2.1 Data transmission

• Data packets?

  Made up of?

  - data is broken up into a packet before being transmitted

  - packet header, payload, packet trailer
• packet header contain?

  - IP address of sending station + receiving station

  - sequence number of the packet, so it can be reassembled

  - packet size , so receiving station knows whole packet has been sent.
• payload contains?

  packet trailer contains?

  - actual data

  
  

  - cyclic redundancy check (CRC) a way of identifying the end of the data packet
• What do data packets allow?

  what do router do?

  what is it called?

  what could happen?

  - data to be sent in manageable chunks along the most efficient route.

  
  

  - routers control the path data takes from sending station to receiving station

  
  

  - called packet switching: data can take different route independent of each other

  
  

  - however could arrive at destination in wrong order.
• how do you sort packets arriving at receiving station in the wrong order?

  what stops packets from bouncing around the router and never getting to the recieving station?

  - a sequence num in packet header allows data to be reassembled in right order.

  
  

  - hop numbers are added to packet headers to ensure they don't 'bounce around' from router to router and become lost.
• 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.
• 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.
• simplex :

  half-duplex :

  full- duplex :

  - data can be sent in one direction only e.g to printer

  - data can be sent in both directions but not same time e.g walkie talkie

  - data can be sent in both directions + same time e.g broadband internet connection
• serial transmission?

  
  

  parallel data transmission?

  - data sent one bit at a time down a single wire/channel

  
  

  - several bits of data are sent down several

  wires/channels at the same time ( each wire transmits each bit)
• features of serial transmission? (5)

  1) < risk of external interference than with

  parallel.

  
  

  2) > reliable transmission ---------->

  distances, so used long D transmisison

  
  

  3) Transmitted bits have no risk being skewed.

  
  

  4) Used if amount of data to be sent is

  relatively small, since transmission rate is

  slower than parallel.

  
  

  5) < expensive than parallel due to fewer

  hardware requirements.
• Features of parallel transmission (7)

  1) Faster rate of data transmission than serial-> preferred method where speed important (e.g internal

  connections in a computer).

  
  

  2) good for shorter distances.

  
  

  3) cause several wires/channels being used, data can be

  skewed over --------> distances (no longer synchronised).

  
  

  4) Easier to program input/output operations when parallel used.

  
  

  5) Preferred method for sending large amounts of data.

  
  

  6) best for time-

  sensitive data

  
  

  7) needs > hardware --its > expensive to

  implement than serial ports.
• What is the Universal serial bus USB-A


  is a form of serial data transmission.

  
  

  It is the industry standard.
• USB benefits

  1) Devices plugged into the computer are automatically detected

  and device drivers are automatically loaded up.

  
  

  2) Connections can only fit one way preventing incorrect

  connections being made.

  
  

  3) Can support different data transmission rates.
• USB drawbacks

  1) Standard USB only supports a maximum

  cable length of 5 m; otherwise, USB

  hubs needed to extend cable length.

  
  

  2) Even though USB is backward compatible, very early USB standards

  (V1) may not always be supported by the

  latest computers.

  
  

  4) Even the latest version 3 (V3) and version

  4 (V4) USB-C systems have a data

  transfer rate which is slow compared

  with, for example, Ethernet connections.
• whats the latest type of USB-C and what kind of system does it use?

  - USB- C

  
  

  1) uses. a totally symmetrical

  24-pin Use connection, so it will fit into a USB port either way round since

  there is no up or down orientation.

  
  

  2) USB-C is actually USB 3.1 but has been

  named USB- to distinguish it from the old format which was known as

  USB-A.
• advantages of USB-C compared to USB- A

  1) much smaller & thinner which suits the current trend in making

  devices much thinner

  
  

  2) it offers 100 watt (20 volt) power connectivity which means full-sized

  devices can now be charged

  
  

  3) it can carry data at 10 Gigabits per second (10 Gbps); this means it can now support 4K video delivery

  
  

  4) the USB-C connection is backward compatible (to USB 2.0 and 3.0)

  provided a suitable adaptor is used.