The ATM Airport - University of Calgary in Alberta
The ATM Airport: VPI / VCI Switching Explained Carey Williamson Department of Computer Science University of Calgary 1 Introduction ATM terminology is confusing (e.g., Virtual Paths, Virtual Channels, VPIs, VCIs, VPCs, VCCs, PVCs...) One way to explain these terms is with the use of a simple analogy: airline travel
2 The ATM Airport Analogy Flight number on a specific airline e.g., AC 1290 Virtual Path Identifier (VPI) e.g., VPI = 23 3
The ATM Airport Analogy Flight number on a specific airline e.g., AC 1290 Virtual Path Identifier (VPI) e.g., VPI = 23 Seat assignment on a specific flight
e.g., 22A Virtual Channel Identifier (VCI) e.g., VCI = 305 4 The ATM Airport Analogy Virtual Channel Connection: an end-to-end concatenation of flights and seat assignments that get you (an individual traffic flow) to your actual destination Example: from Saskatoon to Toronto requires
going Saskatoon-Regina-Winnipeg-Toronto Note that VPIs and VCIs are only locally significant (per hop basis) 5 The ATM Airport Analogy Provides a two-level addressing scheme that uniquely identifies each cell (passenger) on a per-hop basis All VCIs represent individual traffic flows
VPI is a bundle of VCIs all heading in the same direction 6 The ATM Airport Analogy All VCIs on that VPI receive the same grade of service in some sense (e.g., food, cost, arrival time, bumpy flight, crash, etc.) There might be other VPIs between the same two points that offer different quality of service (e.g., other airlines, other flights at different times of day) 7
The ATM Airport Analogy Airlines (and air traffic controllers) only deal with VPIs (i.e., flights) when doing scheduling, takeoff, landing, routing, provisioning, etc (not individual cells) Airlines can add or remove flights (VPIs) on a medium to long term basis, but individual passengers (VCIs) can come and go on a fairly short term basis 8 The ATM Airport (Contd)
Airport terminal Lots of flights and passengers coming in and going out Main goal is to make sure that passengers coming in on flights are sent out on the right outgoing flights 9 ATM switch Lots of cells with
VPIs and VCIs coming in, going out Main goal is to make sure that cells coming in on input ports are switched onto the correct output ports The ATM Airport (Contd) An incoming passenger arrives on seat A of flight B at gate C, and wants to depart on
seat D of flight E at gate F Changing flights and seat: VP/VC switch Changing seats, but not flight: VC switch Changing flight, but not seat: VP switch Same flight, same seat: no switch! 10 Strengths of the Analogy Provides nice explanation for VPIs as bundles of VCIs heading to same place Network management, routing, resource allocation deals with VPIs, not VCIs
Emphasizes locally significant nature of VPI and VCI, but end-to-end notion of virtual channels and virtual paths Explains ATM switching in its role as label multiplexing 11 Weaknesses of the Analogy
VCIs in ATM actually correspond to a traffic flow (stream of cells) not just an individual cell Cells are sent sequentially on ATM links, not in batches like airline flights QOS notions of cell loss, cell delay, and cell delay variation dont really fit analogy well Does not explain why baggage gets lost!!! 12 Summary The ATM Airport offers a clever analogy
for explaining and understanding the role of VPIs and VCIs in ATM networks VPIs correspond to flights VCIs correspond to individual traffic flows Airports are the switching hubs that get you to your proper destination 13
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