Chapter 2-Data Transmission Flashcards
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 particularly 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
Parts of data packets (3)
Packet header
Payload
Trailer
packet headers components (4)
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(4)
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 (3)
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
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 (3)
Max cable length(5)-USB hub needed
Very early versions not compatible with new computers
Slower transfer rate than Ethernet connections
maximum transfer rate of USB
480 Mbps
Node
Stages in a network that can recieve and transmit data packets. (Routers = nodes in communication networks)
Why can errors during data transmission occur (3)
Interference (cable suffers from electrical interference leading to lost data)
Problems during packet switching
Skewing of data (parallel data transmission, can cause corruption if bits arrive out on sync)
How to check for data transmission errors (3)
Parity checks
Check sum
Parity checks
Form of error checking based on the number of 1 bits in a byte of data
Makes use of even parity (even number of 1 bits) or odd parity (odd number of 1 bits)
Parity is determinedvia an agreement made between the sender and reciever before data is transferred therefore it is also a type of transmission protocol
Error of parity checking
If more than one bit is modified during transmission, but the bytes retained even/odd parity no error would be flagged
parity block
Horizontal and vertical parity check on a block of data being transmitted
Checksum
Method to check if data has been changed or corrupted during transmission. The checksum value is calculated from the block of data being sent and sent after each blocj
Check sum process (6)
When a block is about to be transmitted, a checksum is calculated from the data block
Calculation done with an agreed algorhythm between sender and receiver
Checksum transmitted with block of data
Once received check sum is recalculated from the block of data using the algorhythm
The recalculated checksum is compared with the original
If theyre the same, no errors, if their not a resend request is made
Echo check
When data is sent to another device and then back to the sender again who compares the 2 sets of data to check if theres been any transmission errors. Not reliable as you wont know whether it happened ongthe way there or back
Check digits
Method of identifying errors in data entry, the final digit in a code calculated from all thye other digits. Used in International Standard Book Numbers (ISBN), barcodes and Vehicle Identification Numbers.
Types or error identified by checksum (4)
Phonetic (thirteen/thirty)
Incorrect digit (5203/5223)
Ommitted/extra digits (5203/52030)
Transposition errors (right digit wrong order)
Methods used to generate check digit
ISBN 13
Modulo 11
Parity block
Block of data sent and a parity check is done both vertically and horizontally
ARQ’s
Automatic repeat requests
Uses positive/negative acknowledgements and timeouts. Receiving device receives an error detection code, if no error is detected it sends back a positive acknowledgement. If it detects an error it sends a negative acknowledgement requesting for retransmission. Data is automatically resent if time-out limit is reached until a positive acknowledgement is sent back or a predetermined number of positive acknowledgements is reached.
Acknowledgements
Messages sent to the receiver indicating data has/has not been received correctly
Where are ARQs often used
By mobile phone companies 60 guarantee data integrity
Use of encryption
To minimise the risk of an eavesdropper (hacker intercepting data being transmitted over a public network)
Symmetric encryption
Uses the same encryption key to encrypt and decrypt the encoded message
Asymmetric encryption
Designed to overcome security risks associated with symmetric encryption, makes use of a public and a private key