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I DCCN-2023
NETWORK TOPOLOGIES
Unit Structure
Objectives
Introduction
1. An Overview of networking Devices
• Types of networking Devices
2. Overview of Network Topology
▪ The Technical Connotation of Topology
▪ What are the Basic Types of Topologies?
▪ How Is the Physical Topology Classified?
Summary
OBJECTIVES
➢
To understand various network strategies and topologies, you will:
➢
Examine three common strategies used to connect nodes on a
network.
➢
Explore network processing strategies and establish the
differences between centralized and distributed processing.
➢
Identify and compare three common network classifications.
➢
Identify and define three common network topologies.
INTRODUCTION
This chapter presents an outline on Network topology is the layout
pattern of interconnections of the various elements (links, nodes,
etc.) of a computer or biological network. Network topologies may be
physical or logical.
Physical topology refers to the physical design of a network including
the devices, location and cable installation. Logical topology refers
to how data is actually transferred in a network as opposed to its
physical design. In general physical topology relates to a core
network whereas logical topology relates to basic network. This
chapter also presents an insight into the various networking
strategies and the platform needed for networking.
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AN OVERVIEW OF NETWORK
A network consists of two or more computers that are linked
in order to share resources (such as printers and CDs), exchange
files, or allow electronic communications. The computers on a
network may be linked through cables, telephone lines, radio waves,
satellites, or infrared light beams.
Two very common types of networks include:
Local Area Network (LAN)
Wide Area Network (WAN)
Network devices: Definition and significance in networking
A network device is a piece of hardware or software integral to
communication between a computer and an internet network. Network
devices play two roles. The first is establishing a network connection, as a
router or a modem does. The second one is maintaining, protecting and
enhancing that connection, as with a hub, repeater, switch or gateway.
Functions of network devices
Networking devices serve the following general purposes:
• Facilitate data transmission and communication between devices
• Enable efficient and secure network connectivity
• Enhance network performance and optimize traffic flow
• Provide network security by enforcing access control and threat prevention
• Simplify network management and configuration
• Extend network coverage and overcome signal limitations
Types of network devices
Many types of network devices go into creating a network. Some are necessary for
connections, while others are enhancers. Below is a list of network devices that can
play a role in enabling your organization to transfer information as securely as
possible:
Bridges, hubs, NICs, modems (used for analog phone lines) and repeaters are the
earliest types of network devices; they were ubiquitous in the 1990s. DSL modems,
leased line routers, WAPs and firewalls became common in the early years of the third
millennium..
Switches
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A switch is a multiport device that improves network efficiency. Switches maintain
limited routing information about nodes in the internal network, allowing connections to
systems like hubs or routers. Strands of LANs are usually connected using switches.
Generally, switches read the hardware addresses of incoming packets to transmit them
to the appropriate destination.
Switches can be managed or unmanaged. Managed switches let users tweak settings
like LAN traffic and channel prioritization. Unmanaged switches are for basic
connection and do not allow users to control settings.
Bridges
Bridges connect two or more hosts or network segments by storing or hosting frames
between them. Bridges use hardware MAC addresses for transferring frames. They can
forward or block data with the MAC address of the devices connected to each segment.
They can also connect two physical LANs into a larger logical LAN.
Bridges can be set up using two models:
• Local bridging establishes LAN connections with local cables.
• Remote bridging brings two connections together via a wide area network
(WAN).
Bridges have mostly fallen out of favor in recent years, replaced by more functional
switches. In fact, switches are sometimes referred to as “multiport bridges” because of
how they operate.
Routers
The main job of a router is to direct traffic. Routers transfer packets to their destinations
by charting a path through interconnected networking devices using different network
topologies. Routers are intelligent devices that store information about their connected
networks. Most routers also operate as packet-filtering firewalls and use access control
lists (ACLs).
protocols: Routing Information Protocol (RIP), Border Gateway Protocol (BGP) or Open
Shortest Path First (OSPF).
Routers act as your first line of defense. You must configure them to pass only traffic
authorized by network administrators. The routers themselves are configured as static
or dynamic:
• Static routers can only be configured manually and stay that way until changed.
• Dynamic routers use information about nearby routers to build their routing
tables.
Firewalls
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A firewall restricts the internet traffic of a private network, controlling what goes in and
out. They analyze and restrict data packets based on programmed parameters, either
whitelists or blacklists. Whitelists only allow information that falls within a certain set of
parameters, while blacklists deny all information that falls inside the parameters.
Repeaters
A repeater is an electronic device that works at the physical layer of the OSI model to
amplify a received signal. It receives a signal and retransmits it at a higher level or
higher power. As a result, the signal covers longer distances, sometimes more than 100
meters for standard LAN cables.
Repeaters are useful for anyone working in a large facility where the Wi-Fi might be
spotty in the outer reaches of the space. Large office buildings, warehouses,
laboratories and campuses are all locations that can benefit from repeaters.
Gateways
Gateways connect networks operating on different protocols so data can transfer
between destinations. These devices normally work at the Transport and Session layers
of the OSI model. At the Transport layer and above, there are numerous protocols and
standards from different vendors, and gateways help deal with them.
Hubs
Hubs connect multiple computer networking devices, working only on the Physical layer
of the OSI. Hubs do not perform packet filtering or addressing functions. Instead, they
send data packets to all connected devices. A hub also acts as a repeater, amplifying
signals that deteriorate after traveling long distances over connecting cables. A hub is
the most straightforward network connecting device because it connects LAN
components with identical protocols.
There are two types of hubs:
• A simple hub has only one port for connecting a device to other networks.
• Multiple-port hubs allow users to connect many devices, and some expand
modularly.
There are also active and passive hubs. Active hubs boost the signal, as a repeater
would, while passive hubs have no boosting capabilities.
Modems
A modem (modulator-demodulator) converts digital signals into analog signals of
different frequencies and transmits them to a modem at the receiving location. The
receiving modem performs the reverse transformation and provides a digital output to a
device connected to a modem, usually a computer. The digital data is usually
transferred to or from the modem over a serial line through an industry-standard
interface, RS-232.
There are three main types of modems:
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• A DSL modem uses telephone cables and is considered the slowest connection.
• A cable modem transmits information over TV lines and is faster than DSL.
• A wireless modem transfers information between the local network and an
internet service provider; it is the fastest transmitter.
Network interface cards (NICs)
A network interface card is an internal hardware chip that connects a device to the
internet. At the TCP/IP layer, the NIC connects a device to a network. At the physical
layer, the NIC transmits a signal that sends information to the network layer. Then all
data passes through the NIC to the server and back to the device.
There are two main types of NICs:
• An Ethernet NIC comes with an 8P8C socket for connecting an ethernet cable.
• A Wi-Fi NIC connects to a wireless network.
Mobile devices have only a wireless NIC, but most computers still incorporate an
Ethernet chip. Ethernet ports are more reliable but limit a user’s mobility while handling
the device.
Wireless access points (WAPs)
A wireless access point consists of a transceiver (transmitter and receiver) device used
to create a wireless LAN (WLAN). WAPs are separate network devices with a built-in
antenna, transmitter and adapter. WAPs use the wireless infrastructure network mode
to provide a connection point between WLANs and a wired ethernet LAN. They also
have several ports, allowing you to expand the network to support additional clients.
A service set identifier (SSID) name is necessary to connect to a wireless network. The
SID is used to identify all systems belonging to the same network, and client stations
must be configured with the SSID to be authenticated to the WAP.
WHY NETWORKING THESE DEVICES?
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Information storing and sharing.
Computers allow users to create and manipulate information.
Information takes on a life of its own on a network. The
network provides both a place to store the information and
mechanisms to share that information with other network
users.
Connections.
Administrators, instructors, and even students and guests can
be connected using the campus network.
Services.
The institution can provide services, such as registration,
college directories, course schedules, access to research,
and email accounts, and many others. (Remember, network
services are generally provided by servers).
Internet.
The institution can provide network users with access to the
internet, via an internet gateway.
Computing resources.
The institution can provide access to special purpose
computing devices which individual users would not normally
own. For example, an institution network might have high-
speed high quality printers strategically located around a
campus for instructor or student use.
Flexible Access.
Institution networks allow students to access their information
from connected devices throughout the school. Students can
begin an assignment in their classroom, save part of it on a
public access area of the network, then go to the media center
after school to finish their work. Students can also work
cooperatively through the network.
Workgroup Computing.
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Collaborative software allows many users to work on a
document or project concurrently. For example, educators
located at various institution within a county could
simultaneously contribute their ideas about new curriculum
standards to the same document, spreadsheets, or website.
Disadvantages of Installing a Network
Expensive to Install.
Large campus networks can carry hefty price tags. Cabling,
network cards, routers, bridges, firewalls, wireless access
points, and software can get expensive, and the installation
would certainly require the services of technicians. But, with
the ease of setup of home networks, a simple network with
internet access can be setup for a small campus in an
afternoon.
Requires Administrative Time.
Proper maintenance of a network requires considerable time
and expertise. Many schools have installed a network, only to
find that they did not budget for the necessary administrative
support.
Servers Fail.
Although a network server is no more susceptible to failure
than any other computer, when the files server "goes down"
the entire network may come to a halt. Good network design
practices say that critical network services (provided by
servers) should be redundant on the network whenever
possible.
Cables May Break.
The Topology chapter presents information about the various
configurations of cables. Some of the configurations are
designed to minimize the inconvenience of a broken cable;
with other configurations, one broken cable can stop the entire
network.
Security and compliance.
Network security is expensive. It is also very important. An
institution network would possibly be subject to more
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stringent security requirements than a similarly-sized
corporate network, because of its likelihood of storing
personal and confidential information of network users, the
danger of which can be compounded if any network users are
minors. A great deal of attention must be paid to network
services to ensure all network content is appropriate for the
network community it serves.
WHAT IS A TOPOLOGY?
A topology is a description of the layout of a specific region or
area. A network topology is a description of the layout of the region
or area covered by that network.
There are two types of connections that describe how many
devices connect to a single cable or segment of transmission media.
They are: point-to-point and multi-point.
Point-to-point connections provide a direct link between two
devices; for example, a computer connected directly to a printer, or
a modem to a mainframe.
Multi-point connections provide a link between three or more
devices on a network. All computer networks rely upon point-to-point
and multi-point connections.
The Technical Connotation of Topology
The virtual shape or structure of a network is referred as topology.
The pattern or layout of interconnections of different elements or
nodes of a computer network is a network topology that might be
logical or physical.
However, the complete physical structure of the cable (or
transmission media) is called the physical topology. The physical
topology of a network refers to the configuration of cables,
computers, and other peripherals.
The way data flows through the network (or transmission
media) is called the logical topology. A logical topology is the method
used to pass information between workstations.
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What are the Basic Types of Topologies?
There are seven basic topologies in the study of network topology:
1. Point-to-point topology,
2. Bus (point-to-multipoint) topology,
3. Ring topology,
4. Star topology,
5. Hybrid topology,
6. Mesh topology and
7. Tree topology.
The interconnections between computers whether logical or physical
are the foundation of this classification.
Logical topology is the way a computer in a given network transmits
information, not the way it looks or connected, along with the varying
speeds of cables used from one network to another.
On the other hand the physical topology is affected by a number of
factors:
Troubleshooting technique,
Installation cost,
Office layout and
Cables‘ types.
The physical topology is figured out on the basis of a
network‘s capability to access media and devices, the fault tolerance
desired and the cost of telecommunications circuits.
The classification of networks by the virtue of their physical span is
as follows: Local Area Networks (LAN), Wide Area Internetworks
(WAN) and Metropolitan Area Networks or campus or building
internetworks.
How Is the Physical Topology Classified?
Point-to-Point Network Topology
It is the basic model of typical telephony. The simplest
topology is a permanent connection between two points. The value
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of a demanding point-to-point network is proportionate to the number
of subscribers ‘potential pairs. It is possible to establish a permanent
circuit within many switched telecommunication systems: the
telephone present in a lobby would always connect to the same port,
no matter what number is being dialed. A switch connection would
save the cost between two points where the resources could be
released when no longer required.
Bus Network Topology
LANs that make use of bus topology connects each node to a
single cable. Some connector connects each computer or server to
the bus cable. For avoiding the bouncing of signal a terminator is
used at each end of the bus cable. The source transmits a signal that
travels in both directions and passes all machines unless it finds the
system with IP address, the intended recipient. The data is ignored
in case the address is unmatched.
o Linear Bus: If all network nodes are connected to a combine
transmission medium that has two endpoints the Bus is Linear. The
data transmitted between these nodes is transmitted over the
combine medium and received by all nodes simultaneously.
o Distributed Bus: If all network nodes are connected to a combine
transmission medium that has more than two endpoints created by
branching the main section of the transmitting medium.
A linear bus topology consists of a main run of cable with a
terminator at each end (See fig. 1). All nodes (file server,
workstations, and peripherals) are connected to the linear cable. A
bus topology uses one long cable (backbone) to which network
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devices are either directly attached or are attached by using short
drop cables. Because all workstations share this bus, a workstation
checks for any information that might be coming down the backbone
before sending their messages. All messages pass the other
workstations on the way to their destinations
Fig. 1. Linear Bus topology
Advantages of a Linear Bus Topology
Easy to connect a computer or peripheral to a linear bus.
Requires less cable length than a star topology.
Disadvantages of a Linear Bus Topology
Entire network shuts down if there is a break in the main
cable.
Terminators are required at both ends of the backbone
cable.
Difficult to identify the problem if the entire network shuts
down.
Not meant to be used as a stand-alone solution in a large
building.
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Star Network Topology
The topology when each network host is connected to a
central hub in LAN is called Star. Each node is connected to the hub
with a point-to-point connection. All traffic passes through the hub
that serves as a repeater or signal booster. The easiest topology to
install is hailed for its simplicity to add more nodes but criticized for
making hub the single point of failure.
o Extended Star: A network that keeps one or more than one
repeaters between the central node or hub and the peripheral or the
spoke node, supported by the transmitter power of the hub and
beyond that supported by the standard of the physical layer of the
network.
o Distributed Star: The topology is based on the linear connectivity
that is Daisy Chained with no top or centre level connection points.
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.
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Disadvantages of a Star Topology
Requires more cable length than a linear 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.
Ring Network Topology
Ring topology is one of the old ways of building computer
network design and it is pretty much obsolete. FDDI, SONET or
Token Ring technologies are used to build ring technology. It is not
widely popular in terms of usability but incase if you find it anywhere
it will mostly be in schools or office buildings.
Such physical setting sets up nodes in a circular manner
where the data could travel in one direction where each device on
the right serves as a repeater to strengthen the signal as it moves
ahead.
Mesh Network Topology
The exponent of the number of subscribers is proportionate to the
value of the fully meshed networks.
o Fully Connected: For practical networks such topology is too
complex and costly but highly recommended for small number of
interconnected nodes.
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o Partially Connected: This set up involves the connection of
some nodes to more than one nodes in the network via point-to-point
link. In such connection it is possible to take advantage of the
redundancy without any complexity or expense of establishing a
connection between each node.
Hybrid Topology
Hybrid topologies are a combination of two or more different
topologies. WANs sometimes have hybrid topologies because they
connect a variety of LAN topologies. The big advantage of hybrid
topologies is that they connect disparate topologies. However, the
disadvantage of hybrid topologies is that they are potentially complex
to establish and manage.
Tree Network Topology
The top level of the hierarchy, the central root node is connected to
some nodes that are a level low in the hierarchy by a point-to-point
link where the second level nodes that are already connected to
central root would be connected to the nodes in the third level by a
point-to-point link. The central root would be the only node having no
higher node in the hierarchy. The tree hierarchy is symmetrical.
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Advantages of a Tree Topology
Point-to-point wiring for individual segments.
Supported by several hardware and software venders.
Disadvantages of a Tree Topology
Overall length of each segment is limited by the type of cabling
used.
If the backbone line breaks, the entire segment goes down.
More difficult to configure and wire than other topologies.
Considerations When Choosing a Topology
Money. A linear bus network may be the least expensive way
to install a network; you do not have to purchase
concentrators.
Length of cable needed. The linear bus network uses shorter
lengths of cable.
Future growth. With a star topology, expanding a network is
easily done by adding another concentrator.
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Cable type. The most common cable in schools is unshielded
twisted pair,
11.7 SUMMARY
Knowledge of networking topologies is of core importance of
computer networking design. Computer networks can only be
developed using the knowledge about these topologies and
decide to which topology design is best suited according to
the requirement.
A computer network consists of nodes and communication
links whichimplement its protocols. It interconnects a set of
hosts which conform to thenetwork protocols.
A network may be classified as a LAN, MAN, or WAN,
depending on itsgeographic spread, and as private or public,
depending on its accessrestrictions.
It may employ a point-to-point or a broadcast
communication model. A point-to-point model may be based
on circuit switching or packetswitching.
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