\begin{tikzpicture}
.....
\end{tikzpicture}

\documentclass[tikz,border=4mm]{standalone} % specifies the border
\usetikzlibrary{positioning,fit,calc} % used for the efficient working of the positioning system
\tikzset{block/.style={draw, thick, text width=3cm, minimum height=1.5cm, align=center}, 
% the align command is used to align the block diagram at the center
% the height command adjust the height of the block diagram
% here block diagram refers to the whole diagram, not the single block
% the thick command here signifies the border of all the blocks used inside the block diagram. You can change it to thin command if you want the thin edge of the blocks
line/.style={-latex}   % the lesser the width the greater will be the diagram window
}
\begin{document}

\begin{tikzpicture}
  \node[block] (a) {Source};
  \node[block,right=of a] (b) {Transmitter}; % these are the name of the blocks
  % you can use as many blocks by specifying the name and alphabets
  \node[block,right=of b] (c) {Channel};
  \node[block,right=of c] (d) {Reciever};
  \node[block,right=of d] (e) {Destination};
  \node[block] (f) at ([yshift=-3cm]$(b)!1.0!(c)$) {Noise}; 
  % the yshift here specifies the shift in the position of the point on the y-axis. You can change the location according to the requirements.
  % the value mentioned is the distance from (b) to (c). If the value is 0.5, then the block will be at the center of (b) and (c).
  \node[draw,inner xsep=4mm,inner ysep=7mm,fit=(a)(b),label={100:P}](g){}; % it is used to draw the box outside the combination of blocks. Here a block will be drawn outside the block (a) and (b). The xsep and ysep are the dimensions for the box. The label command here is used to set the location of the name of the outer A and B blocks.
  \node[draw,inner xsep=4mm,inner ysep=7mm,fit=(d)(e),label={80:Q}]{};
  
  \draw[line] (a)-- (b);
  \draw[line] (b)-- (c);
  \draw[line] (f) -- (c); % you can change the location of the line to north, west, etc. depending on your requirements.
  \draw[line] (c)-- (d); % -- signifies the line between the two blocks (c) and (d).
 \draw[line] (d)-- (e);
\end{tikzpicture}
\end{document}

\documentclass[tikz, border=5mm]{standalone} 
\usetikzlibrary{positioning, fit, calc} 
\tikzset{block/.style={draw, thick, text width=2cm ,minimum height=1.3cm, align=center}, 
line/.style={-latex}   
}
\begin{document}

\begin{tikzpicture}
\node[block] (a) {A};
\node[block,right=of a] (b) {B}; 
\node[block,right=of b] (c) {C};
\node[block,right=of c] (d) {D}; % the commands used for the different location of different blocks
  \node[block] (e) at ([yshift=-2cm]$(b)!0.5!(c)$) {E}; 
  \node[block] (f) at ([yshift=-2cm]$(c)!0.5!(d)$) {F}; 
  \node[block] (g) at ([yshift=2cm]$(c)!0.5!(d)$) {G}; 
  \node[block] (h) at ([yshift=2cm]$(b)!1.0!(b)$) {H}; 
  \draw[line] (a)-- (b);
  \draw[line] (b)-- (c);
  \draw[line] (c)-- (d);
  \draw[line] (e)-- (f);
  \draw[line] (g)-- (h);
  \draw[line] (f)-- (g);
  \draw[line] (a.east) -- (e.west);
  \draw[line] (h)-- (b);
  \draw[line] (f.east) -- (d.south);
  
\end{tikzpicture}
\end{document}

\documentclass[tikz, border=5mm]{standalone} 
\usetikzlibrary{positioning, fit, calc} 
\usepackage{xcolor}
\tikzset{block/.style={draw, thick, text width=2cm , minimum height=1.3cm, align=center}, 
line/.style={-latex}   
}
\begin{document}

\begin{tikzpicture}
\node[block, fill=olive] (a) {A};
\node[block,right=of a, fill=yellow] (b) {B}; 
\node[block,right=of b, fill= gray] (c) {C};
\node[block, fill=purple] (d) at ([yshift=-2cm]$(a)!0.5!(b)$) {D};
\node[block, fill= orange] (e) at ([yshift=-2cm]$(b)!0.5!(c)$) {E};
\node[draw, fill=pink, fill opacity=0.5,inner xsep=5mm,inner
     ysep=6mm,fit=(d)(e),label={130:A}](f){}; % here label command is used to label the block, which covers block D and E.
\draw[line] (a)-- (b);
\draw[line] (b)-- (c);
\draw[line] (d)-- (e);
\draw[line] (e)-- ($(b)!0.5!(c)$);
\draw[line] (d)-- ($(a)!0.5!(b)$);

\end{tikzpicture}
\end{document}