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CprE 458/558: Real-Time Systems

CprE 458/558: Real-Time Systems. Real-Time MAC Protocols (Contd.). DCR protocol: disadvantages. Poor schedulability because it does not take message deadlines into consideration. DOD-P (Deadline-based DCR protocol). Uses the message class concept

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CprE 458/558: Real-Time Systems

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  1. CprE 458/558: Real-Time Systems Real-Time MAC Protocols (Contd.) CprE 458/558: Real-Time Systems (G. Manimaran)

  2. DCR protocol: disadvantages • Poor schedulability because it does not take message deadlines into consideration CprE 458/558: Real-Time Systems (G. Manimaran)

  3. DOD-P (Deadline-based DCR protocol) • Uses the message class concept • All the messages which have similar (fairly equal) deadlines are classified into one single class • The total number of message classes is a design parameter • A time tree is constructed with each node representing a message class • The time tree is traversed in a pre-order manner • Each node when traversed in turn runs the DCR to resolve collision with messages belonging to that class alone CprE 458/558: Real-Time Systems (G. Manimaran)

  4. DOD-P: Example 1 1 2 3 2 5 Time tree with three message classes Traversed in pre-order manner 3 4 6 7 Perform normal DCR with just the PINK messages Perform normal DCR with just the GREEN messages Perform normal DCR with just the YELLOW messages CprE 458/558: Real-Time Systems (G. Manimaran)

  5. DOD-P Protocol: State Diagram WAIT Channel busy Message arrival Channel idle IDLE READY Successful transmission End of epoch collision TRANSMIT Eligible transmission TIME TREE SEARCH Not eligible for transmission Transmission complete DCR tree search complete collision DCR MODE TRANSMIT Eligible transmission Transmission complete Not eligible for transmission CprE 458/558: Real-Time Systems (G. Manimaran)

  6. Carrier Sense Medium Access (CSMA) Have a message ? YES Channel busy ? NO Transmit message Collision ? NO This step depends on the exact CSMA variation YES Re-transmit later CprE 458/558: Real-Time Systems (G. Manimaran)

  7. Virtual Time CSMA (VTCSMA) • The basic idea is to incorporate a priority mechanism into the CSMA • various priority policies can be incorporated • No coordination between nodes • VT-CSMA uses the following to decide whether or not to attempt message transmission • The state of the channel • The priorities of the messages waiting for transmission • The time according to the synchronized clock • It has no knowledge about the priorities of the messages waiting at the other nodes CprE 458/558: Real-Time Systems (G. Manimaran)

  8. VTCSMA • Maintains two clocks at each node: real clock (RC) and virtual clock (VC) • RC gives real time • VC gives virtual time • RC and VC at a node are synchronized with the corresponding clocks at the other nodes. • VC runs η ( > 1) times faster than the RC • For example, η = 2 means that VC runs twice as fast as the RC when the channel is idle CprE 458/558: Real-Time Systems (G. Manimaran)

  9. VTCSMA • When a channel is busy, the VC stops running • When the channel is idle, the VC is reset according to the priority policy as follows VC = • No change, for FCFS • RC, for EDF & LLF • 0, for SMF CprE 458/558: Real-Time Systems (G. Manimaran)

  10. Virtual time of the message • When a message M arrives at a node, its VSM (virtual start time) is set as follows, depending on the policy: VSM = • AM (msg arrival time) for FCFS policy • DM (msg deadline) for EDF policy • LM (msg laxity) for LLF policy • TM (msg transmission time) SMF policy • A message is (eligible) transmitted only if its VSM <= VC • Multiple messages may be eligible at the same time, which results in collision when they transmit simultaneously CprE 458/558: Real-Time Systems (G. Manimaran)

  11. Modifying VSM • When a node senses the channel is idle and the node has an eligible message, it transmits the message • If the message collides with another message, each node involved in the collision either retransmits its message immediately, with probability “p”, or modifies the VSM of its message, with probability (1 – p), to a random number drawn from an interval (A,B), where (A, B) = • (VC, LM) for FCFS policy • (RC, DM) for EDF policy • (RC, LM) for LLF policy • (0, TM) for SMF policy CprE 458/558: Real-Time Systems (G. Manimaran)

  12. VTCSMA algorithm New message ? YES Initialize VSM Channel busy ? YES NO Initialize VC RC = RC + 1 • Repeat • Discard negative latency messages • RC = RC + 1 • VC = VC + η • Until all messages with VSM less than VC are transmitted • OR the channel becomes busy YES Channel busy ? NO YES Retransmit or modify VSM Collision ? Transmit message NO CprE 458/558: Real-Time Systems (G. Manimaran)

  13. Impact of η • Too largeη • Too many collisions • Too smallη • Too many idle slots CprE 458/558: Real-Time Systems (G. Manimaran)

  14. VTCSMA-LLF: Example Transmission status table for η = 2, transmission time = 16 CprE 458/558: Real-Time Systems (G. Manimaran)

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