Cpr E 458558 RealTime Systems Switched LAN SLAN

Cpr. E 458/558: Real-Time Systems Switched LAN (SLAN) Cpr. E 458/558: Real-Time Systems (G. Manimaran) 1

Switched LAN topology Switch LAN 1 LAN 2 Nodes LAN “k” Cpr. E 458/558: Real-Time Systems (G. Manimaran) 2

Message transmission in SLAN Switch LAN 1 LAN “K” LAN 2 Message path Cpr. E 458/558: Real-Time Systems (G. Manimaran) 3

Worst case channel access time (WCT) 1. Access the local channel (D 1) 2. Switching delay (D 2) Switch 3. Access the destination channel (D 3) LAN 1 LAN “K” LAN 2 Message path Total delay = D 1 + D 2 + D 3 Cpr. E 458/558: Real-Time Systems (G. Manimaran) 4

Worst case channel access time (WCT) • WCT = Worst Case (source LAN access delay, D 1) + Worst Case (switching delay, D 2) + Worst Case (destination LAN access delay, D 3) • Worst Case (source LAN access delay, D 1): – This is computed for all the outgoing messages (intra-LAN and inter-LAN) • Worst Case (switching delay, D 2): – This is computed only for inter-LAN messages • Worst Case (destination LAN access delay, D 3): – 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) LAN Cpr. E 458/558: Real-Time Systems (G. Manimaran) 5

CR Trees N 1 N 2 N 3 N 1 N 5 N 4 CR tree for worst-case LAN segment (LAN 2) N 2 N 3 N 5 N 4 N 6 N 7 Virtual Nodes (LAN 1, LAN 3) Worst Case CR (WCCR) tree Cpr. E 458/558: Real-Time Systems (G. Manimaran) 6

WCT calculation – calculate D 1+D 2 N 1 The worst case channel access time of this modified tree gives the worst case (D 2 + D 3) N 2 N 3 N 5 N 4 N 6 N 7 Virtual Nodes (LAN 1, LAN 3) Worst Case CR (WCCR) tree Cpr. E 458/558: Real-Time Systems (G. Manimaran) 7

Message Admission test Calculate the worst case local channel access time (WC 1) Construct the worst case tree assuming the worst-case destination LAN By adding virtual nodes (one per LAN) Calculate the Worst case channel access time of the above constructed Tree and call it WC 2 New message “t” is current time (D – t) ≥ (WC 1 + WC 2) YES Accept NO Reject Cpr. E 458/558: Real-Time Systems (G. Manimaran) 8