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. CprE 458/558: Real-Time Systems (G. Manimaran)
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
CprE 458/558: Real-Time Systems (G. Manimaran)

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

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

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