CprE 458/558: Real-Time Systems

1
CprE 458/558 Real-Time Systems

• Switched LAN (SLAN)

2
Switched LAN topology
Switch
Nodes
LAN 1
LAN 2
LAN k
3
Message transmission in SLAN
Switch
LAN K
Message path
4
Worst case channel access time (WCT)
2. Switching delay (D2)
1. Access the local channel (D1)
Switch
3. Access the destination channel (D3)
LAN K
Message path
Total delay D1 D2 D3
5
Worst 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) LAN

6
CR Trees
N1
N1
N2
N5
N2
N5
N3
N3
N4
N4
N6
N7
Virtual Nodes (LAN1, LAN3)
CR tree for worst-case LAN segment (LAN 2)
Worst Case CR (WCCR) tree
7
WCT calculation calculate D1D2
N1
The worst case channel access time of this
modified tree gives the worst case (D2 D3)
N2
N5
N3
N4
N6
N7
Virtual Nodes (LAN1, LAN3)
Worst Case CR (WCCR) tree
8
Message Admission test
Calculate the worst case local channel access
time (WC1)
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 WC2
(D t) (WC1 WC2)
New message
YES
Accept
NO
t is current time
Reject