OCR H446 A-Level Computer Science

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1 Name: Class Teacher: Date: OCR H446 A-Level Computer Science MARK SCHEME 1.3 EXCHANGING DATA Content in H446 A-Level Computer Science: 1.1 The characteristics of contemporary processors, input, output and storage devices 1.2 Software and software development 1.3 Exchanging data 1.4 Data types, data structures and algorithms 1.5 Legal, moral, cultural and ethical issues 2.1 Elements of computational thinking 2.2 Problem solving and programming 2.3 Algorithms

2 EXAM QUESTIONS QUESTION 1 a. The office workers of a large company each use a stand-alone computer. The company is considering networking the machines. Describe what is meant by a LAN. Local Area Network (Over) a small geographical area Use of hard-wiring/wireless communication State two advantages of having a LAN instead of a set of stand-alone machines. Not: Shared processing power Communication between machines/workers Any machine can be used Easier to maintain/add/delete software/files Sharing of peripheral devices/software/data Monitoring of workers Simplifies backup procedures b. Describe the following types of data transmission: Serial data transmission Individual data bits sent one after another along the communication medium/one wire/as a single stream Full duplex Communication can be done in both directions at the same time

3 QUESTION 2 Laser Tag is a game where teams of players move round an arena shooting each other with infrared guns. Players wear sensors that keep track of how many times they have been hit by the laser. This is known as being tagged. Below is an extract from a Laser Tag company s website: a. The web page is written in HTML. Write some HTML code which could have been used to produce this extract. You can assume it is already inside <body> tags. Up to 4 marks 1 mark for each correct step in process. Example: Appropriate tags to me Reasons to Choose Us subheading (e.g. h1, h2, h3). Accept b or strong if accompanied by attribute to make increase font size (1) Laser tag italicised (e.g. i or em) (1) ul used for bulleted list (1) li to mark out elements of list (1) 4 marks

4 b. The website also includes JavaScript. Describe what is meant by the term JavaScript. A programming language that runs in a web browser (1) that can be embedded into HTML (1) with <script> tag (1) to add interactivity to a page (1) Explain why it is usually the case that JavaScript is interpreted rather than compiled. JavaScript is likely to be run on a variety of machines (1) with different processors/architectures (1). Compiled code is machine dependent (1) whereas interpreters run on high level code (1) which is machine independent (1)

5 QUESTION 3 A software company decides to release a duplicate file finder which it has names De- Duplicator. Duplicate files are files that are exactly the same (bit for bit identical). Space is often wasted on computers by having multiple versions of the same file. Duplicate file finders are programs that find and identify duplicate files on a hard drive so that they can be removed. Every time the program encounters a file it takes a hash of the file and checks it against a list. If the hash exists in the list, the file is marked to be deleted. If the hash does not exist, it is added to the list. a. Explain two characteristics you would look for in a hashing algorithm for this purpose. One mark for each correct identification up to a maximum of two identifications. One mark for each valid explanation up to a maximum of two explanations. Low chance of collision (i.e. different inputs giving same output) (1) to reduce risk of different files being marked as the same (1) Quick to calculate (1) as lots of files need to be hashed/needs to be quicker than a bitwise comparison to make it worthwhile (1) 4 marks Provides a smaller output than input (1) so quicker to compare hashes than original data (1) b. After running the program, a user finds that they still have apparent duplicates of some of their images. Explain why these apparent duplicates might still be present. Accept any other sensible examples of changes to images that might not be immediately apparent to someone viewing the image Hashing works on the data/bits (1) and so two images may appear the same but not be identical at a bit level (1). This could be because they are different file types (1)/different sizes (1). Even the change of a single bit may result in a completely different hash (1).

6 QUESTION 4 The Big Brains exam board has produced a website that allows students to access revision videos. All pages in the site contain the following tag in the head section. <link href= themes/standard.css rel= stylesheet type= text/css /> Describe one advantage of storing the CSS in an external file rather than it being embedded within HTML files. Content and formatting are kept separate (1). Changes can be made to the external style sheet and affect the whole site (1) saving time (1) and ensuring consistency (1). Stylesheets can be changed for different themes, or different devices (1) 3 marks The exam board wants to limit access to those students with a school account (i.e. one ending sch.uk). When students sign up, JavaScript is used to check that the address they have entered is from a school account. The address is checked again when it reaches the server before login details are sent to the address. Explain why it is important to check the address with JavaScript and again when it reaches the server. The JavaScript check is carried out client-side/in browser (1) meaning address can be checked and stopped prior to reaching server (1) reducing unnecessary load on the server (1) JavaScript can, however, be amended and circumvented (1) therefore address must be checked at the server to ensure this has not happened (1) 3 marks The video table consists of the following fields: VideoID, VideoName, Presenter, Topic. Describe what is meant by the term primary key. A field that has a unique value/a unique identifier (1) for every record in that table (1) in this case VideoID (1) 4 marks

7 Write an SQL query that finds the name and presenter of all videos on the topic of The CPU. Do not award first mark if any other field or SELECT * SELECT VideoName, Presenter FROM Video WHERE Topic= The CPU SELECT VideoName, Presenter FROM Video WHERE Topic= The CPU 4 marks The exam board wants to use a database to keep track of which videos each student has viewed. The structure it plans to use is shown below: Identify one reason why this structure would not be suitable. Many to Many relationships are not allowed/in 3NF 1 mark Draw a new version of the structure to solve this problem. Table added between student and video (1) Student to middle table 1:M relationship (1) Middle table to video M:1 relationship (1) 4 marks

8 QUESTION 5 A school has three independent computer networks. One for student use. One for teacher use. One for use in the administration offices. The three networks are going to be combined to allow data to be stored in one location. The data will be accessible at any computer if the correct access codes are used. Explain how the following devices may be used in the above situation. A bridge 2 nd mark must be in context Provides a link between (local area) networks To connect the student and/or teacher and/or administration networks together A gateway Accept connect a LAN to a WAN for 1 st bullet point, 2 nd mark must be in context Provides a link between two dissimilar networks Links to school networks to the Internet

9 QUESTION 6 A company sells garden furniture. It has decided to create a relational database. A first, incomplete database design includes two tables PRODUCT and ORDER. PRODUCT (ProductID, ProductType, Size, Price, ) ORDER (OrderID, OrderDate, ProductID, ) For example, the product which has ProductID is a large bench which has a price of 150. State one additional piece of data which should be included in PRODUCT and give one reason why it is needed. Marks for single example with reason only, accept other relevant examples E.g. NoInStock to check stock levels/allow re-ordering Location (in warehouse) to find item when needed You should use only the data given above. Explain the use of a primary key in this database. Unique identifier ProductID identifies a product/orderid identifies an order Explain the use of a foreign key in this database. Primary key from one table used as an attribute in another table To link tables/represent relationship ProductID (is foreign key) in ORDER to show which product has been ordered 4 marks

10 A CUSTOMER table is added. An entity-relationship (ER) diagram is shown. Explain why this design would be inefficient for customers. Only one product can be on an order Customer would have to make a separate order for each product required Some of the Structured Query Language (SQL) for this database is SELECT Surname, Title, PhoneNo FROM CUSTOMER WHERE Town = Coventry ORDER BY Surname Describe the purpose of this code and give one situation in which it may be used. Accept other relevant purposes. Allow A-Z/alphabetical Lists attributes Surname, Title, PhoneNo From the table CUSTOMER For all customers in Coventry In ascending order of Surname E.g. for local promotions/new store opening 4 marks

11 QUESTION 7 A programmer spends her spare time contributing to an open source application that converts video files from a range of formats to one which uses lossy compression. Describe what is meant by the term lossy compression. (An algorithm that) makes a file storage space/size smaller but accuracy with which it represents data is reduced/information is lost in the process

12 QUESTION 8 Every bank account has an account number and sort code. The sort code identifies the bank branch (location of the bank) with which the account is held and the account number uniquely identifies the bank account. An extract from a bank s database table is shown in Fig 5.1. State why the table in Fig 5.1 is not in Third Normal Form. Branch name depends on Sort Code (i.e. there is a transitive relationship) 1 mark Explain how the database could be put into Third Normal Form. Create another table for Branches which should include sort-code and branch name (1). Make sort code the primary key of the BRANCH table/add a primary key to BRANCH (1). Remove Branch name from Customers, leave sort-code as primary key/remove sort-code and branch name from customers and add the primary key values from BRANCS as the foreign key (1) ALTERNATIVE ANSWER (ER DIAGRAM) Two tables CUSTOMER and BRANCH (or similar names) (1) Link from CUSTOMER to BRANCHES is Many (1) to One (1) 3 marks

13 A bank needs to ensure the data stored in its database is accurate at all times including when customers deposit or withdraw funds. Discuss how the bank can ensure the accuracy of its data and the importance of doing so. Low (1-3 marks) Candidate demonstrates basic knowledge of Object Oriented Programming/may contain some inaccuracies. Medium (4-6 marks) Candidate demonstrates reasonable knowledge/material is generally accurate but at times underdeveloped. High (7-9 marks) Candidate demonstrates thorough knowledge/material is generally accurate and detailed. Transactions should be Atomic; they should either succeed or fail but never partially succeed Consistent: the transaction should only change the database according to the rules of the database Isolated: each transaction shouldn t affect/overwrite other transactions concurrently being processed Durable: once a transaction has been started it is remains no matter what happens Records should be locked when in use. If one transaction is amending a record, no other transaction should be able to until the first transaction is complete Transactions should maintain referential integrity. Changes to data in one table must take into account data in linked tables. 9 marks Data should have redundancy if part of a database is list it should be recoverable from elsewhere Data entered must be accurate in the first place Security measures need to be in place to prevent malicious tampering of data Data entered should be validated (automatically checked it is sensible) and verified (checked that the data entered matches the original) Ensuring the accuracy of transactions will be partly down to the DBMS and partly down to the code accessing the DBMS. Referential Integrity is often enforced by the database management system. Redundancy can be provided in a number of ways. This could be a RAID setup or mirroring servers.

14 Bank may use validation and verification when data is input. Security procedures may include firewall, enforcement of sensible passwords and enforced user access rights. Validation may include range checks, list checks, presence checks etc. Verification may include double entry and proof reading, It is essential the bank follows the precautions discussed. Verification and validation help ensure the data is initial data is sound (garbage in = garbage out) If they make mistakes with their financial data, they may lose money or overcharge customers and lose business/find themselves in legal trouble. Without redundancy data could be lost. Without careful transaction processing, one transaction could accidentally overwrite another or half complete leading to inaccurate data. Under the Data Protection Act they have an obligation to keep personal data accurate. Verification and Validation.

OCR H446 A-Level Computer Science

Name: Class Teacher: Date: OCR H446 A-Level Computer Science REVISION BOOKLET 1.4 DATA TYPES, DATA STRUCTURES AND ALGORITHMS Content in H446 A-Level Computer Science: 1.1 The characteristics of contemporary