This is an implementation of the GPRS Serving GPRS Support Node (SGSN). As such it implements the GPRS Mobility Management (GMM) and SM (Session Management).

The SGSN connects via the Gb-interface to the BSS (like the osmo-pcu or an ip.access nanoBTS), and it connects via the GTP protocol to a Gateway GPRS Support Node (GGSN) like openggsn.