Numbers of subchapters

Hi,

i am a beginner and this is my first question about docbook.
I want to nummer chapters and subchapters serially and automatically like
this:

1. Chapter
1.1. Subchapter
1.1.1. Subsubchapter
2. Chapter
2.1. Chapter

I looked in serveral tutorials but i have not found an
example of that. I've tried this code:

<?xml version="1.0" encoding="UTF-8"?>
<!DOCTYPE book SYSTEM "docbook_xml/docbookx.dtd">
<book lang="de">
<book>
    <chapter>
        <title>Chapter 1</title>
        <para>...</para>
        <section>
            <title>Section 1</title>
            <para>...</para>
            <section>
                <title >Section 2</title>
                <para>...</para>
            </section>
        </section>
    </chapter>
    <chapter>
        <title>Chapter 2</title>
        <section>
            <title>Section1</title>
            <para>...</para>
        </section>
    </chapter>
</book>

It works fine, but there are no numbers in the subchapters :

1. Chapter 1
        Section1
            Section2
2. Chapter 2
        Section1

Can someone help me? 

Thanks, Katrin.