\documentclass{article}
\usepackage{tikz}
\usetikzlibrary{positioning}

\begin{document}

\begin{tikzpicture}[node distance=0.5cm, every node/.style={rectangle, draw, minimum width=2cm, minimum height=0.8cm, align=center}]

  % Top Level Node
  \node (bs) {Base Station System};

  % First Level Nodes
  \node (bsc1) [below=of bs] {BSC1};
  \node (bsc2) [right=of bsc1, xshift=2cm] {BSC2};
  \node (omc) [right=of bsc2, xshift=2cm] {OMC};

  % BSC1 Branch
  \node (bts11) [below=of bsc1] {BTS11};
  \node (bts12) [below=of bts11] {BTS12};
  \node (bts13) [below=of bts12] {BTS13};
  \node (bts14) [below=of bts13] {BTS14};
  \node (bts15) [below=of bts14] {BTS15};
  \node (bts16) [below=of bts15] {BTS16};
  \node (bts17) [below=of bts16] {BTS17};
  \node (bts18) [below=of bts17] {BTS18};
  \node (bts19) [below=of bts18] {BTS19};
  \node (bts110) [below=of bts19] {BTS110};
  \node (bts111) [below=of bts110] {BTS111};
  \node (bts112) [below=of bts111] {BTS112};
  \node (bts113) [below=of bts112] {BTS113};
  \node (bts114) [below=of bts113] {BTS114};
  \node (bts115) [below=of bts114] {BTS115};
  \node (bts116) [below=of bts115] {BTS116};
  \node (bts117) [below=of bts116] {BTS117};
  \node (bts118) [below=of bts117] {BTS118};
  \node (bts119) [below=of bts118] {BTS119};
  \node (bts120) [below=of bts119] {BTS120};
  \node (bts121) [below=of bts120] {BTS121};

  % BSC2 Branch
  \node (bts21) [below=of bsc2] {BTS21};
  \node (bts22) [below=of bts21] {BTS22};
  \node (bts23) [below=of bts22] {BTS23};

  % OMC Branch
  \node (rnc) [below=of omc] {RNC};

  % Connect Nodes - with side connections to the left (Adjusted to -1)
  \draw (bs) -- ++(-3, -1) -| (bsc1.west);
  \draw (bs) -- ++(-3, -1) -| (bsc2.west);
  \draw (bs) -- ++(-3, -1) -| (omc.west);

  \draw (bsc1) -- ++(-3, -1) -| (bts11.west);
  \draw (bsc1) -- ++(-3, -1) -| (bts12.west);
  \draw (bsc1) -- ++(-3, -1) -| (bts13.west);
  \draw (bsc1) -- ++(-3, -1) -| (bts14.west);
  \draw (bsc1) -- ++(-3, -1) -| (bts15.west);
  \draw (bsc1) -- ++(-3, -1) -| (bts16.west);
  \draw (bsc1) -- ++(-3, -1) -| (bts17.west);
  \draw (bsc1) -- ++(-3, -1) -| (bts18.west);
  \draw (bsc1) -- ++(-1, -1) -| (bts19.west);
  \draw (bsc1) -- ++(-1, -1) -| (bts110.west);
  \draw (bsc1) -- ++(-1, -1) -| (bts111.west);
  \draw (bsc1) -- ++(-1, -1) -| (bts112.west);
  \draw (bsc1) -- ++(-1, -1) -| (bts113.west);
  \draw (bsc1) -- ++(-1, -1) -| (bts114.west);
  \draw (bsc1) -- ++(-1, -1) -| (bts115.west);
  \draw (bsc1) -- ++(-1, -1) -| (bts116.west);
  \draw (bsc1) -- ++(-1, -1) -| (bts117.west);
  \draw (bsc1) -- ++(-1, -1) -| (bts118.west);
  \draw (bsc1) -- ++(-1, -1) -| (bts119.west);
  \draw (bsc1) -- ++(-1, -1) -| (bts120.west);
  \draw (bsc1) -- ++(-1, -1) -| (bts121.west);

  \draw (bsc2) -- ++(-1, -1) -| (bts21.west);
  \draw (bsc2) -- ++(-1, -1) -| (bts22.west);
  \draw (bsc2) -- ++(-1, -1) -| (bts23.west);

  \draw (omc) -- ++(-1, -1) -| (rnc.west);

\end{tikzpicture}

\end{document}