1

CP2073 Networking

Lecture 5

CP2073 Networking 2

Introduction Physical and Logical Topologies Topologies

Bus Ring Star Extended Star Mesh Hybrid

CP2073 Networking 3

Physical vs. Logical Topology

The actual layout of a network and its media is its Physical Topology

The way in which the data access the medium and transmits packets is the Logical Topology

A glance at a network is not always revealing. Cables emerging from a Hub does not make it necessarily a Star Topology – it may actually be a bus or a ring

CP2073 Networking 4

Physical vs. Logical Topology (2)

Your choice of Logical Topology will affect the Physical Topology – and vice versa

Design carefully – it may be difficult to change part way through the installation

Your choice will determine cable installation, network devices, network connections, protocols (and where you will drill holes in the building !)

CP2073 Networking 5

Factors

Cost Scalability Bandwidth Capacity Ease of Installation Ease of fault finding and

maintenance

CP2073 Networking 6

Bus Topology

CP2073 Networking 7

Bus Topology (2) Network maintained by a single cable Cable segment must end with a

terminator Uses thin coaxial cable (backbones

will be thick coaxial cable) Extra stations can be added in a

daisy chain manner

CP2073 Networking 8

Bus Topology (3) Standard is IEEE 802.3 Thin Ethernet (10Base2) has a maximum

segment length of 200m Max no. of connections is 30 devices Four repeaters may be used to a total

cable length of 1000m Max no. of nodes is 150

CP2073 Networking 9

Bus Topology (4) Thick Ethernet (10Base5) used

for backbones Limited to 500m Max of 100 nodes per segment Total of four repeaters , 2500m,

with a total of 488 nodes

CP2073 Networking 10

Bus Topology (5)Advantages Inexpensive to install Easy to add stations Use less cable than

other topologies Works well for small

networks

Disadvantages No longer recommended Backbone breaks, whole

network down Limited no of devices can be

attached Difficult to isolate problems Sharing same cable slows

response rates

CP2073 Networking 11

Ring Topology

CP2073 Networking 12

Ring Topology (2) No beginning or end (a ring in fact !!) All devices of equality of access to media Single ring – data travels in one direction only, guess what

a double ring allows !? Each device has to wait its turn to transmit Most common type is Token Ring (IEEE 802.5) A token contains the data, reaches the destination, data

extracted, acknowledgement of receipt sent back to transmitting device, removed, empty token passed on for another device to use

CP2073 Networking 13

Ring Topology (3)Advantages Data packets travel

at great speed No collisions Easier to fault find No terminators

required

Disadvantages Requires more

cable than a bus A break in the ring

will bring it down Not as common as

the bus – less devices available

CP2073 Networking 14

Star Topology

CP2073 Networking 15

Star Topology (2) Like the spokes of a wheel (without the

symmetry) Centre point is a Hub Segments meet at the Hub Each device needs its own cable to the Hub Predominant type of topology Easy to maintain and expand

CP2073 Networking 16

Star Topology (3) Advantages Easy to add devices as the

network expands One cable failure does not

bring down the entire network (resilience)

Hub provides centralised management

Easy to find device and cable problems

Can be upgraded to faster speeds

Lots of support as it is the most used

Disadvantages A star network requires

more cable than a ring or bus network

Failure of the central hub can bring down the entire network

Costs are higher (installation and equipment) than for most bus networks

CP2073 Networking 17

Extended Star Topology

A Star Network

which has been

expanded to include an additional

hub or hubs.

CP2073 Networking 18

Mesh Topology (Web)

CP2073 Networking 19

Mesh Topology (2) Not common on LANs Most often used in WANs to interconnect

LANS Each node is connected to every other node Allows communication to continue in the

event of a break in any one connection It is “Fault Tolerant”

CP2073 Networking 20

Mesh Topology (3)Advantages Improves Fault

Tolerance

Disadvantages Expensive Difficult to install Difficult to

manage Difficult to

troubleshoot

CP2073 Networking 21

Hybrid Topology

CP2073 Networking 22

Hybrid Topology (2) Old networks are updated and replaced,

leaving older segments (legacy) Hybrid Topology – combines two or

more different physical topologies Commonly Star-Bus or Star-Ring Star-Ring uses a MAU (Multistation

Access Unit (see later slide)

CP2073 Networking 23

Types of Logical Topology Previous slides showed Physical Topologies Only two Logical Topologies (Bus or Ring) Physical Bus or Ring easy to conceptualise Physical Star could be either a Bus or Ring in

logical terms Confused ? See next slides

CP2073 Networking 24

Logical Bus

•Modern Ethernet networks are Star Topologies (physically)

•The Hub is at the centre, and defines a Star Topology

•The Hub itself uses a Logical Bus Topology internally, to transmit data to all segments

CP2073 Networking 25

Logical BusAdvantages A single node failure

does not bring the network down

Most widely implemented topology

Network can be added to or changed without affecting other stations

Disadvantages Collisions can occur

easily Only one device can

access the network media at a time

CP2073 Networking 26

Logical Ring Data in a Star Topology can transmit

data in a Ring The MAU (Multistation Access Unit)

looks like an ordinary Hub, but data is passed internally using a logical ring

It is superior to a Logical Bus Hub – see later slide

CP2073 Networking 27

Logical Ring (2)

CP2073 Networking 28

Logical Ring (3)Advantages The amount of

data that can be carried in a single message is greater than on a logical bus

There are no collisions

Disadvantages A broken ring will

stop all transmissions

A device must wait for an empty token to be able to transmit

CP2073 Networking 29

Summary Bus Topology Ring Topology Star Topology Other Topologies Logical Topologies Questions and Answers