12C Computer Science Notes

Topology is the term which refers to the way that computers are connected in a network. Each different network topology has advantages and disadvantages. Each topology suits different tasks and the choice of topology may depend on:

* Type & number of equipment used * Planned application of the network

* Data rate (speed) required from network * Required response times

* Cost of the network

The FOUR main topologies:

* Bus * Ring * Star * FDDI

Most network software supports all of these topologies.

BUS Topology

Bus Multiport Medium, stations attach through a tap, to a linear transmission medium or bus.

· Full duplex operation between station and tap.

· A transmission from any station travels the length of the medium in both directions and can be received by all stations.

· At each end of the bus is a terminator which absorbs any signal, removing it from the bus.


a) Need to indicate to whom message is intended.

b) Need to regulate transmission to prevent collision.

c) One station may transmit continuously for a long time, halting network for other stations.

Solutions: Data is transmitted in small blocks – frames.

Each station has a unique 48 bit hardware address (MAC address)



Header = Sender and Recipient MAC address Data = files etc.

CSMA/CD = Carrier Sense Multiple Access / Collision Detection.

Stations then listen to the address and if it matches its own address it will process that frame, if not the frame is ignored.

Three commonly used wiring implementations for bus networks:

· 10Base2 (Thin-net), CheaperNet) 50-ohm cable using BNC T connectors, Cards provide transceiver

· 10Base5 (ThickNet) 50-ohm cable using 15-pin AUI D-type connectors and external transceivers

· 10BaseT (UTP) UTP cable using RJ45 connectors and a wiring center (hub NOT switch)



Easy to implement

Limits on cable length and station Numbers

Low Cost

Difficult to isolate network faults


A cable fault affects all stations


As the number of stations increase, the speed of the network decreases.


RING Topology








· The networks consist of a set of repeaters joined by point to point links in a closed loop.

· The links are unidirectional – data circulates around the loop in one way only.

· Each station attaches at a repeater and can transmit data onto the network through the repeater.

· As the frame circulates past Stations, the destination Station recognizes the address and copies the frame into a local buffer as it goes by.

· It continues until it gets to the source station where it is removed.

· The MAC address is needed to determine when each station may transmit (MAC = Medium Access Control).



Faulty stations/cables can be bypassed

Costly Wiring

Equal access for all users

Difficult to connect

Each station has full speed access to the ring

Expensive NIC’s (Network Adapter Card)

As more stations are added performance will begin to decrease slightly



STAR Topology

Each station is connected directly to a central node referred to as a Star Coupler via 2 point – to – point links, one for transmission in each direction.

The two alternatives for central node are:

a) A central node operates in a broadcast fashion: A transmission from 1 station is transmitted on all out going links (Physically a star but logically a bus). This is received by all other stations and only one station may transmit successfully.

b) Central node acts as a frame switching device:

An incoming frame is buffered in the node and then retransmitted on an outgoing link to only the destination station.

The most common wiring implementation for a star network is:

10BaseT UTP or STP Cable using RJ45 connectors and a wiring center (hub or switch)



Easy to add new Stations

Hub or Switch failure disconnects all stations connected to that Hub or switch.

Centralized control

Hubs are slightly more expensive than thin Ethernet

Centralized network/hub monitoring


Faulty station will not affect other stations



FDDI Topology (Fiber Distributed Data Interface)

· FDDI is based on two counter rotating 100-Mbps token-passing rings. The Rings consist of point to point wiring between nodes which repeat the data as it is received.

· Normally 100Mbps

· Usually Implemented over fiber-optic (fast-Ethernet)

· Consists of two Rings, Primary ring is for data transmission; the secondary ringis used also for data transmission or to backup the primary ring in case of a link or station failure.

· Automatic bypassing and isolation of faulty nodes.

FDDI Station Types

There are two main types of stations, class A which attach directly to dual rings; or class B which attaches to a station acting as a concentrator.

A concentrator is a specialized workstation that attaches to the ring and has multiple ports that allow attachment of other devices in a physical star configuration. These may be cascaded.


Logical Networks vs. Physical Networks

A logical network describes how the network operates. A physical network describes how the network has been cabled. It is thus possible to have a physical star network with a logical bus network. In other words, the network operates as a bus network, but the cabling has been implemented using star topology. The logical handling of a network is done by a protocol such as Ethernet 802.3.


Ethernet 802.3: Carrier Sense Multiple Access with Collision Detection (CSMA/CD)

This protocol is commonly used in bus and star (Ethernet) implementations.

Multiple access means that all the stations have access to the one common cable.

Rule 1, Carrier sense means that the each station will listen for network activity and wait for it to stop before sending its own data.

In bus systems, all stations have access to the same cable medium. It is therefore possible that a station may already be transmitting when another station wants to transmit. Rule 1 is that a station must listen to determine if another station is transmitting before initiating a transmission. If the network is busy, then the station must back off and wait a random time interval before trying again.

Rule 2, Collision detection means that if two stations try to transmit data at the same time then it will be detected.

A station which is transmitting must monitor the network to see if another station has begun transmission. This is a collision, and if this occurs, both stations must back off and retry after a random time interval. As it takes a finite time for signals to travel down the cable, it is possible for more than one station to think that the network is free and both grab it at the same time.

CSMA/CD models what happens when a group of people are having a conversation. Using these same rules (monitor and wait).

Data which has collided must be re-sent, this can slow down the network, one way to fix or improve this problem is through the use of a:

Switch which channels data to only the destination station thus allowing data to be sent anytime.

Bridge which splits a network into two parts.

IEEE 802.5: Token Ring

This protocol is used mainly in ring networks to manage access to the ring. A short message (called a token) is circulated around the ring; being passed from station to station (The token originates from the master station which inserts the token into the network). Only one station may have this token at any given time.

A station which wants to transmit data waits for the token to arrive. When the token arrives, the station changes it from a token to a connector message, and appends its message. This new message is then placed on the outgoing side of the ring.

Each station passes on received tokens if they have nothing to transmit. They monitor connector messages to see if the message is addressed to them. If connector messages are addressed to them, they copy the message, modify it to signify its receipt, and then send it on around the ring. Connector messages which are not addressed to them are passed directly on to the next station in the ring.

When the connector message travels the full circle and arrives at the original sending station, it checks the message to see if it’s been received. It then discards the message and replaces it with a token.

Physical Network Cabling


10Base2 Thin Ethernet Network Layout (Physical BUS)

Lengths and No. of Stations:

· Max number of trunk segments = 5

· Max trunk segment length = 185 meters (607 feet)

· Max network trunk cable = 925 meters (3035 feet)

· Max number of stations on a trunk segment = 30

· Min distance between T connectors = 1.5 feet (0.5 meters)


· Each end of the trunk must be terminated in 50-ohms. Terminators

· One of the terminators must be grounded.

· Connector splices must be kept to a minimum.

Hardware: T Connector

· BNC-T type connectors

· RG58-AU 50-ohm cable, 0.2” RG58-AU Coax







Fig 1

10Base5 Thick Ethernet Network Layout (Physical BUS)

Lengths and No. of Stations:

· Max number of trunk segments = 5

· Max trunk segment length = 500 meters (1640 feet) Terminators

· Max network trunk cable = 2500 meters (8200 feet)

· Max number of stations on a trunk segment = 100 RG-11 Coax

· Min distance between transceivers = 2.5 meters (8 feet)

· Max transceiver cable length = 50 meters (165 feet) Transceiver


· Each end of the trunk must be terminated in 50-ohms.

· One of the terminators must be grounded.

· Connector splices must be kept to a minimum.


· Transceivers 802.3

· 50-ohm cable RG-11

· Male DIX connector

Fig 2

10BaseT UTP Network layout (physical STAR)

Lengths and No. of Stations:

· Max segment length of 100 meters

· Hub to hub or repeater to repeater links limited to 100 Meters


· Star Topology

· 4 repeater/% segment rule of 10Base5 is retained Hub or Switch

· Only two nodes per segment are allowed


· RJ45 Connectors Cat 3 – 5 UTP

· Category 3 UTP Minimum, Category 5 for 100Mps & better quality.









Fig 3

Token Ring Network Layout (Physical RING)

Lengths and No. of Stations:

· Max number of workstations = 96

· Max number of 8228 MAU’s = 12 (Multiple Access Unit)

· Max patch cable distance between an 8228 MAU & a station (not including 8’ adapter cable = 45 meters (150 feet)

· Max patch cable distance between two 8228’s = 45 meters (150 feet)

· Max patch cable connecting all 8228’s = 120 Meters (400 feet)


· Stations are connected into the jacks of 8228 units

· Patch cables interconnect R0 to R1 for 8228 units

· The last R0 is connected to the first R1 to form a ring.


· Patch cables generally type 6 (26 awg) or type 1 (22 awg)

· Type 1 for lengths > 20 meters (66 feet)

· IBM 8310574 MIC connectors

· Alternatively, UTP with RJ45 connectors

Diagram Fig 3.

Cabling Summary Chart




Segment Length

Total Length

Physical Topology

Logical Topology


Coaxial Cable







Coaxial Cable



2500m (2.5km)







Star, Extended Star



Optical Fibre


2000m (2km)










Optical Fibre












Glossary of terms & Cable Types:

UTP Unshielded Twisted Pair, a type of cable, most common is Cat 5.






STP Shielded Twisted Pair

Coax Cable








Leave a Reply

Your email address will not be published. Required fields are marked *