Cpr E 458558 RealTime Systems Chapter 7 RealTime

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

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

Performance metrics • Bandwidth of the connection • End-to-end delay: The total time the packet experienced from source to destination • 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 Cpr. E 458/558: Real-Time Systems (G. Manimaran) 3

Performance metrics • Delay M 3 Delay, D 2 M 2 Delay, D 1 M 1 • Delay jitter M 4 D 3 M 3 D 2 M 2 D 1 M 1 • Delay-jitter = Max_delay – Min_delay – In the example, Delay-jitter = (D 1 – D 3) Cpr. E 458/558: Real-Time Systems (G. Manimaran) 4

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

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 Cpr. E 458/558: Real-Time Systems (G. Manimaran) 6

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 Cpr. E 458/558: Real-Time Systems (G. Manimaran) 7

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. Cpr. E 458/558: Real-Time Systems (G. Manimaran) 8

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

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

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

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

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) Cpr. E 458/558: Real-Time Systems (G. Manimaran) 13

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

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

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 Cpr. E 458/558: Real-Time Systems (G. Manimaran) 16

Peak-rate model: Illustrative example CBR 1 0 1 6 1 12 1 18 1 24 1 30 Source Minimum inter-arrival time (Ti) = 6 sec Maximum message rate (1 / Ti) = 1 / 6 = 0. 16 message/sec Maximum message length (μi) = 1 kbits Exact B/W Bandwidth required = 1 / 6 = 0. 16 kbits/secrequirement Cpr. E 458/558: Real-Time Systems (G. Manimaran) 17

Peak-rate model: Illustrative example Burst VBR 1 0 1 6 1 12 1 14 1 16 3 18 1 21 23 1 30 Source 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 Cpr. E 458/558: Real-Time Systems (G. Manimaran) An overstatement of the B/W requirement 18

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 Cpr. E 458/558: Real-Time Systems (G. Manimaran) 19

LBAP model: Illustrative example Burst VBR 1 0 1 6 1 12 1 14 1 16 3 18 1 21 23 1 30 Source 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 Cpr. E 458/558: Real-Time Systems (G. Manimaran) An overstatement of the B/W requirement 20