Cpr E 458558 RealTime Systems RealTime Networks LAN

Channel model - Shared medium (Bus, Ring) Node 1 Node 2 Node 3 ………….

Messages (Packets) • Periodic messages (streams) • Aperiodic messages • Dynamically arriving periodic messages

Bus scheduling vs. Processor scheduling • Task Preemption – Tasks can be preempted, but

Real-Time MAC protocols • Access arbitration process – Decides when a particular node should

RT-MAC design issues • Predictability • Timing correctness • Protocol overhead • Channel utilization

End-to-end message delay • Sender: – Application layer: processing and queuing delays – MAC

Real-Time guarantees • Static guarantees – Periodic messages (streams) • Dynamic guarantees – Aperiodic

Inadequacies of traditional LAN protocols • 802. 3 (CSMA/CD) • 802. 4 (token bus)

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

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? Cpr. E 458/558: Real-Time Systems (G. Manimaran) 2

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

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 Cpr. E 458/558: Real-Time Systems (G. Manimaran) 4

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 Cpr. E 458/558: Real-Time Systems (G. Manimaran) 5

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

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

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

Inadequacies of traditional LAN protocols • 802. 3 (CSMA/CD) • 802. 4 (token bus) • 802. 5 (token ring) • 802. 11 (Wi-Fi) Cpr. E 458/558: Real-Time Systems (G. Manimaran) 9