TikZ picture arrow overlapping

Question

I want something like this:

but the arrows get overlapped in TikZ!

I have tried the following code, but is there a better way than this?

\documentclass{article}
\usepackage{color}
\usepackage[dvipsnames]{xcolor}
\usepackage{tikz}


\begin{document}
\begin{tikzpicture}
    \path (0,0) node(a) [rectangle,draw, fill=blue, text=white] {\LARGE EP};
    \path (2,3) node(b) [rectangle,draw, fill=Cyan] {\LARGE GK};
    \path (7,3) node(c) [rectangle,draw, fill=Cyan] {\LARGE GK};
    \path (9,0) node(d) [rectangle,draw, fill=blue, text=white] {\LARGE EP};

    \draw[thick,<-] (a.north) -- (b.west) node[midway, below right=0.3cm]{\parbox{2cm}{1. Request permission to place call}};
    \draw[thick,->] (a.east) -- (b.south) node[midway, above left=0.3cm]{\parbox{2cm}{4. Grant permission to place call}};    
\end{tikzpicture}
\end{document}

Answer 1

My answer (following the picture in the question) leads to:

Here is the code:

\documentclass{article}
\usepackage[dvipsnames]{xcolor}
\usepackage{tikz}

\tikzset{block/.style={rectangle,draw,minimum height=1cm,minimum width=1cm}}

\begin{document}
\begin{tikzpicture}
    \path (0,0) node(a) [block, fill=blue, text=white] {\LARGE EP};
    \path (2.5,3) node(b) [block, fill=Cyan] {\LARGE GK};
    \path (6.5,3) node(c) [block, fill=Cyan] {\LARGE GK};
    \path (9,0) node(d) [block, fill=blue, text=white] {\LARGE EP};

    \draw[thick,stealth-] (a.120) -- (b.150) node[midway, below right=0.4cm,text width=1.75cm ]{1. Request permission to place call};
    \draw[thick,-stealth] (a.70) -- (b.210) node[pos= 0.3, above left,text width=1.75cm]{4. Grant permission to place call};    
\end{tikzpicture}
\end{document}

I created a style called block where there are also the minimum width and minimum height defined. Moreover to set the starting and arrival point of arrows I used angles of the boxes and instead of adopting parboxes I preferred to set the text width option.

Answer 2

I remove color because xcolor loads color

\documentclass{article}
\usepackage[dvipsnames]{xcolor}
\usepackage{tikz}

\begin{document}
\begin{tikzpicture}
    \node [draw, fill=blue, text=white,minimum size=24pt,outer sep=2pt] (a) at (0,0) {\LARGE EP};
    \node [draw, fill=Cyan,minimum size=24pt,outer sep=2pt]             (b) at (3,2) {\LARGE GK};

    \draw[thick,->] ([xshift=-6pt]a.north) -- ([yshift=+4pt]b.west) 
                   node[midway, below right=5pt,align=left,font=\small]
                   {1. Request \\permission  \\to place call};
    \draw[thick,<-] ([xshift=+6pt]a.north)  -- ([yshift=-4pt]b.west) 
                   node[midway, above left=5pt,align=left,font=\small]
                   {4. Grant \\ permission\\ to place call};    
\end{tikzpicture}
\end{document}