TCAP has no actor identification system of its own. 6 Applying AMLn TCAP does not support associations the way AMLn and the ACSE do. TCAP does, however, support a partial association function: a negotiation procedure that checks if peer TCAP layer elements can handle the same application proto- TCAP has an elaborated function for dialogue handling, which combines an application protocol negotiation function with a dialogue between TC users. This feature has no correspondence in OSI. TCAP satisfies the first requirement of the bullet list in the beginning of this chapter by a dialogue-handling operation called TC-UNIT. The operation TC- BEGIN has to be used when a TC user wants to establish a dialogue before the TC users start communicating. The handling of dialogues in TCAP does not comprise other functions, such as authentication and charging. Thus, TCAP is not designed for "public use." Basically, TCAP uses the primitives for invoking remote operations that are defined by ROSE, which implies that the LPM-LSM separation defined by AMLn is not supported by the TCAP standard. TCAP compensates for real-time problems that are inherent in ROSE operation classes. TCAP also compensates (to some degree) for the fact that it runs on a connectiv- ity layer other than the OSI presentation layer. The sublayers of TCAP represent an implementation model, used as a method for explaining how TCAP works. There is no need to realize these sublayers in real TCAP implementations, i.e., any other implementation model will do as well. Unfortunately, the standard does not inform the reader about that. Luckily, however, most implementors do realize that, and develop their own TCAP implementation models. ATM Cell Switching 6.4.1 Introduction Asynchronous transfer mode (ATM) is a stratum that switches virtual circuits (VC, in the ATM standard called virtual channel). This switching technique can support both data applications and media service (such as voice connections) with the same quality as in PSTN/ISDN (often called "carrier grade"