L9 - RT Communication

Question 1

What are the key requirements for Real Time Communication (RTC) Systems?

Answer 1

• predictability (deterministic latencies)
• synchronism (small jitter)
• reliability (few faults & fault handling)

Question 2

What is needed for very fast RTS?

Answer 2

small latencies and guaranteed bandwidth

Question 3

Name the three important layers of Open System Interconnection (OSI) that are essential for RTC.

Answer 3

1. Application Layer
2. Data Link Layer
3. Physical Layer

Question 4

Physical Layer

Answer 4

• Physical Connections (cables + plugs)
• line codes
• network topologies

Question 5

What is speed of light in class and how long does it take light to travel 100m in a glass wire?

Answer 5

2*10^8 m/s —> 100m in 0.5us

Question 6

Define Hub.

Answer 6

• forwards messages to all connected nodes
• all connected devices are in one collision domain.

Question 7

Switch

Answer 7

A Switch reads an address and forwards only to the addressed node.

Question 8

What network topology is wide spread in RTS? Why?

Answer 8

Bus Topology (small wiring effort and simple node adding/removing)

Question 9

What does NRZ stand for? Name a disadvantage.

Answer 9

Non-Return To Zero (Bit Encoding in Physical Level). Differentiation between two bits of the same type is difficult because only the opposing bit forces a transition.

Question 10

NRZ-L

Answer 10

Non-Return To Zero Level: Standard positive logic signal format used in digital circuits where 1 forces a high level and 0 forces a low level.

Question 11

NRZ-M

Answer 11

Non-Return To Zero Mark: 1 forces transition and 0 does nothing (<>NRZ-S).

Question 12

NRZ-S

Answer 12

Non-Return To Zero Space: 1 does nothing and 0 forces a transition (<>NRZ-M).

Question 13

In Biphase Encoding, there is always a transition at the ______________ of a bit period.

Answer 13

In Biphase Encoding, there is always a transition at the beginning of a bit period.

Question 14

MAC-Methods

Answer 14

MAC (Media Access Control) methods assign medium to a node or resolve simultaneous access.

Question 15

Name three network topologies.

Answer 15

1. Star
2. Ring
3. Bus.

Question 16

Master Slave Communication

Answer 16

Master periodically selects slaves (e.g.: Powerlink). Deterministic Central Control.

Question 17

Name the two types of media access control.

Answer 17

1. Distributed MAC
2. Random MAC

Question 18

On what can Distributed Media Access Control based on?

Answer 18

• Token-Based: Control via message (token);
• Time-Based: Control via time-slices (Time Division Multiple Access)

Question 19

Which are the access types in Random Media Access Control?

Answer 19

• Collision Detection: Not collision free (collisions detected and signalled with Jam signal)
• Collision Avoidance: Collision free (select node with highest priority)

Question 20

Distributed Media Access Control

Answer 20

Deterministic Control: every node has an assigned time slot (offline assignment).

Question 21

Collision Detection and Collision Avoidance are two concepts of _______________.

Answer 21

Random Media Access Control

Question 22

CAN

Answer 22

Controller Area Network

Question 23

Application Layer

Answer 23

• upmost layer in the OSI model
• provides an easy-to-use interface
• makes input/output visible to the user.

Question 24

How can errors in RT protocols be detected?

Answer 24

Via checksums or parity bits.

Question 25

Client Server Model

Answer 25

• 1:1 connection
• Message can follow confirmation

Question 26

Main characteristics of communication protocols

Answer 26

• bandwidth
• number of nodes
• physical connection
• cost availability

Question 27

Name types of field buses.

Answer 27

• CAN
• Profibus
• Flexray
• LIN

Question 28

Types of industrial ethernet

Answer 28

• Profinet
• EtherCAT
• Powerlink

Question 29

Time triggered is good for ____________________.

Answer 29

deterministic behavior

Question 30

Trend for RTC

Answer 30

Trend toward ethernet based protocols

Question 31

RC-232 (I/O protocol)

Answer 31

• RC 232 is related to ground
• provides point to point
• still used as industrial port.

Question 32

RS-422 (I/O protocol)

Answer 32

RS-422 is technical standard and uses voltage difference between two cables. Extends RC-232.

Question 33

What are the advantages of Real Time Ethernet? Name a problem that occurs with the use of RT Ethernet.

Answer 33

• Standard Ethernet communication (cheap & available)
• Problem: Standard Ethernet does not support real-time (Collisions: nodes stop transmissions; Jam signals)

Question 34

Powerlink

Answer 34

• Mixture of polling and timeslot
• managing Node controls enforces cycle timing and sends poll requests
• all other devices act as Controlled Nodes and answer with poll responses

Question 35

EtherCAT

Answer 35

• Messages send by master are forwarded by the slaves
• Process on-the-fly

Question 36

Profinet

Answer 36

Pre-defined reserved time slots to transmit real-time data.

Question 37

With which techniques can Ethernet be extended to meet the needs of a RTS?

Answer 37

• Powerlink
• EtherCAT
• Profinet

In general: Ethernet non-deterministic, not desirable for RTS

Question 38

BUS

Answer 38

• message-based
• typically used in automotive and automation tasks
• max. bitrate depends on bus length; CSMA/CA; NRZ-L with bit-stuffing