Computer science revision notes Flashcards
Reasons for a new system:
Reasons for a new system:
Old system is inefficient
Old system is no longer suitable for its original purpose or is outdated
To increase productivity and quality of output
To increase efficiency and minimize costs
Extent of a new system update depends on
Extent of a new system update depends on how much time, software, hardware, people needed and the immediate environment. May need to train employees to use the system, fire employees (e.g. secretary not needed if salespersons input orders from home PC), get more hardware (e.g. employees need PC and network need to be implemented), change server location, etc.
To decide whether the project is worth pursuing
To decide whether the project is worth pursuing (Compatibility issues/ strategies for mergers/ data migration/ hosting system/ installation processes are issues to be considered in the planning stage, once the project is deemed worth pursuing), use:
Telos
Change managment
Technical feasibility: Is available technology sufficient and advanced enough to implement the system?
Economic feasibility: Is the new system cost effective? Are funds sufficient?
Legal feasibility: Are there conflicts between the system and laws/ regulations?
Operational feasibility: Are existing organisational procedures sufficient to support maintenance and operation of the new system?
Schedule feasibility: How long will it take to implement?
Change management: Shifting people, departments and organisations from one state to the desired state. Need to maximise benefits and minimise impact of change on people so that stakeholders accept the change in environment. Issues regarding planning the system must be resolved, e.g. students informed of double-sided printing to make use of the new feature
Compatibility issues
Compatibility issues
Business mergers: Two businesses combine. Need to ensure systems are compatible. Incompatibility can arise from…
Language differences: Communication issues and different interpretations. Software incompatibility: Different software/ systems can’t operate well on the same computer or same network.
Legacy systems: Old tech, hard/software, computer system or program. Some still satisfy user needs and data cannot be converted to newer formats, or applications upgraded, so organisation continues use of legacy systems even if newer technology is available.
Strategies for merging:
Strategies for merging:
1. Keep both systems, develop to have same functionality (high maintenance cost)
2. Replace both systems with a new one (high initial cost)
3. Combine best systems from both companies (hard for employees to work with system from another company)
4. Only use one company’s info systems (policy issues)
Using client hardware VS hosting systems remotely
Locally hosted system:
Using client hardware VS hosting systems remotely
Locally hosted system: Software is installed and operated on client’s own hardware/ infrastructure. Like paying to buy the product/ software package and owning it. E.g. set up open source message board system on your own web server.
PROS:
Best for large, complex systems.
Only pay once, excluding maintenance (if you don’t pay it, can still continue to use the software but not updated versions).
Can control the data yourself in a secure data centre, less risk of data loss as you yourself can use redundancy to whatever extent you want.
CONS:
Higher initial cost than remote
Harder to predict total cost (maybe more expensive in long run with maintenance payments)
Have to maintain yourself (hire IT personnel).
Remote hosted system: Software As A Service (SaaS) solution. Hardware is elsewhere, updated centrally. Users can access data and operate software from the cloud and pay for service on a subscription basis. e.g. sign up for message board system where others take care of maintenance. PROS:
Lower initial cost
Can predict overall cost easier
Best for when organisation doesn’t have necessary hardware already
You don’t have to maintain it yourself
Data secure in data centre
CONS:
Relying on a third party= risk of data loss if provider shuts down
Legislation in country of provider may be weaker than in user’s country Performance generally lower than on-premise solutions
Remote host in different time zone, can be inconvenient for end-users? Depends on internet connection
Using client hardware VS hosting systems remotely
Definiton, pros and cons for both
Using client hardware VS hosting systems remotely
Locally hosted system: Software is installed and operated on client’s own hardware/ infrastructure. Like paying to buy the product/ software package and owning it. E.g. set up open source message board system on your own web server.
PROS:
Best for large, complex systems.
Only pay once, excluding maintenance (if you don’t pay it, can still continue to use the software but not updated versions).
Can control the data yourself in a secure data centre, less risk of data loss as you yourself can use redundancy to whatever extent you want.
CONS:
Higher initial cost than remote
Harder to predict total cost (maybe more expensive in long run with maintenance payments)
Have to maintain yourself (hire IT personnel).
Remote hosted system: Software As A Service (SaaS) solution. Hardware is elsewhere, updated centrally. Users can access data and operate software from the cloud and pay for service on a subscription basis. e.g. sign up for message board system where others take care of maintenance. PROS:
Lower initial cost
Can predict overall cost easier
Best for when organisation doesn’t have necessary hardware already
You don’t have to maintain it yourself
Data secure in data centre
CONS:
Relying on a third party= risk of data loss if provider shuts down
Legislation in country of provider may be weaker than in user’s country Performance generally lower than on-premise solutions
Remote host in different time zone, can be inconvenient for end-users? Depends on internet connection
Installation processes (adv/dis)
the 4 changeovers
Installation processes (adv/dis)
Implementation/ conversion: Putting new system online and retiring old one. Types… Parallel: Both systems run parallel to each other at first to compare outputs until satisfied
with the new system, terminate old one. If new system fails, can revert to old one = less risk, ideal for critical systems e.g. nuclear power station. But higher cost. Not efficient if systems have different input/outputs/ processes. Workers may be trained to use new system for nothing.
Direct/ Big Bang: Set up new one and terminate old one at the same time. Preferred if system not critical, due to higher risk as system might not function well. Less costly. Pilot: In organisations with multiple sites. New system is introduced in one of the sites (pilot site/group) then introduced to others if successful. Less risk. Worker training Phased: Convert one module at a time e.g. per department. Training period and implementation takes longer
Problems with data migration
Problems with data migration
Data migration: Transferring data between formats, storage types and/or computer systems when switching to a new system/ changing/ upgrading/ merging. Problems/ risks… Incompatibility with file formats in the new system- could lead to incomplete or incorrect data transfer
Non-recognisable data structures and formats- result in a mismatch of data, e.g. in customer records
Data lost or corrupted during transfer due to transmission faults/lack of adequate storage. Not usable at destination
Data misinterpretation due to conventions in different countries e.g. date, measurements, currencies
Different validation rules between companies- could lead to inconsistent/incorrect results Might not be able to use data while transferring, problem if it’s large and takes long
Types of test
Types of testing
Types of testing
Testing is important because it identifies problems to be fixed, areas for improvement and determines whether system/ software fulfils requirements. If not done properly, inadequate system= inadequate employee productivity, reduced efficiency and output, increased costs= end user dissatisfaction.
1. Alpha testing: Offering early development version to other developers before available to general public, get feedback.
2. Beta testing: Provide version to select group of users outside of company (closed beta) or to public (open beta) and receive real-world feedback. But user report is not always best quality, and there are many reports of the same bugs.
3. User acceptance testing: Usually last stage, provided to clients as a last-minute check that the product satisfies target audience
4. Debugging: Systematically finding and correcting bugs/errors. Some programs do it automatically = cheaper and faster.
User documentation
User documentation
Important so users can understand, use and make the most of the system. Ensures users can quickly adapt to the software/ system with minimal costs/inefficiencies. Can include… Requirements- identify attributes, characteristics and functions
Technical- details on how to install and configure the product
End user- manuals for end user, support staff and system administrators. Details on how to
use the product
Marketing- how to market the product, analysis of market demand
User training methods (adv/dis)
User training methods (adv/dis)
Self-instruction: People can use resources like manuals, websites, video tutorials etc. to learn on their own. Easiest and cheapest with more flexible time for user but usually only used for easy/ common use programs with sufficient documentation as effectiveness depends on user motivation and ability to work on their own.
Formal classes: Classroom setting, free discussion. Students can exchange ideas, direct interaction with expert. But may be harder for members who work better on their own and self-assured students may dominate discussions
Remote/ online training: Larger variety of courses online, can access any time, easier to set up and include new members = cheaper. But excludes those without infrastructure/ internet or IT skills to use it, might not be as effective (especially with dependent learners)
Causes of data loss:
what are the consequences
Measures to prevent data loss
Causes of data loss:
User error: Accidental deletion, closing before saving
Natural disasters: Fire, flood, earthquake
Malicious activities: Someone purposefully deleting/ altering/ stealing data (can be employee or external hacking)
Computer viruses: Destruction/ corruption of data
Power failure
Consequences can be serious e.g. hospital: puts lives in danger, may have to repeat tests and procedures. In other situations, can cut into revenue if dissatisfied customer tells others e.g. customer makes reservation but there’s no record of it or free rooms so they have to go elsewhere.
Measures to prevent data loss
1. Regular backups: On hard disks/ magnetic tape, online or on removable media (e.g. USB, CDs) for fast backup and storage
2. Offsite storage: Data backups stored in different geographical location 3. Firewall and antivirus: Prevent virus infections
4. Failover systems: Computer system that system can switch to in case of hardware/ software/ network failure. Often switches automatically to reduce time
Software deployment
It’s important that users can install updates because otherwise they might not have fixes for bugs and errors or be able to benefit from added features/ improvements leading to performance issues. Especially for organisations with different locations- different sites could have different versions of the software, leading to incompatibility.
Causes of data loss:
User error: Accidental deletion, closing before saving
Natural disasters: Fire, flood, earthquake
Malicious activities: Someone purposefully deleting/ altering/ stealing data (can be employee or external hacking)
Computer viruses: Destruction/ corruption of data
Power failure
Consequences can be serious e.g. hospital: puts lives in danger, may have to repeat tests and procedures. In other situations, can cut into revenue if dissatisfied customer tells others e.g. customer makes reservation but there’s no record of it or free rooms so they have to go elsewhere.
Measures to prevent data loss
1. Regular backups: On hard disks/ magnetic tape, online or on removable media (e.g. USB, CDs) for fast backup and storage
2. Offsite storage: Data backups stored in different geographical location 3. Firewall and antivirus: Prevent virus infections
4. Failover systems: Computer system that system can switch to in case of hardware/ software/ network failure. Often switches automatically to reduce time
Software deployment
It’s important that users can install updates because otherwise they might not have fixes for bugs and errors or be able to benefit from added features/ improvements leading to performance issues. Especially for organisations with different locations- different sites could have different versions of the software, leading to incompatibility.
Causes of data loss:
User error: Accidental deletion, closing before saving
Natural disasters: Fire, flood, earthquake
Malicious activities: Someone purposefully deleting/ altering/ stealing data (can be employee or external hacking)
Computer viruses: Destruction/ corruption of data
Power failure
Consequences can be serious e.g. hospital: puts lives in danger, may have to repeat tests and procedures. In other situations, can cut into revenue if dissatisfied customer tells others e.g. customer makes reservation but there’s no record of it or free rooms so they have to go elsewhere.
Measures to prevent data loss
1. Regular backups: On hard disks/ magnetic tape, online or on removable media (e.g. USB, CDs) for fast backup and storage
2. Offsite storage: Data backups stored in different geographical location 3. Firewall and antivirus: Prevent virus infections
4. Failover systems: Computer system that system can switch to in case of hardware/ software/ network failure. Often switches automatically to reduce time
Software deployment
It’s important that users can install updates because otherwise they might not have fixes for bugs and errors or be able to benefit from added features/ improvements leading to performance issues. Especially for organisations with different locations- different sites could have different versions of the software, leading to incompatibility.
Types of updates (i.e. reasons for updates)
Types of updates (i.e. reasons for updates)
Patches: Used to fix known bugs and vulnerabilities. May introduce new bugs though Updates: Fix bugs, add minor functionalities, usually free
Upgrades: As well as bug fixes, add new major functionalities or characteristics, often need to be bought
Releases: Final, working applications gone through testing
Strategies for alerting users about updates
Strategies for alerting users about updates
Automatic: Cookie is placed on the machine when software is registered and installed, communicates with the developer automatically when software is started up. If update is available, messages and alerts are sent back to the machine
Sending an email: User registers email and other details when installing software. Email sent to the registered user with a link to download the update
Roles of a computer in a networked world
Roles of a computer in a networked world
Client: Piece of computer hardware/software that accesses services made available by server, by sending requests to server
Server: Program/host computer that fulfils requests from client programs or computers across network and shares info to clients
Email server: Message transfer agent that transfers electronic messages from one computer to another in a network
DNS (Domain Name Server): Server that translates web addresses written in letters (more memorable for humans) to the numeric IP (Internet Protocol) address
Router: Connects networks together to forward data packets between networks, deciding where to send information so it is received by one network and then sent to another until it reaches its destination
Firewall: Controls incoming and outgoing network traffic, determining what data packets should b
Ethical/ social issues with the networked world
Ethical/ social issues with the networked world (interconnecting computers): (also ethical issues associated with introduction of new IT systems)
Security: Protecting hardware, software, peripherals, data and networks from unauthorized access
Privacy: Controlling how and to what extent data is accessed and used by others, to protect identity e.g. GPS location services on phone, data sold to companies. But there’s also problems with anonymity e.g. cyber bullying, hacking, terrorism etc.
Censorship: Some info may be deemed inappropriate. Network manager could make sure no other computers can access it. e.g. China blocking sites.
People and machines: Easier communication, more information and efficiency etc. BUT addiction, real life neglect, lack of sleep, health problems, car accidents, technical unemployment, digital alterations (e.g. fake videos, fake news etc).
Digital divide and equality of access: Inequalities regarding use and access to computer systems in different environments/ countries, leads to inequality in info and education access
Surveillance: Monitoring people e.g. for law enforcement, employers, traffic control etc. Ethics of privacy and knowledge/ consent to surveillance
Globalisation & cultural diversity: Spread info and reduce political, geographical, financial boundaries. BUT diminishing of traditional cultures
Ethical/ social issues with the networked world (interconnecting computers):
Ethical/ social issues with the networked world (interconnecting computers): (also ethical issues associated with introduction of new IT systems)
Security: Protecting hardware, software, peripherals, data and networks from unauthorized access
Privacy: Controlling how and to what extent data is accessed and used by others, to protect identity e.g. GPS location services on phone, data sold to companies. But there’s also problems with anonymity e.g. cyber bullying, hacking, terrorism etc.
Censorship: Some info may be deemed inappropriate. Network manager could make sure no other computers can access it. e.g. China blocking sites.
People and machines: Easier communication, more information and efficiency etc. BUT addiction, real life neglect, lack of sleep, health problems, car accidents, technical unemployment, digital alterations (e.g. fake videos, fake news etc).
Digital divide and equality of access: Inequalities regarding use and access to computer systems in different environments/ countries, leads to inequality in info and education access
Surveillance: Monitoring people e.g. for law enforcement, employers, traffic control etc. Ethics of privacy and knowledge/ consent to surveillance
Globalisation & cultural diversity: Spread info and reduce political, geographical, financial boundaries. BUT diminishing of traditional cultures
Environment
How the system will benefit the company
Practical issues to consider when networking:
Practical issues to consider when networking:
Practical issues to consider when networking:
Reliability: How consistently a computer system functions according to its specifications, with minimal system failure. Having a long mean time between failures. Failure can = data/time/revenue loss, injury etc.
(Data) integrity & consistency: Maintenance of accuracy and consistency of data. Must be complete, up-to-date, unaltered. Is inconsistent if there’s different versions of data (duplication)
Standards and protocols: Rules followed in development of systems, including proprietary standards (e.g. computers compatible with Microsoft operating system), industry standards (formally decided, e.g. USB), de facto standards (e.g. QWERTY keyboard)
Prototype:
Iteration
Illustrating system requirements:
Data Flow Diagrams (DFD) show
Prototype: Early sample, version or model of a system/software/hardware, displaying the minimum necessary features, used to test and gather feedback on a new concept or system from clients. Clients can follow development closely and see the changes as they are made. Iteration (iterative design): Where solutions/code/prototypes are designed, developed, tested and evaluated in repeated cycles. With each iteration, additional features may be added until there is a fully functional software.
This involves end user participation. Failure to involve end user in design process can lead to software not suitable for its intended use because of lack of feedback- has adverse effect on user productivity, efficiency etc. Need effective collaboration and communication between client, developer and end-user.
Illustrating system requirements:
Flowcharts show flow of data through program, can show all types of processing and can refer to hardware as well as programs, files, databases etc.
Data Flow Diagrams (DFD) show how data is stored and moved through the system, but not type of data or storage.
Central Processing Unit (CPU):
Control Unit (CU):
Arithmetic Logic Unit (ALU):
Registers:
Central Processing Unit (CPU): Component of a computer system which interprets/ processes and executes program instructions.
Control Unit (CU): Fetches data/ instructions from memory, decodes it into commands and controls transfer of data/instructions among other units of CPU.
Arithmetic Logic Unit (ALU): Performs arithmetic and logical calculations/operations
Registers: Small, very fast circuits that store intermediate values from calculations or instructions inside the CPU
Memory Address Register (MAR): Stores address of next instruction to be decoded and executed. Connected to RAM through Address Bus
Memory Data Register (MDR): Stores data of instruction most recently taken from RAM, i.e. instruction about to be executed. Connected to RAM through Data Bus
Primary memory:
Read Only Memory (ROM):
Primary memory: Only storage directly accessible by CPU
Random Access Memory (RAM): Stores data and instructions of program currently being run. Data stored in memory locations that each have a unique address that the CPU uses. Data constantly moved and overwritten. RAM is volatile- all data/ instructions are deleted once there’s no power
Read Only Memory (ROM): Stores permanent instructions and data of programs used to boot and operate the computer (e.g. Basic Input Output System). Cannot be overwritten. Non-volatile, data and instructions remain even when there’s no power. Smaller than RAM.
Cache memory: Type of memory that is smaller and accessed faster than RAM since closer to the CPU. Stores most frequently used data and instructions so CPU has to access the slower RAM less often and performance speeds up. Computer checks if there’s a copy of data in cache before accessing main memory.
secondary memory
Virtual memory:
Examples: HDD, USB, SD, magnetic tape, floppy disk
Or auxiliary storage. Relatively slow but higher capacity than primary memory. Can be written to but non-volatile. Known as persistent storage because contents are persistent, not wiped out after process is finished, or when there’s no power.
Data and instructions are copied from secondary storage to main memory (RAM) before being executed by CPU. Because RAM has limited capacity & processed data needs to be stored somewhere non-volatile, processed data and instructions are moved to secondary memory to make space for new ones.
Without persistent storage, computer would only be able to perform basic operations and all data re-inputted every time. Need to store processed data so we can use it again later, e.g. editing a saved file.
Virtual memory: Part of secondary memory used as if it is main memory. If RAM is too full to load new program, another program/ part of another program is loaded on virtual memory to make room for the new program. If the other program is needed again, it’s swapped back out
Operating system:
Roles of an operating system:
Operating system: Set of software that controls computer’s hardware resources and provides services for programs.
Roles of an operating system:
1. Peripheral communication: OS controls hardware and provides interface for applications to communicate with peripherals
2. Memory management: Manages how memory is used by applications. OS allocates sections of memory to each program being run, keeps track of what data is in which location. If there’s too many processes for RAM, uses virtual memory to run programs simultaneously
3. Data management: Accessing and storing data. Keeping track of files and location in memory to make best use of memory available and provide reliability
4. Resource allocation: Manages resources and multitasking so tasks share CPU time, including memory and processor time
5. Security: Provides measures such as password authentication, magnetic cards, access rights to prevent unauthorised access. Log files keep track of activity
Binary representation
Binary digit (bit): .
Byte:
. Kilobyte:
Megabyte:
Decimal/ denary:
Binary representation
Binary digit (bit): Basic unit of info, 1 or 0. One digit in a binary number.
Byte: 8 bits. Kilobyte: 1024 bytes. Megabyte: 1024 kilobytes.
Decimal/ denary: Positional system that uses 10 digits to represent a number. Base-10. Binary: Uses 2 digits to represent a number. Base-2
Hexadecimal: Positional system that uses 16 digits to represent a number. Base-16 0,1,2,3,4,5,6,7,8,9,A,B,C,D,E,F
10110112 = base 2, therefore binary
10410 = base 10, therefore decimal
8416 = base 16, therefore hexadecimal
Converting
128 64 32 16 8 4 2 1 1 0 1 1 1 0
^Binary representation of [00]101110. Add up all the values where there’s a 1. Converting from Dec to Binary could involve algorithm: divide by 2, retain remainders until you’re dividing 0 by 2. Rest of the digits are 0 if you haven’t reached 8 digits. Read result last to first to get binary number left to right.
e.g. 104/2 = 52 remainder 0
52/2 = 26 remainder 0
26/2 = 13 remainder 0
13/2 = 6 remainder 1
6/2=3 remainder 0
3/2 = 1 remainder 1
1/2 = 0 remainder 1
0 (add a zero to make 8 digits) 104= 01101000
Constructing truth tables
Constructing truth tables
Example: Tania won’t go to play if it is cold and raining, or if she has not done her homework.
Won’t play = (Cold AND Raining) OR (NOT Homework)
Network:
Node:
Server:
Hub:
Router:
Local Area Network (LAN):
Virtual Local Area Network (VLAN): (virtual can be wireless and)
Virtual is about what is connected
Wlan Is about how its connected.
Wireless Local Area Network (WLAN):
Wide Area Network (WAN):
Storage Area Network (SAN):
Personal Area Network (PAN):
Peer-to-Peer Network (P2P):
Internet:
Extranet:
Virtual Private Network (VPN)
Network: Set of computers or computer systems that are interconnected and share resources and data
Node: Device on network
Server: Computer system or software package that provides service to other devices in network, e.g. ability to file share. Fulfils requests from client programs or computers. Client: Device that requests service from server in the same network, e.g. email software client requesting email server software to fetch new emails
Hub: Connection point for all devices on network, devices connected through Ethernet cables. Data from device goes to the hub, then sent to all devices.
Hub takes data and distributes it to other devices, that is sent through the server.
Switch only sends to device that it needs. (security)
Target device receives, others ignore Switch: Connection point but can identify which device is connected to which port, so can send data to target device
Router: Device that connects network to another e.g. home network to internet.
Types of networks
Local Area Network (LAN): Network devices are connected within limited geographical area, e.g. a room, a home, a school, office building etc. Data and peripherals can be shared. Usually client-server, connected with hub/ switch and Ethernet cabling
Virtual Local Area Network (VLAN): Devices or LANs from different segments in a LAN connected logically, e.g. splitting a hospital LAN into VLANs according to department so each department can only access certain data. Usually via software, not hardware
Wireless Local Area Network (WLAN): Connects nodes in limited geographical area but with no wires. Allows for mobile devices and easier connection/ removal, although less secure
Wide Area Network (WAN): Connects different computer systems or LANs from different geographical areas, can span over a city, country or the world, e.g. internet, different sites of organisations connected
Storage Area Network (SAN): Type of LAN designed to handle large data transfers. Each client is connected to a server that provides access to storage devices
Personal Area Network (PAN): Network covering individual’s working area, very small space. e.g. PC, phone, tablet sharing data. Can be wired (e.g. USB) or not (e.g. Bluetooth) Peer-to-Peer Network (P2P): Different to client-server, no centralised server. All nodes have equal privileges and responsibility for processing and sending data. All are client and server at same time, consuming and supplying resources from and to other peers
Extranet: is a wide area network
Internet: A WAN. Connect to it through internet service provider (ISP) for monthly fee. Not owned by single entity, no central server. All computer systems are independent to share resources. Internet provides services such as support for email, file transfer, and the WWW, which consists of webpages
Extranet: External extension to a LAN- Part of a network that uses internet protocols to allow
controlled access by specific users to LAN or WAN. e.g. business wants to share some data/ information with clients or partners but not all, so extends part of network (extranet) available for access but with security/ privacy measures (e.g. firewall) so outsiders cannot access secure data
Virtual Private Network (VPN)
Allows clients from distant locations to connect, that otherwise wouldn’t be able to connect with LAN (too far for cables) or WAN (too far for signal to be picked up). e.g. if worker goes abroad but still needs access to the network.
Technologies needed…
Can use hardware for public networks like the internet through tunnelling, which allows the network to send data via other network’s connections as if connected to LAN Hardware/ software requirements like internet access, VPN software, routers Protocols to encrypt and authenticate traffic to ensure secure transfer over virtual tunnel, e.g. IPSec (Internet Protocol Security)
Advantages:
Info can be accessed in remote places
No need for long-distance leased lines = lower cost
Enhanced security through encryption, can hide your IP address
Disadvantages:
Needs a professional with detailed understanding of security issues and configuration to ensure sufficient security and protection
Reliability of VPN is not directly under the organisation’s control, but under ISP Not all VPN products are compatible across different vendors
VPN can change working patterns, e.g. changes in costs, flexibility to employees and remote offices, global networking opportunities, less time travelling for remote users
VPN vs Extranet
VPN authenticates sender before establishing tunnel
VPN access and transmission are always encrypted, whereas extranet has limited encryption
VPN users have access to everything wh
Network communication layers
Open Systems Interconnection (OSI) model
Network communication layers
An application goes through different layers to send data between systems. Data packets go through different layers with different protocols before being reassembled at the other application.
Open Systems Interconnection (OSI) model: Model that defines layers of network interaction. Each layer deals with one aspect/ abstraction of network communication
1. Physical- e.g. cabling system components
2. Data link- e.g. Network Interface Card (NIC)
3. Network- Routing
4. Transport- Transmission- error detection
5. Session- Retransmission of data if not received by device
6. Presentation- Encryption and decryption of message for security
7. Application- The end-receiver application, e.g. e-mail
Data packet:
Packet switching:
Protocol:
Data packet: Unit of data for transmission with a format, it is part of a message made into a single package. Contains address and data.
Packet switching: Network communication method: routers direct data packets transmitted along different paths through networks to the best next step closer to destination address (where there’s not too much traffic, or no down devices blocking transmission). Eventually the packet reaches the router that knows the address of the destination device. Packets assemble there
Protocol: Sets of rules to facilitate a process being carried out correctly.
In case of data transfer, protocols are rules that ensure data is transferred correctly between systems. A protocol recognised as the standard for a certain type of transfer is called standard protocol.
Necessity of protocols
Necessity of protocols
Data integrity: Ensures data is not changed or corrupted during transmission Flow control: Controlling flow/load of resources due to limited bandwidth. Transport layer uses protocols, e.g. to request sending application to slow down data flow rate Deadlock prevention: Prevents situation where two or more competing programs or
actions are sharing a resource and preventing each other from using it
Congestion management: Prevents requests on network resources exceeding capacity Error checking
Speed of data transmission
Data compression
Speed of data transmission
Bandwidth: Amount of data that can be transmitted over a certain period of time, i.e. theoretical rate of data transfer or speed. Measured in bps or kbps
Throughput: Actual transfer rate of data
Bottleneck: Created where there are slower segments in a network due to different factors below. Throughput becomes only as fast as the slowest link or path
Data is often compressed (encoding data using fewer bits than the original) to take up less bandwidth and reduce transmission time. Receiver has a program to decompress the file
transmission media
Wireless networking
ransmission media
Wireless: e.g. microwave/ radio signals, satellites, infrared (short distance). Compared to metal cabling and fibre optics, is least reliable and slowest but cheapest
Metal conductor: e.g. copper cable, UTP cable, Coaxial cable. Faster, more reliable and expensive than wireless but less than fibre optic.
Fibre optics: Fine optical fibres carrying beams of light as signals. Fastest and most secure but also most expensive.
Wireless networking
Wireless devices communicate with no cables, e.g. with radio signals. Cell phones, tablets,
Modem:
Wireless router:Wireless repeater: Host Configuration Protocol (DHCP):
Wireless Application Protocol (WAP):
Firewall:
Modem: Allows you to connect to internet
Wireless router: Same as router but also has Wireless Access Point (WAP) to connect to existing wired network and allow wireless data transfer between device and network Wireless repeater: Receives signal from router or access point and rebroadcasts it Dynamic Host Configuration Protocol (DHCP): Allows server to automatically assign IP address to client device.
Wireless Application Protocol (WAP): Provides protocols for accessing info over a mobile wireless network, used in mobile devices to access internet, emails etc.
Firewall: Controls what data comes in and out of network
Encryption:
Symmetric-key encryption:
Public-key encryption:
MAC addresses: Medium Access Control.
User ID:
Firewall:
Encryption: Altering a message into form not understandable to unauthorised person. Only person with the correct key can decode the message and read it.
Symmetric-key encryption: Or single/secret key. Device has a key to encrypt packets, the receiving device has the same key to decrypt. E.g. Data Encryption Standard (DES)
Public-key encryption: Or asymmetric key. Has a public key for encryption and a private one for decryption, both mathematically linked. Secure Socket Layer (SSL) and Transport Layer Security (TLS) encryption protocols used on internet
MAC addresses: Medium Access Control. Every device in a network (their NIC) has a unique MAC address of 12 characters. Network permits access only to specified, trusted devices based on their MAC address. MAC address of an untrusted is device blocked so it cannot connect to the network.
User ID: Use password to access device and/or password to access web interface used to set up wireless routers or access points
Firewall: Either software installed on each device, or router hardware firewall that protects from hackers accessing devices through wireless connection. Filters incoming traffic and can block some messages coming through, as well as control/ limit user’s own access to the internet. Extent of firewall use depends on an organisation’s policy, e.g. one organisation may only allow communication between users and outside the network via email but not accessing websites etc.
