1. CprE 458/558: Real-Time Systems
Real-Time Networks – LAN
Basic concepts
CprE 458/558: Real-Time Systems (G. Manimaran)

2. Channel model - Shared medium (Bus, Ring)
Node 1
Node 2
Node 3
Node n
………….
Shared Bus
What is the difference between Task Scheduling & Medium Access Control (MAC) in LAN?
What is the difference between Packet Scheduling in WAN and MAC in LAN?
CprE 458/558: Real-Time Systems (G. Manimaran)

3. CprE 458/558: Real-Time Systems (G. Manimaran)
Messages (Packets)
Periodic messages (streams)
Aperiodic messages
Dynamically arriving periodic messages
CprE 458/558: Real-Time Systems (G. Manimaran)

4. Bus scheduling vs. Processor scheduling
Task Preemption
Tasks can be preempted, but packets cannot be
Granularity of priority levels
Task scheduling:
potentially an arbitrary number of priority levels
Packet scheduling:
fixed number of priority levels  priority inversion
Buffering problems
Unique to packet scheduling
Shortening packet deadline (changing the priority) based on buffer availability
CprE 458/558: Real-Time Systems (G. Manimaran)

5. Real-Time MAC protocols
Access arbitration process
Decides when a particular node should transmit
This impacts the throughput, channel access delay, deadline, and fairness characteristics
Transmission control process
Decides for how long a particular node continue to transmit
Static: the channel holding time is predetermined
Dynamic: holding time is determined at run-time
This impacts the throughput, deadline, and fairness characteristics
CprE 458/558: Real-Time Systems (G. Manimaran)

6. CprE 458/558: Real-Time Systems (G. Manimaran)
RT-MAC design issues
Predictability
Timing correctness
Protocol overhead
Channel utilization
Fairness
Stability during transient overloads
CprE 458/558: Real-Time Systems (G. Manimaran)

7. End-to-end message delay
Sender:
Application layer: processing and queuing delays
MAC layer: queuing delay
Channel access time
Transmission delay
Propagation delay
Receiver:
Application layer: queuing and processing delays
CprE 458/558: Real-Time Systems (G. Manimaran)

8. CprE 458/558: Real-Time Systems (G. Manimaran)
Real-Time guarantees
Static guarantees
Periodic messages (streams)
Dynamic guarantees
Aperiodic messages, Aperiodic streams
Hybrid of static and dynamic guarantees
Best effort
CprE 458/558: Real-Time Systems (G. Manimaran)

9. Inadequacies of traditional LAN protocols
802.3 (CSMA/CD)
802.4 (token bus)
802.5 (token ring)
(Wi-Fi)
CprE 458/558: Real-Time Systems (G. Manimaran)