CP5 Computing June 2003

A multi-user system manages file access.

(i)

Describe one security method the operating system could use to restrict users to accessing only their own files.

[1]

access to a user's files may only be through password protection.

(ii)

Several users may be able to access the same file.

I. Explain why it may be important to record which user has accessed a file.

[1]

if a file is maliciously damaged it would be possible to find out who was responsible.

II. Describe one method that would allow the operating system to keep track of this type of file access.

[1]

the operating system would keep a log of users, the files which they have accessed and the time at which they did so.

2(a)

A wide area network can use either packet switching or circuit switching to send data.

(i)

Explain in detail the term circuit switching.

[2]

the path between transmitter and receiver has a number of switches ...
...which must all be set before the data can be sent.

(ii)

Give an example of the type of information that would be contained in a packet and state one reason why packet switching may be preferred.

[2]

a packet would consist of data and the address to which the data is to be sent.
data is more secure (harder to intercept) as it is split into a number of different packages.

(b)

Two network topologies are bus and ring.

(i)

Give one benefit of each of these types of network topology.

[2]

minimal cabling costs for a bus network
ring network - less likely to have collision problems

(ii)

Explain the main features of token passing on a ring network.

[3]

tokens are passed from one computer to the next around the ring.
when a computer wants to send data it attaches it to an empty token and passes it on...
...the receiving computer captures the data from the token when it arrives.

TicketToday has a computerised real-time transaction processing system, which is used to sell tickets for concerts and sporting events. They receive bookings over the telephone. As each sale is made a sales assistant types in the details. A record in the master file is updated to indicate that the seat has been sold and a copy of the sale is stored in a transaction log.

(i)

Several sales assistants may try to access and update the same record to sell a particular seat. Describe how the operating system prevents a seat being double booked.

[2]

as soon as an operator accesses a record for a certain seat, the record is locked so no other operator can change it
when the transaction is completed, the record is unlocked.

(ii)

In a system failure, the master file could be damaged and the data lost. Explain in detail how the transaction log could be used to restore the master file.

[3]

a backup copy of the master file is opened
..this must be from the time of the start of the transaction log
each transaction in the transaction log is processed again

4(a)

Explain the term sequential file access.

[1]

Each record is accessed in order of a keyfield.

(b)

Explain the purpose of the index in an index sequential file and describe in detail the contents of the index.

[2]

an index can be used to find the stored address of a record
the index would consist of a keyfield and an address for each record.

(c)

A particular multilevel index file has a three level index. Explain, with the aid of a diagram, how a record is located using these indexes.

[2]

Looking up a key in the first index results in a second index. Looking up the key in this second index results in a third index. Looking up the key in the third index results in the address of the record.

Max Key	Index	Looking up the record with key 2589, scan the Maximum keys for the first one that is greater than 2589...the result, look in Index I02....
1000	I01
2000	I02
3000	I03
4000	I04

....the index I02....

Max Key	Index	Looking up the record with key 2589, scan the Maximum keys for the first one that is greater than 2589...the result, look in Index I23....
2200	I21
2400	I22
2600	I23
2800	I24

....the index I23....

Max Key	Address	Looking up the record with key 2589, scan the keys for 2589...the result is the address 421899..where the record is stored.
...	...
2588	321466
2589	421899
2590	520578

(d)

Random access files are accessed directly. Explain in detail how a hashing algorithm allows direct access to a record.

[3]

A hashing algorithm is a calculation ...
....performed on the key field of the record.
The result will be the address where the record can be found so the computer can access it directly.

(e)(i)

Explain why a random access file requires an overflow area.

[2]

When the hash address of the record is calculated it may result in an address which is 'full'.
The record will then be stored in an overflow area.

(ii)

How will accessing a record in the overflow area affect the access time?

[1]

The time of access will be slightly lengthened.

5(a)(i)

What is the difference between serial transmission and parallel transmission?

[1]

serial transmission - bits are sent one after the other down one channel.
parallel transmission - bits are sent simultaneously down a number of different channels.

(ii)

State the advantage of using parallel transmission over serial transmission.

[1]

speed of transmission is faster.

(b)(i)

A modem is used to connect a computer to the telephone network.
Describe the role of the modem in a computer system.

[1]

A modem converts the digital signals of a computer into analogue signals so they can be transmitted down the telephone lines. It also changes them back at the receiving end.

(ii)

State a faster method than using a modem for connecting to the Internet.

[1]

using a digital (ISDN) line.

(c)

Explain the need for multiplexing in a computer system

[1]

a multiplexor combines a number of signals before transmitting them to increase transmission speeds.

A multi-user operating system ensures that all users have access to the computer's resources.

(i)

Explain in detail how the operating system uses time slices to allocate processor time.

[2]

Each user is allocated a time-slice of processing time in turn. These may be a fraction of a second.
When the time slice is finished the processor processes the next user and so on...then back to the first one.

(ii)

Describe two different interrupts that could terminate a time slice.

[2]

hardware interrupt - eg printer out of paper
software interrupt - eg division by zero error.

(iii)

Explain in detail the term partitioning when used to allocate main memory

[2]

the operating system allocates an area of memory to a task, which cannot be used by any other task.
when the task is completed, the memory in the partition is released and can be re-allocated to another task.

A local health authority has employed a software company to write an expert system to diagnose illnesses. The general public will be able to access the expert system in their local health centres.

Describe the role the doctors would have in the development of an expert system. Describe one possible benefit for the doctors and one possible benefit for the general public of using this system.

[4]

the doctors contribute the knowledge for the database...
..and advise on the methods of diagnosis.
fewer people will need to see the doctors...
...so people will not have to wait a long time for an appointment...and diagnosis of illness should be more accurate

8(a)

Explain why primary keys are important in a database.

[1]

They uniquely identify each individual record.

(b)

What are foreign keys and why are they used in a relational database?

[2]

foreign field is a primary key field for another table
..and they provide a link or relationship between tables.

(c)

Meaningful names are given to tables in a database, explain why this is important.

[1]

Maintenance is easier. Programmers can only make changes if they understand how a database works - otherwise problems may occur.

(d)

A specialist mountain bike shop purchases parts from different suppliers and assembles them into mountain bikes. The table below shows a sample of bikes, the parts on the bikes and who supplied each part. It should be possible to trace the supplier for every part on every bike. The data are stored in a database.

Bike Code	Bike Name	Bike Cost	Part Code	Part Name	Part Cost	Supplier Code	Supplier Name	Supplier Contact
B235	Extreme	£800	P091	Front Wheel	75	S76	Alloys Direct	Mr Brown
B235	Extreme	£800	P225	Rear Wheel	85	S76	Alloys Direct	Mr Brown
B176	Hill Climber	£500	P088	Seat	35	S54	Hi Tech Supplies	Mrs Orwell
B176	Hill Climber	£500	P454	Gears	190	S82	XL Engineering	Ms Lloyd
B176	Hill Climber	£500	P643	Front Wheel	50	S98	Big Wheels	Mr Brown
B235	Extreme	£800	P287	Seat	65	S11	DGH Parts	Mr Smith
B235	Extreme	£800	P454	Gears	190	S82	XL Engineering	Ms Lloyd
B481	Gorge Jumper	£650	P287	Seat	65	S11	DGH Parts	Mr Smith

(i)

Using a specific example from the data in the table above, describe in detail one problem with duplicated data.

[2]

Eg : Mr Brown may be mis-spelled as Mr Browne in one of the Supplier Contact fields - it would be unclear which one is correct.

(ii)

Restructure this database into third normal form using four tables.
There is no need to copy the items of data.

[10]

BIKES (Bike Code, Bike Name, Bike Cost)
PARTS (Part Code, Part Name, Part Cost, Supplier Code)
SUPPLIERS (Supplier Code, Supplier Name, Supplier Contact)
ASSEMBLIES (Part Code, Bike Code)

In the following question, additional credit (up to 3 marks) will be given if your answer demonstrates skill in written communication.

Engineers use complex software and powerful computers to create flight-training simulators. These simulators are used to train airplane pilots.

Explain why powerful computers are essential for this application and describe any difficulties in producing a realistic simulator. Discuss the benefits of using flight-training simulators to train pilots.

[8+3]

In a flight simulator there are a large number of parameters (height, flight path, wind-speed etc) to be considered and these must be processed very fast in order to give a realistic feel so a powerful computer must be used. Any change made by the trainee must be immediately reflected in the output (eg tilt of plane) - ie it is a real-time processing system. If there were to be any delay between the trainee turning the wheel and the plane tilting then the realism would not be there.

The main benefit of using simulators is that no damage is done if the trainee makes a mistake - there are no expensive planes to replace after a crash! It is also possible to simulate weather conditions which rarely occur naturally eg hurricane conditions, extreme cold etc...