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+======================
+Reference Architecture
+======================
+
+This section describes the reference architecture, the function blocks,
+and the function entities of Escalator for the reader to well understand how
+the basic functions to be organized.
+
+The software stack on each node is generally as shown in the table below.
+
+.. figure:: images/figure2.png
+   :name: figure2
+   :width: 100%
+
+Since the upgrading of control node will not affect the VNFs service in the blue
+box, this chapter will focusing on the upgrading of compute nodes.
+
+
+Precondition of Upgrade
+=======================
+
+Upgrade related modules in VIM
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+
+Upgrade operations are initiated by the user through the VIM. For VIM, upgrade
+management mainly contains the object:
+
+- **Upgrade Manager**
+
+Mainly responsible for control of the upgrade process.The Escalator is required
+to know the software deployment information of the platform and will use these
+information during the upgrading. It will be collected from some place, such
+as the Installer, Deploy Manager and Escalator itself, etc.
+
+- **VIM Interface**:
+
+Mainly responsible for the external interface, include Vi-Vnfm, Or-Vi. This
+module stores VNFO and VNFM external information such as address and
+authentication.
+
+- **Cloud Manager**:
+
+Mainly responsible for virtualization resources management,which might be
+considered made up of Openstack and SDN control node.
+
+- **System Support**:
+
+This layer is the runtime support environment of upper layers, e.g. Cloud
+Manager and VIM interface., including:OS, HA, etc. To upgrade the upper
+software is based on this module.
+
+.. figure:: images/figure3.png
+   :name: figure3
+   :width: 100%