X.25/LAPB
X.25 is a packet-switching technology.
It is an older standard that describes the interface between a DTE (Date Termination Equipment) and a DCE (Data Circuit-Terminating Equipment).
Typically, a DTE is a router.
A DCE constitutes the end of the WAN link at the subscriber’s location but on the provider’s side.
X.25 is the most widely implemented worldwide WAN protocol of this type.
It provides communications over switched virtual circuits (SVC) or permanent virtual circuits (PVC).