This document describes the driver for the xran driver, implementing O-RAN FHI 7.2 interface. This is for developers; for user documentation, consider the [O-RAN 7.2 tutorial instead](../../doc/ORAN_FHI7.2_Tutorial.md). The main source files and their purpose are as follows: - `oaioran.c`: definition of xran-specific callback functions. - `oran-config.c`: reading of 7.2-specific configuration and preparing xran configration. - `oran-init.c`: initialization of xran. - `oran_isolate.c`: main entry point and definition of function pointers for OAI callbacks. The entry point for the driver library is `transport_init()` in `oran_isolate.c`. This function is concerned with setting necessary function pointers for OAI integration, and prepares xran configuration through `get_xran_config()`. This function reads 7.2 specific configuration and deduces some of the parameters (e.g., frequency) from OAI-provided radio configuration in `openair0_config_t`. Actual xran initialization happens in `oai_oran_initialize()` in `oran-init.c`. It calls various helper function in the same file to set up DPDK memory buffer (pools). Notably, these callbacks are installed: - `oai_xran_fh_rx_callback()` through `xran_5g_fronthault_config()`: callback for PUSCH data; used to provide timing data. - `oai_xran_fh_rx_prach_callback()` through `xran_5g_prach_req()`: callback for PRACH data; not currently used and implemented in PUSCH data callback. - `oai_physide_dl_tti_call_back()` through `xran_reg_physide_cb()`: only used to unblock timing in `oai_xran_fh_rx_callback()` and `oai_xran_fh_rx_prach_callback()` upon first xran call. More detailed information about the xran callbacks can be taken from the xran documentation. During normal operation, OAI calls into the driver through functions `oran_fh_if4p5_south_in()` for PUSCH/PRACH and `oran_fh_if4p5_south_out()` for PDSCH. For PDSCH, `oran_fh_if4p5_south_out()` calls `xran_fh_tx_send_slot()` that optionally compresses IQ data, then writes it into IQ buffers of xran. For PUSCH/PRACH, `oran_fh_if4p5_south_in()` calls `xran_fh_rx_read_slot()`/ `xran_fh_rx_prach_read_slot()` that blocks and waits for the next slot. This is done through a message queue which depends on xran calling the callbacks `oai_xran_fh_rx_callback()`/`oai_xran_fh_rx_prach_callback()` as installed during xran initialization. Once unblocked, it reads first PUSCH data, then PRACH data, before returning to OAI.