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CprE 458/558: Real-Time Systems

CprE 458/558: Real-Time Systems. Chapter 7: Real-Time Networks (WAN). Real-Time communications: Introduction. source. destination. Performance metrics. End-to-end delay: The amount of waiting time till the next packet arrival

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CprE 458/558: Real-Time Systems

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  1. CprE 458/558: Real-Time Systems Chapter 7: Real-Time Networks (WAN) CprE 458/558: Real-Time Systems (G. Manimaran)

  2. Real-Time communications: Introduction source destination CprE 458/558: Real-Time Systems (G. Manimaran)

  3. Performance metrics • End-to-end delay: The amount of waiting time till the next packet arrival • Delay jitter: It is the maximum variation in delay experienced by packets that travel across the connection • Packet Loss: Percentage of packets lost • The nature of the applications dictate the kind of performance requirements required CprE 458/558: Real-Time Systems (G. Manimaran)

  4. Performance metrics • Delay • Delay jitter • Delay-jitter = Max_delay – Min_delay • In the example, Delay-jitter = (D1 – D3) Delay, D2 Delay, D1 M3 M2 M1 D3 D2 M4 M3 M2 D1 M1 CprE 458/558: Real-Time Systems (G. Manimaran)

  5. Applications and Guarantee requirements • Interactive applications require • Bound on both delay and delay-jitter • Can tolerate occasional message loss • Examples: continuous media traffic (video or audio playback) • Discrete applications require • Error-free service • Can tolerate both delay and jitter • Examples: File transfer, Image retrieval CprE 458/558: Real-Time Systems (G. Manimaran)

  6. Providing performance guarantees: Issues • Choice of the packet scheduling algorithm at the intermediate node (switch) • The message scheduling algorithms at the switches determine the order in which the packets from different connections are serviced CprE 458/558: Real-Time Systems (G. Manimaran)

  7. Approaches to Real-time Communication • Pure circuit switching: It reserves the entire physical channel for the connection. E.g., telephone networks • Pure packet switching: It can efficiently utilize network bandwidth but cannot provide real-time guarantees. E.g., Internet • Packet-oriented switching: A virtual channel is established before transmission begins, employs statistical multiplexing to utilize bandwidth efficiently. E.g., ATM (Asynchronous Transfer Mode) network CprE 458/558: Real-Time Systems (G. Manimaran)

  8. Types of service • Guaranteed service: (Deterministic or hard guaranteed service). This approach is conservative in resource reservation (for peak workload) and is the simplest method for real-time services. • Predictive service: This service is meant for adaptive applications that can tolerate occasional violation of delay bound. Multimedia playback applications function well with this category of service. • As-soon-as-possible service: This is best-effort service with priorities, the highest to be given to interactive burst traffic and the lowest to asynchronous bulk transfer. This category of service provides no guarantees, and no resources are reserved for it. CprE 458/558: Real-Time Systems (G. Manimaran)

  9. Real-Time Channel • A virtual circuit that provides the required end-to-end QoS guarantees. • QoS parameters: bandwidth, delay, delay jitter, packet loss, etc. CprE 458/558: Real-Time Systems (G. Manimaran)

  10. Life-cycle of a Real-Time Channel • Channel establishment phase • QoS routing • Resource reservation • Data Transmission phase • Traffic policing/shaping • Packet scheduling • Rate adaptation • Channel tear-down phase • Releasing session resources CprE 458/558: Real-Time Systems (G. Manimaran)

  11. Request for a new connection: I need the so and so QoS guarantees Channel Establishment Phase If yes admit the connection Can the current network condition provide the required QoS ?? If NO reject the connection CprE 458/558: Real-Time Systems (G. Manimaran)

  12. Run-time scheduling phase Which flow to send first?? Router Set of Per-flow queues Node 1 2 3 4 Output Link 5 CprE 458/558: Real-Time Systems (G. Manimaran)

  13. Characterization of Real-Time Traffic • The traffic generated by the real-time sources fall in one of the two categories: • Constant bit rate (CBR): In CBR, fixed-size packets are generated at regular intervals. It is smooth and nonbursty. The data generated by sensors (periodic). • Variable bit rate (VBR): (1) fixed sized packets arriving at irregular intervals or (2) variable-sized packets arriving at regular intervals • Voice traffic (talk spurts alternate with periods of silence) • video source (different compression ratios result in variable size packets generated at regular intervals) CprE 458/558: Real-Time Systems (G. Manimaran)

  14. CBR and VBR examples Source CBR 1 1 1 1 1 1 0 6 12 18 24 30 Source VBR 1 1 1 1 1 3 1 1 0 6 12 14 16 18 21 23 30 CprE 458/558: Real-Time Systems (G. Manimaran)

  15. 1 1 0 1 6 1 12 1 18 24 1 1 1 1 1 18 1 16 14 12 6 0 Change in Traffic characteristics Source CBR 1 1 1 1 1 1 1 1 1 1 1 0 6 Source Switch VBR The CBR now becomes bursty because of cross traffic CprE 458/558: Real-Time Systems (G. Manimaran)

  16. Traffic Models • Peak-Rate Model: Most hard real-time systems use the peak-rate model for traffic characterization. The parameters of this model, for a connection i, are • Minimum inter arrival time (Ti) • Maximum message rate (1 / Ti) • Maximum message length (μi) • End-to-end delay bound (Di) • The peak bandwidth requirement of the connection is (μi / Ti) • The peak-rate model is exact only for the CBR traffic and overstates the bandwidth requirement for all VBR sources CprE 458/558: Real-Time Systems (G. Manimaran)

  17. Peak-rate model: Illustrative example Source CBR 1 1 1 1 1 1 0 6 12 18 24 30 • Minimum inter-arrival time (Ti) = 6 sec • Maximum message rate (1 / Ti) = 1 / 6 = 0.16 message/sec • Maximum message length (μi) = 1 kbits • Bandwidth required = 1 / 6 = 0.16 kbits/sec Exact B/W requirement CprE 458/558: Real-Time Systems (G. Manimaran)

  18. Peak-rate model: Illustrative example Burst Source VBR 1 1 1 1 1 3 1 1 0 6 12 14 16 18 21 23 30 An overstatement of the B/W requirement • Minimum inter arrival time (Ti) = 2 sec • Maximum message rate (1 / Ti) = 0.5 messages/sec • Maximum message length (μi) = 3 Kbits • Peak bandwidth required = 3/2 = 1.5 Kbits/sec CprE 458/558: Real-Time Systems (G. Manimaran)

  19. Traffic Models (contd.) • Linear Bounded Arrival Process (LBAP) Model • This model uses an additional parameter representing the maximum burst size (Bi) • In this model, the number of bits transmitted during any interval of length t is bounded by Bi + (t / Ti) • This model can guarantee deterministic delay bounds CprE 458/558: Real-Time Systems (G. Manimaran)

  20. LBAP model: Illustrative example Burst Source VBR 1 1 1 1 1 3 1 1 0 6 12 14 16 18 21 23 30 An overstatement of the B/W requirement • Average inter-arrival time (Ti) = 6 sec • Maximum message rate (1 / Ti) = 0.16 messages/sec • Burst size (Bi) = 7 Kbits • Maximum message length (μi) = 3 Kbits • Bandwidth required = 3/6 + 7 = 7.5 Kbits/sec CprE 458/558: Real-Time Systems (G. Manimaran)

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