CprE 458/558: Real-Time SystemsSwitched LAN topologyMessage transmission in SLANWorst case channel access time (WCT)Slide 5CR TreesWCT calculation – calculate D1+D2Message Admission testCprE 458/558: Real-Time Systems (G. Manimaran) 1CprE 458/558: Real-Time SystemsSwitched LAN (SLAN)CprE 458/558: Real-Time Systems (G. Manimaran) 2Switched LAN topologySwitchLAN 1 LAN 2 LAN “k”NodesCprE 458/558: Real-Time Systems (G. Manimaran) 3Message transmission in SLANSwitchLAN 1 LAN 2LAN “K”Message pathCprE 458/558: Real-Time Systems (G. Manimaran) 4Worst case channel access time (WCT)SwitchLAN 1 LAN 2LAN “K”Message path1. Access the local channel (D1)2. Switching delay (D2)3. Access the destination channel (D3)Total delay = D1 + D2 + D3CprE 458/558: Real-Time Systems (G. Manimaran) 5Worst case channel access time (WCT)•WCT = Worst Case (source LAN access delay, D1) + Worst Case (switching delay, D2) + Worst Case (destination LAN access delay, D3)•Worst Case (source LAN access delay, D1):–This is computed for all the outgoing messages (intra-LAN and inter-LAN)•Worst Case (switching delay, D2):–This is computed only for inter-LAN messages•Worst Case (destination LAN access delay, D3):–This is computed at the source.–It is the worst case channel access time of the destination LAN–If the destination is not known, this will be the worst case access channel time for the largest (worst case) LANCprE 458/558: Real-Time Systems (G. Manimaran) 6CR TreesN1N2 N5N3N4N1N2 N5N3N4N6N7Virtual Nodes (LAN1, LAN3)CR tree for worst-case LAN segment (LAN 2)Worst Case CR (WCCR) treeCprE 458/558: Real-Time Systems (G. Manimaran) 7WCT calculation – calculate D1+D2N1N2 N5N3N4N6N7Virtual Nodes (LAN1, LAN3)Worst Case CR (WCCR) treeThe worst case channel access time of this modified tree gives the worst case (D2 + D3)CprE 458/558: Real-Time Systems (G. Manimaran) 8Message Admission testCalculate the worst case local channel access time (WC1)Construct the worst case tree assuming the worst-case destination LANBy adding virtual nodes (one per LAN)Calculate the Worst case channel access time of the above constructedTree and call it WC2(D – t) ≥ (WC1 + WC2)“t” is current timeNew
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