Library architecture

The Modem library is the application interface to the nRF9160 modem firmware, which contains a full IP and DTLS/TLS stack as well as GNSS. It provides the standard Socket APIs, to facilitate application development and additional APIs to manage the modem and perform full-modem firmware updates.

The library implements a communication interface between the Application and Modem cores on the nRF9160 through the RPC protocol, using the Inter Processor Communication (IPC) peripheral and a shared region of RAM.

The following figure shows a simplified Modem library architecture:

Modem library architecture diagram

Modem library architecture diagram

Shared memory configuration

The Modem library implements a communication interface between the application core and the modem core by using a shared memory area to exchange data. The shared memory area can be located anywhere within the first 128 kilobytes of RAM (lowest addresses) and it is logically divided into the following four regions:

  • Control

  • TX

  • RX

  • Trace

The application can configure the size and location of these regions. The application is responsible for reserving the memory area by setting the values of these parameters before passing them onto the library. The library accepts these values as parameters to the nrf_modem_init() function through nrf_modem_shmem_cfg.


The size of the Control area is fixed.

The application can adjust the size of these regions based on the needs to minimize the RAM requirements of the library.

For nRF Connect SDK users, the Partition Manager will automatically reserve some RAM for each region during linking, according to the size of each region as specified in the glue.

Control area

The control area contains the data structures that are used to setup the communication between the modem core and the application core. Among the sizes of the memory regions, only the size of the control area is fixed, and it is exported in the nrf_modem_platform.h file.

For nRF Connect SDK users, the build system and the glue implementation will automatically take care of passing the correct size to the nrf_modem_init() call.

TX area

The TX area contains the data payload of messages that are sent to the modem. The size of this area affects the largest buffer that nrf_send() can send in a single call, and the size of the longest AT command that can be sent to the modem. When provisioning the TLS certificates, the size of the TX area must be large enough to accommodate the TLS certificate and the AT command that is used for provisioning. The library OS abstraction layer defines the following functions to allocate and free data in this memory region:

RX area

The RX area is entirely managed by the modem and this area contains all the incoming data from the modem. The incoming data includes GNSS data, AT command responses, and IP traffic. The size of this area determines the maximum amount of incoming data from the modem that the application core can buffer. If the area is full and the application has not read the data yet, new data cannot be buffered in this area.

An example of an operation that requires a large RX area is the reading of a TLS certificate associated with a security tag. The size of the RX area must be as large as the size of the TLS certificate that is being read, and the AT command that is used to read the certificate.

Trace area

The trace area contains the trace output from the modem core. This area of memory is optional, and the area size can be configured to be zero, to disable the trace output.

Library heap

The Modem library dynamically allocates memory during some operations, for example tracing and DNS lookups. This memory is never shared with the modem core and hence, it can be located anywhere in the application core’s RAM instead of the shared memory regions. The library OS abstraction layer defines the following functions to allocate and free up dynamic memory for the library: