Chapter 2: Data Transmission Flashcards
Definition of data
data is raw facts and figures
Definition of information
Data that has been given context and meaning
Definition of algorithm
A logical, step-by-step process for solving a problem
Definition of data transmission
The movement of data (bits) between at least two digital devices
Definition of data packet
a small part of a message/data that is transmitted over a network; after transmission all the data packets are reassembled to form the original message/data
Definition of a router
A device that enables data packets to be moved between different networks, for example to join a LAN to a WAN
Definition of streaming
Sending data, usually audio or video, in a way that allows it to start being processed before it is complete
Definition of 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
Definition of encryption
The process of making data meaningless using encryption keys; without the correct decryption key the data cannot be decoded (unscrambled)
Definition of hacking
Attempting to bypass a system’s security features to gain unauthorised access to a computer
Definition of protocol
A set of rules that governs transmission of data
Definition of node
Stages in a network that can receive and transmit data packets; routers are nodes in communication networks
Definition of 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 pakcets
Definition of packet trailer
the part of a data packet that indicates the end of the data packet and cyclic redundancy check error check
Definition of cyclic redundancy check (CRC)
an error checking method in which all the 1-bits in the data packet payload are added and the total is stored in the packet trailer; the same calculation is repeated at the receiving station
Definition of payload
the actual data being carried in a data packet
Definition of packet switching
a method of transmission in which a message is broken into many data packets which can then be sent along pathways independently of each other
Definition of hopping/hop 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
Definition of simplex
data that can be sent in one direction only
Definition of half-duplex
data that can be sent in both direction but not at the same time
Definition of full-duplex
data that can be sent in both directions at the same time (simultaneously)
Definition of serial data transmission
sending data down one channel/ wire one bit at a time
Definition of parallel data transmission
sending data down several channels/wires several bits at a time (usually 1 byte)
Definition of skewed (data)
data that arrives at the destination with the bits no longer sychronised
Definition of universal serial bus (USB)
a type a serial data transmission which has become the industry standard for connecting computers to devices via a USB port
Definition of 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)
Definition of 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
Definition of parity block
a horizontal and vertical parity check on a block of data being transmitted
Definition of 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
Definition of 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
Definition of automatic repeat request (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
Definition of acknowledgement
a message sent to the receiver indicating that data has been received correctly (used in the ARQ error detection method)
Definition of timeout
the time interval allowed to elapse before an acknowledgement is received (in the ARQ error detection method)
Definition of 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
Definition of 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
Definition of eavesdropper
another name for a hacker who intercepts data being transmitted on a wired or wireless network
Definition of plaintext
the original text/message before it is put through an encryption algorithm
Definition of ciphertext
encrypted data that is the result of putting a plaintext message through an encryption algorithm
Definition of encryption algorithm
a complex piece of software that takes plaintext and generates an encrypted string known as ciphertext
Definition of symmetric encryption
a type of encryption in which the same encryption key is used both to encrypt and decrypt a message
Definition of asymmetric encryption
a type of encryption that uses public keys and private keys to ensure data is secure
Definition of public key
a type of encryption key that is known to all users
Definition of private key
a type of encryption key which is known only to single computer/user
Definition of quantum computer
A computer that can perform very fast calculation; 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 considerable more data than existing
How big are data packets?
Usually quite small, typically 64KiB
Data packet structure
A typical packet is split up into:
A packet header
the payload
a trailer
What is in the header of a data packet?
- IP address of the source device (sender)
- IP address of the destination device (receiver)
- Sequence number of the packet (allowing correct reassembly)
- Size of the packet (in bytes)
What is in the payload of a data packet?
The actual data in the packets (around 64KiB) - the payload
What is in the trailer of a data packet?
- Method of identifying the end of the packet
- Some form of error checking to ensure packet arrives error-free
What are 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
What are the disadvantages 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
What are the factors that need 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)
Simplex data transmission
Simplex mode occurs when data can be sent in ONE DIRECTION ONLY (for example, from sender to receiver). An example of this would be sending data from a computer to a printer
Half-duplex data transmission
Half-duplex mode occurs when data is sent in BOTH DIRECTIONS but NOT AT THE SAME TIME (for example, data can be sent from ‘A’ to ‘B’ and from ‘B’ to ‘A’ along the same transmission line, but they can’t both be done at the same time). An example of this would be a walkie-talkie where a message can be sent in one direction only at a time; but messages can be both received and sent
Full-duplex data transmission
Full-duplex mode occurs when data can be sent in BOTH DIRECTIONS AT THE SAME TIME (for example, data can be sent from ‘A’ to ‘B’ and from ‘B’ to ‘A’ along the same transmission line simultaneously). An example of this would be a broadband internet connection
What happens when a device is plugged into a computer using a USB port?
- the computer automatically detects that a device is present (this is due to a small charge 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 for permission to connect to the device website)
8 Benefits of USB systems
- devices plugged into the computer are automatically detected and device drivers are automatically loaded up
- connections can only fit one way preventing incorrect connections being made
- it has become an industry standard, which means considerable support is available
- can support different data transmission rates (from 1.5Mbps to 5 Gbps)
- no need for external power source since cable supplies +5V power
- USB protocol notifies the transmitter to re- transmit data if any errors are detected; this leads to error-free data transmission
- it is relatively easy to add more USB ports if necessary, by using USB hubs
- USB is backward compatible (that is, older versions are still supported)
Why can errors 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)
How can data be checked for errors following transmission?
- parity checks
- checksum
- echo check
What is 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
Automatic Repeat Requests (ARQs) process
- ARQ uses positive and negative acknowledgements and timeout
- the receiving device receives an error detection code as part of the data transmission (usually 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 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 the time limit, it automatically re-sends the data until a positiv 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
Serial data transmission
- less risk of external interference than with parallel (due to fewer wires)
- more reliable transmission over longer distances
- transmitted bits won’t have the risk of being skewed (that is, out of synchronisation)
- used if the amount of data being sent is relatively small since transmission rate is slower than parallel (for example, USB uses this method of data transmission)
- used to send data over long distances (for example, telephone lines)
- less expensive than parallel due to fewer hardware requirements
Parallel data transmission
- faster rate of data transmission than serial
- works well over shorter distances (for example, used in internal pathways on computer circuit boards)
- since several channels/wires used to transmit data, the bits can arrive out of synchronisation (skewed)
- preferred method when speed is important
- if data is time-sensitive, parallel is the most appropriate transmission method
- parallel ports require more hardware, making them more expensive to implement than serial ports
- easier to program input/output operations when parallel used
Why are parity checks used?
To check whether data has been changed or corrupted following data transmission
Echo check process
- 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
Name four types of error check digits can identify
- an incorrect digit entered
- transposition errors where two numbers have changed order
- omitted or extra digits
- phonetic errors
