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CprE 458/558: Real-Time Systems

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Title: PowerPoint Presentation Author: ecpegen Last modified by: Manimaran Govindarasu Created Date: 1/18/2000 12:20:14 AM Document presentation format – PowerPoint PPT presentation

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Title: CprE 458/558: Real-Time Systems


1
CprE 458/558 Real-Time Systems
  • Backplane MAC protocol
  • (combined scheduling)

2
Backplane scheduling
  • Guarantees periodic messages
  • Admission test and scheduling
  • Aperiodic messages
  • Admission test and scheduling

3
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

4
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
5
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.

6
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
7
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

8
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)

9
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)

10
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

11
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
12
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)
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