ling model. These formats support the mapping of ASPs on messages. The AML univer- sal message format is designed to carry any communication event that is described by the AML abstract language. The universal message format assumes that the transport service used by the logical node (perhaps supported by a TAH function) is reliable, error free and accepts unrestricted size SDUs. These requirements are basically the same as for the message format defined by ROSE. Abstract protocol designers also should not have to invent data types and provide encoding methods. A natural choice is there- fore to use ASN. 1 data types, combined with BER. These standards are designed specifically for those conditions. The ASN. syntax and predefined data types are designed to facilitate message design. Among others things, types exists for a number of commonly used alphabets and for time. The syntax strongly supports the specification of user-defined data types. Together with language constructs like suppression of type information, identifiers, value definitions, tags, default and optional values, ASN.1 can be used to provide message specifications on a higher semantic level than other languages (as, e.g. IDL). 5.7 Protocol Specification Methods The ROSE message format and the AML universal message format differ in two regards: The differences in abstract language: the operation classification systems are very different. Most significant is that AML allows provider-initiated and spon- taneous operations (OFFER, C_REPORT, etc.) which do not exist in ROSE. There are also differences in how operations are specified. For example, ROSE regards the correspondence to Refuse primitives as a separate operation. ROSE also includes something called "linked operations". This may have been an attempt to support operation classes like the AML OFFER, etc., which, how- ever, are not defined as part of the ROSE operation classification system. ROSE messages are operation oriented, while the AML universal message for- mat has an optional support for 