\begin{tikzpicture}[xscale=1.5, yscale=1]

%% nodes
\node (1) at (1, 0) {$0$};
\node (2) at (2, 0) {$1$};
\node (3) at (3, 0) {$2$};
\node (4) at (4, 0) {$3$};
\node (5) at (5, 0) {$4$};
\node (6) at (6, 0) {$5$};
\node (7) at (7, 0) {$6$};
\node (8) at (8, 0) {$7$};

%% a-edges
\path (1) edge [bend left=65] node [near start, above=1pt] {$a_1 = a$} (8);
\path (1) edge [bend left=60] node [near start, below=-1pt] {$a_2$} (8);
\path (2) edge [bend left=55] node [near start, above=-1pt] {$a_3$} (8);
\path (3) edge [bend left=50] node [near start, above=-1pt] {$a_4$} (8);
\path (5) edge [bend left=45] node [near start, above=-1pt] {$a_5$} (8);
\path (5) edge [bend left=40] node [near start, below=-1pt] {$a_6$} (8);
\path (7) edge [bend left=20] node [near start, above=-1pt] {$a_7$} (8);

%% b-edges
\draw (1) -- (2) node [midway, below] {$b_1$};
\draw (2) -- (3) node [midway, below] {$b_2$};
\draw (3) -- (4) node [midway, below] {$b_3$};
\draw (4) -- (5) node [midway, below] {$b_4$};
\draw (5) -- (6) node [midway, below] {$b_5$};
\draw (6) -- (7) node [midway, below] {$b_6$};
\draw (7) -- (8) node [midway, below] {$b_7 = b$};

\end{tikzpicture}