Multiplexed protocols, such as X.25 and Frame Relay, present a slightly different face to the application. These layers actually consist of two parts: the major and minor services.
The major service is in overall control: it controls application access to its minor services and minor service access to the physical link. Like non-multiplexing layers, the major service is opened, pushed, and connected. However, data is never exchanged through a major service. Hence, other layers cannot be pushed on top of it. Data is exchanged only through minor services associated with the major service.
Minor services correspond to Permanent Virtual Circuits (PVCs) and Switched Virtual Circuits (SVCs) in X.25 and to Data Link Connection Identifiers (DLCIs) in Frame Relay. Minor services must be separately opened and connected. They represent independent "virtual" connections which share a single physical link. They cannot be pushed on top of other layers. They "inherit" the push from the major device. However, as independent logical connections, they may have other layers pushed on top of them. For instance, in the Annex G protocol, X.25 traffic is carried by Frame Relay DLCIs. In this situation, the LAPB layer of the X.25 device is pushed onto a Frame Relay virual circuit.
Note that a major service should not be disconnected until all minor services have been disconnected and cannot be closed until all its minor services have been closed.