2. Data transmission Flashcards
Define a data packet
a small part of a message/data that is transmitted over a network
Define packet header
the part of the data packet that contains the IP addresses of the sender and receiver, and includes the packet number which allows reassembly of the data packets
Define packet trailer
The part of a data packet test indicates the end of the data packet and CRC.
Define CRC
An error checking method in which all the 1-bits I’m the data packet payload are added and the total is stored in the packet trailer.
Define payload
The actual data being carried on a data packet
Define node
Stages in a network that can receive and transmit data packets
Define packet switching
A method of transmission in which a message is broken into many data packets which can be sent along pathways independently to each other
Define router
A device that enables data packets to be moved between different networks
Define real time streaming
the transmission of data over a network for live events where the data is sent as soon as it is received or generated
Define hop/hopping number
a number in a data packet header used to stop data packets that never reach their destination from ‘clogging up’ the data paths/routes
Define simplex
data that can be sent on one direction only
Define half-duplex
data that can be sent in both directions but not at the same time
Define full duplex
data that can be sent in both directions at the same time (simultaneously)
Define serial data transmission
sending data down one channel/wire one bit at a time
Define parallel data transmission
sending data down several channels/wires several bits at a time (usually 1 byte)
Define skewed data
data that arrives at the destination with the bits no longer synchronised
Define USB
a type of serial data transmission which has become the
industry standard for connecting computers to devices via a USB port
Define parity check
a method used to check if data has been transferred correctly; it makes use of even parity (an even number of 1-bits) or odd parity (an odd number of 1-bits)
Define parity bit
a bit (either 0 or 1) added to a byte of data in the most significant bit position; this ensures that the byte follows the correct even parity or odd parity protocol
Define parity block
a horizontal and vertical parity check on a block of data being transmitted
Define parity byte
an extra byte of data sent at the end of a parity block; it is composed of the
parity bits generated from a vertical parity check of the data block
Define checksum
a verification method used to check if data transferred has been altered or corrupted; calculated from the block of data of data being sent; the checksum value is sent after each data block
Define ARQ
a method of checking transmitted data for errors; it makes use of acknowledgement and timeout to automatically request re-sending of data if the time interval before positive acknowledgement is too long
Define acknowledgment
a message sent to the receiver indicating that data has been received correctly (used in the ARQ error detection method)
Define timeout
the time interval allowed to elapse before an acknowledgement is received (in the ARQ error detection method)
Define echo check
a method used to check if data has been transferred correctly; data is sent to a receiver and then immediately sent back to the sender; the sender then checks if the received data matches the sent data
Define check digit
an additional digit appended to a number to check if the entered number is error-free; check digit is a data entry check and not a data transmission check
Define eavesdropper
another name for a hacker who intercepts data being transmitted on a wired or wireless network
Define encryption
the process of making data meaningless using encryption keys; without the correct decryption key the data cannot be decoded (unscrambled)
Define plaintext
the original text/message before it is put through an encryption algorithm
Define cipher text
encrypted data that is the result of putting a plaintext message through an
encryption algorithm
Define encryption algorithm
a complex piece of software that takes plaintext and generates a ciphertext
Define symmetric encryption
a type of encryption in which the same encryption key is used both to encrypt and decrypt a message
Define asymmetric encryption
a type of encryption that uses public keys and private keys to ensure data is secure
Define public key
a type of encryption key that is known to all users
Define private key
a type of encryption key which is known only to the single computer/user
Define quantum computer
a computer that can perform very fast calculations; it can perform calculations that are based on probability rather than simple 0 or 1 values; this gives
a quantum computer the potential to process considerably more data than existing computers
What does a packet header consist of
» the IP address of the sending device
» the IP address of the receiving device
» the sequence number of the packet (this is to ensure that all the packets can
be reassembled into the correct order once they reach the destination)
» packet size (this is to ensure the receiving station can check if all of the
packets have arrived intact).
What does packet trailer consist of
» some way of identifying the end of the packet; this is essential to allow each packet to be separated from each other as they travel from sending to receiving station
» an error checking method; cyclic redundancy checks
How does CRC error checking method work
– this involves the sending computer adding up all the 1-bits in the payload
and storing this as a hex value in the trailer before it is sent
– once the packet arrives, the receiving computer recalculates the number of
1-bits in the payload
– the computer then checks this value against the one sent in the trailer
– if the two values match, then no transmission errors have occurred;
otherwise the packet needs to be re-sent.
Describe packet switching
» each packet will follow its own path (route)
» routers will determine the route of each packet
» routing selection depends on the number of packets waiting to be processed
at each node
» the shortest possible path available is always selected – this may not always
be the shortest path that could be taken, since certain parts of the route may
be too busy or not suitable
» packets can reach the destination in a different order to that in
which they were sent.
State the benefits of packet switching
» there is no need to tie up a single communication line
» it is possible to overcome failed, busy or faulty lines by simply re-routing
packets
» it is relatively easy to expand package usage
» a high data transmission rate is possible.
State the drawbacks of packet switching
» packets can be lost and need to be re-sent
» the method is more prone to errors with real-time streaming (for example, a
live sporting event being transmitted over the internet)
» there is a delay at the destination whilst the packets are being re-ordered.
How are packets lost
they keep ‘bouncing’ around from router to router and never actually reach their destination.
How does hopping work
Each packet has a maximum op number to start with.
Once a hop number reaches zero, and the packet hasn’t reached its destination,
then the packet is deleted when it reaches the next router.
The missing packets will then be flagged by the receiving computer
a request to re-send these packets will be made.
What are the factors needed to be considered when transmitting data
» the direction of data transmission (for example, can data transmit in one direction only, or in both directions)
» the method of transmission (for example, how many bits can be sent at the same time)
» how will data be synchronised (that is, how to make sure the received data is in the correct order).
What happens When a device is plugged into a computer using one of the USB ports
» the computer automatically detects that a device is present (this is due to a small change in the voltage on the data signal wires in the USB cable)
» the device is automatically recognised, and the appropriate device driver software is loaded up so that the computer and device can communicate effectively
» if a new device is detected, the computer will look for the device driver that matches the device; if this is not available, the user is prompted to download the appropriate driver software (some systems do this automatically and the user will see a notice asking for permission to connect to the device website).
State errors that can occur during data transmission
» interference (all types of cable can suffer from electrical interference, which can cause data to be corrupted or even lost)
» problems during packet switching (this can lead to data loss – or it is even possible to gain data!)
» skewing of data (this occurs during parallel data transmission and can cause data corruption if the bits arrive out of synchronisation).
Describe the checksum process
» when a block of data is about to be transmitted, the checksum is calculated from the block of data
» the calculation is done using an agreed algorithm (this algorithm has been agreed by sender and receiver)
» the checksum is then transmitted with the block of data
» at the receiving end, the checksum is recalculated by the computer using the
block of data (the agreed algorithm is used to find the checksum)
» the re-calculated checksum is then compared to the checksum sent with the
data block
» if the two checksums are the same, then no transmission errors have occurred;
otherwise a request is made to re-send the block of data.
Describe echo check
» a copy of the data is sent back to the sender
» the returned data is compared with the original data by the sender’s computer
» if there are no differences, then the data was sent without error
» if the two sets of data are different, then an error occurred at some stage
during the data transmission.
What kind of errors can be detected by check digit
» an incorrect digit entered, for example 5327 entered instead of 5307
» transposition errors where two numbers have changed order, for example 5037
instead of 5307
» omitted or extra digits, for example 537 instead of 5307 or 53107 instead
of 5307
» phonetic errors, for example 13 (thirteen), instead of 30 (thirty).
Describe ISBN-13 method
1) add all the odd numbered digits together
2) add all the even numbered digits together and multiply the result by 3
3) add the results from 1 and 2 together and divide by 10
4) take the remainder, if it is zero then use this value, otherwise subtract the remainder from 10 to find the check digit.
Recalculating ISBN-13 method
1) add all the odd numbered digits together, including the check digit
2) add all the even number of digits together and multiply the result by 3
3) add the results from 1 and 2 together and divide by 10
4) the number is correct if the remainder is zero.
Define modulo-11
1 each digit in the number is given a weighting of 8, 7, 6, 5, 4, 3 or 2 starting from the
left (weightings start from 8 since the number will become eight-digit when the
check digit is added)
2 the digit is multiplied by its weighting and then each value is added to make a total
3 the total is divided by 11
4 the remainder is then subtracted from 11 to find the check digit (note if the
remainder is 10 then the check digit ‘X’ is used).
Recalculateing modulo-11
1 each digit in the number is given a weighting of 8, 7, 6, 5, 4, 3, 2 or 1 starting from
the left
2 the digit is multiplied by its weighting and then each value is added to make a total
3 the total is divided by 11
4 the number is correct if the remainder is zero
Describe ARQ
» ARQ uses positive and negative acknowledgements and timeout
» the receiving device receives an error detection code as part of the data transmission (this is typically a Cyclic Redundancy Check ; this is used to detect whether the received data contains any transmission errors
» if no error is detected, a positive acknowledgement is sent back to the sending device
» if an error is detected, the receiving device now sends a negative acknowledgement to the sending device and requests re-transmission of the data
» a time-out is used by the sending device by waiting a pre-determined amount of time ….
» if no acknowledgement of any type has been received by the sending device within this time limit, it automatically re-sends the data until a positive acknowledgement is received ….
» or until a pre-determined number of re-transmissions has taken place
» ARQ is often used by mobile phone networks to guarantee data integrity.
How do DOS attacks work?
Large number of requests sent to server at once
Useless traffic floods the server
Server will come to a stop trying to deal with the traffic
Prevents users gaining access to the web server
What do cookies do?
Saves log-in information
Provides customized page for user
Enables target advertisements
One-click purchasing
3 functions of browsers
− Allows user to view web pages
− Renders HTML
− Allows user to bookmark/favourite web pages
− Provides navigation features
− Allows (multiple) tabs
− Stores cookies
− Records history of pages visited
− Has a homepage
− Runs active script
− Allows files to be downloaded from website/internet
− Sends a request to the IP address/web server (to obtain the contents of a web page)
− Sends URL to DNS
− Manages HTTP/HTTPS protocol
Location of a device on a network
IP address
Number that uniquely identifies device
MAC address
What are data packets+ 2 other names
Packets, datagrams
A small part of a message/data that is sent over a network which after transmission reassemble to form the original message/data
Why are data packets used
They are usually 64KiB, †hey are much easier to control and splitting them up means that they can be sent along different routes
When would data packets be beneficial
When a particular transmission route is out of action or blocked
What is a drawback of packets
The packets all need to be reassembled at the destination
What are packets split up into
Packet header
Payload
Trailer
What do packet headers consist of
IP address of the sender
IP adress of the reciever
Sequence number of packet
Packet size
What does a payload consist of
The actual data in the packet
Usually 64 KiB
What does a packet trailer contain
A method of identifying end of packet- allows them to separate from eachother
A method of error checking
What are CRC’s
Cyclic redundancy checks
Sending: adds all the one bits in payload and stores as hex value in the trailer
Recieve: recalculates no of bits in payload
Compares to trailer
Asks to resend
What does a router do
Receives data packet and based on info in header decides where to send it next
Describe the stages of packet switching
The message is split up into smaller data packets.
These packets are then independently sent over the network
Along the transmission route are nodes containing router which use the header/ IP address to determine where to send the packet
The shortest possible route is chosen
Packets reach destination in the wrong order
What are the benefits of packet switching
No need to tie up single communication line
Possible to overcome faulty lines by rerouting
Relatively easy to expand package usage
High data transmission rate is possible
What are the benefits of packet switching
No need to tie up single communication line
Possible to overcome faulty lines by rerouting
Relatively easy to expand package usage
High data transmission rate is possible
What are the drawbacks of packet switching
Packets can be lost and need to be resent
Delay while packets are being reordered
More prone to errors with real-time streaming
What is hopping
Hopping is the use of a hop number that decreases by one each time a packet leaves a router
What 3 factors need to be considered when transmitting data
By what are they considered
Direction of data(1 or 2 directions)
Method of transmission(how many bits at a time)
How will data be synchronised (to end up in the right order)
Communication protocols
When does simplex data transmission occur
When data can only be sent in One direction
Eg. Computer toPrinter
When does Half Duplex data transmission occur
When data is sent in Both directions but not at the same time
Eg. Walkie talkie
When does Full-duplex data transmission occur
When data can be sent in both directions at the Same time
Eg. Broadband network connection
When does Full-duplex data transmission occur
When data can be sent in both directions at the Same time
Eg. Broadband network connection
What is serial data transmission
Benefit
Drawback
Eg
When data is sent over a single wire one bit at a time in a single stream(can be simplex etc.)
Data arrives in order
Can be slower than parallel
Computer->USB->printer
What is parallel data transmission
Benefit
Drawback
Eg
When several bits of data are sent over multiple Chanel’s/wires at the same time
Good over short, faster than serial
Can become skewed 20m+
Internal circuits in computer
What is skewed data
Data that is out of order
What is a USB
a type of serial data transmission which has become the industry standard for connecting computers to devices via a USB port
What kind of data transmission is used by USB
Half and full duplex
What does a USB cable consist of
A 4-wire shielded cable with a red and black cable for power and green and white for data transmission
Red-5V
Black-ground
White+
Green- -
What happens when a device is plugged into a computer using a USB port
Computer detects there is a device-voltage change in data signalers
Device is recognized- appropriate software is loaded to communicate effectively
Device is not recognized-user is prompted to download appropriate software
What are the benefits of USB systems(8)
Devices are automatically detected and software is loaded
Only fits in one way -prevents incorrect connection
Support available
Can support different data transmission rates
No need for external power source
Error-free data transmission(is resent)
Easy to add more USB ports with USB hubs
Backward compatible(old versions supported)
Drawbacks of USB
Max cable length(5)-USB hub needed
Very early versions not compatible with new computers
Slower transfer rate than Ethernet connections
What is the maximum transfer rate of USB
480 Mbps
Methods of transmission
Copper cables using electrical pulses
Radio frequency (wireless)
optical fibre (light)
infrared
Use of fibre optic cables
Carry almost 100% of worldwide internet and phone traffic
Bandwidth
Amount of data that can be carried at one time
Bandwidth
Amount of data that can be carried at one time
Serial Transmission
Method:
Advantages:
Bits are sent one signal at a time
Advantages over parallel:
smaller and simpler connectors
crosstalk creates interference between parallel lines and can result in corrupted bytes which would need to be retransmitted
serial links are much more reliable over greater distances than parallel
Parallel Transmission
Signals (bits) are sent simultaneously over a number of parallel wires
Crosstalk
Crosstalk occurs when a signal from one line gets transferred to another line
Skew
Each wire in a parallel cable has slightly different properties so the signals travel down the wires at slightly different speeds. This means the transmission must be over very short distances to avoid this problem
Simplex Transmission
Data travels in one direction only
Half duplex transmission
Data can travel in both directions along a single cable, but not simultaneously (not in both directions at the same time)
(Full) Duplex transmission
Data can travel in both directions simultaneously using two cables
Bit rate and Baud rate
The speed of data is measured in bits per second
The baud rate is the rate at which the signal changes
Baseband means one signal (bit) is sent at a time down a serial connection
In standard baseband, bit rate = baud rate
Why serial transmission can be faster than parallel
With higher bandwidths, more than one bit can be coded in a signal, increasing the bit rate
Bit rate of channel = baud rate x number of bits per
signal
Transmission errors
When data is transmitted, it doesn’t always arrive in the same format that it was sent
Electric interference
Power surges
Synchronisation
Wear and tear on the cable or connectors
Error checking
Computers need methods to check for data transmission errors caused by interference
These methods include the use of:
Parity bits, blocks
Check digits
Check sums
Automatic Repeat reQuests (ARQ)
Parity bits
Computers use either odd or even parity
When sending a byte of data one bit is used as a parity bit
This is bit is set to a 1 or a 0 to make the total number of 1s and 0s in the byte (including the parity bit) odd or even depending on the machine
Using Parity for error detection
When data is transmitted, the parity bit is set at the transmitting end and parity is checked at the receiving end
If the wrong number of bits are ‘on’, an error has occurred
The receiving computer notifies the transmitting end and the data is resent
Check digits
A check digit is an additional digit at the end of a string of other numbers designated to check for mistakes in input or transmission
Printed books and other products have a unique barcode with an ISBN (International Standard book number) EAN (European Article Number)
The first 12 digits of the barcode are the unique item number, the 13th is the check digit
This can be calculated using the Modulo 10 system
Check sums
A check sum works similar to a check digit
The checksum of all bytes in a data transmission is calculated using an algorithm
The check sum is sent with the data
The receiving computer recalculates the checksum based on the data it received and compares it with the checksum received or a known, expected value
If it does not match, the data may have been altered or corrupted during transmission and the data is resent
Automatic Repeat reQuests
Computers using ARQs will automatically return an acknowledgement that the data was correct
The simplest stop and wait ARQ will resend the data if an acknowledgement is not received within a specific period of time period, it will assume that there was a flaw in the data or transmission error
Connecting to the internet
PC > Router > Modem > ISP > Internet
router provides access to local area network
modem connects to your internet service provider (ISP)
- modem no longer used in modern technology
ISP connects you to the internet
Describe serial data transmission
Most reliable
Data is sent one bit at a time down a single wire
Bits are kept separate and low chance of data being skewed
Used in USB
Describe parallel data transmission
Data is sent multiple bits at a time down multiple wires
Quite fast used integrated circuits
Used short distance else high chance of data being skewed
Data sent from computer to printer
Synchronous and Asynchronous data transmission
Synchronous- data transmitted as a continuous stream, ensures data send and received are in sync
Asynchronous- data transmitted in an agreed pattern using control bits, stops data from being mixed up
Describe USB
Asynchronous serial data transmission
When plugged in computer, automatically detected, device driver is loaded and communication can occur
Advantages-
Can’t be inserted incorrectly
Supports different transmission speeds
Backward compatible
Robust and portable
Describe integrated circuits
-Uses parallel data transmission
-IC require high speed
Internal clock used to correct timing of data making it synchronised
-Uses many wires
Simplex, duplex, half-duplex
Simplex:
-Data transmission in only one direction
Duplex
-Data transmission both directions simultaneously
Half-duplex
-Data transmission both directions but not at the same time
Why choose serial over parallel
More accurate over long distance
Less chance of interference that will skew or distort data
Data will arrive in order
Cheaper to purchase/install/maintain
Why choose parallel over serial
Faster transmission speed
Can send lots of data at one go
More efficient short distances
data transmission types (5)
simplex
duplex
half-duplex
full-duplex
serial
parallel
simplex
data transmitted one way only
duplex
data transmitted in both way
half-duplex
data transmitted in both ways but not at the same time
full-duplex
data transmitted in both ways at the same time
serial(2)
one bit sent at a time over a single wire (eg. computer to printer)
used to send data externally between devices
parallel (2)
multiple bits sent at the same time using multiple wires
- used to send data internally
why use serial instead of parallel when connecting device to PC
single wire reduces the cost
single wire means less chance of corruption
more reliable over time
bits still synchronised after transmission
asynchronous transmission (2)
Data is sent in form of byte or character
- time interval of transmission is random
synchronous transmission (2)
Data is sent in form of blocks or frames
- time interval of transmission is constant
USB (7)
universal serial bus
data transmission method
uses serial transmission
device automatically detected
only fits one way; it cant fit incorrectly
industry-standard
backwards-incompatible
methods of error-checking (4)
parity check
checksum
ARQ; automatic repeat request
check digit
how does parity check work (5)
checks byte of data
a parity bit is added
counts num of 1s
can be odd or even
if parity incorrect, error is detected
when will parity check not work (2)
multiple errors in same byte
## 2 bits interchanged
how does ARQ (automatic repeat request) work (6)
uses acknowledgement
check performed on receiving data
if error detected request sent to respond data
resend request repeated until data sent correctly
send an acknowledgement that data is received
if acknowledgement not received in set time data is resent
checksum (4)
values calculated from the data
it is transmitted with the data
value recalculated after transmission
values compared after transmission to check for error
What is simplex data transmission?
data transmission in one direction only
What is half-duplex data transmission?
data transmission in two directions but not at the same time
What is full-duplex data transmission?
data transmission in two directions at the same time
What is serial transmission?
data sent one bit at a time over a singe wire or channel
What is parallel transmission?
several bits of data sent down several wires/channels at the same time
What is asynchronous transmission?
data being transmitted in an agreed bit pattern
What is synchronous transmission?
a continuous stream of data which is accompanied by timing signals generated by an internal clock
What is a Universal serial bus? (USB)
an asynchronous serial data transmission method which has become an industry standard
What are the advantages of a USB?
the device is automatically detected
can only be entered one way around (prevents false connections)
device drivers are automatically loaded
What are parity checks?
Allows us to see if any bits have been skewed in data transmission
can either be odd or even number of 1’s (parity complete with parity bit)
What are Automatic repeat request (ARQ)?
Uses an acknowledgement to indicate data is received correctly
A timeout is used which is the time allowed to elapse before an acknowledgement is received
What is checksum?
an additional value known as checksum is sent at teh end of the block of data
the checksum is based on the number of bytes in the block
if the checksum does not match on the receivers end, then an error has occoured
What is an echo check?
the data is returned to the sender
the sender then compares the data received with the data sent
if they are different then an error in data transmission has occoured
What is the issue with echo checks?
the data could become skewed when transferred back and not have been affected when sent to the receiver
Define data transmission
data transmission is the movement of data bits between at least 2 digital devices over short and long distances
what is serial transmission
data is transmitted one bit at a time over one single wire or channel, it is synchronous, it is slower then parallel. it is for long distances
e.g USB
what is parallel transmission
data transmitted multiple bits at a time over multiple wires/channels. it is asynchronized, it is expensive, it is fast, its for short distances
e.g Integrated Circuit
what is simplex transmission
it is in 1 direction
e.g computer to printer
what is half-duplex transmission
this is data transmitted in both directions but not simultaneously
e.g walkie talkie
what is full-duplex data transmission
it is in both directions and simultaneously
e.g phone call
what is synchronous data transmission
it is transmitted in a continuous stream of data
the data is accompanied by timing signals
ensures sender and receiver are synchronized
timing must be accurate
faster than asynchronous
what is asynchronous data transmission
it is transmitted in discrete packets
each packet of data is sent with control bits
so the receiver knows when the data starts and ends
prevents data being mixed up
sent with time intervals
slower than synchronous
What happens when a device is plugged into a computers using one of the USB ports?
The device is automatically detected.
a device driver is loaded.
describe parity checking
this is used to check whether data has been changed or corrupted following transmission from one device to another
even parity - even number of 1s
odd parity - odd number of 1s
method of parity checking
both parties agree on protocol
sender adds parity bit to data
sends the data
receiver checks that it is the parity bit agreed
it is incorrect, the receiver requests for the data to be resent
describe automatic repeat requests
Uses acknowledgement and time out
Check performed on received data
If error detected, request sent to resend data
if acknowledgement sent data is correct
If no acknowledgement is sent that data is received
Data is resent / Resend request repeated, till data is resent correctly
or request times out // limit is reached
describe checksum
Sending device creates value from calculation on data
Value is transmitted with the data
Receiving device performs same calculation
Values are compared after transmission
If values do not match an error is detected
describe echocheck
receiver sends a copy of data immediately back to the sender for comparison - you cant tell when error occurred
What are the parts of a data packet?
Packet header
Payload
Trailer
What are the parts of a data packet?
Packet header
Payload
Trailer
What does the packet header contain?
Destination IP address
Sender’s IP address
Packet number
What are the steps of packet switching?
Data is broken into packets and given a sequence number
Each packet will take a different route to the next depending on the quickest route
The router controls the route that the packet takes
When packets arrive, they are reordered based on their packet number
What is serial transmission?
Data bits are sent one at a time down one cable at high speeds
What is serial transmission used in?
USB
What are the disadvantages of serial transmission?
Lower transmission speeds
What are the advantages of serial transmission?
Lower interference
More reliable over long distances
Smaller, cheaper connections to produce
What are the advantages of USB?
Backwards compatible
Cannot be inserted upside down
Do not need an additional power source ~ 5V power
Industry standard
What are the disadvantages of USB?
Only supports cable lengths of 5m
Slower data transfer speeds
Limitations of backwards-compatability
What is parallel transmission?
Several bits of data sent simultaneously across several wires
What is parallel transmission used for?
Printer cables or internal device connections
Disadvantages of Parallel transmission?
Data corruption commonly occurs over long distances
Skewing
Can only be used for short distances
What is skewing?
When data bits travel parallel to each other can move at different speeds causing them to arrive out of sync
Advantages of Parallel transmission?
Very fast data transfer
Can send data in both directions at the same time
What is simplex transmission?
Data travels only in one direction down a single cable
What is an example of simplex transmission?
Sending data from the computer to a monitor as it never needs to go the other way
What is half-duplex transmission?
Data can travel in both directions along a single cable, but not simultaneously
What is an example of half-duplex transmission?
Printer parallel cables
Text is sent to a printer, and the printer can send ‘Out of paper’ messages back; but only when the computer has finished sending text
What is an example of half-duplex transmission?
Printer parallel cables
Text is sent to a printer, and the printer can send ‘Out of paper’ messages back; but only when the computer has finished sending text
What is duplex transmission?
Data can travel in both directions simultaneously using two communication channels
What is an example of duplex transmission?
Networking or Internet cables to transmit and receive at the same time
How does data transmission work?
Data in digital computers consist of numbers. To transmit numbers across a channel the numbers have to be converted to a physical form such as electrical currents or pulses of light. Numbers in computers are in binary (base 2) so only two physical states are required for transmission e.g. 0v and 5v.
Communication may be in guided (e.g. along a wire) or unguided (e.g. broadcast to air) channels: in both cases the data are encoded as electro-magnetic signals. Signals decrease in strength with distance and need repeating and error checking mechanisms to ensure revcovery of the data.
What is serial data transmission?
Bits are sent via an interface one bit at a time over a single wire from the source to the destination
Everything is sent in order
When is serial data transmission used?
Fibre optics
USB’s (universal SERIAL bus, duh)
What is parallel data transmission?
Several bits are sent simultaneously over a number of parallel wires
More risk of bits not arriving in order, so error detection and correction is necessary
When is parallel data transmission used?
Printer using a parallel port
Integrated Circuits (IC)
Why would you choose serial or parallel data transmission?
Long distance communication uses serial transmission because it is easier to regenerate the signal on a single line (by repeaters). It is also easier to route single lines through switches and cheaper than using multiple cables.
Parallel transmission is confined to short distances because of the problem of skewing, the weight and expense of the cable and the difficulties of switching multiple lines through exchanges or routers.
Describe what is meant by ‘encryption’.
The process of converting data into a format that cannot be understood by an unauthorised viewer, unless you have access to the key needed to decrypt it
(conversion of plaintext into ciphertext)
What is meant by ‘plaintext’?
Plaintext describes the original unencrypted text
What is the need for encrypting data?
To protect the data’s confidentiality
Give examples of two forms of encryption.
Symmetric encryption
Asymmetric encryption
Describe the difference between symmetric and asymmetric encryption.
Symmetric uses the same key to encrypt and decrypt the data
Asymmetric uses two different keys - public key to encrypt the data and a private key, which only the recipient knows, to decrypt it
Describe how a Caesar cipher works.
Realigns the alphabet, substituting each letter of the original message with a letter further down the alphabet
(systematic shift, all letters affected in the same way)
(Exam-style question)
Explain one reason why the Caesar cipher is a weak encryption algorithm. (2)
The number of keys are limited, making it easy to use brute force to decrypt
Bit
Binary digit
Image file size formula
File size(bits) = width(px) x height(px) x colour depth(bits)
Hexadecimal
Used to avoid errors when manipulating numbers
Represend binary numbers using smaller digits (1 hexadecimal digit = 1 nibble)
Computers don’t process hexadecimal
Units
kilobit - 1000 bits
megabit - 1000^2 bits
gigabit - 1000^3 bits
kilobyte - 8 x 1000 bits
megabyte - 8 x 1000^2 bits
gigabyte - 8 x 1000^3
kibibyte - 1024 bits
mebibyte - 1024^2 bits
gibibyte - 1024^3 bits
Compression
Reduce file sizes by repackging or removing some data
Compression
Reduce file sizes by repackging or removing some data
Lossless compression
Reduce file size while retaining the exact meaning of the original data.
Lossy compression
It reduces file size by permanently deleting some of its data.
How is text represented
Characters in texts
How are characters in text represented
Using the ASCII table, each character has a unique bit pattern.
Is a 7-bit code, there are 128 sequences/patterns.
How do computers record sound
With a microphone (or a recording device) a computer will take samples at a fixed interval (usually thousands of samples per second), it will then convert these values into binary to be stored in a file.
Audio file size formula
file size (bits) = sample rate(Hz) x bit depth(bits) x recording length (seconds)
Signed and Unsigned integer
An unsigned integer has no positive or negative sign, a signed one does.
Signed: -8, +8
Unsigned: 8
Flowchart symbols
“Sausage” - Start/Stop
Rectangle - Process
Rhombus - If/elif/else (conditionals)
Parallelogram - Input / Output
Rectangle with vertical borders - Subprograms
Iteration
Repetition of code
Decomposition
Breaking down a big problem into smaller parts
Flowchart symbols
Terminal/”Sausage” - Start/Stop
Rectangle - Process
Diamond- If/elif/else (conditionals)
Parallelogram - Input / Output
Rectangle with vertical borders - Subprograms
Iteration
A repetition of code
Decomposition
Breaking down a bigger problem into smaller parts
Abstraction
Removing unnecessary details so that only the most important details remain
Error types
Syntax
Logic
Runtime
What device sends packets across a network?
Routers
What is a data packet broken down into?
Header:
- Number of data packets
- Packet number
- Source and Destination Addresses
- Protocol
Payload:
-Data
Trailer:
- Any error checking information
Packet switching
Method of transmitting data packets across a network where other transmissions are happening at the same time. The router uses a lookup table to find the most efficient route, if it can’t find where to forward it, it will ask other routers to use their lookup table until the destination is found
Serial data transmission
Data is sent one after another on the same channel
Parallel data transmission
Data is sent simultaneously along multiple channels.
Why is parallel data transmission unreliable over longer distances?
Data can arrive at different times due to different speeds
Interference can be caused between the different channels
USB 3 advantages
Low power consumption
Easy to use
Can charge connected device
Parity bit how does it detect for errors?
A parity bit is set, even or odd.
After transmission, the amount of ones are counted and if it doesn’t follow the parity bit, we know there was an error during transmission.
Why might parity bit not detect a transmission error?
If bits are interchanged, it won’t detect an error since the amount of ones would technically remain the same after transmission.
Check sum what is it?
An additional piece of data added onto the data being transmitted which is also derived from that data.
What is check digit?
An extra digit added to the end of some numeric data. It is calculated from the other digits in a way that can be replicated, which allows it to be checked after transmission.
Echo check steps
Data is transmitted from A to B
B transmits a copy of the received data back to A
A checks data received matches original data sent
If data matches, A sends acknowledgement to B
If data does not match, A resends original data
what is data transmission?
Is the transfer of data of a point to point (or multipoint) communication channel
What is serial data transmission?
is a form of data transmission were bits of characters are sent one at a time along a communication path
serial data transmissions travel over a single wire in one direction
what are advantages and disadvantages of serial data transmission?
advantage - travels over a single wire at much less cost
disadvantage - Slower than parallel data transmission
what is parallel data transmission?
Is a transmission of data where several bits are transmitted simultaneously, each along its own separate channel or multiple channels
what are advantages and disadvantages of parallel data transmission?
Advantage - faster then serial
disadvantages - not appropriate for long distance. Data transmission
- can be expensive.
