\def\layersep{1.5cm}
\def\outsep{0.7cm}
\def\dy{1.25}

\begin{tikzpicture}[->, >=stealth, shorten >= 0pt, draw=black!50, node distance=\layersep, font=\sffamily]
    \tikzstyle{node}=[circle,fill=black,minimum size=2pt,inner sep=0pt]
    \tikzstyle{block}=[draw=black,rectangle,fill=none,minimum size=1.5cm, inner sep=0pt]
    \tikzstyle{annot} = []

	\node[node] (xc) at (0, -\dy cm) {};
    \node[block, text width = 2.7cm, align= center] (DSP) at (2*\layersep, -\dy cm) {Discrete-Time \\ System};
	\coordinate (yc) at (4*\layersep, -\dy cm) {};
		
    \path (xc) edge (DSP);
    \path (DSP) edge (yc);
    
    \node[above = 0.5mm of xc] {$x[n]$};
    \node[above = 0.5mm of yc] {$y[n] = T\{x[n]\}$};
    

\end{tikzpicture}