definitions. descriptions etc, paper 2

paper 2, including as and a-level theory questions (+ old spec that are relevant to current spec)

Describe how an instruction is fetched from main memory during the fetch stage of the fetch-execute cycle. Your description should cover the use of registers and buses, together with the role of main memory. [4 marks]

• Contents of the PC are transferred to the MAR

• Address bus used to transfer this address to main memory

• Fetched instruction transferred using the data bus

• Contents of addressed memory location loaded into the MBR

• Transfer content in the MBR to the CIR

During the decode and execute stages of the fetch-execute cycle the instruction that is being processed is stored in the CIR. Explain why the instruction could not be processed directly from the MBR. [2 marks]

• to execute the instruction, other data may need to be fetched from main memory

• further memory fetches would overwrite the contents of the MBR

Explain why the Harvard architecture is sometimes used in preference to the von Neumann architecture. [2 marks]

• instruction and data can be accessed simultaneously

• avoids possibility of data being executed as code (can be manipulated by hackers)

State two conditions that must be met for the Vernam cipher to offer perfect security. [2 marks]

• the key must be at least as long as the plaintext

• the key must not be reused

• the key must be truly random

Explain the difference between a symmetric and an asymmetric cipher system. [1 mark]

• symmetric: the same key is used for encryption and decryption

• asymmetric: different (but related) keys are used for encryption and decryption

The system sends the data over a long distance using serial communication. Explain why serial communication is more appropriate in this instance than parallel communication. [2 marks]

• data might skew if parallel is used

• the longer the distance the likelihood of data skew

• to avoid problems of interference between wires

• hardware is cheaper to manufacture for serial communication

Explain the general purpose of a D-type flip-flop. [1 mark

• used to store state of data input

One input to a D-type flip-flop is a data signal. State what the other input to a D-type flip-flop is and what it is used for. [2 marks]

• input is: clock, trigger, enable

• used for: output is updated to reflect current status of input

State the purpose of the operand part of an instruction and explain how the addressing mode is related to this. [2 marks]

• an operand is data that will be used by the operation

• the addressing mode indicates how the value in the operand is a memory address or an immediate value

Discuss the advantages and disadvantages of programming using a high-level language compared to programming using assembly language. [4 marks]

advantages:

• program code is easier to maintain/debug

• faster development time

• programs are more portable

• built in support for data structures

• language is problem oriented

disadvantages:

• assembly language code may execute more quickly

• assembly language enables direct manipulation of memory

• assembly language code may use less memory (R: reference to no translations needed)

Explain what a higher-order function is. [1 mark]

A function that takes a function as an argument

Explain how MIDI represents music and the advantages of using MIDI for representing music instead of using sampled sound. [4 marks]

• music is represented as a series of MIDI events

• eg) channel, pitch, volume

• MIDI messages are usually 2-3 bytes long

• status bytes are split into a command and channel number (4 bits each)

• more compact resolution

• no data lost about musical notes

• simple method to compose algorithmically

Explain the difference between a physical topology and a logical topology. [2 marks]

• physical: the physical layout/architecture of the cabling

• logical: how the data packets flow around the network

Explain the differences between client-server and peer-to-peer networking. [4 marks]

client server:

• resources stored on the server

• clients access resources from the server

• centralised/ improved security management

• configuration requires expertise

peer-to-peer:

• resources stored on each individual computer

• any computer can access resources from any other

• each computer has an equal status

• management of security can be more difficult

• no dependence on a server

Describe how a vector graphic is represented. [2 marks]

• image is composed of objects

• properties of object are stored

Describe how a row of pixels could be represented in compressed form by using run length encoding. [2 marks]

• a run is a sequence of pixels of the same colour

• pairs of values would be stored, which would consist of the run length and the colour of the pixels in the run

Explain:

• the purpose of the DHCP system

• why the DHCP system is used

• what will happen during this communication. [4 marks]

• to automate the configuration of hosts connecting to a TCP/IP network

• reduces the need for expert knowledge when configuring a host

• Communication:

  • Host sends request to discover a DHCP server

  • DHCP servers offer configuration to host

  • Host accepts offer to configuration

  • DHCP server confirms configuration has been allocated to host

Describe the principles of operation of an optical disk drive that is used to read data from an optical disk such as a CD-ROM or DVD-ROM. [6 marks]

Reading mechanism:

• laser / light is shone at the beam

• light is focused on spot on the track

• some light reflects back

• light sensor detects reflection

• disc spins at constant linear velocity

Data representation:

• data is stored on one spiral/track

• land reflects light whereas pits scatter light/do not reflect light

• land represents 1, pit 0

USB Flash Drives (a type of SSD) are a more popular choice for transferring files such as images and word processed documents from one computer to another than CD-Rs (a type of optical disk). Explain why this is the case. [1 mark]

• flash drives have a higher storage capacity

• flash drives have higher read/write times

• flash drives can be reused

• no drive is required to use a flash drive

State two reasons why database designs, such as this one, are usually normalised. [2 marks]

• minimise data duplication

• eliminate data inconsistency

• eliminate insertion inconsistency

• eliminate update inconsistency

• eliminate deletion inconsistency

Explain some of the challenges that face legislators in the digital age. [3 marks]

• information can be processed and transferred in a way that was not possible before

• technology evolves quickly (so difficult for law to keep up with changes)

• methods such as encryption make it difficult to monitor criminal activities

• resources required to enforce legislation may be unavailable

• some crimes may be committed by states, not individuals

Describe what an ordinal number is and what an ordinal number could be used for in the context of an array. [2 marks]

• shows order

• the ordinal numbers could represent the position/ index of values in the array

Describe two security measures to ensure a network’s wireless access point is secure and explain how these security measures will make wireless connections to the access point more secure. [2 marks]

• use WPA / WPA 2

  • to encrypt transmission, so that intercepted transmissions cannot be understood by someone that does not have a key

• disable broadcast of SSID

  • so that the network is harder to discover

  • must know the network’s SSID to connect

• use a MAC address whitelist

  • so that only devices with a known MAC address can connect

Explain how the CSMA/CA access method with RTS/CTS will be used during this transmission. [6 marks]

• computer with data to send monitors for data signal

• if data signal present, continues to wait

• when data signal not present, computer sends a Request To Send

• WAP responds to the RTS with a Clear To Send signal

• RTS/CTS signal blocks any other transmissions from nodes in range

• if CTS not received continue to wait (until transmission ends)

• receiver sends acknowledgement / ack after (all) data received

• after transmitting, the transmitter waits to receive acknowledgement packet (to confirm data received and not corrupted)

• if no acknowledgement/ack received (within reasonable time period) then

  • wait a time period

  • then listen again / retransmit

• the acknowledgement/ack also notifies other computers that they can transmit again

• waiting periods are (often) random

One characteristic of a data set that might result in it being classified as Big Data is that it contains a variety of different forms of information. Describe two other characteristics that might result in a data set being classified as Big Data. [2 marks]

• there is a high volume of data

• data is generated/ must be processed at a very high velocity

Describe two different types of resource management that an operating system is responsible for. [2 marks]

• allocate processors to processes

• allocate memory to processes

• allocate I/O devices

• allocate space on a storage device to files

• installation of new software

Explain how the transport layer of the TCP/IP stack determines which application layer software on the server should deal with a received request. [1 mark]

• transport layer adds the port number to the data packet to determine which server/software should deal with the request

Describe one function of the network layer of the TCP/IP stack. [1 mark]

• adds source/ destination IP addresses to packets

• performs routing

• creates checksum for packet header

• splits data into datagrams

Explain the key difference between lossless and lossy compression methods. [1 mark]

• the data can be fully recovered if lossless

• original data cannot be fully recovered if lossy

Explain why dictionary-based compression is not very effective for compressing small amounts of text for transmission. [1 mark]

• for small texts, there is very little repetition

• (so compressed file size will be similar to the original)

Two methods that can be used to manage concurrent access are:

• record locks

• timestamp ordering.

Select one of these methods and describe how it manages concurrent access. [2 marks]

record locks:

• when a user starts to edit a record, an exclusive lock is set on the record

• other users cannot edit the database until the lock is released

timestamp ordering:

• timestamps are generated for each transaction (indicating the order that the transaction occurs)

• database records timestamp of last read/write time for each record

• database server applies rules to determine if processing a transaction will result in loss of data integrity or data inconsistency, and aborts the transaction if so

Explain the difference between asynchronous and synchronous communication. [1 mark]

asynchronous:

• receiver and transmitter are not synchronised by a common clock

• receiver’s clock is synchronised to transmitter’s each time a start bit is received

• receiver and transmitter clocks are synchronised for the duration of a transmission

synchronous:

• receiver and transmitter continuously synchronised by a common clock

• timing information transmitted within the data

• receiver and transmitter clocks are continuously synchronised

Explain how the wider data bus has resulted in sped up execution of programs. [1 mark]

• increases the amount of data that can be transferred at one time

Discuss the advantages and disadvantages of representing an image as a vector graphic instead of as a bitmap. In your answer, include an example for which it would be most appropriate to use a vector graphic and an example for which it would be most appropriate to use a bitmap. [6 marks]

advantages of vector graphics:

• individual components of the image can be manipulated independently

• the image can be enlarged without loss of quality

• vector graphics are resolution independent

• images saved as vector graphics typically take up less storage space, so can be transmitted more quickly than an equivalent bitmap

• eg) chart, logo, map

advantages of bitmaps:

• can represent images with complex textures

• can represent images that are not composed of regular shapes

• eg) images captured by nature

Explain what it means for a cipher to be described as being computationally secure. [1 mark]

• the cipher cannot be cracked by any known method in a reasonable amount of time

Explain what the key exchange problem is, in relation to a symmetric cipher. [2 marks]

• how to pass the key (from the sender to the receiver)

• without it being intercepted

Describe the steps that must be completed to translate and execute a program written in an imperative, high level language. Your description should include:

• why translation is necessary

• the differences between how a compiler and an interpreter would translate the program

• how the machine code instructions that are used to carry out the program will be fetched and executed by the processor from main memory. [12 marks]

why translation is necessary:

• processor can only execute machine code instructions

• high level instructions cannot be executed directly

differences between compilation and interpreting:

• compiler analyses program as a whole

• interpreter analyses program on a line-by-line basis

• compiler produces object code/executable file

• interpreter calls subroutines within its own code to carry out commands

• compile will not translate any of the program if it encounters an error

• interpreter executes program until the first error is encountered

• once translated, compiled code does not need the compiler present to run

• once compiled, code will only run on one type of processor

how machine code is fetched and executed:

• PC contents copied to MAR

• address bus used to transfer this to memory

• transfer of main memory content uses the data bus

• contents of addressed memory location loaded into MBR

• PC incremented

• MBR copied to CIR

• CIR instruction decoded by control unit

• instruction is split into opcode and operand

• if necessary, data is fetched, data is stored in memory

• result stored in accumulator/register

• status register updates

Describe an example of a problem that could occur if no system were in place to manage concurrent access to the database. [3 marks]

• two users read and edit a record/the same data simultaneously

• one user writes the record, and thn the other user user writes the rceord

• one user’s update is lost

Discuss the improvements that will occur in the communication system as a result of using

• a majority voting system instead of the parity bit

• unicode to encode the characters to be transmitted instead of ASCII

and any disadvantages that will result from them. [4 marks]

improvements:

• errors can be sometimes corrected as well as detected

• multi-bit errors can be detected

• a greater range of characters can now be transmitted

• elimination of problems caused by different versions if ASCII sets

disadvantages:

• each character will require more bits

• each bit will be sent multiple times

• the (effective) rate at which information / (useful) data can be transmitted will be reduced

Name two fields typically included in a packet. [2 marks]

• port numbers

• sequence numbers

• time to live

• packet size/length

• IP version

• flags

• window size

Explain what the checksum is used for and outline how the checksum’s value will be determined. [2 marks]

checksum used for:

• to check if packet contents have been corrupted/changed

outline how checksum is calculated:

• calculated from packet payload(contents)

Describe the role of a router in packet switching. [2 marks]

• connects two networks together

• router determines which outgoing link to send packet to

• router must determine the shortest path possible

• router updates routing tables to represent congestions/failures/network changes

• router modifies MAC addresses for next hop

State two conditions that must be met for the Vernam cipher to offer perfect security. [2 marks]

• the key must be at least as long as the plain text

• the key must be truly random

• the key must be kept securely

• the key must be only used once

Describe how the fetch-execute cycle is used to carry out machine code instructions and how the hardware of a computer could be improved so that programs can be executed more quickly. [12 marks]

fetch-execute stage 1:

• contents of PC copied to the MAR

• address bus used to transfer this address to main memory

• read signal sent along the control bus

• transfer of main memory content uses data bus

• contents of addressed memory location loaded into MBR

• PC incremented

• contents of MBR copied into CIR

fetch-execute stage 2:

• instruction to decode held by CIR

• control unit decodes the current instruction

• instruction split into opcode and operand

fetch-execute stage 3:

• if necessary, data is fetched

• opcode identifies type of operation

• operation performed by control unit

• status register updated

improving hardware:

• replace processor with one with more cores

• replace processor with one with more cache memory

• use a parallel processor architecture

• increase clock size of processor

• install more RAM

• replace motherboard with one that has more lines in the data bus

Describe what an interrupt is and explain the purpose of interrupts. [2 marks]

• a signal/request sent to the processor from hardware or program

• so that a device/hardware that needs the immediate attention of the processor can be serviced/dealt with

• so that the currently executing task can be suspended

State two reasons why JSON might have been chosen to encode data instead of XML, assuming that the software supports both methods. [2 marks]

• more compact

• quicker to parse

• structure understood directly in some languages (eg. Java)

• easy for humans to read/write/understand

State two features of functional programming languages that make it easier to write code that can be distributed to run across more than one server. [2 marks]

• immutable data structures

• statelessness // functions do not have side-effects

• order of execution can be decided at run-time

• functions can be distributed to servers and executed on data sets and then results can be combined

Two characteristics of Big Data are that the volume of data means it is too big to fit on a single server and the data comes in a variety of forms. Describe the third characteristic of Big Data. [1 mark]

• the data is generated at a very high velocity

Describe what synchronous data transmission is. [1 mark]

• receiver and transmitter are continuously synchronised by a common clock

Describe one limitation of the use of parity bits for managing errors. [1 mark]

• errors that change an even amount of bits cannot be detected

Describe the role that will be played by the transport layer of the TCP/IP stack in the transmission of the email from a computer to an email server. [3 marks]

• establish an end-to-end connection between servers

• perform error detection

• use port number to pass the data onto the correct server application in the application layer

• split data up into segments

• perform flow control

Describe the additional functionality of a gateway, beyond that of a router. [1 mark]

• protocol conversion

State the name and purpose of two application layer protocols that will be used to transfer the email from one computer to another computer. Each protocol must have a different purpose. [4 marks]

• SMTP

  • to send email to another server

• POP3

  • retrieve emails on a server

• IMAP

  • retrieve/manage emails on a server

• HTTP(S)

  • access emails over the web

Explain what a well-known port is and why an email server must use one. [2 marks]

• a reserved port that has a specific purpose

• the communication is initiated by the sender and must be the same for all initial email communications

Describe the operation of RFID. [6 marks]

• RFID tag contains (transmission) circuitry and antenna

• memory on tag stores data

• RFID transmits signal

• signal induces current in RFID tag

• RFID tag transmits data by radio wave

• RFID reader converts radio wave bac into binary data

• RFID tag is a passive device (on a card)

• RFID transmits over very short range

Describe the operation of a barcode scanner. [6 marks]

• a laser is directed at the barcode

• moving mirror moves light beam across barcode

• light is reflected back

• black/white bands reflect different amounts of light

• photodiode measures the amount of reflected light

• light reflected is converted into an electrical signal

What properties have to be true for a normalised database? [3 marks]

• each attribute in a relation is dependent on the primary key

• each attribute in a relation is dependent only on the primary key; it is not also dependent on any other attribute in the relation

• there are no repeating groups (or equivalently each attribute is atomic)

Describe the main purpose of the DNS system and how it works. [3 marks]

• translates FQDNs into IP addresses

• DNS stores a database of FQDNs and corresponding IP addresses

• DNS is a distributed database of mappings

• DNS servers are organised into a hierarchy

• if one DNS server cannot resolve a lookup the query will be passed to another (DNS server)

Explain why a peer-to-peer system would be most appropriate to use in the house. [3 marks]

• small number of users/devices

• likely that users will trust each other

• avoid additional cost of buying server

• individual users have the ability to choose which files/resources they share and with who

Explain why desktop computers usually have secondary storage devices. [2 marks]

• to store data/programs while the computer is turned off

• as the contents of RAM are lost when the computer is turned off

• to transfer data/programs between computer

• allow storage of data sets/files that could not fit in RAM

Which type of secondary storage device is best suited for storing a small file (50KB) which is to be transferred from one computer to another? [1 mark]

• USB flash drive

Which type of secondary storage device is best suited for distributing the contents of a sound library (3GB) to 2000 owners of a piece of music software? [1 mark]

• DVD ROM

Which type of secondary storage device is best suited for Storing a backup of the data from a server (2TB)? [1 mark]

• magnetic tape

Explain the principles of operation when:

• reading from a CD-RW

• writing to a CD-RW. [3 marks]

reading:

• low power laser is used to read data back from the disk

writing:

• to write data a high powered laser makes sections less reflective

general:

• the difference between reflective and non-reflective parts / pits and lands indicates the 1s and 0s

• the data is stored as a continuous spiral track;

Give two differences between primary and secondary storage. [2 marks]

• secondary storage is non-volatile whereas primary storage is volatile

• secondary storage is not directly accessible to the processor whereas primary storage is

• capacity of primary store is limited by the width of the address bus whereas there is no limit on the capacity of secondary storage

Explain the principles of operation of a hard disk drive. [6 marks]

• bits (1s and 0s) represented by magnetising spots on a disk

• disk divided into tracks and sectors

• disk head can move in and out

• disk continually spins

• disk spins at high speed

• data is read/written as correct sector passes under the read/write head

• data transferred in sectors

• may be multiple platters

• use of cache to speed up data

What is secondary storage? [1 mark]

• non-volatile data storage not immediately accessible to the CPU

Give reasons why a systems analyst might recommend use of a hard disk in a particular situation. [3 marks]

• read/write

• direct access

• high speed

• high capacity

• secure

Give reasons why a systems analyst might recommend use of a tape streamer in a particular situation. [3 marks]

• read/write

• low cost

• high speed

• portable

• high capacity

• secure

Give reasons why a systems analyst might recommend use of a CD ROM in a particular situation. [3 marks]

• read only

• portable

• low cost

State one reason why IPv6 has been introduced to replace IPv4. [1 mark]

• there are not enough unique addresses in IPv4

• eliminate need for NAT

• simplified / more efficient routing is possible

• improved facilities for multicasting

• automatic configuration possible without DHCP

• allows bigger packet sizes

• devices can roam between location can keep the same IP address

• improved support for prioritising traffic by type

State one advantage of using the DHCP system. [1 mark]

• reduces the need for expert knowledge when configuring a host

• reduces the time required to configure hosts

• facilitates efficient use of a limited pool of IP addresses

• avoids errors eg) programming incorrect subnet mask, duplicating IP addresses

Explain how two or more computers connected to the Internet can have the same IP address and still communicate with each other. [2 marks]

• the computers have private/non-routable IP addresses

• NAT will be performed so that computers can communicate on the Internet

Explain four different ways that a firewall can protect computers on a LAN. [4 marks]

• block/allow traffic from certain IP addresses

• block/allow certain types of packets

• firewall maintains information about current connections and only allows packets relevant to these connections through

• act as a proxy server

• identify unusual behaviour from a host

A packet of data is to be transmitted across the Internet from Computer A in the UK to Computer B in Belgium. A checksum will be used to attempt to detect if any errors have occurred during the transmission. Explain how:

• Computer A will use a subnet mask to determine whether or not it can send the packet directly to Computer B across the LAN or if the packet must be sent via the Internet

• the packet will be routed across the Internet

• the checksum can be used to determine if the received packet has been changed during the transmission. [12 marks]

determining if on LAN:

• AND operation of subnet mask with Computer A’s IP address

• AND operation of subnet mask with Computer B’s IP address

• result of each AND operation is the network ID

• compared results, as both are different data must be sent via the Internet

• if they were the same, could send to Computer B directly

routing across internet:

• hierarchal organisation of routers

• example of hierarchical organisation of routers eg) passed up to a national router, transferred internationally and then passed back down a hierarchy

  • path to take selected by each router

  • route may change as a result of congestion

  • (possible) repackaging of packet to use different protocol (as gateway may change protocol)

  • route determined using the (Network ID part of the destination) IP address

  • use of router tables to determine next hop

  • router decrementing "time to live" of packet

  • source and destination MAC addresses changed at each router // MAC addresses used for each “hop”

  • NAT will occur at router(s).

checksum:

• checksum produced when packet transmitted by Computer A

• MOD operation used to limit magnitude of checksum

• checksum appended to packet

• computer B recalculates checksum

• received and calculate checksum compared

• if these match packet contents/data are accurate.

A web server has a non-routable IP address, it cannot be accessed directly from outside the network. Therefore, access to the web server will be facilitated by an External Router, which supports Network Address Translation (NAT) and port forwarding. Explain how the External Router will have been configured so that the web server can be accessed by computers outside the network. [2 marks]

• traffic arriving on the HTTPS port

• must be forwarded by the external router to the IP address of the web server

What is meant by the term low-level language? [1 mark]

• a language that is close to the hardware

• language that interacts with basic hardware

• commands map directly to the processor instruction set

• an architecture dependent language // language that is not portable

Describe two ways in which high level languages make programs easy to maintain and understand than programs written in a low-level language. [2 marks]

• high level languages allow several machine code statements to be replaced by one high level statement

• high level program expressed in language that is human-oriented/uses English-like keywords

• allows programmers to use:

  • meaningful identifier names

  • procedures/functions/subroutines/libraries

  • use data structures

• easier to see logic

• can maintain one codebase for use across multiple architectures

Describe:

• the role of a translator

• the differences between a compiler and an interpreter

• a situation where you would use a compiler

• a situation where you would use an interpreter. [6 marks]

• the role of the translator is to take source code and translate it into machine code

• a compiler takes the whole source code and translates it

• compiled code will execute more quickly

• compiled code will produce an executable file

• an interpreter translates program source code line-by-line

• interpreters call routines built into the interpreter to execute commands

• Interpreting code is slower than running compiled code

• Can run (parts of) a program using an interpreter even if it contains syntax

  errors

• Source code is required for the program to be interpreted

• interpreted code the interpreter is always required

situations for compiler:

• so that source code cannot be accessed by users

• when creating an executable file for distribution

• where speed of execution is important

• where targeting a device with a small amount of memory

situations for interpreter:

• to allow execution on a wide range of processors

• when prototyping and testing/debugging code

• when no compiler yet exists for the processor

What is the second generation of programming language? [1 mark]

• assembly

What has to be done to make an executable form of a program written in a second generation programming language, which can be directly executed by this computer, and what would be used, typically, to do this? [2 marks]

• has to be translated into machine code

• by an assembler

What is meant by the term protocol? [1 mark]

• a set of rules to allow data exchange/communication between devices

How does a high level language interpreter work? [2 marks]

• analyses statement by statement each line of source code

• calls routines to carry out each instruction

What is meant by the term stored program concept? [3 marks]

• instructions and data are stored in main memory

• program is run by fetching, decoding, and executing instructions from main memory in sequence

• program can be replaced by loaded another program into main memory

• contents of main memory interpreted as either data or insructions

Describe the principles of operation of a digital still camera. [3 marks]

• lens focuses light onto image sensor

• image sensor is a CCD

• CCD used ADC to convert measurement of light intensity into binary

• image is captured when the shutter is pressed

• large pixels collect more electrons than small pixels so produce better quality images

• shutter speed can be adjusted to cope with varying lighting conditions

• image is recorded as array/group of pixels

Explain what is meant by the term imperative high level language. [2 marks]

imperative:

• instructions are executed in a programmer defined sequence

HLL:

• a language that uses more meaningful keywords

• has structures for assignment

• one instruction maps to several machine code instructions

Give one reason for there being so many third generation programming languages. [1 mark]

• languages used for a specific problem domain

• access to specific data types

• competition between companies that develop languages

• providing different function libraries

• languages developed for visual applications

Give two limitations of using assembly language to code a program. [2 marks]

• very hard/difficult to understand

• very easy to make mistakes

• hard to find any errors

• time consuming to develop software in assembly language

• lack of in-built procedures/functions

Describe four measures that can be used to reduce the threat posed by viruses. [4 marks]

monitoring:

• firewall could block packets from sources/computers known to be high-risk

• proxy server could receive/check downloaded files

• spam filters can block emails from suspicious sources

• digital certificates can be used to verify the source of a downloaded file

• digital signature/checksum can be used to verify that a file has not been changed

protection:

• enable automatic update of applications

• use a virtual machine to execute program

• set access rights to minimise risk of viruses being able to access/modify important data

• encrypt files so data cannot be extracted

• backup data and keep offline so that it can be recovered

• disable the use of external drives

• use MAC address allow list so only known devices can join a network

• use a computer with Harvard architecture to prevent data being executed as code

code quality:

• ensure code does not allow buffer overflow

• test software for security issues

• carry out a code review so that code is independently checked by another programmer

• use code analysis software to identify flaws

• use latest versions of libraries

Explain why the voltages produced by photosensors, when taking a photograph on a digital camera, are considered to be analogue and why the pixel is considered to be digital. [2 marks]

• the voltages are continuously variable and reflect the structure of any original light data

• the pixel data is discrete // the colour of each pixel is stored in a fixed number of bits

Describe what synchronous transmission is. [1 mark]

• receiving and transmitting components share a common clock

Explain why serial communication is used for connecting peripherals to a computer via USB connections, and why parallel transmission is used by the data bus inside the computer. [3 marks]

difference between communicating between peripherals and components inside the computer:

• data has to travel further to a peripheral

• position of internal components is fixed

• more data is transmitted between internal components than to peripherals

why internal buses are parallel:

• multiple bits can be transmitted simultaneously

why serial is used for connecting peripherals:

• data skew cannot occur

• crosstalk cannot occur

• cabling is cheaper

• cables can be longer

• cabling allows for more flexibility over positioning

• data transmission speed on one wire can be higher

Describe the role of an I/O controller. [2 marks]

• allows processor to control/communicate with a peripheral using an I/O port

• translates signals/data received from a peripheral into a form that can be processed by the computer

• allows new peripherals to be added without having to redesign the processor

• to check that data received from peripherals are not corrupted

• implements protocols used by I/O devices for communication

• generates an interrupt when data is ready to be transferred from an I/O device

State an example of how the control bus is used when the processor stores data into main memory. [1 mark]

• synchronise operation of processor and memory

• indicate that a memory write is happening

• send signal to request use of system bus

• receive signal granting use of system bus

• indicate number of bits being transferred

Describe:

• how data for a file would be stored on a magnetic hard disk and read from it by the file server

• how the individual layers of the TCP/IP stack in the file server would be used to transmit the file onto a network

In your response, you do not need to describe how the file would be transmitted across the network, or how the TCP/IP stack would be used in the computer that has requested the file. You only need to write about how the data would be put onto the network by the file server [12 marks]

how data is stored on and read from a magnetic hard disk:

• disk is coated in a magetisable material

• magnetising a spot in one direction could represent a 0 and the other direction could represent 1

• disk divided into rings called tracks

• tracks divided into sectors/blocks

• read/write head moves in/out radially

• disk spins at high speed

• read/write head sense the magnetic field and converts into 0s and 1s

how the TCP/IP stack is used in the file server:

• four layers are application, transport, network, link

• file is passed through each layer in turn

application:

• file server software will operate in the application layer

• file transfer may use FTP protocol

transport:

• establishes end-to-end connection between file-server and computer

• splits data into segments

• receives file/data on a port // adds source and destination port numbers to segments

• adds checksum to segment

network:

• adds source and destination IP addresses to datagram

link:

• physical interface to network communications medium

• uses device drivers // uses network interface card

• adds MAC address of destination

State on advantage and one disadvantage of purchasing a file server that uses SSDs compared to one that uses magnetic hard disks. [2 marks]

advantage:

• lower power consumption

• faster access times

• smaller (physically)

• generates less hear

• lower failure rate

disadvantage:

• higher cost

• higher error rate

Describe how domain names are organised. [2 marks]

• hierarchal organisation

• eg) top level domains :

  • .com

  • .org

  • .uk

  • .fr

• eg) second level domains

  • .co.

  • .ac

• domains can have subdomains created for them

Explain the service provided by Internet registries and why they are needed. [2 marks]

• registering domains to people/organisations/companies

• to enter domain name (to IP address mappings) into the DNS system

• to ensure that domain names are unique

• to ensure domain names are not used by more than one person/organisation/companies

Explain how a computer located outside the LAN can access the web server, despite the fact that the web server is identified by a non-routable IP address. [3 marks]

• computers outside the LAN will use the public IP address of the router

• router maintains a port mapping table

• router must identify traffic arriving on the HTTP port

• relevant traffic must be forwarded by the router to the IP address of the web server

Explain why it might be undesirable to allow the network settings of the web server to be configured by a DHCP server. [1 mark]

• the IP address of the web server might be changed by the DHCP server

• if the IP address of the web server changes the router will not be able to forward data to it