The ATM Airport: VPI VCI Switching Explained

1
The ATM AirportVPI / VCI Switching Explained

• Carey Williamson

Department of Computer Science University of
Calgary
2
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

3
The ATM Airport Analogy

• Flight number on a specific airline
• e.g., AC 1290

• Virtual Path Identifier (VPI)
• e.g., VPI 23

4
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

5
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)

6
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

7
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)

8
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

9
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

• 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

10
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!

11
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

12
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!!!

13
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