Add further information to the README

README.md

@@ -55,7 +55,7 @@ base station then replies with an encrypted packet containing a random salt
 value, and the device uses this salt for all further communication until the next
 reset, increasing the salt value by one for each packet sent. This scheme removes
 the need for the device to generate random numbers, as microcontrollers commonly
-are not capable of generation of high-quality ranodm numbers.
+are not capable of generation of high-quality random numbers.
 
 The packet sent from the device to the base station to request a salt consists of
 a single byte with the sender ID and is recognized by the base station by its
@@ -93,7 +93,7 @@ packet as the salt.
    3. Values: Reply to a `GetValues` packet containing the requested information.
 
    The 3 most significant bits of the field encode the element count for packets
-   which can holt multiple elements (as described below).
+   which can hold multiple elements (as described below).
 
 4. Packet payload (always 21 bytes, padded with zero bytes)
 
@@ -106,21 +106,42 @@ packet as the salt.
      possible value types and the corresponding value size are listed below.
    * GetValues: The payload consists of a list of value types. It is the sender's
      responsibility to ensure that the resulting response fits into one packet.
-     If too many values are requested, the base station will ignore the request.
-   * Values: The payload consists of a list of (value type, report time, value)
-     tuples. The report time is the unix time of the last report of the
-     corresponding sensor node.
+     If too many values are requested, the base station will ignore the request. 
+
+     TODO: Describe the network topology above and specify which payload type is
+     used by which device for which purpose.
+
+     TODO for GetValues: There should be a way for a device to request
+     the value from a certain source (e.g. the termperature of the living room).
+
+     TODO for GetValues: There should be an answer packet to signal that the
+     requested source is not available.
 
 5. CRC-16-CCITT checksum
 
 ### Value types:
 
-0. Time (4 bytes): The value is a unix time in seconds.
+0. Time (8 bytes): The value is a unix time in seconds.
 1. Temperature (2 bytes): The value is a temperature value in tenths of degrees
    celsius.
 2. Pressure (4 bytes): The value is a pressure in pascal.
 3. Humidity (2 bytes): The value is the relative humidity in tenths of a percent.
 
+# Data handling:
+
+The base station collects all needed data from the sensors and other sources and
+acts as a server for all devices.
+
+- The base station shall contain a list of all devices on the network.
+- The base station shall have an interface where devices can be added and removed
+  from the network.
+- Every sensor shall be registered using a (module id, location) tuple.
+- The base station shall monitor the update rates of the devices.
+- Every device shall be assigned a threshold of missed updates after which it is
+  considered to be offline.
+- The base station shall indicate that a device is offline if another device
+  requests a report from the offline device.
+
 # License
 
 The hardware contained in this repository is licensed under the [CERN Open Hardware Licence Version 2 - Weakly Reciprocal](cern_ohl_w_v2.txt), and the software is licensed under the [GNU General Public License v3](gpl-3.0.md).