While this article focuses on one of them, it is helpful to keep in mind the context of the entire system. Figure 1 shows the systems architecture as described in the baseline specification. These functional groups represent collections of capabilities that need to be implemented together. The specification describes in detail the required behaviors for each functional group. The functional groups are used so that the designer is not constrained by whether to implement one or many functional groups in the same device.
Most internetworking protocols operate in terms of the network or subnet.
MPOA is a little different. The IASG represents those aspects of the subnet that MPOA supports. Thus, it includes the notion of broadcast scope, and of an aggregation, which is at the bottom of the routing hierarchy. However, in MPOA, ATM-attached hosts no longer need to be cognizant of the address aggregation. And direct communication can be used between hosts that are actually in different IASGs.