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CprE 458/558: Real-Time Systems. Backplane MAC protocol (combined scheduling). Backplane scheduling. Guarantees periodic messages Admission test and scheduling Aperiodic messages Admission test and scheduling. Backplane Bus: Introduction.
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CprE 458/558: Real-Time Systems Backplane MAC protocol (combined scheduling) CprE 458/558: Real-Time Systems (G. Manimaran)
Backplane scheduling • Guarantees periodic messages • Admission test and scheduling • Aperiodic messages • Admission test and scheduling CprE 458/558: Real-Time Systems (G. Manimaran)
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 CprE 458/558: Real-Time Systems (G. Manimaran)
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 CprE 458/558: Real-Time Systems (G. Manimaran)
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. CprE 458/558: Real-Time Systems (G. Manimaran)
A closer look Module i Module i+1 BIU BIU RQ PQ RQ PQ BG BR BG BR BG BR BG BR Packet Bus Bus Arbiter Bus Manager Operating System Central Unit CprE 458/558: Real-Time Systems (G. Manimaran)
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 CprE 458/558: Real-Time Systems (G. Manimaran)
The working of the BA • The entire N slots of the bus service time are partitioned among the periodic and aperiodic messages as follows: α = Number of slots dedicated for aperiodic messages Q = number of slots reserved for already admitted periodic messages N ≥ (Q + α) CprE 458/558: Real-Time Systems (G. Manimaran)
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 / (N*t)) – 2) • And the periodic message is admitted if: (Ci + Q) ≤ (N - α) CprE 458/558: Real-Time Systems (G. Manimaran)
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” CprE 458/558: Real-Time Systems (G. Manimaran)
The BA scheduling algorithm For every time slot n > q n ≤ q YES YES If RQ request exists If PQ request exists YES NO YES NO Grant bus to RQ (n--); Grant bus to PQ (q--); (n--); Grant bus to PQ (q--); (n--); Grant bus to RQ (n--); If (n==0) { n = N; and q = Q; } CprE 458/558: Real-Time Systems (G. Manimaran)
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) CprE 458/558: Real-Time Systems (G. Manimaran)
