CprE 458/558: Real-Time Systems

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CprE 458/558 Real-Time Systems

• Backplane MAC protocol
• (combined scheduling)

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Backplane scheduling

• Guarantees periodic messages
• Admission test and scheduling
• Aperiodic messages
• Admission test and scheduling

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Backplane Bus Introduction

• Consider a multimedia application consists of a
  number of hardware modules, such as
• a network interface module
• a storage module
• a video display module
• an image processing module
• All connected by a packet bus
• Note this scenario applicable to LAN

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A typical backplane bus of a workstation
The central bus arbiter control the access to the
common packet plane bus through the control
connections
Module 1
Module 2
Module 3
Module 4
Module 5
Module 6
Packet Bus
Bus Arbiter
Control connections
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The backplane bus model

• Each hardware module has to handle a combined
  load of both periodic and aperiodic messages.
• Every module has a bus interface unit (BIU) that
  connects it to the packet bus.
• The output buffer of the BIU consists of two
  parts
• A periodic queue (PQ)
• An aperiodic queue (RQ)
• Each queue has its own bus request (BR) and bus
  grant (BG) signals that the BIU uses for
  arbitration.
• The central bus arbiter (BA) resolves any
  contention among the units for the packet bus.

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A closer look
Module i
Module i1
BIU
BIU
RQ
PQ
RQ
PQ
BR
BG
BR
BG
BR
BG
BR
BG
Packet Bus
Bus Arbiter
Bus Manager Operating System
Central Unit
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The backplane bus model

• The BA periodically executes a service algorithm
  with a period of N time slots (each slot t is
  the time taken to transmit one packet), called
  the bus service cycle time
• The bus service time is much smaller than the
  period of any periodic message

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The working of the BA

• The entire N slots of the bus service time are
  partitioned among the periodic and aperiodic
  messages as follows
• a Number of slots dedicated for aperiodic
  messages
• Q number of slots reserved for already admitted
  periodic messages
• N (Q a)

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Admission test for periodic messages

• Whenever a new periodic message stream of message
  size Mi and period Ti arrives,
• The Bus Manager in the central unit calculates
  the smallest integer ci that satisfies the
  following condition
• Mi Ci (ceil (Ti / (Nt)) 2)
• And the periodic message is admitted if
• (Ci Q) (N - a)

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The BA scheduling algorithm

• The BA scheduling algorithm maintains two runtime
  variables n and q defined as follows
• n remaining number of slots in a given service
  cycle
• q number of slots reserved for the periodic
  messages that have not yet been used during the
  current service cycle.
• The BA does its job in every time slot according
  to the current values of n and q

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The BA scheduling algorithm
For every time slot
n gt q
n q
YES
YES
If RQ request exists
If PQ request exists
YES
NO
YES
NO
Grant bus to PQ (q--) (n--)
Grant bus to PQ (q--) (n--)
Grant bus to RQ (n--)
Grant bus to RQ (n--)
If (n0) n N and q Q
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RT-LAN MAC protocols -- Summary

• Scheduling aperiodic messages
• DCR, DoD-P, VTCSMA, Demand-adaptive
• Scheduling periodic streams
• TDM, Demand-adaptive protocol
• Combined scheduling (periodic aperiodic)
• Backplane Bus algorithm
• FieldBus and CAN bus (industry automation)