How to draw this two rectangles in this picture :

I tried like this :
\documentclass[border=10pt]{standalone}
%\usepackage[margin=1cm]{geometry}
\usepackage{tikz,xcolor}
\usetikzlibrary{shapes.geometric,arrows,positioning,fit,calc,}
\tikzset{
b/.style={draw, rectangle, rounded corners=2ex,minimum height=0.5in, minimum width=2in,align=center},
c/.style={draw, rectangle, rounded corners=2ex, minimum height=0.5in, minimum width=2in,align=center, rotate=-90},
ar/.style={rounded corners=2ex,->,>=latex },
myarrow/.style args={#1 colored by #2 and #3}{
-stealth,line width=#1,#2,postaction={draw,-stealth,#3,line width=(#1)/3,
shorten <=(#1)/3,shorten >=2*(#1)/3},
}
}
\begin{document}
\begin{tikzpicture}
\node (client) [c] {Client};
\node (serveur) [c, below=3 cm of client] {Serveur};
\end{tikzpicture}
\end{document}
Here's the result :

I have a problem with the positioning. Please explain your code and don't just give the code.


leftnotbelow:\node (serveur) [c, left=3 cm of client.east] {Serveur};– May 18 '14 at 11:12