This document provides information about network topologies and media access methods. It begins by defining a network topology as the physical layout of computers on a network. The four primary network topologies are then described as star, bus, ring, and mesh. Communication methods for bus, ring, and star topologies are explained. Hybrid topologies that combine elements of different primary topologies are also introduced. The document concludes by discussing two common media access methods: CSMA/CD and CSMA/CA.

1. UNIT OF COMPETENCE
DETERMINE BEST-FIT
TOPOLOGY
Learning Outcomes
 Identify key information source
 Determine user needs
 Develop best topology

2. Determine Best-Fit Topology
Unit Descriptor
 This unit defines the competence required to
determine the most appropriate way of
networking computers to meet user needs and
business requirements.

3. LO1: Identify key information
source
3
Introduction to Network Topologies
 The term topology, or more specifically, network topology,
refers to the arrangement or physical layout of computers,
cables, and other components on the network.
 “Topology” is the standard term that most network
professionals use when they refer to the network’s basic
design
Types of Network Topology:
 Physical Topology: is A network topology that describes the
physical layout of the wire and devices as well as the paths used
by data transmissions
 Logical Topology: are describe how information or data are
transmitted or moved through the physical topology across the
network.

4. Introduction to Network
Topology
4
There are four types of standard Physical
topologies, such as
1. Bus
2. Star
3. Ring
4. Mesh

5. Bus Topology
5

6. Bus Topology
6
What is Bus Topology?
 It is often referred to as a “linear bus” because the
computers are connected in a straight line. This is the
simplest and most common method of networking
computers.
 In a bus topology, all computers are connected to a shared
communication line, called a trunk or a backbone.
 The computers are connected to the backbone using
T-connectors.
 Both ends of the backbone use terminators in order to
prevent reflection of signals or bounced signal.
 If the terminator is missing or is deliberately removed, the
data transmissions are disrupted.
 There is no central device or any special configuration.
 A bus topology is normally implemented with coaxial cable.

7. Bus Topology
7
Communication on the Bus
 In a bus topology, signals are broadcast to all stations. Each
computer checks the address on the signal (data frame) as it passes
along the bus. If the signal’s address matches that of the computer,
the computer processes the signal. If the address doesn’t match, the
computer takes no action and the signal travels on down the bus.
 Only one computer can ‘talk’ on a network at a time. A media
access method (protocol) called CSMA/CD is used to handle the
collisions that occur when two signals are placed on the wire at the
same time.
 The bus topology is passive topology. In other words, the computers
on the bus simply ‘listen’ for a signal; they are not responsible for
moving the signal along.

8. Bus Topology
8
Network Expansion in bus topology
 As the physical size of the site grows, the network will need to grow
as well. Cable in the bus topology can be extended by one of the
two following methods
 A component called a barrel connector can connect two pieces of
cable together to make a longer piece of cable. However,
connectors weaken the signal and should be used carefully.
 One continuous cable is preferable to connecting several smaller
ones with connectors. Using too many connectors can prevent the
signal from being correctly received.

9. Bus Topology
9
Network Expansion in bus topology…
 A device called a repeater can be used to connect two
cables.
 A repeater actually boost the signal before it sends the
signal on its way.
 The following figure shows a repeater boosting a weakened
signal. A repeater is better than a connector or a longer
piece of cable because it allows a signal to travel farther
and still be correctly received.

10. Bus Topology
10
Advantages of Bus Topology
 Use of cable is economical.
 Media is inexpensive and easy to work with.
 System is simple and reliable.
 Bus is easy to extend.
 Doesn't require a hub
Disadvantages of Bus Topology
 Network can slow down in heavy traffic.
 Problems are difficult to isolate.
 Cable break can affect many users.
 Increasing the number of computer will slow down the
performance of the network
 Less fault tolerant

11. Star Topology
11

12. Star Topology
12
What is Star Topology?
 In the star topology, cable segments from each computer
are connected to a centralized component called a
hub.
 The HUB offers a common connection for all stations on
the network. Each station has its own direct cable
connection to the hub. In most cases, this means more
cable is required than for a bus topology. However, this
makes adding or moving computers a relatively easy
task; simply plug them into a cable outlet on the wall.

13. Star Topology
13
Continued…
 If a cable is cut, it only affects the computer that
was attached to it. This eliminates the single point
of failure problem associated with the bus
topology. (Unless, of course, the hub itself goes
down.)
 Star topologies are normally implemented using
twisted pair cable, specifically unshielded twisted
pair (UTP). The star topology is probably the most
common form of network topology currently in
use.

14. Network Topology
14
Advantages of a Star Topology
 Easy to install and wire.
 No disruptions to the network when connecting or
removing devices.
 Easy to detect faults and to remove parts.
Disadvantages of a Star Topology
 Requires more cable length than a Bus topology.
 If the hub, switch, or concentrator fails, nodes
attached are disabled.
 More expensive than linear bus topologies because
of the cost of the hubs, etc.

15. Ring Topology
15

16. Ring Topology
16
What is Ring Topology ?
 The ring topology connects computers on a single
circle of cable.
 Data travels in the ring in a circular fashion from one
computer to another, forming a logical ring.
 Unlike the bus topology, there are no terminated ends;
the signal travels around the circle in a clockwise (or
anticlockwise) direction.
 The signals travel around the loop in one direction and
pass through each computer, which can act as a
repeater to boost the signal and send it on to the next
computer. This made ring topology to be called active
topology.

17. Ring Topology
17
Continued…
 Note that while this topology functions logically as
ring, it is physically wired as a star.
 The central connector is not called a hub but a
Multistation Access Unit or MAU.
 Rings are normally implemented using twisted pair
or fiber-optic cable.

18. Ring Topology
18
Communication on the Ring Topology
 Under the ring concept, a signal is transferred
sequentially via a "token" from one station to the next.
 When a station wants to transmit, it "grabs" the token,
attaches data and an address to it, and then sends it
around the ring. The token travels along the ring until it
reaches the destination address.
 The receiving computer acknowledges receipt with a
return message to the sender. The sender then releases
the token for use by another computer.
 Each station on the ring has equal access but only one
station can talk at a time.

19. Ring Topology
19
Advantages of Ring Topology
 This type of network topology is very
organized. Each node gets to send the data
when it receives an empty token. This helps
to reduces chances of collision. Also in ring
topology all the traffic flows in only
one direction at very high speed.
 Growth of system has minimal impact on
performance
 All stations have equal access.
 Additional components do not affect the

20. Ring Topology
20
Disadvantages of Ring Topology
 Each packet of data must pass through all the
computers between source and destination. This makes
it slower than Star topology.
 If one workstation or port goes down, the entire network
gets affected.
 Network is highly dependent on the wire which connects
different components.
 MAU’s and network cards are expensive as compared to
Ethernet cards and hubs.

21. Mesh Topology
21
Mesh Topology

22. Mesh Topology
22
What is Mesh Topology?
 A mesh topology network offers superior redundancy and
reliability.
 In a mesh topology, each computer is connected to every
other computer by separate cabling.
 This configuration provides redundant paths throughout the
network so that if one cable fails, another will take over the
traffic.
 While ease of troubleshooting and increased reliability are
definite pluses, these networks are expensive to install
because they use a lot of cabling.
 You can calculate the number of cables used
in mesh topology using the following formula:
 No of Cable= n (n-1)/2, where n is no of
computer used in mesh topology.

23. Mesh Topology
23
Advantages of Mesh topology
 Data can be transmitted from different devices
simultaneously.
 This topology can withstand high traffic.
 Even if one of the components fails there is always
an alternative present. So data transfer doesn’t get
affected.
 Expansion and modification in topology can be done
without disrupting other nodes.

24. Mesh Topology
24
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 very
complex.

25. Hybrid Topology
25
Hybrid Topology
 It is combination of any two or
more network topologies.
 These instances can occur
where two basic network
topologies, when connected
together, can still retain the
basic network character, and
therefore not be a hybrid
network.
 Star-Bus
 Star-Ring

26. Hybrid Topology
26
Star Bus
 The star bus is a combination
of the bus and star topologies.
 In a star-bus topology, several
star topology networks are
linked together with linear bus
trunks.
 If one computer goes down, it
will not affect the rest of the
network. The other computers
can continue to communicate.
If a hub goes down, all
computers on that hub are
unable to communicate. If a
hub is linked to other hubs,
those connections will be

27. Hybrid Topology
27
Star Ring
 The star ring (sometimes
called a star-wired ring)
appears similar to the star
bus. Both the star ring
and the star bus are
centered in a hub that
contains the actual ring or
bus.
 Linear-bus trunks connect
the hubs in a star bus,
while the hubs in a star
ring are connected in a
star pattern by the main
hub.

28. Choosing network topology
 A network's topology affects its capabilities.
 The choice of one topology over another will
have an impact on the:
 Type of equipment the network needs.
 Capabilities of the equipment.
 Growth of the network.
 Way the network is managed.

29. Summary on Network
Topologies
 The physical layout of computers on a network is called a
topology.
 There are four primary topologies: star, bus, ring, and mesh.
 Topologies can be physical (actual wiring) or logical (the way
they work).
 In a bus topology, the computers are connected in a linear
fashion on a single cable.
 Bus topologies require a terminator on each end of the cable.
 In a star topology, the computers are connected to a
centralized hub.
 Mesh topologies connect all computers in a network to one
another with separate cables.
 In a token-ring topology, the computers are connected
physically in a star shape, but logically in a ring or circle. The
data is passed from one computer to another around the circle.
 Hubs are used to centralize the data traffic and localize failures.
If one cable breaks, it will not shut down the entire network.

30. Summary on Network
Topologies
30
Topolog
y
Advantages Disadvantages
Bus Use of cable is economical.
Media is inexpensive and easy to work
with.
System is simple and reliable.
Bus is easy to extend.
Network can slow down in
heavy traffic. Problems are
difficult to isolate.
Cable break can affect
many users.
Ring System provides equal access for all
computers. Performance is even despite
many users.
Failure of one computer
can impact the rest of the
network. Problems are hard
to isolate.
Network reconfiguration
disrupts operation.
Star Modifying system and adding new
computers is easy. Centralized
monitoring and management are
possible.
Failure of one computer does not affect
the rest of the network.
If the centralized point
fails, the network fails.

31. Self check Questions on network topology
31
1) Physical or logical arrangement of network is
a) Topology b) Routing c) Networking d) None
2) In this topology there is a central controller or
hub
a) Star b) Mesh c) Ring d) Bus
3) This topology requires multipoint connection
a) Star b) Mesh c) Ring d) Bus
4) Data communication system spanning states,
countries, or the whole world is
a) LAN b) WAN c) MAN d) None
5. Calculate the number of cables used for 11
computers in mesh topology ?
A. 110 B. 55 C. 50 D. 11

32. Self check Questions on network topology
32
Exercise 1.1: Case Study Problem
 A small, independent, business/home/life insurance company
consisting of an owner, a business manager, an administrator,
and four agents decides to implement a network. The company
occupies half of a small building in an office park. Their volume
of business had been stable for the past three years, but
recently it has been increasing. To handle the increased
business volume, two new agents will be hired.
The Following Figure illustrates the current arrangement.
Figure 1 Case study model

33. Self check Questions on network
topology
Continued…
 Everyone in the company has a computer, but the business manager has
the only printer. These computers are not connected by any form of
networking. When agents need to print a document, they must first copy the
file to a flash disk, then carry it to the business manager's computer, where
they are finally able to print it. Similarly, when staff members want to share
data, the only means available is to copy the data on one computer to a
flash disk and insert the disk in another computer.
 Recently, problems have arisen. The business manager is spending too
much time printing other people's documents; and it is frequently unclear
which copy of a given document is the current and authoritative version.
 Your task is to design a network for this company. To clarify the task of
choosing a solution, you ask some questions. Circle the most appropriate
answers to the following questions:
 Which type of network would you suggest for this company?
 Peer-to-peer
 Server-based
 Which network topology would be most appropriate in this situation?
Bus Ring Star Mesh Star bus Star ring

34. LO1: Identify key information
source
Overview of Media Access Methods
 A media access method refers to the manner in which a
computer gains and controls access to the network’s
physical medium (e.g., cable).
 Common media access methods include the following:
 CSMA/CD
 CSMA/CA
 Token Passing
 Demand priority
 One of the primary concerns with media access is how to
prevent packets from colliding when two or more
computers try to transmit simultaneously.
 If two stations are transmitting at the same time, it is
possible that the signals may overlap, resulting in garbled
data. Such overlapping is referred to as a "collision."

35. CSMA/CD
 CSMA/CD stands for Carrier-Sense Multiple Access with
Collision Detection. It is a media access method which
means it defines how the network places data on the cable
and how it takes it off.
 CSMA/CD specifies how bus topologies such as Ethernet
handle transmission collisions. A collision occurs when two
or more computers transmit signals at the same time.
 "Carrier Sense" means that each computer on the LAN continually
listens to the cable for the presence of a signal prior to transmitting.
 "Multiple Access" means that there are many computers attempting
to transmit and compete for the opportunity to send data (i.e., they are
in contention).
 "Collision Detection" means that when a collision is detected, the
stations will stop transmitting and wait a random length of time before
retransmitting.
 Because CSMA/CD works to control or manage collisions
rather than prevent them, network performance can be

36. CSMA/CA
 CSMA/CA stands for Carrier-Sense Multiple Access
with Collision Avoidance and is a media access
method very similar to CSMA/CD.
 The difference is that the CD (collision detection) is
changed to CA (collision avoidance).
 Instead of detecting and reacting to collisions,
CSMA/CA tries to avoid them by having each
computer signal its intention to transmit before
actually transmitting. In effect, the transmitting
computer gives a 'heads up' prior to transmitting.
 Although CSMA/CA can prevent collisions, it comes
with a cost in the form of the additional overhead
incurred by having each workstation broadcast it's
intention prior to transmitting. Thus, CSMA/CA is
slower than CSMA/CD.

37. TOKEN PASSING
 Token passing is a media access method
by which collisions are prevented.
 Collisions are eliminated under token
passing because only a computer that
possesses a free token (a small data frame)
is allowed to transmit.
 The token passing method also allows
different priorities to be assigned to different
stations on the ring. Transmissions from a
stations with higher priority take precedence
over stations with lower priority.
 Token passing is used on Token Ring
networks

38. Demand Priority
 Demand priority is a media-access method works with
a star topology.
 In this method, a node that wishes to transmit indicates
this wish to the hub and also requests high- or regular-
priority service for its transmission. After it obtains
permission, the node begins transmitting to the hub.
 The hub is responsible for passing the transmission on
to the destination node; that is, the hub is responsible
for providing access to the network.
 By letting the hub manage access, the architecture is
able to guarantee required bandwidths and requested
service priority to particular applications or nodes.
 Demand priority is a media-access method used in a
100 megabit per second (Mbit/s) 100 Ethernet
implementation

39. 1. Which Media Access Method works with Star
Topology?
A. CSMA/CD B. Token Passing C.
Demand Priority D. A and B
2. One of the primary concerns with media
access is
A.
What is the primery concerns with media
access method?

40. LAN Architectures
 Network architecture refers to the manner in
which the hardware and software is structured.
The architecture includes the cable access
method (transmission), topology, and lower level
protocols.
 The most common types of LAN architectures
are
 Ethernet(Fast Ethernet, Gigabit Ethernet) and
 Token Ring

41. Ethernet (Fast Ethernet, Gigabit
Ethernet)
 Ethernet is a popular, relatively inexpensive, easy-to-install LAN
architecture with the following characteristics:
 Uses the CSMA/CD media access control.
 Data transmission normally occurs at 100 Mbps (10Mbps in the early forms
and 10Gbps in the most recent forms).
 Typically implemented in a star topology (early versions used bus topology
as well).
 Ethernet LANs are normally distinguished by the type of cable they use
Twisted Pair (Thinnet or Thicknet were also used in the past).
 An Ethernet LAN is often described in terms of three parameters:
transmission rate, transmission type, and segment distance or cable
type.
 "100baseT" means:
 100 - transmission rate or through put of 100Mbps
 base - transmission type is baseband rather than broadband network (i.e., the
signal is placed directly on the cable, one signal at a time)
 T – the cable type (e.g. Twisted pair)
 Few types of Ethernet: 10Base2, 10Base5, 10BaseT and 10BaseF,
Few types of Fast Ethernet 100BaseT, 100BaseF, Few types of Gigabit
Ethernet 1000BaseT, 1000BaseF etc..

42. Token Ring
 Token ring is a relatively expensive LAN
architecture that is strongly influenced by IBM. It is
very stable and can be expanded without a
significant degradation in network performance.
 Token ring uses the token passing media access
control. Data transmission normally occurs at 4 or
16 Mbps depending on the cable.
 Token ring is normally implemented in a logical
ring/physical star topology with a MAU (Multistation
Access Unit) as the hub. The maximum number of
stations on one ring is 260 for shielded twisted pair
and 72 for unshielded twisted pair (UTP). There can
be up to 33 MAUs per ring.
 Token Ring LANs normally use shielded twisted pair
(STP) but may also use unshielded twisted pair
(UTP) or fiber-optic cable. The maximum distance to
the MAU from the workstation depends on the cable
and varies from 45 meters for UTP to 100 meters for
STP.

43. Internetworking Devices
43
Hub
Repeater
Bridge
Switch
Router
Internetworking Devices and their symbol

44. Internetworking Devices
Why Interconnect?
 To separate / connect one corporate
division with another.
 To connect two LANs with different
protocols.
 To connect a LAN to the Internet.
 To provide a security wall between two
different types of users.
 To connect WLAN to LAN

45. Repeater
 Repeaters amplify a weak signal so that the signal stays
as strong as the original one.
 Used when a network’s cabling extends beyond its
capability.

46. Hub
 A hub interconnects two or more workstations
into a local area network.
The Cloud

47. Hub
 When a workstation transmits to a hub, the hub
immediately resends the data frame to all
connecting links. This will increase network
traffic.
 To overcome this drawback of hubs, Switches
were introduced.
 Hubs expand one Ethernet connection into many.
For example, a four-port hub connects up to four
machines
 Simply a multi-port repeater.

48. Types of Hub
 Passive Hub
 Passive hubs used in a LAN environment; special form of repeater
that allow multiple devices to be wired into a central location and
share the same media; do not regenerate (repeat) the signal
 Active Hub
 An Ethernet hub, active hub, network hub, repeater hub, multiport
repeater or hub is a device for connecting multiple Ethernet
devices together and making them act as a single network
segment. ...
 A central connecting device in a network that regenerates signals
on the output side to keep the signal strong. Also called a
"multiport repeater.
 And it is more intelligent than passive Hub

49. HUB
 Hubs inexpensive devices. However, hubs can
considerably increase network traffic because
they broad cost data to the device connected
on all the ports resulting in increase network
traffic.
 To overcome this drawback of hubs, Switches
were introduced.
CP2073 Networking
49