n:= n + 1 waitUA waitUA waitUA waitUA Figure 2.10 Symbols legal after a state process DLC ready L_EstabReg SABME SABME SABME TRANSITION n:=n+ 1 n:= n + 1 n:= n + 1 waitUA TRANSITION TERMINATOR waitUA Figure 2.11 Transition structure Figure 2.11 shows the structure of a transition: the symbols after an input, or a spontaneous transition, or a priority input or a continuous signal are called the transition body. The transition body must not contain any input or continuous signal. Following the transition body we find the transition terminator, which can be a nextstate, a stop, a join (lab1 in the figure) or a return (for procedures only). 2.3.2 Start Every process must contain exactly one start symbol. When a process instance is created, the first transition ready to be executed is the transition beginning from the start symbol. Quick Tutorial on SDL 15 process process1_1 n Integer; TIMER T607:= 10.0; SABME SET (T607) Figure 2.12 Start transition Figure 2.12 shows an example of start transition: n is set to 0, signal SABME is transmitted and timer T607 is started before going to state disc. 2.3.3 States States must be defined using a state symbol. A common mistake is to confuse the notions of state and nextstate: Figure 2.12 is incorrect, because state disc is not defined; Figure 2.13 is correct, because state disc is defined. process DLC This is a state This is a nextstate Figure 2.13 State disc defined When the character "-" is entered in a nextstate, as in Figure 2.14, it means that after executing the transition, the state will remain unchanged. process process1_1 V76frame Figure 2.14 Dash nextstate 2.3.4 Input Signals reaching a process instance are stored into its FIFO queue. When performing an input, the first signal in the queue (the oldest one) is removed from the queue, and the values of the signal parameters, if any, are assigned to the variables specified in the input symbol. 16 Validation of Communications Systems with SDL Note, that if the first signal in the queue is not present in any input below th