c modeling that distinguishes between market transactions for access to the network and governance transactions within the network. This important distinction will suggest a Coasian theory of networks, discussed in the next section. Market transactions for network services are voluntary by their very nature. Accordingly, market participants engage in access transactions only when they anticipate obtaining gains from trade. This contrasts with regulatory mandates that place requirements on the provision of access by network firms. Such transactions are not voluntary in nature and thus may be expected to differ from what would be observed in the marketplace in the absence of regulation. Mandated access to networks is a regulatory term of art. The definition of mandated access has its roots in the antitrust law of essential facilities and is spelled out at length in the 1996 Telecommunications Act and decisions by regulatory agencies. The classification system that we propose is useful for analyzing regulation and public policy toward networks. The five types of access to networks are represented in Figure 1.13. Access requires a combination of market transactions and the necessary physical interfaces and technological standards to carry out the transactions. In the figure, networks are represented by solid nodes connected by solid lines. These indicate a network as a unified facility owned by a particular company. In the figure, the dotted lines represent the combination of market transactions and network interfaces that form access to networks. The critical issue turns on who is seeking access - customers, competitors, or suppliers of complements. There are five main types of access: (1) retail access, (2) wholesale access, (3) interconnection access, (4) platform access, and (5) unbun- dled access. Retail access is provided to final customers, whereas wholesale and unbundled access are provided to competitors. Platform access means that the network facilities conform to a standard that allows other companies to p