2.1 Types & Methods of Data Transmission
Data Packets
Data sent over long distances is generally broken down into data packets. This is because small packets are easier and quicker to send over the internet. These packets are sent via TCP (Transmission Control Protocol). TCP is used for organising data once it has been sent over the internet.
Packet Structure
A typical packet is split up into 3 parts; a header, payload, and a trailer.
Header
The IP address of the sending device
The IP address of the receiving device
The sequence number of the packet (may also include the total number of packets)
The packet size
Payload
Contains the actual data of the packet (usually 64 KiB)
Trailer
This is a way to identify the end of the packet.
Used as an error checking method ()
Packet Switching
This is a way to send data packets individually from the start to the end point and then being reassembled into the correct order once they've arrived at their destination.
A message is split up into data packets
These packets are all labelled with their headers and trailers
They are then sent into the internet
Routers will route every individual packet based on the information in the header
The routing selection depends upon the number of packets waiting at each node
Each packet will travel independently as fast as it can to reach the destination
The packets are then resorted into the correct order using the information in the trailer
There is no need to tie up a single communication line
More prone to errors with real-time streaming
Possible to overcome failed routes by rerouting
There can be a delay while packets are re-ordered
Relatively easy to expand the network
Packets can get lost
High data transmission rate is possible
Packets can need resending
To overcome the issue of lost packets hogging up the system. Each packet has a hop number in its header. Every time a packet visits a router, that number decreases by 1.
CRC's
This stands for a cyclic redundancy check. These are used to check data packets:
The sending computer adds up all of the 1-bits in the payload and stores this as a hexadecimal value in the trailer of a packet before it's sent.
Once a packet arrives, the receiving computer recalculates the number of 1-bits in the payload.
The computer then checks this with the trailer's hexadecimal value.
If both match, then no transmission errors have occurred. Otherwise, the packet is re-sent.
Data Transmission
There are 3 main types of data transmission modes:
Simplex
Half-duplex
Full-duplex
Simplex
Transmissions are unidirectional (travel in one direction only)
(Serial simplex) -> one wire transmits one bit at a time in one direction
(Parallel simplex) -> multiple wires transmit one bit at a time in one direction
Half-duplex
Transmissions are bidirectional (travel in both directions) but not simultaneously
(Serial half-duplex) -> one wire transmits one bit at a time one direction at a time
(Parallel half-duplex) -> multiple wires transmit multiple bits one direction at a time
Full-duplex
Transmissions are bidirectional (travel in both directions) at the same time
(Serial full-duplex) -> one wire transmits one bit at a time in both directions at the same time
(Parallel full-duplex) -> multiple wires transmit send multiple bits of data in both directions at the same time
Cheap
Cheaper than simplex
Expensive
Slow data transmission
Slow data transmission
Fast data transmission
Needs 2 wires for bidirectional transfer
Serial & Parallel
You also have serial and parallel data transmission. The difference between the two is that serial data transmission occurs when data is sent one bit at a time over a single wire as a single stream and is generally used for long distances. However, parallel data transmission occurs when several bits of data are sent at once through several wires all at the same time. It is generally used for shorter distance as for distances above 20m, data can become skewed (arrive unsynchronised).
Data will arrive in the order it is sent
Fast data transmission
Less likely to have errors
Expensive over short distances
Cheap over short distances
Very expensive over long distances
Slow data transmission
Delays if data arrive unsynchronised
Expensive over long distances
Buffers to store data while waiting for bits
USB's
It stands for Universal Serial Bus and is a way to send unsynchronised serial data between devices. There are a few types such as, A, B and C.
When a device is connected to a USB, the computer automatically detects that the device has been connected and the appropriate driver is loaded so that the device can communicate with the computer.
If the device is new, the computer will look for a matching device driver. If one cannot be found, then the user must download and install and appropriate driver manually.
Devices automatically detected
Maximum length of cable is 5m
Driver automatically loaded
Older versions have limited transmission rates
Only fit it one way so ensures compatible data transmission
Very old USB standards may not be supported in the near future
USB usage is standardised
Different transmission rates supported
Newer standards are backwards compatible
Exam Questions
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