Project Scope

Introduced as part of MS-side GPRS osmocom-bb project
Open source GPRS "modem" implementation for OsmocomBB
SDR GSM mobile phone (GPRS in Calpyso PHY is out of scope and not implemented)
ARDC (Amateur Radio Digital Communications) approved grant of funds to DARC
DARC (Deutschen Amateur-Radio-Club) contracted sysmocom to perform the related work

Reminder: GPRS Stack

L2 + L3: modem, osmocom-bb/src/host/layer23/src/modem/
L1: osmo-trx-ms-blade, osmo-trx/Transceiver52M/ms/

Block diagram

Interfaces between layers "specified" in 3GPP specs

libosmo-gprs: Introduction

Contains several .so, one per layer:
- libosmo-gprs-*: csn1, rlcmac, llc, sndcp, gmm, sm
- Helper libosmo-csn1 (the CSN.1 codec from osmo-pcu.git)
Layer communication through primitives
Follows 3GPP "specified" interfaces as close as possible
Each layer can be used independently
App in charge of forwarding primitives between layers through callback hooks
Support multiple concurrent subscribers

libosmo-gprs-<layer>: Public API

include/osmocom/gprs/<layer>/<layer>.h:
- osmo_gprs_<layer>_init(cfg)
- osmo_gprs_<layer>_set_log_cat(enum logc, int num)
  - Support multiple log categories
  - Dynamic mapping of log categories done by app
- Layer related public data types
include/osmocom/gprs/<layer>/<layer>_prim.h:
- struct osmo_gprs_<layer>_<if>_prim: Holds all primitives for each interface
- struct osmo_gprs_<layer>_prim: osmo_prim_hdr + struct osmo_gprs_<layer>_<if>_prim
- osmo_gprs_<layer>_prim_alloc()
- osmo_gprs_<layer>_prim_set_{up,down}_cb()
- osmo_gprs_<layer>_{upper_down,lower_up}()
include/osmocom/gprs/<layer>/<layer>_private.h:
Private APIs and data types

libosmo-gprs-sm

SMREG:

Used by app directly to manage PDP Contexts
eg. Tx PDP_ACTIVATE.req, Rx PDP_ACTIVATE.cnf

GMMSM:

Trigger implicit attach/detach during PDP Context creation/deletion

libosmo-gprs-gmm

GMMSM:

Trigger implicit attach/detach during PDP Context creation/deletion

GMMREG:

Explicit attach/detach by app
SIM AUTH required towards app (Primitive osmocom specific)

LL:

Rx/Tx GMM PDUs to LLC layer

LLGMM:

LLC-SUSPEND, LLC-STATUS, TRIGGER (Cell Notification, READY timer)

GMMRR:

Assing new negotiated TLLI to lower layers (RLC/MAC)
Page indications
LLC transmitted notifications (timers, counters)

libosmo-gprs-sndcp

Internal implementation ported from osmo-sgsn + heavy clean up

SN:

Used by app to Tx/Rx user data (IP packets from tundev)
App configures, negotiates SNDCP XIDs

SNSM:

Coordination with GPRS SM layer
eg. PDP Context was (de)activated

LL:

Rx/Tx SNDCP PDUs to LLC layer

libosmo-gprs-llc

Internal implementation ported from osmo-sgsn + heavy clean up

LL:

Forward GMM/SNDCP PDUs to/from upper layers
LLE (per SAPI)

LLGMM:

Already covered in previous slide (LLC-SUSPEND/STATUS/TRIGGER)
LLME (shared among all SAPIs)

GRR: (MS role)

Tx/Rx LLC frames to/from RLC/MAC

BSSGP: (SGSN role)

Tx/Rx LLC frames to/from BSSGP

libosmo-gprs-rlcmac

Some parts of the implementation ported from osmo-pcu + heavy clean up

GRR:

Forward LLC frames to/from upper layers (LLC)

GMMRR:

Assing new negotiated TLLI to lower layers (RLC/MAC)
Page indications
LLC transmitted notifications (timers, counters)

L1CTL:

Tx RACH.req (Access Burst)
Rx CCCH_DATA.ind (DL CCCH/BCCH blocks)
Tx PDCH_{ESTABLISH,RELEASE.req (PDCH activation/deactivation)
Tx CFG_{UL,DL}_TBF.req (UL/DL TBF configuration in a PDCH)
Rx PDCH_DATA.{ind,cnf}, Tx PDCH_DATA.req (PDCH blocks)

`modem` Application

Takes care of CS management (cell sync, etc.)
- No cell scan / automatic selection, requires manual ARFCN specification
Manages SM and MM state through SMREG and GMMREG primitive interfaces
Manages the tundev for each APN
Provides callbacks to forward primitives:
- Between the different libosmo-gprs-* libs
- To/from L1CTL provider (unix socket, trxcon + osmo-trx-ms-blade)

Example: `modem` establishing a PDP Context

modem: Configure APNs (VTY)
modem: Modem syncs to cell (CCCH)
modem → SM: SMREG-PDP_ACT.req
SM → GMM: GMMSM-ESTABLISH.req
GMM → LLC: LL-UNITDATA.req(GMMAttachReq)
LLC → RLC/MAC: GRR-UNITDATA.req(llc_frame[GMMAttachReq])
RLC/MAC needs a TBF to transmit data
1. RLC/MAC → L1CTL: L1CTL-RACH.req
2. L1CTL → RLC/MAC: L1CTL-CCCH_DATA.ind(ImmAss[ts, fn, usf, tfi])
3. RLC/MAC → L1CTL: L1CTL-PDCH_EST.req
4. RLC/MAC → L1CTL: L1CTL-CFG_UL_TBF.req
5. L1CTL → RLC/MAC: L1CTL-PDCH_RTS.ind(fn, ts, usf)
RLC/MAC → L1CTL: L1CTL-PDCH_DATA.req(rlc_blk[llc_frame[GMMAttachReq]])
L1CTL → RLC/MAC: L1CTL-PDCH_DATA.ind(rlc_blk[llc_frame[GMMAttachAcc]])
RLC/MAC → LLC: GRR-UNIT_DATA.req(llc_frame[GMMAttachAcc])
LLC → GMM: LL-UNIT_DATA.req(GMMAttachAcc)
GMM → SM: GMMSM-UNITDATA.req(SM-ActivatePDPContextReq)
…

trxcon

Project info

MS side L1 implementation (burst scheduling, Viterbi decoding)
Started as a hobby project back in 2016
Rx/Tx capable PoC presented at 34c3 back in 2017
- part of the SDR PHY for osmocom-bb initiative
- worked with grgsm_trx, GNU Radio gr-gsm based transceiver
Actively used by ttcn3-bts-test (conformance/regression tests for osmo-bts)

commit 8975b437ed476a1655e19b68d113bc93c468b087
Author: Vadim Yanitskiy <axilirator@gmail.com>
Date:   Thu Jun 30 00:41:58 2016 +0600

    host/trxcon: introduce a new 'trxcon' application

trxcon

Project architecture (before GPRS MS)

Very simple proxy between the L1CTL and TRXC/TRXD interfaces
No clear component separation, direct function calls
No state tracking (FSMs), no timeouts…
PoC level code quality

trxcon

Project architecture (after GPRS MS)

Proper separation between components
- libraries: libtrxcon, libl1sched, l1gprs
- … can be re-used by other apps
trxcon_fsm - the "heart" and the "brain"
libgprs: GPRS support!

trxcon / trxcon_fsm

Event flow

Figure 1. Cell selection

trxcon / trxcon_fsm

Event flow

Figure 2. DCCH establishment & release

trxcon / trxcon_fsm

Event flow

Figure 3. DCCH burst Rx/Tx

trxcon / l1gprs

Performance requirements

Unlike GSM, GPRS imposes tight performance requirements
- an UL BLOCK.req must be sent at specific Tn/Fn, not earlier or later
TDMA gives us 5.77ms to handle a DL BLOCK.ind and prepare UL BLOCK.req
- see https://people.osmocom.org/fixeria/pdch.txt
Solution 1: move the whole RLC/MAC to the lower layers
Solution 2: move part of RLC/MAC to the lower layers
- we decided to follow this approach
- the part of RLC/MAC is l1gprs

trxcon / l1gprs

The l1gprs library

RLC/MAC (lower part) as per 3GPP TS 44.060
Intentionally separate library with abstract API
- already used by the virtphy app (might be broken, though)
- can hopefully be integrated with the layer1 firmware for Calypso
  - ancient copy of libosmocore.git is a blocker (see OS#2378)
Relevant files: src/shared/l1gprs.c and include/l1gprs.h
Depends on libosmocore and libosmogsm

trxcon / l1gprs

New L1CTL messages

L1CTL_GPRS_{UL,DL}_TBF_CFG_REQ - UL/DL TBF configuration Req
L1CTL_GPRS_{UL,DL}_BLOCK_{REQ,IND} - UL/DL PDTCH block Req/Ind
L1CTL_GPRS_RTS_IND - Ready-to-Send Ind (from PHY)
L1CTL_GPRS_UL_BLOCK_CNF - UL PDTCH block Confirmation

API

l1gprs_logging_init(logc)
l1gprs_state_alloc() / l1gprs_state_free()
l1gprs_handle_{ul,dl}_tbf_cfg_req()
l1gprs_handle_ul_block_{req,cnf}()
l1gprs_handle_dl_block_ind()
l1gprs_handle_rts_ind()

trxcon / l1gprs

Event flow

Figure 4. PDCH block Rx/Tx

osmo-trx-ms

The MS side transceiver

osmo-trx was so far the BTS side transceiver
- ms branch: early/unfinished work by Tom Tsou (2014)
osmo-trx-ms the MS side transceiver by @Hoernchen
- partly based on the work by Tom Tsou
- partly based on burst detector from gr-gsm by Piotr Krysik
- based on libtrxcon + libl1sched + libl1gprs
- requires the clock source to work

Limitations

SDR support: bladeRF only
- -blade variant only, -uhd is untested (likely broken)
no VTY interface, hard-coded logging config
may hang/crash sometimes, requires restart

osmo-trx-ms

How to build?

$ git clone https://git.osmocom.org/osmo-trx
$ git submodule init # <1>
$ git submodule update # <1>
$ autoreconf -fi
$ ./configure --with-mstrx --with-bladerf

Fetch the osmocom-bb git submodule

Debian package(s)

$ apt install osmo-trx-ms-blade

Future work

libosmo-gprs:
- Cleanup API: No need 1 alloc API per primitive+operation type
- Declare its API stable
libosmo-gprs-rlcmac:
- Support TBFs with >1 TS
- Support EGPRS
- Tons of other features missing
osmo-sgsn:
- WIP: Port to libosmo-gprs-{llc,sndcp}, OS#5853 (branch pespin/libosmo-gprs)

GPRS support in osmocom-bb

Project Scope

Reminder: GPRS Stack

Block diagram

libosmo-gprs: Introduction

libosmo-gprs-<layer>: Public API

libosmo-gprs-sm

libosmo-gprs-gmm

libosmo-gprs-sndcp

libosmo-gprs-llc

libosmo-gprs-rlcmac

`modem` Application

Example: `modem` establishing a PDP Context

trxcon

trxcon

trxcon

trxcon / trxcon_fsm

trxcon / trxcon_fsm

trxcon / trxcon_fsm

trxcon / l1gprs

trxcon / l1gprs

trxcon / l1gprs

trxcon / l1gprs

osmo-trx-ms

osmo-trx-ms

Future work

Know more

GPRS support in osmocom-bb

Project Scope

Reminder: GPRS Stack

Block diagram

libosmo-gprs: Introduction

libosmo-gprs-<layer>: Public API

libosmo-gprs-sm

libosmo-gprs-gmm

libosmo-gprs-sndcp

libosmo-gprs-llc

libosmo-gprs-rlcmac

modem Application

Example: modem establishing a PDP Context

trxcon

trxcon

trxcon

trxcon / trxcon_fsm

trxcon / trxcon_fsm

trxcon / trxcon_fsm

trxcon / l1gprs

trxcon / l1gprs

trxcon / l1gprs

trxcon / l1gprs

osmo-trx-ms

osmo-trx-ms

Future work

Know more

`modem` Application

Example: `modem` establishing a PDP Context