How to build the finite state machine of the whole system by each component's sub FSM

[LUMING DONG]

Dear Grphwalker team, I'm now studying  how to apply Graphwalker in real world intrusitrial system test case design.

However, not like examples of  interactive applications such as  Amazon shopping cart,  many industrial systems are more complicated which consist of  sevral distributed components that interact with one another to fufill a dedicate functions. 

For example, the following diagram is a ATM system consists of three entities, end user,

ATM machine, and the bank.   

In my opinion, each entity has its own state variables forming

concret state machine as following:

Then, the whole system's FSM state variables equals   < <user's state vector>, <ATM's state vector>, <Bank's state vector> >, whose state space is  the  Cartesian Product

of the three components.

However, I know the above modelling method is not easy to do, maybe I should  abstract

 the model in a appropriate granularity,   but  I still have no idea how to get start， can

you give me some hints?