Computer Systems

Computer Systems

1. System Architechture

   • CPU
     • Carries out the instructions for the FDE cycle
     • ALU: Performs logical and Arithmetic calculations
     • CU: Provides timing and control signals
     • Cache: to store instructions that are going to be reused
     • Factors that effect the CPU
       • Clock speed
       • Cache sze
       • Number of cores
     • MAR: Holds the memory address of the data that needs to be accessed
   • FDE Cycle
     • Fetch-decode-execute cycle
       • MDR: Holds actual data or instructions
       • Accumulator: Stores the immediate result of a calculation
       • PC: Holds the address of the next instruction in the cycle
     • Fetch
       • 1. Copies memory address from the PC to the MAR.
       • 3. Increases the PC to the point of the address of the next instruction
       • 2. copies the instruction in the MAR to the MDR
     • Decode
       • The MDR instructions are decoded by the CPU
     • Execute
       • The instruction is performed
   • Embedded System
     • To perform a single task from within a larger piece of equipment
     • Washing machines, dishwashers, cameras and printers

2. Memory

   • RAM
     • Random Access Memory
     • Volatile
     • Stores all data that is currently being used
     • Slower than the CPU cache
     • Faster than the Secondary storage
   • Virtual Memory
     • Opened when the RAM is full
     • A.K.A: Secondary storage
     • Can make the computer slower
   • ROM
     • Holds the BIOS
     • Read only
     • Non-volatile
     • Uses Flash Memory
       • Stores data in electrical circuits
       • Used in SD cards, USB sticks and SSD'S

3. Storage

   • Secondary Storage
     • Non-volatile
     • Read/write speed is slower than primary storage
     • Magnetic Hard disks
       • internal storages for PC and laptops much cheaper HDD have high data storage amount long lasting and reliable can be damaged by large impacts
     • Solid State Drives
       • no moving parts faster read/write speed than HDD used for the same thing as HDD portable
     • Optic Disks
       • 3 forms:Read-only, Write-once, Rewriteable.CD - 700 MBDVD - 4.7 GBBlue-ray - 25 GBuse is declining: streaming services availiable, phones and tablets don't have optic disk drives. Cheaper per GB, portable, no damage by water but do scratch easily.
     • Magnetic Tape
       • used for archiving. greater storage capacity lowest cost per GB need a specialized device to read/write. very slow when finding data fast read/write speed when data is found.

4. Networks

   • LAN
     • Local Area Network one location network e.g. a house
   • WAN
     • Wide area network made of lots of LANs e.g. the internet, businesses with multiple offices
   • Factors affecting the performance of networks
     • Bandwidth Choice of hardware and network topology
   • Client-server
     • files and software stored in one place on the server server stores important data such as passwords, usernames, and access information Advantages: easy to keep track of files easy to perform backups easy to install and update sotware easy to manage network security servers are always on disadvantages: expensive and needs an IT specialist to maintain it if the server goes down all data is lost server may not work as well if too many clients are using the server
   • Peer-to-Peer
     • Don't use servers files are stored on the individual device advantages: Easy to maintain if one device goes down the rest aren't affected Disadvantages: no centralised management easy to loose trak of where a file is stored machines are less reliable prone to slow down
   • Hardware
     • NIC
       • Network interface controller allows the device to connect to a network for both wired and wireless connections
     • Switches
       • Connect devices to a LAN recieves data and transmits the data to the the device on the network with the correct MAC address
     • Routers
       • Transmits data between networks always connected to 2 different networks connects a LAN to the internet direct data on the internet
     • Cables
       • CAT 5e/6 : 4 twisted pairs of copper cables twisted together to avoid interference used for Ethernet Coaxial: made from a single copper wire surrounded by plastic for insulation as well as a metal mesh which reduces interference. Fibre Optic: data is transmitted as light very expensive don't suffer from interference can transmit over large distances without loosing signal quality 
   • Wireless network
     • Uses radio waves to transmit data you need a wireless access point to do this  
     • WI-FI
       • Uses radio frequency bands 2.4 GHz and 5GHz bands are split into channels the channels in the 2.4 GHz band overlap performance is effected by interference Advantages of: 2.4GHz: greater range goes through solid walls easier 5GHz: faster over short distances less chance of interference
   • The internet
     • DNS
     • Hosting
     • The Cloud
   • Virtual Networks
     • Entirely software based multiple virtual networks can be on the same physical network virtual networks have their own security

5. Network topologies, protocols and layers

   • Topologies
     • Star
       • All devices are connected to a central server or a switch can be wired or wireless advantages: -if a device fails the rest of the network is unaffected. -easy to add more devices to the network -better performance disadvantages: - the whole network goes down if the server or switch is effected - can be expensive if it is a wired network
     • Mesh
       • all devices are connected to each other no single point where the network can fail as there is always an alternative route can be expensive as lots of wire is needed
   • Protocols
     • HTTP
       • Hyper Text Transfer Protocol web browsers use it to access websites and communicate with web servers
     • HTTPS
       • HTTP Secure encrypts all information that has been sent or recieved
     • FTP
       • File Transfer protocol used to move, edit, and access files from many devices on a network
     • POP
       • Post Office Protocol retrieves emails from a server and holds emails until it is downloaded then the protocol deletes the email from the server
     • IMAP
       • Internet Message Access Protocol retrieves emails from a server holds the email until you delete it.
     • SMTP
       • Simple Mail Transfer Protocol used to send emails used to transfer emails between servers
     • TCP/IP
       • It controls how data is sent between networks it is made up of 2 protocols: Transmission Control Protocol- sets up the connection to a network splits data up into packets reassembles the packets Internet protocol- controls packet switching
   • MAC addresses
     • An identifier so it can be found on a network unique to the device and cannot be changed uses either 48 or 64 bit binary numbers mainly used by the Ethernet protocol
   • IP addresses
     • Static IP
       • permanent addresses used when connecting to devices in a LAN as well as hosting websites on the inernet
     • Dynamic IP
       • are assigned whenever a device accesses a network different IP address every time you log on
   • Layers
     • A group of protocols that have similar functions
     • Layer 1
       • Data Link Layer Passes th edata over physical networks. e.g. ethernet
     • Layer 2
       • Network layer Makes connections with other networks directs packets handles traffic e.g. IP
     • Layer 3
       • Transport layer controls data flow splits data into packets checks that they have been sent and delivered e.g. TCP
     • Layer 4
       • Application layer turns data into websites and other applications. e.g. HTTP, FTP,SMTP
   • Packet switching
     • 1. the device sending the data splits it into packets and gives it a number
     • 2. routers read the packets header so it knows which way to send it
     • 3. the receiving device rearranges the packets according to the packet number
     • 4. If a packet hasn't arrived then the receiving device requests for the data to be sent again
     • 5. if all the data has been received then a confirmation message is sent to the sending device
   • Ethernet
     • the way devices connect to the internet

6. System Security

   • Forms of attack
     • Passive attack
       • an attack where the hacker monitors data going to and from a network. they are hard to detect but can be prevented by encrypting your data
     • Active attack
       • an attack that uses malware firewalls are the main defence can be easily detected
     • Insider attack
       • someone exploiting their own network access to steal information
     • Brute force attack
       • an attack that uses software to crack passwords can be revented by locking devices after a certain number of failed attempts or just having a strong password 
     • Denial-of-service attack
       • the stopping of someone from accessing certain parts of a network by increasing data flow and making is very slow to access a network
   • Threats to networks
     • Malware
     • Phishing