Data Communications Networking Devices - 4th Ed.pdf

2.7 ATM _________________________________________________________________________ 263onto as well as a new connection identi®er to be placed into the cell header. Thenew connection identi®er is then used for routing between the next pair of switchesor from a switch to an endpoint.Types of switchesThere are two types of ATM switches, the differences being related to the type ofheader ®elds read for establishing cross-connections through the switch. A switchlimited to reading and substituting VPI values is commonly referred to as a VPswitch. This switch operates relatively fast. A switch that reads and substitutes bothVPI and VCI values is commonly referred to as a Virtual Channel VC) switch. AVC switch generally has a lower cell operating rate than a VP switch as it mustexamine additional information in each cell header. You can consider a VP switch asbeing similar to a central of®ce switch, while a VC switch would be similar to endof®ceswitches.Using connection identi®ersTo illustrate the use of connection identi®ers in cell switching, consider Figure2.57, which illustrates a three-switch ATM network with four endpoints. Whenswitch 1 receives a cell on port 2 with VPI = 0, VCI = 10, it uses the VPI and VCIvalues to perform a table lookup, assigning VPI = 1, VCI = 12 for the cell headerand switching the cell onto port 1. Similarly, when switch 1 receives a cell on port 3with VPI = 0, VCI = 18 its table lookup operation results in the assignment ofVPI = 1, VCI = 15 to the cell's header and the forwarding of the cell onto port 1. Ifwe assume switch 2 is a VP switch, it reads and modi®es only the VPI, thus, theVCIs are shown exiting the switch with the same values they had upon entering theswitch. At switch 3, the VPC is broken down, with virtual channels assigned toroute cells to endpoints C and D that were carried in a common virtual path fromswitch 1 to switch 3.Figure 2.57Cell switching example