 It communicates with the carrier's network management system (NMS), by which we mean all management functions supported by the carrier. Therefore, the CNM agent usually supports only the portion of these management functions that pertain to the service provided to the customer. The interface between a CNM agent and NMS should be based on open standards, in order to ensure that the CNM agent may take advantage of new functions that are added to the NMS. The CNM service initially supported by carriers targeted Cell Relay Service customers. CNM services are therefore built based on Cell Relay Service and ATM network requirements. 3.3.3 M4 Interface The M4 interface specifies requirements for managing individual network elements. It defines a protocol-independent MIB that identifies the information items to be exchanged between ATM devices and the system that manages them. This MIB is a logical information tree, consisting of managed entities, and it is intended as a framework for the development of protocol-specific MIBs, such as those based on SNMP or CMIP, that would potentially permit the management of ATM networks with heterogeneous devices. Several aspects of the M4 interface are similar to the five-layer TMN model. M4 addresses the interactions between the lower three element, element management and network management layers. The M4 specification also addresses the fault, per- formance, configuration and security areas of ATM management. Thus, M4 is a complete specification that accommodates the management of current and future public ATM networks. Early versions of the M4 interface did not, in practical terms, significantly impact ATM network management. This may have been due to the complexity of the specification, which made it hard for M4 to challenge SNMP-based implementations. 3.4 Wireless ATM A wireless ATM network (W-ATM) comprises mobile terminals, base stations, ATM switches and concentrators (collectively, "nodes") and fixed terminals. Mobile terminals access the network through a radio link. Base 