Computer Science Paper 1

What is the purpose of a computer?

• The purpose is to take data, process it, then output it.
• A computer system consists of hardware and software.
• Hardware is the physical stuff that makes up your computer system.
• Software is the programs or applications that a computer system runs.

What is an embedded system?

• Embedded sytems are computers built into other devices such as a dishwasher or TV.
• they are usually systems dedicated to a single task.
• The benefits of an embedded system is its easy to design, cheaper to produce and more efficient than a general purpose computer

Power supply- Supplies power to the motherboard, optical and hard drives, and other hardware

Case cooling fan- Extracts hot air from the cooling case.

CPU heat sink and cooling fan- Keeps the CPU at a steady tempreature.

CPU- Does all the processing (it is situated under the heat sink.)

Graphics card- Used for handling graphics and image processing relieving the processing load on the CPU.

Motherboard- Main circuit board in the computer, wher all the hardware is connected.

Hard disk drive- Internal secondary storage.

RAM sticks- Computer memory.

Optical drive- For read/write of optical discs.

What is the CPU?

The central processing unit (CPU) is the brain of the computer system. It processes all the data and instructions that make the system work. The processing power of a CPU depends on characteristics like clock speed, the number of cores and cache size.

The architecture of a CPU describes the main componensts of the CPU and how they interact and work with each other. Von Neumann and Harvard are the two main types but you only need to know about Von Neumann.

The three main parts:

• The Control Unit (CU)- Has overall control of the CPU. Main job is executing program instructions. Controls the flow of data inside and outside the CPU.

• The Aritmetic Logic Unit- Does all the calculations. Completes simple addition and subtraction.Contains the accumulator register. Also, it performs logic operations, binary shifts, comparisons, multiplication and division.

• The Cache- Very fast memory inside the CPU, however, it is slower than registers but faster than RAM. Stores regularly used data so the CPU can access it quicker the next time its needed. Has low capacity, expensive compared to RAM.

Von Neumann architecure describes a system where the CPU runs programs stored in memory. Programs consist of data and instructions which are stored in memory addresses.

PC- Holds the memory address of the instruction for each cycle.

ACC- stores the intermediate results of calculations in the ALU.

MAR- Holds any memory address about to be used by the CPU. The address may point to Data or a CPU instruction.

MDR- Holds the actual data or instruction.

Fetch instruction: Copy memory address from the program counter to the MAR.

Copy the instruction stored in the MAR addres to the MDR.

Icrement the program counter to point the address of the next instruction, ready for the next cycle.

Decode instruction: The instruction in the MDR is decoded by the CU. The CU may then prepare for the next step by loading values into the MAR and MDR.

Execute instruction: The instruction is performed. This could be:

load data from memory, write data to memory, do a calculatin or logic operation.

RAM- Random access memory is main memory which can be read and written. It is also volatile.

Volatile- Temporary memory which requires power to retain its data.

Non-volatile- Permanent memory which keeps contents when power is off.

Virtual memory- When RAM is full virtual memory is needed as secondary storage.

ROM- Read only memory is non-volatile and can only be read.

               It is also used in SSDs, SD cards and USB sticks.ROM chips use flash memory, very common in non-volatile.

Clock speed- The number of instructions 1 processor core can carry out per sec (Hz). Higher the clock speed, the more instructions carried out at each second.

Number of cores- Each core in a CPU can process data independtly of the rest. The more cores, the more instructions it can carry out.

Cache size- Much faster than RAM. Larger, it is the faster the CPU can access the data.

GPU- specialised circuits for handling graphics and image processing. Relieves the processing load on the CPU.

Primary storage- Memory areas that the CPU can access very quickly. Mostly volatile.

Secondary storage- Where the dat not in use is stored. It is non-volatile.

Hard disk drives (HDDs)- Multiple magnetic disks stacked up on top of eachother, where the data stored is in sectors of the disks.

Solid state drives(SSDs)- SSDs have no moving parts, use flash memory and like HDDs are used for internal storage.

Advantages of HDDs:

• cheaper
• higher capacity than SSDs
• longer read/write life than SSDs.

Advantages of SSDs:

• faster
• doesnt need defragmenting 
• more shockproof
• SSDs are silent, HDDs make some noise.

Optical discs- CDs, DVDs, and Blu-ray. very cheap per GB, shock proof and waterproof.

Magnetic tape- Much greater storage capacity than HDDs. Low cost per GB. Used by large companies for archiving.

Optical disadvantages- Cant compete with flash storage due to its low capacity. Modern devices such as phones do not have optical drives. Internet speeds have also increased, with that the rise of streaming sites have put optical discs on the decline.

Magnetic disadvantages- Very slow when finding specific data stored.

The OS (operating system) communicates with the internal and external hardware via the device drivers. Every piece of hardware is connected through the operating system.

• It allows multi-tasking.
• Handles file and disk manageent.
• Operating systems deal with user accounts.
• Communicates with internal and external hardware via device drivers.
• Provides a platform for different applications to run. 
• Manages system security and user accounts.

Defragementation- Files are stored on a hard disk. Files are moved, deleted and change size. When writing files to the disk, the OS splits files into smaller blocks to fill up gaps. Overtime a disk can become fragemnted which makes read/write files slowr. Defragmentation reoraganises data on the hard drive to put fragmented files back together. Moves files to collect all free space.

Backup Utillities- A backup is a copy of a systems files and setting stored externally. It means if data loss occurs it can be recovered. A backup utilliy schedules regular backups which are either full or incremental:

Full Backup- A copy of every file on the system is taken. It uses a lot of storage space. Can take a long time to create but can be faster to restore from.

Incremental- Only files that were created or edited since the last backup are copied. Use less storage and are much faster to create. Full system restore is slow as the last full backup must be restored then all the incremental ones since tht point.

Open source- This is software where its source code is freely avaliable. It is legally modified by users to create their own spin-off software.

Advantages

• Usually free
• Made for the greater good/non-profit
• Can be adapted to fit user needs
• Can be more creative and innovative
• Very reliable and secure

Disadvantages

• Wont get many regular updates
• can be buggy
• No warranties if something goes wrong
• No customer support

Proprietary- Only the compiled code is release, source code is a closley guared secret. It has licenses to restrict modification of the code. 

Advantages

• Comes with warranties, documentation and user support
• Well tested and reliable
• Usually cheaper for the company

Disadvantages

• Can be expensive
• May not fit the users exact needs
• The comoany may not maintain older software after warranties expire.

LANs- Covers a small geographicla arear on a small site. All the hardware is owned by the company that uses it. It can be wireless or wired.

Why choose LANs?- Sharing files is easier. internet connection shared on all devices. Communicates with LAN users cheaply and easily. 

WANs- Connects LANs in different geographical areas. Organisations hire the infastructure Unlike LANs. Can be connected with fiber or copper lines, satellite links or radio links.

Factors Affecting performance-

Bandwidth- How much data is shared between users on a netwrok. The greater the bandwidth the better a network performs.

Wired connections- Generally faster and more reliable than wireless. Fiber optic cables can give better performance than copper ones. Wireless performance depends on signal quality which is affected by range of device, interferences from other devices and physical obstructions.

NIC- An internal piece of hardware which connects a device to a network.

Switches- Connects devices on a LAN. Recieves data and transmits it to the device with the correct MAC address

Routers- Transmits data between networks. Theyre always connected to at least two different networks.

Ethernet cables- Used to connect devices in a LAN. CAT 5e and CAT 6 are the most common types.

Coaxial cables- Made of a single copper wire surrounded by a plastic layer for insulation.

Fiber Optic cables- Transmit data as light. High performance cables that dont suffer interference and can travel long distances without signal loss.

Client-server- Managed by a server and the devices connected to it are clients.Files stored centrally on the server rather than on individual client devices. Clients send requests to the server.

Pros-

• Easier to keep track of files.
• Easier to perform backups
• Easier to install and update software
• Easier management of network security
• Servers are reliable and always on.

Cons-

• Expensie to set up, needs IT specialists to maintain it
• Server dependance 
• Server may become overloaded

P2P- All devices are equal, connecting directly to each other without a server. Files stored on individual devices and share them with others. You may use one at home.

Pros-

• Easy to maintain
• No dependence on server

Cons-

• No centralised management
• Copying files between creates duplicate files
• Peer machines are less reliable and data may be lost if one fails
• Machines are prone to slow down when other devices access them

Star Topology- All devices are connected to a central switch or server that controls the network. The central switch switch allows many devices to access the server simultaneously.It can be either wired or wireless.

Bus- All devices are arranged in a line, connected to a single backbone cable. Devices send data both directions. Can cause data collisions, which slows down a netwrok.

Ring- Data moves around in one direction, preventing collisions. Only one device can send data at a time and data passes through many devices before getting to its destination.

Mesh- It is a decentralised and works by sending data along the fastest route from one device to the other. It has no single point where the network can fail: if one device fails the data will go along a different route.  full mesh is where every device is connected to every other device. partial meshes not all devices are fully-connected.

Protocol- It is a set of rules for how devices communicate and how data is transmiited across a network.

MAC addresses - They are unique identifiers assigned to network enabled devices by a manufacturer. They are unique and cant be changed. They're 48 or 64-bit binary numbers but are converted to hex to make it easier. MAC adresses are mainly used by ethernet cables or LANs. LAN switches read the MAC adresses which are used to direct data to the right device.

IP adresses- They are used when sending data between TCP/IP networks. They arent assigned to hardware, they are assigned manually or automatically before the device accesses the network. It can be either 32 or 128-bit.

Satic- Permanent addresses which  are used to connect Printers on a LAN and for hosting a website. Can be very expensive.

Dynamic- An address which changes everytime it connects. ISPs commonly use this type of IP as it is cost effectiveand can be reused.

Data is sent between networks in packets.

Every packet has a header which contains the control info which includes the packets destination address, source code and packet number. The packets payload is the thing the person receiving it is likely to read. Then finally the checksum number is used to validate wether the payload is corrupted or not, if the values match then the data has been received correctly.

Packet switching is used to direct the data.

Data splits into chunks (packets). Each packet has a from and to address and also a payload. Packets sent onto the network, moving from router to router taking different paths. Each packets journey time can differ. Once arrived they are re-ordered. Message sent from recipient to sender saying the message has been recieved. If there is no such message, the sender transmits data again.

TCP/IP is the most important protocol

This is the protocol which dictates how data is sent between networks. It is made up of two protocols.

Transmission Control Protocol- Sets the rules for how devices connect on a network.

Internet protocol (IP)- Is responsible fro packet switching.

HTTP- Used by web browsers to access websites and communicate with web servers.

HTTPS- A more secure version which encrypts all info sent and received.

FTP- Used to access, edit and move files between dvices on a network

POP3- Used to retrieve emails from a server which holds the email until you download it and then the server deltes it.

IMAP- Used to retrieve emails froma server which holds it until you actually delete it you only download a copy.

SMTP- Used to send emails Also used to transfer emails betwee servers.

Layer 1-Data link layer- Passes data over the physical network. Responsible for how bits sent as electrical signals over cables, wireless and other hardware. Ethernet.

Layer 2-Network layer- Making connections between netwroks, directing data packets and handling traffic used by router. IP.

Layer 3-Transport layer- Controlling data flow. TCP.

Layer 4-Application layer- Turning data into websites and other applications and vice versa. HTTP, FTP AND SMTP.

The Internet- It is a network of a network. Its a WAN which connects devices and networks from all oveer the world. It si based around a TCP/IP model. The WWW is a collection of websites that are hosted on web servers and accessed through the HTTP model. CAan be accesed by URLs and addresses. A DNS translates a websites domain name into an IP address.

The cloud uses the internet to store files and applications

It acts like an extension of a traditional client-server network where user files are stored centrally on a network server.

Pros- Users can access files from any connected device. Easy to increase amount of avaliable storage. No need to buy expensive hardware to store data.

Cons- Needs a connection to the interent to access files. Dependant on a host for security and backups. Data in the cloud can be vunerable to hackers.

Virtual network- It is a network which is entirely software-based. They're created by partitioning off some of the physical network's bandwidth to form a seperate network.

Passive attack- This is where someone monitors data travelling on a network and intercepts any sensitive info they find. The best defense against it is data encryption.

Active attack- This is when someone uses malware or other planned attacks. They are more easily detectable. Best defense is a firewall.

Insider attack- When someone in an organisation exploits their network access to steal info.

Brute Force Attack- An attack which attempts to gain unathorized access by cracking passwords via trial and error using a special type of software.To prevent hava a stronger password or lock account after a certain number of attempts.

Denial of Service attack- Stops user form accessing a part of network or website. It involves flooding the network with traffic.

Malware is software which can harm devices

Viruses attach themselves to certain files. Worms are like viruses but they self-replicate without user help. Trojans are malware disguised as legitimate software.

Social Engineering

A way of gaining access to sensitive information or illegal access by influencing people such as an employee from a large company. It can usually happen over the phone with the person ringing up this employee posing as a network administrator or something like that and ask for sensitve info such as a password or info which is then stolen.

PhishingThis is where criminals send emails or texts claiming to be from a well kown business like a bank. The user will be asked to enter their personal info into a spoof site where the criminals will steal it. 

SQL Injection

SQL injection is a code injection technique, used to attack data-driven applications, in which malicious SQL statements are inserted into an entry field for execution.

 A good network policy will include:

• Regular network tests to find and fix security weaknesses. 
• Use password to prevent unauthorised people from accessing the network
• Enforce user access levels
• Install anti-malware and firewall software
• Encrypt sensitive data

Penetration testing- An organisation employs legal hackers to simulate potential attacks.

User access levels- it controls which parts of a network which different groups can access.

Anti-malware software- Designed to stop and prevent malware from damaging an organisations network and devices that are connected.

Encryption- This is when data is translated into a code which only someone with the correct key can access.