Chapter 2 Flashcards
What are the 3 groups networks are classified into?
- LAN
- WAN
- MAN
What is a LAN?
• A network covering a small geographical area
(e.g. a building) connecting computers & shared devices
• Uses dedicated infrastructure
What are devices connected to in a LAN?
• Computers and devices connected to hubs or switches
What is a hub?
• Hardware used to connect together a number of
devices to form a LAN that directs incoming data
packets to all devices on the LAN
All packets will be sent to every device
• If the address in packet matches the deviced address ➔ packet is accepted; otherwise it is rejected
What is a switch?
• Hardware used to connect together a number of
devices to form a LAN that directs incoming data
packets to a specific destination address only.
- It stores the MAC addresses of devices on a
network and filters data packets to see which
devices have asked for them.
- This makes a switch more efficient when demand
is high.
• More powerful/secure
• Packets will only be sent to devices where the address matches the recipient address
What is the hub/switch connected to and for what reason?
• One of the hub/switch is connected to a router and/or modem to allow the network to connect to the Internet
What is a router?
- Connects devices to form a LAN
- Connects different networks together
- Homes and businesses use it to connect to the Internet
- Can often incorporate a modem within the hardware
What is a modem (Modulator/Demodulator)?
- Enables a computer to connect to the Internet over a telephone line
- Converts digital signals from a computer to analogue signals that are then sent down the telephone line
- A modem on the other end converts the analogue signal back to a digital signal which another computer can understand
What is a WLAN (Wireless Local Area Network)?
- Wireless network communications over short distances
* Uses: Infrared/Radio signals
WAP – Wireless Access Point
- Connects the devices to the wired network
- Uses
- Spread spectrum technology – wideband radio frequency
- Infrared – very short range & easily blocked
Where can a LAN be applied?
- Sending and receiving email
- A company or school centrally storing files
- Using a print server
- Using a file server
What is a WAN (Wide Area Network)?
• Network covering a very large geographical area
(worldwide)
• Multiple LANs are joined together using a router/modem to form a WAN
• Consists of LANs connected via:
-Telephone lines
- Satellites
What do WANs use to connect to each other?
• Public communication networks (Broadband & Fibre)
OR
• Dedicated/leased lines
+ Faster connection / transmission of data
+ Usually more consistent transmission speed
+ More secure (less risk of hacking)
- More expensive to set up/maintain
- Disruption will leave no alternative
What are the characteristics of telephone lines used to transmit data?
- Transmission of data using telephone lines (PSTN)
- The PSTN consists of many different types of communication lines
- Data is transmitted in both directions at the same time // (full) duplex data transmission
- The communication passes through different switching centres
What is a MAN (Metropolitan Area Network)?
- Network which is larger than a LAN but smaller than a WAN, which can cover several buildings in a single city, such as a university campus
- Size restriction – a single city
+ of networking computers?
+ Devices can be shared (e.g.: printers) ➔ costs are reduced
+ Data and files can be shared
+ Licences for software on network are cheaper than buying licences for the same number of individual stand alone computers
+ Access to reliable data from a central source (e.g.: file server)
+ Data & files can be backed up centrally at the end of the day
+ Users can communicate using email and instant messaging
+ Apply/restrict access to certain files and/or networks (e.g. Internet)
- of networking computers?
- Cabling and servers can be an expensive initial cost
- Managing a large network can be complex and difficult
- Malfunction of devices (e.g.: file servers) can affect the whole network
- Malware and hacking can affect entire networks (although afford some protection)
Characteristics of public networks?
- Owned by a communications carrier company
- Many organisations use it
- Usually no specific password requirements to enter it
- Sub-networks may be under security management
Characteristics of private networks?
- Owned by a company/organisation
- LANs/intranets
- With restricted user access, passwords are required to enter
What are the two types of Networking Models?
- Client-Server
* Peer-to-peer
How does a client-server model work?
• Uses separate dedicated servers and client computers connected to the servers
• A server:
- provides services to other computers on the network (e.g.: sending and receiving of email - to a number of “client” computers on the network)
- provides a resource to the other machines on the network (e.g.: File server, Print server)
• A client is a computer that relies on servers to provide and manage data
What do typical servers include?
- file servers - hold and maintain user files
- applications servers - allow programs to be run over a network
- web servers - hold and share web pages
- print servers - manage printing across a network
- mail servers - handle emails between users
Characteristics of a client-server model?
• Users can access files stored on dedicated servers
• Server dictates which users can access which files
• Allows installation of software onto a client’s computer
• Server is responsible for the authentication of user log-ons
• More secure as clients have access to only those resources and files assigned by the network
administrator
• Can be as large as you want – easier to scale up
• If a shared resource is deleted form the server, the back-up would restore it
• Can become bottlenecked if there are several requests at the same time
Where can client-server models be used?
- A web browser is the client software which makes a request to the server, the server finds the website on a webserver and it sends the website to the browser
- An email client is another client program (Client is the front end of an application): provides the interface between the user and the application
• Bank:
When checking your bank account from your computer: your client computer forwards a request to a server program at the bank, the program may then turn forward a request to its own client program, which then sends a request to a database server at another bank computer and once your account balance has been retrieved from the database, it is returned back to the bank data client, which in turn serves it back to your personal computer, which then displays the information to you
How does a peer-to-peer model work?
- All computers have equal status - no computer (node) has control over the network
- There are no servers or clients. Instead, each computer is known as a peer: peers are both a client and a server
- Peers store their own files, which can be accessed by all other peers on the network – no central storage
- No requirement to authenticate users
- It is best suited to smaller organisations that have fewer computers, or where fewer computers need access to the same data
- No more than 10 computers
Where can a peer-to-peer model be used?
- Fairly small network of users
- Ideal for sharing files
- When no need for robust security
- When they need workstation-based applications rather
- Unsuitable for a service such as booking tickets, as one server needs to keep track of how many tickets are left
Client-server VS P2P:
Client server:
The server controls the security of the network
The server manages the network: needs a dedicated team of people to manage the server
Clients are dependent on the server
The server can be upgraded to be made more powerful to cope with high demand
Data is all backed up on the main server
P2P
No central control over security
No central control over network: anyone can set up
Clients are independent
If machines on the network are slow they will slow down other machines
Each computer has to be backed up: data can be easily deleted by users
What is a thick client?
• A full computer that can work online & offline and does not require processing from the server
What is a thin client?
• Connect to and interact with a server.
The device often has a small hard drive or none at all, as data and programs are primarily stored in cloud / on the network’s main servers
• Thin clients have limited functionality and most features do not function when connected to the network
Thick VS Thin Client:
Thin client software:
• Always relies on a connection to a remote server or computer for it to work
• Requires very few local resources (such as
SSD, RAM memory or computer processing time)
• Relies on a good, stable and fast network connection for it to work
• Data is stored on a remote server computer
Thick client software
• Can run some of the features of the software even when not connected to a server
• Relies heavily on local resources
• More tolerant of a slow network connection
• Can store data on local resources such as HDD or SSD
+ of thick clients:
- More robust (device can carry out processing even when not connected to server)
- Clients have more control (they can store their own programs and data)
- of thick clients:
• Less secure (relies on clients to keep their own data secure)
• Each client needs to update data & software individually
• Data integrity issues, since many clients access
the same data which can lead to inconsistencies
+ of thin clients:
• Less expensive to expand (low-powered and
cheap devices can be used)
• All devices are linked to a server (data updates and new software installation done centrally)
• Server can offer protection against hacking and malware
- of thin clients:
• High reliance on the server; if the server goes
down or there is a break in the communication
link then the device cannot work
• Despite cheaper hardware, the start-up costs
are generally higher than for thick clients
What are the 4 ways of connecting computers (network topologies)?
✓ Bus
✓ Star
✓ Mesh
✓ Hybrid
Characteristics of a Bus Network Topology:
!P2P – for small number of devices: company/office!
• Use of a central cable – all devices are connected
to the cable
• Easy to expand
• Requires little cabling
• Data travels in one direction
• While data is sent between devices then other
devices cannot transmit
• Terminators at each end to prevent signal reflection
• Each node looks at each packet and compares the
address – if the recipient’s address matches the
node address packet is accepted
+ of a bus network topology:
- If one node fails, the other nodes continue to function
* Easy to increase the size of the network (adding nodes on the cable)
- of a bus network topology:
- If main cable fails, then the whole network goes down
- Heavy loading lowers the performance
- Each packet passes through every node ➔ not secured network
Characteristics of a Star Network Topology:
!Client-server!
• Each computer/device is connected to a central hub/switch
• Data going from host to host is directed through the central hub/switch
• Useful for networks where
• Devices are frequently added/removed
• There is heavy data traffic
+ of a star network topology:
- Reduced data collision
- More secure b/c packets can only travel to nodes with the correct address (switch)
- Easy to improve – upgrade the hub
- If one connection fails, it only affects one node
- of a star network topology:
- High initial installation costs
* If central hub/switch fails, whole network goes down
What are the 2 types of Mesh Network Topology:
• Type of P2P
- Routing
- Data is directed to its destination by the shortest route; it is re-routed if one node has failed - Flooding
- Data is sent via all nodes
What are the applications of a mesh network topology?
• Internet, MANs/WANs
• Industrial monitor & control (use of sensors)
✓ Medical monitoring of hospitalised patients
✓ Electronics interconnectivity
✓ Modern vehicles use wireless mesh topology to control/monitor all the components of the vehicle
+ of a mesh network topology:
- Easy to identify where faults on the network have occurred
- Broken links do not affect the other nodes
- Good privacy and security, since packets travel along dedicated routes
- Relatively easy to expand
- of a mesh network topology:
- Large amount of cabling is needed ➔ expensive, time consuming
- Set-up & maintenance is difficult and complex
What is a Hybrid Network Topology?
- Mixture of two or more different topologies
- Packets handling depends on which of the topologies are used to make up the hybrid structures
• Application example
- 3 hotel chains, having different topologies
- If all 3 chains are taken over by another company
Hybrid topology can be used to connect all 3 chains
+ of a hybrid network topology:
- Depends on the topologies used
- They can handle large volumes of traffic
- It is easy to identify where a network fault has occurred
- Ideal for creating larger networks
- of a hybrid network topology:
- Can be very complex to install, configure and maintain
* Depends on the topologies used
What is cloud computing?
• Cloud computing is the on-demand availability of computer system resources, without direct active management by the user • Data centers available to many users over the Internet • Covers: - Storage - Databases - Networking - Software - Analytical services using the Internet
What is cloud storage?
• A method of data storage where data is stored on off-site servers
- Same data stored on many servers → access data in case of maintenance/repair
• Owned & managed by a hosting company
What are the three common cloud storage systems?
- Public Cloud – customer & provider are different companies
- Private Cloud – customer & provider is a single entity
- Hybrid Cloud – combination (sensitive data resides in the private cloud
+ of using cloud storage:
- Customer/client files stored on the cloud can be accessed at any time from any device anywhere in the world provided internet access is available
- No need for a customer/client to carry an external storage device with them, or use the same computer to store and retrieve information
- Provides the user with remote back up of data to aid data loss and disaster recovery
- Recovers data if a customer/client has a hard disk or back up device failure
- Offer almost unlimited storage capacity
- of using cloud storage:
- If the customer/client has slow or unstable internet connection they would have problems accessing or downloading their data/files
- Costs can be high if large storage capacity is required
- Expensive to pay for high download/upload data transfer limits wiht the customer/client internet service provider (ISP)
- Potential failure of the cloud storage company is possible - this poses a risk of loss of all back-up data
• Potential data loss when using cloud storage
• Important & irreplaceable data could be lost
• Hackers accessing accounts/pharming could lead to
✓ Data loss
✓ Corruption of data
What are the different wired networking?
- Twisted pair cables
- Coaxial cables
- Fibre optic cables
What are the different wireless networking?
- Wi-Fi
- Bluetooth
- Satellites
How devices connect using bluetooth?
- Sends & receives radio waves in a band of 79 different frequencies
- Devices automatically detect & connect to each other
- The communication parts constantly change channels (frequencies) → less risk of interference with other devices
- Creates a secure WPAN (Wireless Personal Area Network) based on key encryption
When is bluetooth used?
- When devices are less than 30m apart
- When the data transmission speed is not critical
- When using low bandwidth apps (sending music files from smart phone to headset)
What spread spectrum technology seems the best overall to use?
Out of radio waves, microwaves and infrared, microwaves seem to offer the best compromise:
- Reasonable bandwidth, penetration, attenuation
Why are satellites used?
• To overcome the problem that the curvature of the Earth prevents microwaves and radio waves transmitting data globally
How do satellites work?
- Signal is beamed from antenna A to satellite
* Signal is boosted by the satellite and is then beamed back to Earth & picked up by antenna B
Characteristics of twisted pair cables:
- Most common in LANs
- Has the lowest data transfer rate
- Suffers from external interference
- The cheapest option
- Shielded – commercially
Unshielded – used by residential users - The security of transmitted signal is not guaranteed
- Flexible
Characteristics of coaxial cables:
- Most common in MANs & cable television companies
- Better data transfer rate
- Less affected by external interference
- Higher cost than twisted pair cables
- Higher transmission capacity
- Suffers from the greatest signal attenuation → the signal degrades
- Offers the best anti-jamming capabilities
- The security of transmitted signal is not guaranteed
- Flexible
Characteristics of fibre optic cables:
- Most common over long distances
- The best data transfer rate
- Very high resistance to external interference
- High cost
- Single- or multi- mode
- The smallest signal attenuation ➔ signal does not degrade as fast
- Security of transmitted signal is guaranteed
- It cannot be bent b/c it will break
- Only transmits data in one direction
What are the + of fibre-optic cables over copper
cable?
- Less interference in signal
- Greater bandwidth // Faster transmission speeds possible
- More difficult to hack // more secure
- Signal does not degrade as fast // Needs less signal boosting
- Consumes less power
- Easier to install b/c it is lighter in weight
What are the - of fibre-optic cables over copper
cable?
- (Initial) installation cost is higher // Cable / hardware is more expensive to buy (per metre)
- Fibre-optic cables can break when bent
- Specialists / trained personnel needed to install maintain
- Only transmits data in one direction
- If a fibre-optic cable connection fails, many more services can be affected
- Difficult to terminate // Electronics at both ends are more complex
Wired Vs. Wireless:
WIRELESS:
• Devices have increased mobility, provided they are
within the range of the WAPs
• Increased chance of interference from external
sources
• Data transfer rate is slower
• Data is less secure than with wired systems; is
easier to intercept radio waves and microwaves (essential to protect data transmissions using encryption)
• Signals can be stopped by thick walls (e.g.: in old
houses) and signal can vary, or ‘drop out
WIRED:
• Devices are not mobile; they must be close enough to allow for cable connections
• More reliable and stable network
• Data transfer rates tend to be faster with no ‘dead spots’
• Tends to be cheaper overall, in spite of the need to buy and install cables
• Lots of wires can lead to tripping hazards, overheating of connections (potential fire risk) and disconnection of cables during routine office cleaning
What are the hardware requirements of networks?
- Hub
- Switch
- Repeater
- Bridge
- Router
- Gateway
- Modem
- NIC
What is a repeater?
• Devices added to the transmission systems to boost
the signal so it can travel greater distances
• They amplify signals for both analogue (copper cable)
& digital (fibre optic cable) communication links
• To prevent ‘dead spots’ in the Wi-Fi zone
• They plug into electric wall sockets & sent out
booster signals
• Non-selective devices (non-logical) → they boost all
signals
What is a bridge?
• Wired or Wireless
• Used to connect two or more network segments together or two separate LANs that use the same protocol
- the two networks are bridged together in order to work as a single network
What is a router?
- Sit behind a firewall
- Main function: to transmit internet and transmission protocols between 2 networks and allow private network to be connected
- Can be wired/wireless
What is the role of routers?
• Restrict broadcasts to a LAN
• Act as a default gateway
• Can perform protocol transmission
- E.g.: allowing a wired network to communicate with a wireless (Wi-Fi) network – the router can take an Ethernet data packet, remove the Ethernet part and put the IP address into a frame recognised by the wireless protocol (protocol conversion)
• Can move data between networks (forward packets from one network to another)
• Can calculate the best route to a network destination address
How does a router send a data package?
- It inspects the IP address
- Each computer on the same network has the same part of an IP address, so the router is able to send the data packet to the appropriate switch
- The data packet will then be delivered using the MAC destination address
- If the MAC destination address doesn’t match any device on the network, it passes on to another switch on the same network until the appropriate device is found
What is a gateway?
- Device that connects LANs which use different protocol
- Converts data packets from one protocol to another
- Gateways can also act as routers, firewalls, servers
- It functions as an entry/exit point to the network
- Can be wired/wireless
What is the NIC (Network Interface Card)?
• Needed to allow a device to connect to a network
• Contains the MAC address assigned by the manufacturer
• Wireless NICs
- Use an antenna
- Plug into a USB port / can be internal integrated circuit plug in
- Work in two modes
-> Infrastructure mode: requires WAP
-> Ad hoc mode: no need of WAP; devices can interface with each other directly
What is ethernet?
• A protocol used by many wired LANs to send data along this network cable
A network using Ethernet is made up of?
- A node
- Medium (e.g.: Ethernet cable)
- Frame (data is transmitted in frames which is made up of source address and destination address – MAC addresses)
What are the conflicts when using ethernet?
• When using Ethernet, possible for IP addresses to conflict.
- When devices on the same network have the same IP addresses
HOW:
• When a device is given a temporary dynamic IP addresses
• And another devices using static IP address has the same IP address
• This is a problem b/c you can not connect to a network unless you have a
unique IP
SOLUTION:
• Restarting the router – new dynamic addresses will be given which could resolve the problem
What is the Carrier Sense Multiple Access with Collision Detectio (CSMA/CD)?
• Method to detect collisions and resolve the issue
What is bit streaming?
• A contiguous sequence of digital bits sent over the Internet/network that requires a high speed data communication link
• Data is compressed before transmission
• Actual playback is (a few seconds) behind the time the data is received by computer
• Buffering is used to ensure smooth playback (no pausing) of the media files
- As data for a file is downloaded it is held in the buffer temporarily. As soon as enough data is in the buffer the file will start playing
- When buffer is emptied, it fills up again so that viewing is continuous
• Buffer: temporary computer storage area
How is a video is sent using bit streaming?
- Data is compressed before transmission
- Data is sent continuously as a series of bits
- Data is hosted on a media server
- The server sends data to the buffer of the client computers
- The user’s software (media player) receives bit stream from the buffer
What should be considered?
• Data transmission from the file server to buffer must be greater than the rate at which data is transmitted from buffer to media player
+ of bit streaming?
- No need to wait for whole video/music file to be downloaded before the user can watch or listen
- No need to store large files on your device
- Allows video files/music files to be played on demand
- No need for any specialist software
- Affords piracy protection (more difficult to copy streamed files than files stored on a hard drive)
- of bit streaming?
• Cannot stream video or music file if
broadband connection is lost
• Video/music files will pause to allow the data being streamed to ‘catch up’ if there is insufficient buffer capacity or slow broadband connection
• Streaming uses up a lot of bandwidth
• Security risks associated with downloading files from the Internet
• Copyright issues
What is On Demand bit streaming?
- Existing digital are converted to a bit streamed format and stored on a server
- A link to the encoded video/music file is placed on the web server to be downloaded
- The user clicks on the link and the video/music file is downloaded in contiguous bit stream
- The streamed video/music is broadcast to the user as and when required
- It is possible to pause, rewind and fast forward the video/music
What is Real Time bit streaming?
- An event is captured live by camera & microphone and is sent to a computer
- The video signal is converted (encoded) to a streaming media file
- The encoded file is uploaded from the computer to the dedicated video streaming server
- The server sends the encoded live video to the user’s device
- It is live, so it is not possible to pause, rewind or fast forward
What is meant by Internet?
• Interconnected Network
• Network of networks; the global connection of interconnected computer networks (Internet is the
infrastructure)
• Makes use of TCP/IP (Transmission Control Protocol/Internet Protocol)
What is mean by WWW?
- A collection of interlinked multimedia web pages & other documents stored on websites
- Uses http(s) protocols to transmit data
- The WWW uses the Internet to access information from servers and other computers
- Web pages are written using HTML (HyperText Mark-up Language)
- URLs (Uniform Resource Locators) specify the location of all web pages
- Web resources are accessed by web browsers
What hardware/software are needed to connect to
Internet?
Computer/tablet/etc
ISP (Internet Service Provider) – hardware & software
Telephone line connection or mobile phone network connection (or wirelessly using wireless router)
Web browser
Router
How is a user’s voice converted to digital packages?
Using Voice over Internet Protocol (VoIP):
• Packet switching
• Sent over the network via the fastest route
• The conversation is split into data packages – each packet has
- Sender’s address
- Receiver’s address
- Order number of packet
• At the receiving end the packets are reassembled into the original state
Phone call using PSTN:
• Uses telephone connected to a telephone line
• Telephone line connection always open
(even when no one talks). It remains active even during a power cut
• Modern phones are digitised systems and use fibre optic cables
• Dedicated like is used between two points for the duration of the call
• 10mins call will transmit around 10 MB of data → big waste of capacity
• Existing phone lines use circuit switching
Phone call using the Internet:
• Uses internet phone or microphone, speakers & webcam
• The internet connection is only ‘live’ while data (sound/video) is being transmitted
• VoIP converts sound to digital packages which are sent over the Internet; only sent as it is needed so there is no dedicated line. Files undergo some
compression to reduce the amount of
data being transmitted
• 10mins call will transmit around 3 MB of data – b/c the link only exists while data is being transmitted
• Use packet switching
What are protocols used by Internet?
- Rules that must be agreed by senders & receivers on the Internet
- TCP/IP protocols
What does the IP address provide?
- Gives each device on a network an ID (// used to locate a device on a network)
- Each address is unique within that network
- Allows a device to send data to the correct destination
Characteristics of IPv4:
• The most common type of addressing on the Internet is IP version 4 (IPv4)
• Uses 32 bits giving 232 possible addresses
• Consists of four numbers in the range 0–255 separated by full stops
8 bits ➔ 2
8 = 256 (0-255)
possible numbers
Example 253.0.127.76
Problems with IPv4:
• IPv4 system provides insufficient address range
Characteristics of IPv6:
- To allow the Internet to grow in terms of number of hosts and the potential amount of data traffic
- Uses 128 bits giving 2128 possible addresses
- Consists of eight numbers in hexadecimal notation separated by colon. Each number has 4 hex digits
Example A8FB:7A88:FFF0:0FFF:3D21:2085:66FB:F0FA
IPv6 benefits over IPv4:
- Has no need for network address translation
- Removes risk of private IP address collisions
- Has built in authentication
- Allows for more efficient routing
What is zero compression?
- Way to shorten IPv6
* Replace groups of zeros 0000:0000 by a double colon :: (only once)
What is sub-netting (IPv4)?
• Practice of dividing network into two or more sub-networks
- Helps reduce network traffic
- Can hide the complexity of the overall network
Characteristics of private IP addresses:
- IP address reserved for internal network use behind a router & it cannot be reached across the Internet
- Assigned by the router of the network
- More secure
- They are unique within their network, but can be duplicated within other networks
- Allow for separate addresses within a network
- Access to the network without taking up a public IP address space
Characteristics of public IP addresses:
• Address assigned by the user’s ISP to identify the location of their device on the Internet
• Accessible from anybody using the Internet
• Less secure
• They are unique to the Internet
• IP addresses from the private address space are never assigned as public
• Used by:
- DNS servers
- Network routers
- Directly-controlled computers
Characteristic of a Static IP Address:
• A node given a static address always keeps the same address
• Even when a computer disconnects and rejoins a
network
• Address is assigned by the server/ISP
Characteristic of a Dynamic IP Address:
• A node given a dynamic address has a different address assigned to it each time it connects to the network
- This method is known as dynamic host configuration protocol (DHCP)
• Address is assigned by the network OS
+ of Static IP Address:
• Makes it easy for network managers to know which device is which on a network
• Since there is no change of IP address, this is suitable for applications which use Voice over Internet Protocol
(VOIP),gaming applications, and Virtual Private Network(VPN)
+ of Dynamic IP Address:
• Allows more devices to connect than there are available addresses
- When a device disconnects from the network, its address is freed up for another device to use. When the first device reconnects it is assigned any free address
• Relatively lower security risk (as your computer is assigned a new IP address each time you log on)
• Dynamic IP addressing should not be used for VOIP, VPN, playing online games as Dynamic IP addressing is less reliable and could cause the service to disconnect while you are on a VOIP, VPN or gaming
What is the Uniform Resource Locator (URL)?
- Specifies location of a web page
* Website address: http://www.grammarschool.ac.cy
What are the parts of a URL?
protocol://website address/path/filename
protocol:
• http:/https:
website address: • Domain host (www) • Domain name (name of website) • Domain type ✓ .com ✓ .org ✓ .net ✓ .gov , etc • Country code ✓ .uk ✓ .de ✓ .cy, etc
path:
• Web page
filename:
• The item from the web page
What is the Domain Name Service (DNS)?
- Gives domain names for Internet hosts and is a system for finding IP addresses of a domain name
- Domain names eliminate the need for a user to memorise IP addresses
- DNS process involves converting a host name into an IP address the computer can understand
- DNS server contain a database of URLs with the matching IP addresses
What happens when the DNS searches for a website?
• The user opens their web browser and types in the URL and the web browser asks the DNS server 1 for the IP address of the website
• The DNS server 1 can’t find the URL in its database or its cache and sends out a request to DNS server 2
• DNS server 2 finds the URL and can map it to 107.162.140.19; the IP address is sent back to DNS server 1 which now puts the IP address and
associated URL into cache/database
• This IP address is then sent back to the user’s computer
• The computer now sets up a communication with the website server and the required pages are downloaded. The web browser interprets the HTML
and displays the information on the user’s screen
JavaScript Code Characteristics:
- JavaScript file is saved with .html extension
- Written in a separate file which is then called from within the HTML
- Contained within the HTML itself, within script tags
When would a developer want to use JavaScript?
- Whenever the developer wants the user to have processing carried out on the client computer
- Any process which does not involve the software running on the server
- More often the JavaScript is used for collecting data which is to be used by a program running on the server
- In particular, data validation and verification can be handled using JavaScript
PHP Code Characteristics:
• The difference is that any PHP script is processed on the server
• PHP can be:
- Contained in a separate file accessed by the HTML
- Written inside the file containing the HTML. In this case the HTML file must be named with a .php extension rather than the usual .html extension
When would a developer want to use PHP?
- An application will not run quickly if it is constantly transmitting data back and forward between the client computer and the server
- For the particular case of a database application it is imperative that the database remains on the server (or within the system of which the server is part) and that only the results of queries are displayed on a browser screen
- Also any SQL associated with the use of the database needs to be running on the server not on the client
- An example of this will be considered as data extracted from Database in SQL via Query and only result displayed to the user
What is the difference between client-side scripting and server-side scripting?
- Client-side script is run on the computer making the request (makes use of JavaScript)
- Server-side script is run on the web server (makes use of PHP)
- The results are sent to the computer that made the request
What are data tansmission errors?
• All data that gets transmitted may be corrupted or changed in some way (bits to flip from 1s to 0s and vice-versa)
What are some error detection methods?
- Check digits
- Check sums
- Parity checks
- Automatic Repeat reQuests (ARQ)
How does a check digit work?
- A check digit is the final digit of a set of digits, calculated using all other digits of the code; the computer recalculates the check digit after the number has been input
- If there is a match → correct transmission
When is check digit used?
• It is used for error detections on identification codes, such as
✓Barcodes
✓ISBNs
✓Vehicle Identification Numbers (VIN)
What errors can check digit catch?
✓an incorrect digit entered:
ex: 5327 entered instead of 5307
✓transposition errors where two numbers have changed order:
ex: 5037 instead of 5307
✓omitted or extra digits:
ex1: 537 instead of 5307
ex2: 53107 instead of 5307
✓phonetic errors:
ex: 13 (thirteen) instead of 30 (thirty)
How does checksum work?
• Data is sent in blocks and an additional value, the checksum, is also sent at the end of the block of data
• The receiving end recalculates, following an algorithm, the checksum from the data sent
• This value is compared to the checksum transmitted
- If same values => no errors in transmission
- If different values => request for retransmitting the data
How does parity check work?
• Data is sent in bytes
- One bit is allocated as a PARITY BIT and the 7 remaining bits are the actual data
- Parity bit is allocated before transmission takes place
What are the two types of parity checking?
✓EVEN PARITY ensure that an even number of 1-bits is present in each byte
✓ODD PARITY ensure that an odd number of 1-bits is present in each byte
• Before transmission, an agreement is made between sender & receiver regarding which of the parity type is used
Calculating the parity bit:
• Parity bit calculated at transmitting site for each byte
• Recalculated and compared at receiving end
- If matched the data is correct
- Otherwise data is corrupt & must be re-transmitted
What issues arise with parity check?
• It cannot detect if disturbance has affected a byte that would stillm have the same odd or even calculation, but the bits themselves have changed
• Example: Odd parity is used The byte 1001100 is sent The byte 1100100 arrives →No error would be detected
• Solution → Use PARITY BLOCKS
How do parity blocks work?
• A block of data is sent and the number of 1-bits are totaled horizontally and vertically
- So, parity check is done in both horizontal and vertical directions
What can parity blocks check?
1) Identifies that an error has occurred and
2) Indicates where the error is
How does Automatic Repeat reQuest (ARQ) work?
• ARQ uses acknowledgement and timeout
• When the receiving device detects an error following data transmission, it asks for the data packet to be resent
• If no error is detected, a positive acknowledgment is sent to the sender
• The sending device will resend the data package if:
- It receives a request to re-send the data
OR
- A timeout has occurred
• Whole process is continuous until the data packet received is correct or until the ARQ time limit (timeout) is reached
• Often used by mobile phone networks
