Network topology types (Bus, Star, Ring, Mesh, Hybrid, Logical, Physical) | TechTerms


Today we will learn what is network topology,
its types, and how they differ from one another. Network topology is how computers connect/relate
to one another in a computer network. It is of two types: Physical topology
Logical topology Where physical topology describes the way
computers connect with the help of cables, logical topology describes the way data flows
from one computer to another within a computer network. The most common computer network used to interconnect
physically distributed computers is Local Area Network which has three main topologies:
Star Topolog Bus Topology, and
Ring Topology Others are: Mesh Topology, and
Hybrid Topology Please note, the data in the network layer is in the form IP packets. IP packets are packed into frames in the data
link layer. Physical layer then transmits the frames into
the transmission media. For example, networking cables. So in a computer network, the data moves from
one computer to another in the form of frames. The same term will be used in further discussion. Starting with Bus topology
In a bus topology, computers connect to a shared central cable, called a bus, with taps
and drop lines. Shared means that all connected computers
use the same cable for data frame transmission. Taps are the connectors, and drop lines are
the cables running in between the computers and the bus. In this topology, if a computer sends data
frames to a second computer, all other computers connected to the same central cable also receive
the frame, i.e. other computers can hear what the first computer is saying. However, only the target computer accepts
it; others reject the frame by checking the destination MAC address in the received frame. Suppose you are distributing prizes as per
the results of a competition. You asked Ram to come on the stage for his
reward. So, only Ram will go and collect the prize. Same is in the case of bus topology. Only that computer accepts data frame to whom
it is addressed to. Since bus topology requires less cabling,
so it is easy to install and less expensive to implement as compared to other topologies. However, with the increase in the length of
the central cable and increase in the count of taps, the strength of the signal decreases,
so only a limited number of computers can be connected in bus topology. In this topology, all computers depend on
the central cable for data transmission. So, if the central cable fails, it paralyzes
the whole network, i.e. bus topology has very little fault tolerance. Here, a security risk also exists because
all computers can hear what other computers are saying on the shared media. In a bus topology, only one computer can transmit
data at a time. So while one computer is sending a file to
a printer, other computers have to wait. If other computers too start sending data
at the same time, it will collide and corrupt the whole data. In such a case, data should be re-transmitted. Therefore, if more computers connect to the
same central cable, the network will become slower and also increase the chances of data
collisions. It is one of the reasons why bus topology
is rarely used in modern computer networks. The collisions can be avoided by using an
access control protocol. For eg. Carrier Sense Multiple Access with Collision
Detection (CSMA/CD). Example of bus topology is
Ethernet LANs Next is
Star topology In Star topology, the computers connect to
a central device, a switch or a hub, with point-to-point communication links. Point-to-Point connection means that there
is a dedicated link/cable between the two devices. Other devices can not use it. In this topology, if one computer wants to
send some data frame to another computer, it is first routed to the central device. The central device, then, either broadcast
or unicast the received data frame towards the destination computer based on the type
of central device used. Broadcast means the transmission of data to
all connected devices while unicast means transmission of data to the target device
only. If the central device is a hub, it broadcasts
the received frame to all of the connected computers, i.e. a hub is a multiport repeater. The frame has a destination MAC address which
is unique to every computer present in a network, so only the target computer accepts the frame,
others reject it. Since a hub broadcasts the received data,
so it increases unnecessary data traffic in the network. To overcome this limitation, a switch is used
as a central device. Every computer has a unique MAC address. A switch stores the MAC address of the devices
connected to its ports. It is called a switch table. Since the data frame it receives has a destination
MAC address, and the switch knows that the device with MAC address MAC 2 is connected
to port P2, so the switch forwards the received data frame to port P2 only. Hence, instead of broadcasting, the switch
unicasts the frame. Suppose your computer is connected to a printer
with a star topology network. Now, you click on PRINT on your computer to
print a file. The file is sent to the central device. If the central device is a hub, the file is
forwarded to all the connected computers. However, only the printer accepts it. On the other hand, if the central device is
a switch, the file is sent to the printer only, i.e. unicast. Moreover, when the computer is busy in sending
a file to the printer, computers A and B can also communicate with each other without affecting
the computer – printer link, i.e. switched connection allows simultaneous communication. On the other hand, a hub allows only one device
to communicate at a time. Please note if the destination MAC address
in the frame is Ethernet broadcast address, then the switch also broadcast the received
frame. In star topology,
Only one input/output port and one cable is needed for each device to connect to several
devices. It makes it less expensive than the mesh topology. It is also easy to reconfigure because we
can add or remove devices simply by connecting or disconnecting one cable. If one cable connecting to the central device
fails, only one communication link goes down and not the entire network. So star topology has good fault tolerance. Fault detection is also easy because we only
need to locate a computer which is not receiving data. However,
If the central device goes down, the whole network is paralyzed. One more limitation in star topology is The
number of computers in the network is limited by the number of input/output ports in the
central device.` Example of star topology
High-speed LANs. Now Ring topology
In a ring topology, each computer connects to two adjacent computers to form a ring. Data transmitted by one computer moves from
one computer to another in a circular fashion to reach its final destination. the advantages of ring topology are
Easy installation and less cabling, compared to mesh topology. In this topology, data moves in one direction,
it reduces the chances of data packet collisions. The ring topology is relatively easy to troubleshoot
because we only need to locate a computer which stops receiving data from its upstream
neighbour. However,
As each data frame has to pass all computers between the source and destination, it makes
data transmission slower than the star topology. Since all computers connect to form a closed
loop, one fault paralyzes the whole network. It is difficult to reconfigure because we
need to break the ring to add or remove the computers. Due to this reason, the physical ring topology
is rarely used, instead logical ring topology is used. Example of ring topology is
Token ring Token Ring does not use a physical ring topology. Instead, it uses a physical star and logical
ring topology in which the data moves in a circular fashion. Token ring uses a token-passing protocol,
where a frame, called token, keeps on circulating on the ring. If one computer has data frames to transmit,
it holds the token and transmits the frame. Once the transmission is done, the token is
released into the network. A token ring network is deterministic, meaning
each connected computer is given access for transmission at fixed time intervals. Therefore, a network can have one physical
topology and an entirely different logical topology at the same time. Mesh topology
In a fully connected mesh topology, each device has a point-to-point link to every device
in the network. Therefore,
· If the number of devices in a network is 4
· Then the number of links/cables each device have is 4-1, i.e. 3
Now, we have 4 devices, · Therefore, total number of links/cables
4 devices have is 4 (4-1), i.e,=12 Note that these are simplex links. In simplex links, data can move in one direction
only. So, one link is used for sending data, and
the other is used to receive data from the adjacent computer. Duplex links are the ones where data can move
in both directions. Therefore, we can replace two simplex links
with one duplex link. Hence, the total number of duplex links in
the mesh topology is 4(4-1)/2, i.e. 6. In general, if there are n devices, 
the total number of simplex links are n (n-1), the total number of duplex links are n (n-1)/2,
and the total number of input/output ports in
each device is n-1. So, if the total number of computers to be
connected is 10, then the total number of duplex links will be 45, and each device should
have 9 input/output ports, which would be difficult to manage and also increase the
cost. However, The dedicated point-to-point link
eliminates traffic problems which are encountered if a link is shared among several devices. · The dedicated point-to-point link maintains
privacy and security of the messages shared between two devices because other computers
cannot hear what the computer A is saying to computer B. · If one link fails, it does not affect the
whole network, i.e. good fault tolerance, · Since each device is connected to every
device, so installation is difficult. · Multiple input/output ports and a large
number of cables increase the cost and make it expensive. · More cables in mesh topology consume large
space too. Example of mesh topology is
The connection between regional telephone offices. All topologies are interconnected to form
a hybrid topology. Every topology has it’s merits and demerits,
so while choosing a physical topology for a network, we should always consider:  
It’s Cost Ease of installation  
Ease of maintenance, and Cable fault tolerance

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