Modern embedded computing systems tend to be heterogeneous in the sense of being composed of subsystems with very different characteristics, which communicate and interact in a variety of ways-synchronous or asynchronous, buffered or unbuffered, etc. Obviously, when designing such systems, a modeling language needs to reflect this heterogeneity. Today's modeling environments usually offer a variant of what we call amorphous heterogeneity to address this problem. This paper argues that modeling systems in this manner leads to unexpected and hard-to-analyze interactions between the communication mechanisms and proposes a more structured approach to heterogeneity, called hierarchical heterogeneity, to solve this problem. It proposes a model structure and semantic framework that support this form of heterogeneity, and discusses the issues arising from heterogeneous component interaction and the desire for component reuse. It introduces the notion of domain polymorphism as a way to address these issues.

Abstract.A structured use of control, communication and computing techologies in vehicles and in the highway can lead to major increases in highway capacity. Our context is an automated highway system (AHS) in which traffic is organized in platoons of closely spaced vehicles under automatic control. The AHS control tasks are arranged in a three-layer hierarchy. At the top or link layer a centralized controller assigns to each vehicle a path through the highway and sets the target size and speed for platoons to reduce congestion. The remaining two layers are distributed among controllers on each vehicle~ A vehicle's platoon layer plans its trajectory to conform to its assigned path and to track the target size. The plan consists of a sequence of elementary maneuvers: merge (combines two platoons into one), split (separates one platoon into twv), and change lane (enables a single car to change lane). Once the protocol layer determines that a particular maneuver can safely be initiated, it instructs its regulation layer to execute the conmponding precomputed feedback control law which implements the maneuver. This paper focuses on the design of the platoon layer. In order ot ensure that it is safe to initiate a maneuver, the platoon layer controller en|ers into a negotiation with its neighbors. This negotiation is implemented as a protocol--a structured sequence of message exchanges. After a protocol terminates successfully, the movement of the vehicles involved is coordinated and the maneuver can be initiated. A protocol is designed in two stages. In the first stage, the protocol is described as an informal state machine, one machine per vehicle. The informal state machine does not distinguish between actions and conditions referring to the vehicle's environment and those referring to the protocol itself, In the second stage this distinction is enforced and the protocol machines are specified in the formal language COSPAN. COSPAN software is then used to show that the protocol indeed works correctly. One can now be reasonably confident that, properly implemented, the protocol designed here will work as intended.Key Words: protocol specification, protocol verification, intelligent control, hybrid systems, smart cars, IVHS Overview of the AHSHighway congestion imposes an intolerable burden on many urban residents. Because congestion occurs when the demand for travel exceeds highway capacity, a sound approach to reducing congestion will involve a mix of policies affecting demand and capacity depending on local circumstances and priorities. These policies include building more highways, reducing demand by raising tools, promoting mass transit and higher vehicle occupancy (car-pooling), and developing a high-speed communication network which, for many

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