Some initial investigation on GPRS support

Hi! I've started to analyze GPRS and was actually even starting to write some code for it, but then have given up for the time being - it's much more work than anticipated. Given the long todo list of OpenBSC right now, I think I'll have put aside GPRS for some time :( Based on looking at protocol traces, I have figured out the nanoBTS GPRS/EDGE implementation roughly looks as follows: * make sure we allow the BTS to activate the GPRS software components in abis_nm / OML activation! * BTS will use a UDP connection on port 23000 for the GPRS related frames. The GSM specs will consider this type of connection between the PCU (part of the nanoBTS) and the SGSN. The establishment/configuration of the UDP port number and SGSN ip address has not yet been identified. Probably similar to how the RSL link is activated via OML. The protocol stack looks like: IP : UDP : NSIP : BSSGP : LLC : higher-layer IP and UDP you should know and/or not care about ;) NSIP is a IP-enabled version of NS as specified in TS 08.16 BSSGP is specified in TS 08.18 LLC is as specified in TS 04.64 the higher-layer depends on the SAPI value of the LLC and can be * GMM (GPRS Mobility Management as specified in 04.08) * User Data (actual IP packets, e.g.) * SMS So what is weird about this is that the GPRS MM is actually part of 04.08, but it is not terminated at the BSC but rather at the SGSN. Also, the deep stack comprised of many headers is really weird. Furthermore, it seems that a lot of the packet scheduling and timeslot allocation is happening inside the nanoBTS - very unlike the GSM side of things. I have not yet managed to figure out how to allocate/dedicate resources to GPRS.. after all, the BTS needs to know how many timeslots it can use for it, etc. If anyone wants to dig deeper, you're most welcome to do so. A list of relevant specs: 01.61 GPRS cipher algorithm requirements 03.60 Overall GRPS logical architecture (above RL and MAC) 03.64 GPRS radio interface 04.60 RLC/MAC on PDCH 04.64 MS-SGSN LLC spec (on top of RLC/MAC) 04.65 SGSN SNDCP 08.14 BSS SGSN Gb Layer 1 08.16 BSS SGSN Gb Layer 2 08.18 BSS SGSN BSS GPRS protocol 09.95 Interworking between modified PLMN supporting legacy GPRS and GPRS mobiles 22.060 GPRS Service Spec 23.060 GPRS Radio Service Spec 29.016 SGSN - VLR Interface Gs network interface spec 29.018 SGSN - VLR Interface Gs layer3 interface spec 29.060 GPRS Tunneling (GTP) over Gn and Gp Happy hacking, -- - Harald Welte <laforge(a)gnumonks.org> http://laforge.gnumonks.org/ ============================================================================ "Privacy in residential applications is a desirable marketing option." (ETSI EN 300 175-7 Ch. A6)