Hi again, Great! Let me explain some basics first. In current OsmocomBB we have an osmocon application, which forwards L1CTL (GSM Layer 1 Control Messages) between host software (GSM L2 & L3 applications such as mobile) and phone's custom firmware through serial link (sercomm). Since we are going to use SDR based PHY for GSM L1, similar application is still required. I named one as 'trxcon'. Just like osmocon, it acts as a bridge between existing L2 & L3 applications and another application (modified OsmoTRX or GR-GSM based TRX), providing an UNIX socket (/tmp/osmocom_l2). layer23 app <-> trxcon <-> transceiver <-> SDR I am currently working on trxcon application, doing some code refactoring and integration of osmo-fsm (Finite State Machine) framework. One consists of: - L1CTL link - UNIX socket handlers; - L1CTL handlers - REQ / RSP / IND handlers; - TRX interface - CTRL / DATA interface for TRX; - TDMA scheduler - WIP right now... Basically this application is similar to OsmoBTS. Regarding to transceiver, we can use OsmoTRX here, but some code modifications are required in this case (see 'ms' branch). For now I decided to use GR-GSM blocks from Piotr Krysik, and have written a simple transceiver implementation, which you can find here: https://github.com/axilirator/gr-gsmtrx As there are currently no GR-GSM blocks, which allow to transmit UL data, we are limited to RX only mode. But I had a great discussion with the author, and he is going to work on required blocks. That is a brief introduction. Feel free to ask me if you have any related questions, and feel free to push your commits for review! You can also find me @fixeria in IRC #osmocom room. With best regards, Vadim Yanitskiy.