This layer defines the backbone transport, switching, and routing between access layer elements.

It is generally transparent to the access portion of the network, except in cases such as IP-FR and IP-ATM, or more advanced backbone intelligent services that communicate with the access, premises, or application layers.

To the user, the backbone can be a carrier-provided switched service. In this case, the backbone is the "network cloud" to which the access devices send and receive traffic. This cloud could be an independent medium such the public internet, an intelligent switched backbone service as in AT &T's INCS ATM service, or just a "big transport pipe" from point A to point B.

One example of a backbone layer service is FR in which the user-access device (a router) interfaces with a backbone node (frame switch).

Backbone layer services include private line (SONET, Packet Over SONET, DWDM, and more), FR, ATM, and IP (transport and Internet access). Note that the backbone layer can provide an access layer with a standard platform interface, protocol (or multiple protocols), architecture, technology, quality of service (QoS), and potentially many other standard services.