Real Time Systems

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Real Time Systems

• Chapter 6 Real-Time Communication

Dr Shamala Subramaniam
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Real-Time CommunicationIntroduction

• Conventional Communication Systems
• Bandwidth and throughput are important
• Long delays reduce quality of service
• Videoconferencing
• Cellular Communication
• Hard Real-Time Systems
• Delivery of content by specific deadline is
  important
• Internal communication (between life support
  system components)
• External communication (remote manipulation)

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Real-Time Sources Generates Traffic

• Constant Rate fixed packet size and at periodic
  intervals
• Variable rate fixed packet size and variable
  interval or variable packet size at fixed
  interval.
• Bursty traffic require greater demands on buffer
  space
• Talkspurts

Silence
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Communications Media

• Electrical Medium
• Optical Fibers
• Wireless

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Network Topologies

• Network Topology Affects the system response time
  Reliability. The following features are
  important
• Diameter the maximum distance (number of hops)
  between any two nodes as a function of the number
  of nodes. Ideally it should increase slowly.
• Node Degree The number of edges adjacent to
  each node and determines the number of I/O ports
  per node and the number of links in the system.
• Fault Tolerance the network can withstand the
  failure of individual links and nodes while still
  remaining functional.

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Network Topology (cont.)

• Point-to-Point
• Dedicated links
• If no connection, utilize the intermediate nodes
• Shared (Broadcast)
• Nodes have access to all communication channels
• Only one node can transmit at any time over a
  channel

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Real-Time CommunicationIntroduction

• Traffic Characteristics
• Constant versus variable rate
• Polled sensor data
• Compressed voice stream
• Common message transfer techniques
• Packet switching
• Circuit switching
• Wormhole routing

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Wormhole Routing

• Is a way of pipelining packet transmission in a
  mulithop network
• Each packet is broken into a train of flits each
  about one or two bytes long.
• The sender transmits one flit per unit time and
• The flits are forwarded from node to node until
  they reach their destination.
• Over time, a train of flits in contiguous
  (Adjacent), forms and makes its way to its
  destination.

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Example

• The network is a 3-dimensional hypercube
• A message is to be sent from node 000 to node 111
• Node 000 breaks up its packet into flits, and
  send them to node 001 at rate of one flit per
  cycle
• Node 001 forwards the flits it receives to node
  011, which forwards to 111
• If the packet consists of six flits, the activity
  is as follows

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Wormhole Routing (cont.)

• Hypercube 3 dimensional
• N-dimensional ? 2n nodes
• An n-dimensional hypercube is formed by taking
  two (n-1) dimensional hyper cubes and connecting
  like nodes

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Wormhole Routing (cont.)
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Protocols Overview
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Real-Time CommunicationProtocols

• Contention-Based
• Each node listens until network is idle
• Nodes transmit only idle network detected
• Collisions occur when multiple nodes transmit at
  the same time
• Stop, random pause, retransmit

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Real-Time CommunicationProtocols

• Ethernet (CSMA/CD)
• All the nodes can monitor the communication
  channel
• When a node wants to transmit, if it observes the
  channel is busy, it will refrain from interfering
  with the ongoing transmission.
• When it senses the channel is idle it will make
  an attempt to transmit, thus there may be
  concurrent transmission
• Upon collisions, they will abort and retransmit
• Difficult to guarantee determinism

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Virtual Time Carrier Sensed Multiple Access
(VTCSMA)

• Single channel broadcast networks
• Bus Ring topology
• In CSMA there is no detection of priorities
• However, simply using the state of the channel,
  the priorities is not enough, the time
  information must also be considered.
• The key to VTCSMA algorithm is that the priority
  can be computed as a function of the current time
  and some other parameter.

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Virtual Time Carrier Sensed Multiple Access
(VTCSMA) cont.

• The algo. Uses 2 clocks at each node
• One is the real-clock (RC) which tells the
  real-time and is synchronized with the clocks
  at the other nodes.
• The 2nd is the virtual clock (VC) which behaves
  as follows
• When the channel is busy, the VC freezes, when
  the channel becomes free, the VC is reset
  (according to a formula) and then runs at rate n
  gt1. That is the VC runs faster than the RC when
  the channel is free, and not when it is busy.

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Virtual Time Carrier Sensed Multiple Access
(VTCSMA) cont.

• Since the real clocks are assumed to be
  synchronized and the virtual times are regularly
  reset with respect to the real-times, the virtual
  times told at the various nodes are the same,
  plus or minus some small skew.
• This is used as the global priority of packets
  transmitted
• Each node computes a virtual time to start
  transmission VSX(M) for every packet M awaiting
  transmission at that node.
• When the virtual time is greater than or equal to
  VSX(M), packet M becomes eligible for
  transmission.

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Real-Time CommunicationProtocols

• Contention-Based
• Work well only when collisions are low
• Sporadic traffic
• High loads can slow contention-based protocols
  down to a standstill
• Old parts of Engineering building network
• Token-Based
• Only the node that has a token can transmit
• (Same as the conch shell in Lord of the Flies)
• Token is passed from node to node
• Can easily be used as a deterministic protocol

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Real-Time CommunicationProtocols

• Token-Based
• Delays and overhead
• Propagation delay and message latency
• Messages are generally passed point-to-point
• Contention-based protocols are generally
  broadcast
• Token transfer time
• Time required to pass token from one node to
  another (no transmission can occur during this
  time)

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Real-Time CommunicationFault-Tolerance

• Token-Based
• Problems
• Loss of token
• Disconnection of a single node
• Options
• Timeout for lost token detection
• Contention-Based
• Problems
• Traffic flooding / denial-of-service
• Options
• Segment isolation (switches / routers / filtering)

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Real-Time CommunicationFault-Tolerance

• Routing
• Messages can be sent over multiple independent
  routes
• Too many duplicates can flood networks
• Too few duplicates may not provide adequate
  fault-tolerance

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Real-Time CommunicationIndustrial Issues

• Low-level communication
• Mechanical -gt Pneumatic -gt 4-20mA
• Medium-level Communication
• Physical Medium
• Twisted pair (differential)
• Coaxial
• Fibre-Optics
• Protocols
• Traditional use of deterministic semi-proprietary
  interfaces between devices and controllers (PLCs)
• DeviceNet (CSMA-based), Profibus (token-based),
  CANOpen, Fieldbus (lower-level)

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Real-Time CommunicationIndustrial Issues

• High-Level Communication
• Increased demand for information and transparency
  to plant floor information
• Use of common general purpose protocols between
  devices/PLCs and HMI/SCADA systems
• Ethernet (and TCP/IP over Ethernet)
• More durable physical connectors (RJ-45 is
  flimsy)
• Topologies supporting redundancy
• Full-duplex switched networks to improve
  determinism
• Interoperability / low-cost / high bandwidth

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Real-Time CommunicationWireless Issues (Digital)

• Code Division Multiplex Access (CDMA)
• Spread-Spectrum
• Tolerant to interference
• Contention-Based
• Time Division Multiple Access (TDMA)
• Similar to Token-Based Protocols
• Dedicated time slots for each user