L9 - RT Communication Flashcards
What are the key requirements for Real Time Communication (RTC) Systems?
- predictability (deterministic latencies)
- synchronism (small jitter)
- reliability (few faults & fault handling)
What is needed for very fast RTS?
small latencies and guaranteed bandwidth
Name the three important layers of Open System Interconnection (OSI) that are essential for RTC.
- Application Layer
- Data Link Layer
- Physical Layer
Physical Layer
- Physical Connections (cables + plugs)
- line codes
- network topologies
What is speed of light in class and how long does it take light to travel 100m in a glass wire?
2*10^8 m/s —> 100m in 0.5us
Define Hub.
- forwards messages to all connected nodes
- all connected devices are in one collision domain.
Switch
A Switch reads an address and forwards only to the addressed node.
What network topology is wide spread in RTS? Why?
Bus Topology (small wiring effort and simple node adding/removing)
What does NRZ stand for? Name a disadvantage.
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.
NRZ-L
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.
NRZ-M
Non-Return To Zero Mark: 1 forces transition and 0 does nothing (<>NRZ-S).
NRZ-S
Non-Return To Zero Space: 1 does nothing and 0 forces a transition (<>NRZ-M).
In Biphase Encoding, there is always a transition at the ______________ of a bit period.
In Biphase Encoding, there is always a transition at the beginning of a bit period.
MAC-Methods
MAC (Media Access Control) methods assign medium to a node or resolve simultaneous access.
Name three network topologies.
- Star
- Ring
- Bus.
Master Slave Communication
Master periodically selects slaves (e.g.: Powerlink). Deterministic Central Control.
Name the two types of media access control.
- Distributed MAC
- Random MAC
On what can Distributed Media Access Control based on?
- Token-Based: Control via message (token);
- Time-Based: Control via time-slices (Time Division Multiple Access)
Which are the access types in Random Media Access Control?
- Collision Detection: Not collision free (collisions detected and signalled with Jam signal)
- Collision Avoidance: Collision free (select node with highest priority)
Distributed Media Access Control
Deterministic Control: every node has an assigned time slot (offline assignment).
Collision Detection and Collision Avoidance are two concepts of _______________.
Random Media Access Control
CAN
Controller Area Network
Application Layer
- upmost layer in the OSI model
- provides an easy-to-use interface
- makes input/output visible to the user.
How can errors in RT protocols be detected?
Via checksums or parity bits.
