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== OsmoBTS software architecture

=== OsmoBTS PHY interface abstraction

The OsmoBTS PHY interface serves as an internal abstraction layer
between given PHY hardware (as provided by the bts_model) and the actual
logical transceivers (TRXs) of a BTS inside the OsmoBTS code base.


==== PHY link

A PHY link is a physical connection / link towards a given PHY.  This
might be, for example,

* a set of file descriptors to device nodes in the /dev/ directory
  (sysmobts, litecell15)
* a packet socket for sending raw Ethernet frames to an OCTPHY
* a set of UDP sockets for interacting with OsmoTRX

Each PHY interface has a set of attribute/parameters and a list of 1 to
n PHY instances.

PHY links are numbered 0..n globally inside OsmoBTS.

Each PHY link is configured via the VTY using its individual top-level
vty node.  Given the different bts-model / phy specific properties, the
VTY configuration options (if any) of the PHY instance differ between
BTS models.

The PHY links and instances must be configured above the BTS/TRX nodes
in the configuration file.  If the file is saved via the VTY, the code
automatically ensures this.


==== PHY instance

A PHY instance is an instance of a PHY, accessed via a PHY link.

In the case of osmo-bts-sysmo and osmo-bts-trx, there is only one
instance in every PHY link.  This is due to the fact that the API inside
that PHY link does not permit for distinguishing multiple different
logical TRXs.

Other PHY implementations like the OCTPHY however do support addressing
multiple PHY instances via a single PHY link.

PHY instances are numbered 0..n inside each PHY link.

Each PHY instance is configured via the VTY as a separate node beneath each
PHY link.  Given the different bts-model / phy specific properties, the
VTY configuration options (if any) of the PHY instance differ between
BTS models.


==== Mapping PHY instances to TRXs

Each TRX node in the VTY must use the 'phy N instance M' command in
order to specify which PHY instance is allocated to this specific TRX.

=== Internal control flow

==== start-up / sequencing during OsmoBTS start

.Control flow at OsmoBTS start-up procedure
[options="header",cols="10%,35%,55%"]
|===
| section | function | description
| bts-specific | main() | Entering main() from glibc
| common | bts_main() | initialization of talloc contexts
| common | bts_log_init() | initialization of logging
| common | handle_options() | common option parsing
| bts-specific | bts_model_handle_options() | model-specific option parsing
| common | gsm_bts_alloc() | allocation of BTS/TRX/TS data structures
| common | vty_init() | Initialziation of VTY core, libosmo-abis and osmo-bts VTY
| common | main() | Setting of scheduler RR priority (if configured)
| common | main() | Initialization of GSMTAP (if configured)
| common | bts_init() | configuration of defaults in bts/trx/s object
| bts-specific | bts_model_init | ?
| common | abis_init() | Initialization of libosmo-abis
| common | vty_read_config_file() | Reading of configuration file
| bts-specific | bts_model_phy_link_set_defaults() | Called for every PHY link created
| bts-specific | bts_model_phy_instance_set_defaults() | Called for every PHY Instance created
| common | bts_controlif_setup() | Initialization of Control Interface
| bts-specific | bts_model_ctrl_cmds_install() | Install model-specific control interface commands
| common | telnet_init() | Initialization of telnet interface
| common | pcu_sock_init() | Initialization of PCU socket
| common | main() | Installation of signal handlers
| common | abis_open() | Start of the A-bis connection to BSC
| common | phy_links_open() | Iterate over list of configured PHY links
| bts-specific | bts_model_phy_link_open() | Open each of the configured PHY links
| common | write_pid_file() | Generate the pid file
| common | osmo_daemonize() | Fork as daemon in background (if configured)
| common | bts_main() | Run main loop until global variable quit >= 2
|===


==== At time of OML establishment

.Control flow at time of OML establishment
[options="header",cols="10%,35%,55%"]
|===
| section | function | description
| bts-specific | bts_model_oml_estab() | Called by core once OML link is established
| bts-specific | bts_model_check_oml() | called each time OML sets some attributes on a MO, checks if attributes are valid
| bts-specific | bts_model_apply_oml() | called each time OML sets some attributes on a MO, stores attribute contents in data structures
| bts-specific | bts_model_opstart() | for NM_OC_BTS, NM_OC_SITE_MANAGER, NM_OC_GPRS_NSE, NM_OC_GPRS_CELL, NMO_OC_GPRS_NSVC
| bts-specific | bts_model_opstart() | for NM_OC_RADIO_CARRIER for each trx
| bts-specific | bts_model_opstart() | for NM_OC_BASEB_TRANSC for each trx
| bts-specific | bts_model_opstart() | for NM_OC_CHANNEL for each timeslot on each trx
| bts-specific | bts_model_change_power() | change transmit power for each trx (power ramp-up/ramp-down)
|===

==== At time of RSL connection loss

.Control flow at time of RSL connection loss
[options="header",cols="10%,35%,55%"]
|===
| section | function | description
| bts-specific | bts_model_abis_close() | called when either one of the RSL links or the OML link are down
|===