ions: it shows a structure of interfaces, and at the same time, how external messages are terminated in internal units. 2.3 Architectural Principles m1::={C1} The function of x3: Receive d(C) from MD1 i1: d(C) Do <something with C1> m2::={C2} The function of x2: Receive d(C) from MD1 M::={A, B, m3::={A,d(B), d(C)} Do <something with C2> C{C1 or C2}} i2: d(C) The function of MD: (message discriminator) Send d(B) to x1 If A=a1, send d(C) to x2 i5: d(B) If A=a2, send d(C) to x3 m4::={B} The function of x1: Receive d(B) from MD Do <something with B> Fig. 2.23 A substructure created by message discrimination. d(M), d(C), etc., are the bits of the values of parameters. M::={...} is the set of parameters of the external message type M The concept "layer" was used in the OSI reference model to describe a structure of seven layers. The SM&AML correspondence to "layering" is based on recursive use of message discrimination functions (MD) to create a hierarchical discrimina- tion structure. This is shown in Fig. 2.24. M::={A, {M1 or M2}} M1::={B, {C or D}} M2::={E, {F or G}} M::={A, {M1 or M2}} {B,d( or D)} M1::={B, {C or D}) i111: d(C) M2::={E, {F or G}} i112: d(D) {A, d(M1 orM2)} i11: d(M1) i12: d(M2) i121: d(F) {E, d({F or G})} i122: d(G) Fig. 2.24 A hierarchical discrimination structure. MD are message discrimination functions If we draw internal interfaces and messages in different directions, we recognize the way OSI-based models are often shown, see Fig. 2.25. 2 Foundations of Sysnet Modelling i111: d(C) i112: d(D) i121: d(F) i122; d(G) M::={A, {M1 or M2}} {B,d({C or D})} M1::={B, {C or D}} {E, di ({F or G})} M2::={E, {F or G}} i11: d(M1) i12: d(M2) {A, d({M1 or M2})} Fig. 2.25 The OSI method to describe hierarchical discrimination structures SM&AML does not use the concept "layer". The reason can be explained by Fig. 2.25. A message discrimination function takes a data flow in one interface and sepa- rates it into subflows in several other interfaces. Recursive use of discrimination for each internal interface creat