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+Terminology of advanced features
+================================
+
+.. _peers:
+
+Peers
+-----
+
+An "Orthanc peer" is another instance of Orthanc, possibly running on
+a remote computer.
+
+Contrarily to a "modality", it is possible to communicate with a peer
+through the **HTTP/HTTPS protocol** (i.e. through the REST API of
+Orthanc), and not through the DICOM protocol. This implies a much
+easier configuration: It is only required to know the URL, the
+username and the password to communicate with an Orthanc peer. This
+contrasts with DICOM query/retrieve, that is quite complex and that
+involves a lot of pitfalls (cf. the FAQ entry about :ref:`troubleshooting
+DICOM communications <dicom>`) that can be bypassed if using HTTP.
+
+Furthermore, as HTTP(S) communications are generally not blocked by
+firewalls (contrarily to the DICOM protocol that is inherently an
+Intranet protocol and that often requires the setup of VPN tunnels),
+it is much easier to setup communications of medical images through
+the Internet with Orthanc peers.
+
+
+.. _recycling:
+
+Recycling/Protection
+--------------------
+
+Because of its focus on low-end computers, Orthanc implements **disk
+space recycling**: The patient that has been stored for the longest
+time inside Orthanc can be automatically deleted when the disk space
+used by Orthanc grows above a threshold, or when the number of stored
+patients grows above a threshold. This feature enables the automated
+control of the disk space dedicated to Orthanc.
+
+Recycling is controlled by the ``MaximumStorageSize`` and the
+``MaximumPatientCount`` options in the :ref:`Orthanc configuration
+file <configuration>`.
+
+It is possible to prevent important data from being automatically
+recycled. This mechanism is called **protection**. Each patient can be
+individually protected against recycling by using the
+``Unprotected/Protected`` switch that is available from Orthanc
+Explorer.
+
+Note that protection is only available at the patient level. It
+protects all the studies/series/instances of the patient against
+recycling. The rationale is that we think it is important to keep
+available all the data related to one patient. Unwillingly losing a
+study/series that is part of the same patient might lead to a loss in
+consistency with respect to the medical history of this patient.
+
+
+.. _compression:
+
+Compression
+-----------
+
+If your disk space is limited, besides :ref:`recycling`, you should
+also consider using **disk space compression**. When compression is
+enabled, Orthanc compresses the incoming DICOM instances on-the-fly
+before writing them to the filesystem, using `zlib
+<http://en.wikipedia.org/wiki/Zlib>`_. This is useful, because the
+images are often stored as raw, uncompressed arrays in DICOM
+instances: The size of a typical DICOM instance can hopefully be
+divided by a factor 2 through lossless compression. This compression
+process is transparent to the user, as Orthanc automatically
+decompresses the file before sending it back to the external world.
+
+Compression is controlled by the ``StorageCompression`` option in the
+:ref:`Orthanc configuration file <configuration>`.