Overview

• Distributed platforms are becoming popular for real time applications
• This since distributed solutions are cost effective because they use several pieces of low cost hardware
• A distributed real time system needs support of real time communication not the traditional communication. This since
  • traditional communication gives best effort service
  • real time communication has specific quality of service demands like maximum possible delay, maximum loss rate and so on

Real time communication

• real time communication is one where once network accepts the connection it guarantees quality of service
• characteristics of a real time communication
  1. timeliness – message must be delivered in predefined deadlines
  2. predictability – must gurantee bounded delay and low jitter for each message
  3. determinism – transmission occurs with predictable timing not affcted by variable system or load
  4. reliability – minimal loss or corruption of data
  5. QoS – minimum quality of service levels
  6. prioritization – messages marked higher priority are delivered first
  7. synchronization – coordinated timing to align communication between distributed compontns
• Quality of service (QoS) – key parameters
  1. Bandwidth
  2. Maximum transmission delay
  3. Delay Jitter
  4. Loss Rate
  5. Blocking Probability
• Real time systems can be of types hard, firm, soft. And defining real time communication QoS for each of these is difficult
• Hence real time communication is classified as
  • Real time communication
    • examples: manufacturing automation, automatic chemical factory, internet banking, video conferencing, internet telephony
  • Soft (non) real time communication
    • example: Email, FTP
    • QoS for soft real time communication – FTP
      • loss free operation – in case of ftp even losing 1 packet will mean entire document becoming corrupted
      • limit to average delay and average throughput
    • QoS for soft real time communication – audio broadcasting, mulit media application
      • stringent limits on jitter but insensitive to delays

QoS Parameters

Delay Jitter

• Delay consists of 3 components
  • propogation – 5ms / km
  • transmission – packet size / link speed
  • queuing delay
• Delay Jitter
  • difference between maximum and minimum packet delay that can be encountered
• Why jitter happens?
  • in case of high load queuing starts happening increasing delay
  • each packet may take different paths to reach destination hence jitter
• How to fix delay jitter?
  • A buffer at the receiver end can control jitter
  • it is important to have right buffer size otherwise buffer overruns may happen
  • buffer size = peak rate of arriving messages X delay jitter
    • video source transmits 30 frames/sec with 1 frame=2MB and delay jitter of 5sec
    • buffer size = 30 x 2MB x 5 = 300MB

Bandwidth

• It is the maximum throughput (bytes per sec) and could define the bounds on end to end delay
• Delay experience is inverse of the bandwidth

Loss Rate

• % of all transmitted packets that are lost
• Loss of packets could be due to
  • delay bound variation – inconsistent maximum transmission delay making packets arrive too late for real time system
  • delay jitter bound variation – excessive jitter can exceed what buffer can compensate leading to loss of packets
  • buffer overflow – in case of burts or variations incoming data the buffer may overrun capacity leading to loss of packets
  • data corruption – unintentional changes to data during transmission say due to noise, hardware faults

Blocking probability

• probability a connection is refused by admission control mechanism
• it may happen due to heavy load or lack of resources

Real time traffic

• Real time traffic is the data generated by a source for real time transmission over a network
• 3 types of Real time traffic
  • Constant bit rate
    • constant data generated by source
    • example – data generated by sensor
    • fixed message sizes are generated over fixed intervals
  • Variable bit rate
    • different rate of data generated at different time by source
    • common VBR traffic is when transmission switches between fixed size packets and idle time
    • example – audio signals generate from speech where during periods of silence there is no traffic
  • Sporadic rate
    • data generate in burst followed by long period of silence
    • its a special case of VBR traffic
    • example – fire alarm system
      • when fire detected large messages are generated
      • when fire is extinguished then it will become idle

Types of Real time networks

• Bus network
  • single line for communication
  • used in small embedded systems
  • example: CAN bus
• Switched network
  • switches to connect nodes allowing multiple simultaneous paths
  • example: ethernet
  • basic model of real time communication
    • nodes and switches – multiple nodes connected via switches providing simultaneous paths
    • real time traffic flows – data organized into flows each with its own timing constraints
    • scheduling and resource allocation – flow transmission scheduling to guarantee timing bounds
    • buffers, protocols – buffers to hold data temporarily with protocols designed to ensure data is not lost, re-ordered or corrupted
• Ring network
  • each node connects to 2 neighbours forming a ring
  • data ciruclates around the ring
• Mesh network
  • nodes are interconnected often with multiple paths between source and destination
• Wireless network
  • used where mobility or ease of installation is required
• Hybrid network
  • combines say ethernet (switched network) with wireless sensor branches