Computer Network - NetworkTopology

Manoj KumarM.Sc.(Computer Science)

Saturday, January 24, 2015 1

A network as a "group of computers and associated devices

that are connected by communications facilities.“

A network provides two principle benefits: the ability to

communicate and the ability to share.

A network supports communication among users in ways that other mediacannot. E-mail, the most popular form of network communication,provides low-cost, printable correspondence with the capability forforwarding, acknowledgment, storage, retrieval, and attachments.

Sharing involves not only information (database records, e-mail, graphics,etc.), but also resources (applications, printers, modems, disk space,scanners, etc.) Through its ability to share, a network promotescollaboration


A network can consist of two computers connectedtogether on a desk or it can consist of many LocalArea Networks (LANs) connected together to form aWide Area Network (WAN) across a continent.


A topology refers to the manner in which the cable is

run to individual workstations on the network.

Two or more devices connect to a link; two or more

links are forms of topology.

The topology of network is the geometricalrepresentation of the relationship of all the links andlinking devices (usually called nodes) one to another.

A topology is concerned with the physicalarrangement of the network components


TOPOLOGY

MESH TOPOLOGY

STAR TOPOLOGY

BUS TOPOLOGY

RING TOPOLOGY

TREE TOPOLOGY

HYBRID TOPOLOGY


In mesh topology, every device has a dedicated pointto point link to every other device. The term dedicatedmeans that the link carries traffic only between thetwo devices it connects.

STATION 1

STATION 2

STATION 3 STATION 4

STATION 5

Fig. : MESH TOPOLOGY


To find the number of physical links in a fullyconnected mesh network with n nodes, we firstconsider that each node must be connected to n-1nodes and finally n must be connected to n-1 nodes.We need n(n-1) physical links. However, if each linkallows communication in both directions, we candivide the number of links by 2 . We need n(n-1)/2duplex node links.


1. The use of dedicated links guarantees that each connectioncan carry its own data load, thus eliminating the trafficproblems.

2. Mesh topology is robust ;if one link becomes unusable itdoes not incapacitate the entire system.

3. There is advantage of privacy or security.

4. There is no need to provide any additional information , i.e.;source address, along with the packet because each nodesare connected to each other.

5. Point to point links make fault identification and faultisolation easy.Saturday, January 24, 2015 8

1. Every device must be connected to every other device.

2. installation and reconnection are difficult.

3. The shear bulk of wiring can be greater than the available space (in walls, ceilings or floors) can accommodate.

4. Not flexible ;

5. Poor expandability ;i.e., to add a new node in network is very cost effective.


A mesh topology is the connection of telephoneregional offices in which each regional office needsto be connected to every other regional office.


In start topology, each device has a dedicated pointto point link only to a central controller, usually calledhub.

The devices are not directly linked to one another.

HUB

STATION

STATION

STATION

STATION

Fig. : STAR TOPOLOGY


A star topology does not allow direct traffic betweendevices.

The controller acts as an exchange : if one devicewants to send data to another, it sends the data to thecontroller , which then relays the data to the otherconnected device.


1. Less expensive than mesh topology, due to less cablingand ports.

2. It is easy to install and reconfigure because each deviceneeds only one link and one I/O port to connect it toany number of other .

3. Provide high speed and great maintainability

4. Robustness: if one link fails, only that link is affectedother link remain active.


1. One big disadvantage of star topology is thedependency of whole topology on one single point,the hub.

2. If the hub goes down, the whole system is dead.

3. Often more cabling is required in a star topologythan in some other topologies such as ring or bustopology.


Star topology is used in local-area networks(LANs), high-speed LANs often use a star topologywith a central hub.


A bus topology is multipoint. One long cable acts as a backbone to link all the devices in a network.

Nodes are connected to the bus cable by drops lines and taps.

Station Station

Station station

Cable end Cable end

Fig, :Bus topologySaturday, January 24, 2015 16

A drop line is a connection running between the device and the maincable.

A tap is a connector that either splices into the main cable orpunctures the sheathing of a cable to create a contact with the metalliccore.

Full-duplex operation between the station and the tap allows data tobe transmitted onto the bus and received from the bus.

At each end of the bus there is a terminator, which absorbs anysignal, preventing reflection of signal from the endpoints. If theterminator is not present, the endpoint acts like a mirror and reflectsthe signal back causing interference and other problems.

Each station has a unique address in the network, a station copies apacket only when the destination address of the packet matches withthe self-address.Saturday, January 24, 2015 17

Ease of installation.

Bus uses less cabling than mesh or star topology.

In the bus, redundancy is eliminated.

A shared link is used, hence it is very cost effective.

Great expandability; One can easily add any newnode or delete any node without affecting other nodes.


Difficult reconnection and fault isolation.

A fault or break in the bus cable stops all transmission.

Because of the shared medium, it is necessary to providesome extra information about the desired destination, i.e.to explicitly specify the destination in the packet.

One station is allowed to send data at a time and all otherstations listen to it, as it works in a broadcast mode. Hence,only one station can transfer the data at any given time.


Bus topology used in the design of early LANs,Ethernet LANs.


In a ring topology, each device has a dedicated pointto point connection with only two devices on eitherside of it.

Ring Topology

Station 1

Station 2

Station 3 Station 4

Station 5

Station 6

Station 7


RING TOPOLOGY

SINGLE RING TOPOLOGY

DOUBLE RING TOPOLOGY

Meaning data travels in circular fashion from one computer to

another on the network

Continue…


Station 1

Station 4

Station 3

Station 2Fig. : Single Ring Topology

SINGLE RING TOPOLOGY

Single ring: A signal is passed along the ring in one direction, from device to device,until it reaches its destinations.


DOUBLE(DUAL) RING TOPOLOGY

Station 1

Station 4

Station 3

Station 2Fig. : Dual Ring Topology

Dual ring: A signal is passed along the ring in both direction, from device to device,until it reaches its destinations.


Easy to install and reconfigure.

Fault isolation is simplified, generally in a ring topology, a signal iscirculating at all times.Each computer has equal access to resources

If one device does not receive a signal within a specified period , it canissue an alarm. The alarm alerts the network operator to the problemand its location.

Network is highly dependent on the wire which connects differentcomponents.

Extension needs to affect only two neighbored stations, i.e.; goodexpandability.


Unidirectional traffic

A break in the ring can disable the entire network.

Each packet of data must pass through all the computers between sourceand 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 differentcomponents.

MAU’s and network cards are expensive as compared to Ethernet cards andhubs.


Ring topology was prevent when IBM introduce ,its

LAN token ring.


Tree topology is a combination of Bus and Star topology. A tree topology is a variation of a star. As in a star, nodes in atree are linked to a central hub that controls the traffic to thenetwork. However, not every device plugs directly into the centralhub.

HUB

HUBHUB

STATION 1 STATION 2

STATION 9STATION 8 STATION 7 STATION 12STATION 11STATION 10

STATION 5STATION 4STATION 3

Fig, : Tree topology


Continue… The majority of devices connects to a secondary hub that inturn is connected to the central hub.

The central hub in the tree is an active hub.

An active hub contains a repeater, which is a hardwaredevice that regenerates the received bit patterns beforesending them out.

Repeating strengthens transmissions and increases thedistance a signal can travel.

The secondary hubs may be active or passive hubs. A passivehubs provides a simple physical connection between theattached devices.


It allow more device to be attached to a single centralhub and can therefore increase the distance a signal cantravel between devices.

Point to point connection of devices.

It allows the network to isolate and prioritizecommunications from different computers.

Having different levels of the network makes it moremanageable hence easier fault identification and isolation


Maintenance of the network may be an issue whenthe network spans a great area.

Since it is a variation of bus topology, if thebackbone fails, the entire network is crippled


A good example of this network could be cable TV technology where the main cable from the main office is divided into main branches and each branch is divided into smaller branches and so on. The hubs are used when a cable is divided.


Hybrid topology use a combination of any two or more topologies.

In such a way that the resulting network does not exhibit one of thestandard topologies (e.g., bus, star, ring, etc.)

For example a tree network connected to a tree network is still a treenetwork topology.

A hybrid topology is always produced when two different basic networktopologies are connected.

Two common examples for Hybrid network are: star ring network and starbus network.


HUB

BUS HUB

STATION

STATION

STATION

STATION

RING

STATION

STATION

STATION

STATION

Fig, : Hybrid topology

STAR

STAR

Hybrid topology is a combination of Bus, Star and ring topology


Reliable : Unlike other networks, fault detection and troubleshooting is easy in this type of topology. The part in which fault is detected can be isolated from the rest of network and required corrective measures can be taken, WITHOUT affecting the functioning of rest of the network.

Scalable: Its easy to increase the size of network by adding new components, without disturbing existing architecture.

Flexible: Hybrid Network can be designed according to the requirements of the organization and by optimizing the available resources. Special care can be given to nodes where traffic is high as well as where chances of fault are high.

Effective: Hybrid topology is the combination of two or more topologies, so we can design it in such a way that strengths of constituent topologies are maximized while there weaknesses are neutralized. For example we saw Ring Topology has good data reliability (achieved by use of tokens) and Star topology has high tolerance capability (as each node is not directly connected to other but through central device), so these two can be used effectively in hybrid star-ring topology.Saturday, January 24, 2015 35

Complexity of Design: One of the biggest drawback of hybridtopology is its design. Its not easy to design this type of architectureand its a tough job for designers. Configuration and installationprocess needs to be very efficient.

Costly Hub: The hubs used to connect two distinct networks, arevery expensive. These hubs are different from usual hubs as they needto be intelligent enough to work with different architectures andshould be function even if a part of network is down.

Costly Infrastructure: As hybrid architectures are usually larger inscale, they require a lot of cables, cooling systems, sophisticatenetwork devices, etc.


ANY

QUESTION?


THANK YOU!


Date post:	18-Jul-2015
Category:	Engineering
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Computer Network - NetworkTopology

Engineering