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28.5 Concepts from communication theory 915

To say that not all three transistors fail is the same as saying that one or more transistorswork.

So we could say

A : atleastone transistor works

Hence the probability thatatleastone transistor works is0.97.

EXERCISES28.4

1 Which pairsofevents are complementary?

A:the componentis reliable

B:there is only onecomponent

C:there are less thantwo components

D:more thantwo components are

reliable

E:the componentis unreliable

F:thereismore thanonecomponent

G:most ofthe components are

unreliable

2 EventsA,B,C andDare defined.

A:the lifespanis90 daysorless

B:the machine is reliable

C:allcomponents have been tested

D: atleastthree components from

the sample are unreliable

State the eventsA,B,C andD.

Solutions

1 AandE;CandF

2 A:the lifespanis more than 90days

B:the machine is unreliable

C:some components have not been tested

D:two orfewer components are unreliable

28.5 CONCEPTSFROMCOMMUNICATIONTHEORY

Communicationengineersfinditusefultoquantifyinformationforthepurposesofanalysis.Inordertodosoaveryrestrictedviewofinformationisused.Informationisseenin

termsofknowledgeofaneventoccurring.Ahighlyimprobableeventoccurringconstitutes

more information than an almost certain event occurring. This correlates to some

extent with human experience as people tend to be much more interested in hearing

aboutunlikely events. The information,I, associatedwith anevent, isdefined by

I=−logp

0<p1

where p = probability of an event occurring. Notice that p = 0 is excluded from the

domain of the function as the logarithm is not defined at 0. In practice this is not a

problem because an event with zero probability never occurs. Often logarithms to the

base 2 are used when calculating information as in many cases information arrives in

the form of a ‘bit stream’ consisting of a series of binary numbers. For this caseI has

unitsofbits and isgiven by

I=−log 2

p

Aformulaforevaluating logarithmstothe base2isgiven inChapter2.

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