1.3 Exchanging data

• A byte is eight bits
• The computers in the office are connected to a LAN which is connected to the Internet. The LAN is set up in a client-server network.

  Give one advantage and one disadvantage to the architects’ firm of a client-server set up rather than a peer to peer setup

  Advantage:

  • Centrally administered in one location.
  • One location to back up. Disadvantage:
  • Central point of failure.
  • Can be expensive to maintain/set up (e.g. cabling costs, specialist staff.)
• The LAN is connected to the Internet via a firewall. Describe the term ‘firewall’


  A piece of software that monitors and filters/blocks packets going to and from a network
• State why the architects’ firm would use a firewall

  • Prevent unauthorised access to a network.
  • To restrict applications that are used internally that have internet access.
  • To restrict websites that can be accessed from within the company.
  • To protect the company’s data/intellectual property.
• Often an individual customer’s record needs to be accessed. This is done by searching using the Customer ID.

  Explain why a hash table is better suited than a linked list to store the customer records, particularly as the company acquires more customers

  • A linked list requires every node to be checked (until the desired record is found).
  • A linked list will take longer to search (as more nodes are added).
  • A hash table enables direct access to the location of the record.
  • A hash table will take the same time to search (as more nodes are added)/It takes no longer as more records are added.
• State an advantage to the website’s visitors of the file being compressed

  • Downloads quicker.
  • Saves user money by using less bandwidth/ on data usage.
• Explain why the company should use lossless and not lossy compression.

  • Lossy takes away some of the information from the original.
  • Lossless preserves all the information from the original.
  • With text the loss of small amounts of information will make it unreadable
• Most films are now distributed to cinemas digitally. A studio allows cinemas to download its latest film 5 days before the release date via a private download. It wants to ensure that no cinema shows it before the release date.

  Describe one technical measure the studio could use to ensure that films are not shown early

  Encrypt the film - Send the key/password out on the release date
• Describe what is meant by the term ‘primary key'


  A unique identifier.
• Describe what is meant by the term ‘hash’


  A result generated by applying an algorithm/numeric process to a value.
• Describe one advantage to storing a password as a hash.

  • Hash functions are one way/can’t be reverse
  • If someone gains access to the database they cannot access user’s password.
• Explain the difference between a HTML id attribute and a HTML class attribute.

  • Only one element can have a given id(id is unique. )
  • Class can be used assigned to multiple elements/used multiple times.
• Give one disadvantage of this code being run client side rather than server side.

  • Won’t work if JavaScript is disabled.
  • Shows incorrect message if user’s computer’s clock is wrong/in different time zone.
  • (Source) code is visible allowing it to be copied/modified.
• Explain what happens when a search engine indexes the page.

  • A program called a spider/crawler/bot Traverses the web following the links.
  • It takes each word in the document
  • It adds an entry for the page (under the word) in the index alongside the word’s position on the page.
• Describe what the SQL statement below does.

  SELECT FlightNumber FROM Flight WHERE DestinationCode='JFK'

  • Gets/selects/outputs the flight numbers from the ‘Flight’ table Of flights with the destination JFK
  • It returns OC0089 and OC7750
• The airport wishes to allow airlines to be able to access the data it has on flights via the internet.

  Describe one format or method the airport could use to provide the data to the airlines so they can use it in their own applications

  • SQL/Structured Query Language
  • A language for creating/querying databases
• Describe what is meant by the term ‘WAN’

  • Wide Area Network
  • Collection of connected computers/devices over a large geographical area
  • Often using 3rd party communications channels
• The internet uses a set of protocols referred to as the TCP/IP stack. The TCP/IP stack consists of four different layers, each with its own set of protocols.

  Explain why protocols are important on a network

  • Allowing them to communicate
  • By ensuring all devices follow the same rules/standards
  • So they interpret data/signals in the same way
• State the name of the four layers of the TCP/IP stack.

  • -Application
  • Transport
  • Internet
  • Network Interface/(Data)
  • Link/Physical
• State what is meant by the term ‘compression’


  The process of making a file smaller/take up less storage
• Being able to stream high resolution films is only possible due to improvements in compression.

  Explain why compression is important for the streaming of high resolution films.

  • High resolution videos take up large amounts of memory/RAM
  • Due to the large number of pixels that need to be represented − When streaming, the data being sent is time sensitive/ sufficient data (i.e. the next chunk of video) needs to be received and processed within a given amount of time Otherwise there will be pauses/buffering.
  • Compression reduces the amount of data that needs to be sent/bandwidth needed
• Explain how a hash table can be used to handle collisions

  • Linear probing could be used
  • Move through the structure one space at a time − to find the next free space
  • Chaining could be used − Each location points (to the start of) a linked list.
  • The new item is added to the end of the linked list/free points to an overflow area
  • The new item is stored with the other values in the same area
• Explain what is meant by referential integrity

  • Ensuring that changes are consistent across a database
  • if a record is removed all references to it are removed
  • A foreign key value must have a corresponding Primary key value in another table.
• State what is meant by the term ‘server’


  A device which provides a central point of control/access
• Explain why it is important that the review score that the user entered is also checked server-side

  • Client side processing can be modified
  • And can sometimes be disabled on the browser
  • To prevent malicious code (such as an SQL injection)
  • To prevent a non - validated review (one with a score of over 5) being sent to the server
• Describe what is meant by the term ‘Atomic’ in the context of ACID transactions.

  − A transaction can only be fully completed or not completed / cannot be partially complete
• State what the letters CID refer to in ACID

  • Consistency
  • Isolation
  • Durability
• A hotel uses a computer system to keep track of room bookings. The hotel staff are able to query a database to discover which rooms are booked or which rooms are free. The hotel’s computer network uses a client-server model.

  Describe what is meant by the term ‘client-server’ in this context.

  • Client computers connect to server
  • Server provides access to a resource/service
  • In this case hotel staff use client computers to connect to database on server
• Give two advantages of client-server compared to peer-to-peer

  • only one point of failure
  • easier to manage users/access
  • Easier to backup
  • Easier to keep data secure.
  • Technicians can more easily remotely install / monitor.
• Explain the purpose of a network switch.

  • Joins computers/devices together on a LAN
  • Receives packets/data
  • Recipient’s address is given in packet header/it uses the mac address
  • Send packets/data
  • Out the correct port /to the specific computer device
• Define what is meant by the term ‘foreign key

  A field that links to a (primary) key in a second table
• Describe two different ways that hashing could be used in a database.

  • Hashing for security
  • e.g. hash passwords in database to make sure they cannot be read if they are stolen
  • Hashing for direct access
  • e.g. records can be quickly accessed by using hash of index as address
• How can referential integrity be broken?

  • if primary key is deleted/updated
  • foreign keys are no longer valid
• Describe one problem that would arise with the flat file database structure if a customer wanted to insure more than one car at the same time.

  • Only one customer entry allowed (because of key field)
  • so would not be able to add second entry
  • Customer data already present/would be repeated
  • resulting in redundant data/wasted space
  • resulting in inconsistencies should changes be made
• Describe how the flat file database structure could be altered to efficiently allow each customer to insure multiple cars at the same time.

  • Add in second table for the cars // splitting up cars/customers
  • The primary key of customer is used as a field in the car/vehicle table as a foreign key of cars
  • Create one to many relationship.
• Describe the purpose of HTML and CSS within the code of the website

  • HTML defines the structure of a web page
  • HTML defines the content of a web page
  • Using tags (enclosed in <>)
  • CSS defines the style / appearance
  • Using selectors such as classes / IDs / etc
  • Can be placed within HTML or externally in a file
  • Multiple pieces of CSS can be combined (the more local instances overriding)
• The JavaScript function above is used to show users the booking fee. When users click to buy the tickets, the booking fee is calculated again on the server.

  Explain why server side processing is used to recalculate the booking fee.

  • processing done away from the user’s control/Client side processing could be altered
  • Browser may not support client-side language/ scripting could be turned off
  • Booking fee calculation needs to be correct for all locations
• Explain one advantage of client side processing to either the customer buying the tickets, or to company who own the website.

  • Website will work more quickly for user
  • Reduces load on the server
  • Will need to spend less on processing power/bandwidth
• Describe how a website is indexed by a search engine.

  • Web crawler /spider visits site
  • Either be selecting it from an existing list or following a link.
  • Records information such as text / metatags / etc
  • Records the position of each word within the page
  • Storing them in an index
  • Follows links to other sites
• State what is meant by the term ‘damping factor’.

  • A value between 0 and 1
  • Probability that a user will not follow a link