\begin{tikzpicture}[->,>=stealth,shorten >=1pt,auto,scale=0.6, transform shape]
% x node set with absolute coordinates
\node[state] (U1) at (1,1.5) {$U_1$};
\node[state] (A) at (0,0) {$A$};
\node[state] (B) at (2,0) {$B$};
\node[state] (C) at (1,-1.5) {$C$};
\node[state] (U2) at (-1,-1.5) {$U_2$};

% Directed edge
\path (U1) edge (A);
\path (U1) edge (B);
\path (A) edge (B);
\path (A) edge (C);
\path (U2) edge (C);
% Bidirected edge
%\path[bidirected] (A) edge[bend left=60] (C);
\end{tikzpicture}
