The ATM Forum defined two types of frame relay bearer services (FRBS) for ATM networks. The ATM service is a bearer service because it makes no claims to actually being a FRF- or ITU-T-compliant frame relay service. However, FRBS can transfer frame relay traffic across an ATM network in several frames. Therefore, FRBS is truly a bearer service for frame relay frames.
The visual shows the first type of FRBS internetworking scenario called network interworking. Two separate frame relay networks are connected by an ATM network in the middle. Frames are translated into cells and back at the access points of the ATM network. This is not the only possible place to convert frames to cells, but it is the most common.
Perhaps the most interesting point about this form of FRBS is how major service providers used it. Note that the ATM network could be a single switch. It is then possible to have “frame relay port adapters” (the term varies among ATM switch vendors) that support frame relay UNIs to the customers, but are ATM networks at heart. The endpoints of the frame relay UNIs that feed the ATM switch do not have to be frame relay networks; they can be frame relay access devices (e.g., routers) located at the customer sites. After all, a FR UNI is a FR UNI. So service providers can use FRBS adapters to support frame relay users with an ATM switching network backbone.