Added tl partial description

docs/d2d/link_layer.rst

@@ -2,6 +2,19 @@
 D2D - Link Layer setup
 **********************
 
+Layer 2: Link Layer overview
+----------------------------
+The **Link Layer** also called **Data Link Layer** is responsible for
+transfering data between two devices. It defines how devices gain the
+access to a medium. The **Link Layer** organizes data before
+transmission and verifies whether the data were received or sent
+successully. The best example of it could be a collision detection
+feature - which is supported by the **D2D** libray. The **L2** plays
+an important role in error detection and error recovering. In case
+of an **UART** examplary implementation, **Link Layer** can detect
+medium transmission errors such as **parity** or **overrun**, or
+framing erros such as **CRC** mismatch.
+
 How to setup
 ------------
 After building a basic hardware connection between the Devices (Nodes),

docs/d2d/transport_layer.rst

@@ -4,4 +4,54 @@ D2D - Transport Layer setup
 
 Overview
 --------
+The **Transport Layer** is a layer 4 in the **OSI** model.
+It is reponsible for providing end-to-end reliable communication
+between two logical Nodes. This layer ensures the user that
+data sent between the devices are correct and error-free.
+The **L4** manages a traffic over the medium. It controls
+a bandwidth by imposing speed reductions, controls
+message re-transmissions in case of framing errors and
+enusers data integrity such as maching a **Response** to
+the **Query**.
 
+How to setup
+------------
+There is no need to setup the **Transport Layer** and
+after it's initialization it is ready to go. However, there
+is one module on which the **L4** depends on. It is
+a **Pseudo Random Number Generator** (**PRNG**) module.
+
+Pseudo Random Number Generator setup
+------------------------------------
+The **Pseudo Random Number Generator** module is used by the
+**Transport Layer** in order to generate random back-off times.
+It requires a small setup for a target microcontroller. Required
+setup can be done inside a ``prng_config.c/h`` files. Please take
+a look at the ``prng_config.h`` API.
+
+.. code-block:: C++
+    :linenos:
+
+    void prng_init_bit_generator(void);
+    void prng_start_bit_generator(void);
+    void prng_stop_bit_generator(void);
+    uint16_t prng_bit_generator_get(void);
+    int prng_delay(void);
+    void prng_seed(unsigned int x);
+
+Each function contains a descriptive doxygen comment. It is
+probably more interesting how to provide quick but reliable
+implementation of the above functions.
+
+The random bit generation can utilize an **ADC** microcontroller's
+peripheral.
+
+
+What next
+---------
+. . .
+
+
+**Footnote**
+
+**kaeraz**, 2019/02