cable stops all transmission.
– Signal reflection at the taps can cause degradation in quality
 Advantages of Bus Topology

1. Ease of installation
2. Backbone cable can be laid along the most efficient path,
then connected to the nodes by drop lines of various lengths
In this manner, a bus uses less cabling than mesh or star
3. Consider this example: Four network devices in the same
room require 4 lengths of cable in a star topology reaching all the
way to the hub. In a bus, this redundancy is eliminated. Only the
Backbone cable stretches thro the entire facility. Each drop line
has to reach only as far as the nearest point on the backbone
 Disdvantages of Bus Topology

1. Difficult reconnection and fault isolation

2. A bus is usually designed to be optimally efficient at
installation. It can therefore be difficult to add new devices
3. Signal reflection at the taps can cause degradation in quality
4. Adding new devices may require replacement or
modification of the backbone
5. Also, a fault or break in the bus cable stops all transmission,
even b/w devices on the same side of the problem. The
damaged area reflects signals back in the direction of origin,
creating noise in both directions
Bus topologies were one of the 1st used in design of early LANs
and traditional Ethernet LANs can use a bus topology but they
are less popular now
 Ring Topology

Station A Station B
Repeater Repeater

Repeater Repeater

Station C Station D
 Ring Topology
• Each device has a dedicated point-to-point connection with
only the two devices on either side of it
• A signal is passed along the ring in one direction, from
device to device until it reaches its destination
• Each device in the ring incorporates a repeater
• When a device receives a signal intended for another device,
its repeater regenerates the bits and passes them along
• If one device does not receive a signal within a specified
period, it can issue an alarm. Network operator will address
that problem based on alarm
 Advantages of Ring Topology

1. A ring is relatively easy to install and reconfigure. Each device is

linked to only its immediate neighbours (either physically or
logically)
2. To add or delete a device requires only 2 connections. The only
constraints are media and traffic (max. ring length and no. of
devices)
3. Generally in a ring a signal is circulating at all times. fault isolation
is simplified
Disadvantages of Ring Topology
Unidirectional traffic can be a big disadvantage
In a simple ring, a break in the ring can disable the entire traffic.
To be solved using a dual ring or a switch capable of closing the break
 Networking devices
7. Application Layer Gateway

6. Presentation Layer
5. Session Layer
4. Transport Layer Switch

3. Network Layer Router

2. Data link Layer Bridge

1. Physical Layer Repeater

 Two layer switch
• Two layer switch operates at PHY and data
link layer
• Three layer switch operates at network layer
• Bridge is an example of two-layer switch.
• Bridge with few port can connect a few LANs
• Bridge with many port may be able to allocate
a unique port to each station, with each station
on its own independent entity.
 3-layer switches
• E.g. router.
• Routes packets based on their logical
addresses (host-to-host addressing)
• A router normally connects LANs and WANs
in the Internet and has a routing table that is
used for making decision about the route.
• The routing tables are normally dynamic and
are updated using routing protocols.
 Switches Categorization

• We must clarify the term Switch by adding the level at which the device
operates. We can have a two-layer switch or a three-layer switch.
• A three-layer switch is used at the network layer; it is a kind of router
• The two-layer switch performs at the physical and data link layers.
• A two-layer switch is a bridge, a bridge with many ports and a design that
allows better (faster) performance
• A bridge with a few ports can connect a few LANs together.
• A bridge with many ports may be able to allocate a unique port to each
station, with each station on its own independent entity. This means no
competing traffic (no collision)
 Two-layer / Three-layer Switches

• A two-layer switch, as a bridge does, makes a filtering decision based on

the MAC address of the frame it received
• However, a two-layer switch can be more sophisticated. It can have a
buffer to hold the frames for processing.
• Some new two-layer switches, called cut-through switches, have been
designed to forward the frame as soon as they check the MAC addresses
in the header of the frame.
• A router is a three-layer device that routes packets based on their logical
addresses (host-to-host addressing).
• A router normally connects LANs and WANs in the Internet and has a
routing table that is used for making decisions about the route
• The routing tables are normally dynamic and are updated using routing
protocols
 Hub
A hub is used as a central point of connection among media
segments. Cables from network devices plug in to the ports on
the hub.
Types of HUBS :
– A passive hub is just a connector. It connects the
wires coming from different branches.
– The signal pass through a passive hub without
regeneration or amplification.
– Connect several networking cables together
– Active hubs or Multiport repeaters- They regenerate
or amplify the signal before they are retransmitted.
 Repeater
• A repeater is a device that operates only at the PHYSICAL
layer.
• A repeater can be used to increase the length of the network
by eliminating the effect of attenuation on the signal.
• It connects two segments of the same network, overcoming
the distance limitations of the transmission media.
• A repeater forwards every frame; it has no filtering
capability.
• A repeater is a regenerator, not an amplifier.
• Repeaters can connect segments that have the same access
method. (CSMA/CD, Token Passing, Polling, etc.)
 Bridges
• Operates in both the PHYSICAL and the data link layer.
• As a PHYSICAL layer device, it regenerates the signal it receives.
• As a data link layer device, the bridge can check the
PHYSICAL/MAC addresses (source and destination) contained in
the frame.

• A bridge has a table used in filtering decisions.

• It can check the destination address of a frame and decide if the
frame should be forwarded or dropped.
• If the frame is to be forwarded, the decision must specify the port.
• A bridge has a table that maps address to ports.
• Limit or filter traffic keeping local traffic local yet allow
connectivity to other parts (segments).
 Gateway
• Connect two networks above the network layer of OSI
model.
• Are capable of converting data frames and network
protocols into the format needed by another network.
• Provide for translation services between different
computer protocols.
• Transport gateways make a connection between two
networks at the transport layer.
• Application gateways connect two parts of an
application in the application layer, e.g., sending email
between two machines using different mail formats
• Example: Broadband-modem-router
 Routers and Gateways
• A three-layer switch is a router, but a faster and more sophisticated. The
switching fabric in a three-layer switch allows faster table lookup and
forwarding
• A gateway is normally a computer that operates in all five layers of the
Internet or seven layers of OSI model.
• A gateway takes an application message, reads it, and interprets it. This
means that it can be used as a connecting device between two
internetworks that use different models.
• For example, a network designed to use the OSI model can be connected
to another network using the Internet model.
• The gateway connecting the two systems can take a frame as it arrives
from the first system, move it up to the OSI application layer, and remove
the message.
• Also gateway is used to filter unwanted application-layer messages as it
provides security.