Real-Time Communications Flashcards
What are the standard of the International Organizaiton for Standardization?
- Application Layer
- Presentation Layer
- Session Layer
- Transport Layer
- Network Layer
- Data Link Layer
- Physical Layer
What are the functionalities for Physical Layer
Physical connections
- Cables
- Plugs/ Sockets
Line Codes
Network topologies
What is Line Codes
- Representation of the bits
- voltage between metallic wires
-Differential voltages between metallic wires - Light pulses in optical fibers
Example :
Non Return to Zero 1. positive voltage, 0 zero volt
Return to Zero
- voltage between metallic wires
- 1 high for the first half of the bit period, low for the rest
What does a Hub do
Forwards messages to all connected nodes
What does a Switch do
Reads address, fowards only to the addressed node
What constitues the Data Link Layer?
- Media Access Control (MAC)
- Transmission error detection
What is CSMA/CD?
Carrier Sense Multiple Access/ Collision Detection
- Sending is only allowed if bus is free
- if several nodes start to send at the same time, collision is detected; the nodes restart sending after a random waiting time.
- standard Ethernet
Which of the MAC methods are real-time compatible
Only the Deterministic ones
Methods for Error detection in non real-time protocols
Checksum / CRC
Parity Bits
Functionalities for Application Layer
- Data input and output
- Makes the other layers simpler to use
- functions for the programmer of an embedded system in the network
[Communication Modes] Client Server Model
- One to one connection
- A message can be followed by a confirmation
[Communication Modes] Producer consumer model
- One producer
- All Nodes Listen
- Normally no confirmation
Applied Protocols
IO Protocols
Traditional Ethernet Protocols
Examples of Industrial Ethernet Protocols
EtherNet / IP (Industrial Protocol)
Profinet
EtherCAT
How to decide use which Protocol?
- Required bandwidth
- Maximum number of nodes
- Length of possible physical connection
- Cost
- Availability of components
Is standard Ethernet suitable for hard real-time applications?
Ethernet is not suitable for hard realtime application dur to the fact that the bus access is not deterministic
There can be collisions and random waiting times that can lead to jitter in the message sending period.
This normally requires a master-slave system. Examples are EtherCAT or POWERLINK.
Name the topoloty that is wide-spread in real-time systems
Bus topology
- small wiring effort
-simple node adding
What happens when two nodes start sending at the same time? (CSMA/CD)
- Frame ready?
- bus free/ idle
- start transmission and monitor bus
- collision detected?
– send jam signal
– increment retransmission counter
– abort if max. number of retransmissions reached
– go to frame ready - end of transmission
What does Application Layer do?
- Data input and output
- Make the other layers simpler to use
- Functions for the programmer of an embeddedsystem in the network
The EtherCAT system can use a single Ethernet frame (Ethernet packet) to send data from the master device to several nodes (slaves). At the same time, those nodes send their data to the master device. How can this be achieved?
The message from the master travels through the nodes which extract or insert data in the passing frame (on-the-fly, not copy and repeat)
