The ATM Airport:VPI / VCI Switching Explained Carey Williamson Department of Computer Science University of Calgary
Introduction • ATM terminology is confusing (e.g., Virtual Paths, Virtual Channels, VPI’s, VCI’s, VPC’s, VCC’s, PVC’s...) • One way to explain these terms is with the use of a simple analogy: airline travel
Flight number on a specific airline e.g., AC 1290 Virtual Path Identifier (VPI) e.g., VPI = 23 The ATM Airport Analogy
Flight number on a specific airline e.g., AC 1290 Seat assignment on a specific flight e.g., 22A Virtual Path Identifier (VPI) e.g., VPI = 23 Virtual Channel Identifier (VCI) e.g., VCI = 305 The ATM Airport Analogy
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 VPI’s and VCI’s are only locally significant (per hop basis)
The ATM Airport Analogy • Provides a two-level addressing scheme that uniquely identifies each “cell” (passenger) on a per-hop basis • All VCI’s represent individual traffic flows • VPI is a “bundle” of VCI’s all heading in the same direction
The ATM Airport Analogy • All VCI’s 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 VPI’s between the same two points that offer different quality of service (e.g., other airlines, other flights at different times of day)
The ATM Airport Analogy • Airlines (and air traffic controllers) only deal with VPI’s (i.e., flights) when doing scheduling, takeoff, landing, routing, provisioning, etc (not individual cells) • Airlines can add or remove flights (VPI’s) on a medium to long term basis, but individual passengers (VCI’s) can come and go on a fairly short term basis
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 ATM switch Lots of cells with VPI’s and VCI’s 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 (Cont’d)
The ATM Airport (Cont’d) • 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!
Strengths of the Analogy • Provides nice explanation for VPI’s as “bundles of VCI’s” heading to same place • Network management, routing, resource allocation deals with VPI’s, not VCI’s • 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”
Weaknesses of the Analogy • VCI’s 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 don’t really fit analogy well • Does not explain why baggage gets lost!!!
Summary • The “ATM Airport” offers a clever analogy for explaining and understanding the role of VPI’s and VCI’s in ATM networks • VPI’s correspond to flights • VCI’s correspond to individual traffic flows • Airports are the switching hubs that get you to your proper destination