begin{figure}
 centering
  begin{tikzpicture}
  draw[thick,dashed] (0,0) arc (0:180:2.5cm);
  draw[fill=white] (-2,0) rectangle (-3,-0.4);  node at (-2.5,-0.2){{tiny STP}};
  draw[fill=white] (0,0) circle [radius=0.3cm]; node at (0,0){{tiny P$_{r}$}};
  draw[fill=black] (-0.5,1.5) circle [radius=0.3cm];
  draw[fill=white] (-1.5,2.3) circle [radius=0.3cm];  node at(-1.5,2.3) {{tiny P$_{i}$}} ;
  draw[fill=black] (-2.5,2.5) circle [radius=0.3cm];
  draw[fill=black] (-3.5,2.3) circle [radius=0.3cm];
  draw[fill=white] (-4.5,1.5) circle [radius=0.3cm] ;  node at (-4.5,1.5){{tiny P$_{2}$}};
  draw[fill=white] (-5,0) circle [radius=0.3cm]; node at (-5,0){{tiny P$_{1}$}} ;
 end{tikzpicture}
end{figure}