DLCI's (Data Link Connection Identifier)

DLCI's are a necessary component of every PVC.  The act as placeholders within a map, that tell each switch along the path, how to direct the frame.  Imagine driving from Washington DC to LA  -  each road sign along the way will direct you to another, unique pathway.  DLCI's do the same thing - they are preloaded into each switch and act as road signs to the traveling frames.  In this case, however, the frame contains the DLCI, and as it hits each switch along the way, a new road sign replaces the current one, and is placed into the Frame's contents.  Each switch, opens the incoming frame, reads the DLCI, goes into a lookup table, and assigns a new DLCI accordingly.

Customers can select their own endpoint DLCI's.  For a new Network, this gives them a chance to add DLCI's logically.  If done "correctly"  (actually, anything is correct - this just makes evrything more logical and understandable), each location is given a logical number, and all remote locations that have a PVC there, will number the DLCI accordingly.  When testing at a router, the technician will know which DLCI should be configured, by knowing the far end's "location number".  

The following diagram shows a correctly configured set of 4 Frame Relay nodes.  All the technician needs is a list of location numbers (i.e. Hines 100,  Philly 200, etc).  Suppose for example, that a technician is working with the router config in Hines, and he wants to check the DLCI for Philly.  By knowing just those four numbers, he knows that the DLCI for the PVC going to Philly is 200.  The only site with PVC's going to the Regional offices is Austin, and it has two extra PVC's to connect to those sites.