\section{Outlook}\label{sec:outlook}

In addition to the XEPs covered above, there are a few additional XEPs which can
be implemented to further increase the effectivity of Chatty Things. Especially
the documents XEP-0323 through XEP-0326 (which are currently in Experimental
status) are targeted to the Internet of Things.

\paragraph{Concentrators (XEP-0326)~\cite{xep0326}}
In contrast to sensor nodes which are focused on collecting data, concentrators
can be used to serve as a proxy and control a subset of the network. The XEP
defines messages to query a sensor node for data sources, and subscribing to
them, while subscription is loosely modeled after the Publish-Subscribe
mechanism (XEP-0060). It also specifies how clients can request data or
control certain nodes over a concentrator.

This approach can be practical in large-scale sensor networks, where usually not
every sensor node can be reached directly, and where sensor nodes only have a
very limited amount of storage. Individual concentrators can then be equipped
with larger storage and serve as a facility to aggregate data from sensor nodes.
This structure can be implemented on several levels, forming a hierarchy. A user
interested in specific values then only needs to communicate with a single node
in the network.

\paragraph{Sensor Data (XEP-0323)~\cite{xep0323}}
This XEP specifies a way of reading out values from a
sensor node. It allows to specify multiple data sources (e.~g. temperature,
humidity) as well as multiple types of data (e.~g., momentary values, historical
values, peak values).
As a simple use case, the client sends an IQ stanza containing the request and a
sequence number used to identify the request. The sensor node then rejects or
accepts the request by returning a corresponding IQ stanza. If it has
accepted the request, it reads out the requested data and returns it in a
subsequent message stanza to the client.

An example of this protocol can be seen in Figure~\ref{fig:examplexmpp}: after
both clients have opened their streams, the client requests the momentary values
for power and energy from the node named \emph{Device04}. The device first
acknowledges this request, and, after retrieving the values, sends them back
to the client. Afterwards, both sides close their streams.

\input{fig-example-xmpp-stream.tex}

\paragraph{Control (XEP-0325)~\cite{xep0325}} In this document, a way of
controlling sensor nodes is specified, which allows a client to get and set
control values on the node over message or IQ stanzas. As an example, in this
way a sensor node could be instructed to return data in a different unit or
range, or be put into power-safe mode.

\paragraph{Provisioning (XEP-0324)~\cite{xep0324}}
To protect the integrity of a sensor network and securing the data being
collected, this XEP specifies a way of implementing access rights and user
privileges. Since a single sensor node is usually very restricted in user input and
output, the approach is very simple and can be implemented e.~g. using a button
and an LED for interaction, while presentation of data takes places on a
provisioning server with a rich user interface (which can be, for example, a
concentrator).

When integrating a new sensor node into the network, the user instructs the
provisioning server to generate a \term{friendship} request for the new node.
The node can e.~g. symbolize this request by blinking its LED and requesting a
button press in the next 30 seconds. If the user presses the button, the node
confirms the friendship to the server. The server then remembers this sensor
node and generates a token which must be used in all further communication
between the server and the sensor node, else communication is rejected.

\paragraph{Efficient XML Interchange Format (EXI, XEP-0322)~\cite{xep0322}}
Finally, EXI describes how XMPP stanzas sent between nodes can be compressed,
thereby effectively reducing the overhead in message size introduced by XML.
XMPP nodes can negotiate a compressed stream inside their existing XMPP streams
and exchange \code{<compress>} stanzas which then contain the payload. However,
it is to be noted that this requires further implementation of compression
algorithms as well as additional CPU and memory resources and thus might
decrease message throughput and increase power consumption on embedded systems.

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