doc/manuals/chapters/bts.adoc

[[bts]]
== Reviewing and Provisioning BTS configuration

The main functionality of the BSC component is to manage BTSs. As such,
provisioning BTSs within the BSC is one of the most common tasks during
BSC operation. Just like about anything else in OsmoBSC, they are
configured using the VTY.

BTSs are internally numbered with integer numbers starting from "0" for
the first BTS. BTS numbers have to be contiguous, so you cannot
configure 0,1,2 and then 5.


=== Reviewing current BTS status and configuration

In order to view the status and properties of a BTS, you can issue the
`show bts` command. If used without any BTS number, it will display
information about all provisioned BTS numbers.

----
OsmoBSC> show bts 0
BTS 0 is of nanobts type in band DCS1800, has CI 0 LAC 1, BSIC 63, TSC 7 and 1 TRX
Description: (null)
MS Max power: 15 dBm
Minimum Rx Level for Access: -110 dBm
Cell Reselection Hysteresis: 4 dBm
RACH TX-Integer: 9
RACH Max transmissions: 7
System Information present: 0x0000007e, static: 0x00000000
  Unit ID: 200/0/0, OML Stream ID 0xff
  NM State: Oper 'Enabled', Admin 2, Avail 'OK'
  Site Mgr NM State: Oper 'Enabled', Admin 0, Avail 'OK'
  Paging: 0 pending requests, 0 free slots
  OML Link state: connected.
  Current Channel Load:
                   TCH/F:   0% (0/5)
                  SDCCH8:   0% (0/8)
----

You can also review the status of the TRXs configured within the BTSs of
this BSC by using `show trx`:

----
OsmoBSC> show trx 0 0
TRX 0 of BTS 0 is on ARFCN 871
Description: (null)
  RF Nominal Power: 23 dBm, reduced by 0 dB, resulting BS power: 23 dBm
  NM State: Oper 'Enabled', Admin 2, Avail 'OK'
  Baseband Transceiver NM State: Oper 'Enabled', Admin 2, Avail 'OK'
  ip.access stream ID: 0x00
----

The output can be restricted to the TRXs of one specified BTS number
(`show trx 0`) or even that of a single specified TRX within a
specified BTS (`show trx 0 0`).

Furthermore, information on the individual timeslots can be shown by
means of `show timeslot`. The output can be restricted to the
timeslots of a single BTS (`show timeslot 0`) or that of a single
TRX (`show timeslot 0 0`). Finally, you can restrict the output to
a single timeslot by specifying the BTS, TRX and TS numbers (`show
timeslot 0 0 4`).

----
OsmoBSC> show timeslot 0 0 0
BTS 0, TRX 0, Timeslot 0, phys cfg CCCH, TSC 7
  NM State: Oper 'Enabled', Admin 2, Avail 'OK'
OsmoBSC> show timeslot 0 0 1
BTS 0, TRX 0, Timeslot 1, phys cfg SDCCH8, TSC 7
  NM State: Oper 'Enabled', Admin 2, Avail 'OK'
----


=== Provisioning a new BTS

In order to provision BTSs, you have to enter the BTS config node of the
VTY. In order to configure BTS 0, you can issue the following sequence
of commands:

----
OsmoBSC> enable
OsmoBSC# configure terminal
OsmoBSC(config)# network
OsmoBSC(config-net)# bts 0
OsmoBSC(config-net-bts)#
----

At this point, you have a plethora of commands, in fact an entire
hierarchy of commands to configure all aspects of the BTS, as well as
each of its TRX and each timeslot within each TRX. For a full
reference, please consult the telnet VTY integrated help or the respective
chapter in the VTY reference.

BTS configuration depends quite a bit on the specific BTS vendor and
model. The section below provides just one possible example for the
case of a sysmoBTS.

Note that from the `configure terminal` command onwards, the telnet VTY
commands above are identical to configuration file settings, for details see
<<vty>>.

Starting with `network` as above, your complete sysmoBTS configuration may look
like this:

----
network
 bts 0
  type sysmobts
  band DCS1800
  description The new BTS in Baikonur
  location_area_code 2342
  cell_identity 5
  base_station_id_code 63
  ip.access unit_id 8888 0
  ms max power 40
  trx 0
   arfcn 871
   nominal power 23
   max_power_red 0
   timeslot 0
    phys_chan_config CCCH+SDCCH4
   timeslot 1
    phys_chan_config TCH/F
   timeslot 2
    phys_chan_config TCH/F
   timeslot 3
    phys_chan_config TCH/F
   timeslot 4
    phys_chan_config TCH/F
   timeslot 5
    phys_chan_config TCH/F
   timeslot 6
    phys_chan_config TCH/F
   timeslot 7
    phys_chan_config PDCH
----


=== System Information configuration

A GSM BTS periodically transmits a series of 'SYSTEM INFORMATION'
messages to mobile stations, both via the BCCH in idle mode, was well as
via the SACCH in dedicated mode. There are many different types of such
messages. For their detailed contents and encoding, please see _3GPP TS
24.008_ <<3gpp-ts-24-008>>.

For each of the 'SYSTEM INFORMATION' message types, you can configure to
have the BSC generate it automatically ('computed'), or you can specify
the respective binary message as a string of hexadecimal digits.

The default configuration is to compute all (required) 'SYSTEM
INFORMATION' messages automatically.

Please see the _OsmoBSC VTY Reference Manual_ <<vty-ref-osmobsc>> for
further information, particularly on the following commands:

* `system-information (1|2|3|4|5|6|7|8|9|10|13|16|17|18|19|20|2bis|2ter|2quater|5bis|5ter) mode (static|computed)`
* `system-information (1|2|3|4|5|6|7|8|9|10|13|16|17|18|19|20|2bis|2ter|2quater|5bis|5ter) static HEXSTRING`


=== Neighbor List configuration

Every BTS sends a list of ARFCNs of neighbor cells
. within its 'SYSTEM INFORMATION 2' (and 2bis/2ter) messages on the BCCH
. within its 'SYSTEM INFORMATION 5' messages on SACCH in dedicated mode

For every BTS config node in the VTY, you can specify the behavior of
the neighbor list using the `neighbor list mode` VTY command:

automatic::
	Automatically generate a list of neighbor cells using all other
	BTSs configured in the VTY
manual::
	Manually specify the neighbor list by means of `neighbor-list
(add|del) arfcn <0-1023>` commands, having identical neighbor lists on
BCCH (SI2) and SACCH (SI5)

manual-si5::
	Manually specify the neighbor list by means of `neighbor-list
(add|del) arfcn <0-1023>` for BCCH (SI2) and a separate neighbor list by
means of `si5 neighbor-list (add|del) arfcn <0-1023>` for SACCH (SI5).


=== Configuring GPRS PCU parameters of a BTS

In the case of BTS models using Abis/IP (IPA), the GPRS PCU is located
inside the BTS. The BTS then establishes a Gb connection to the SGSN.

All the BTS-internal PCU configuration is performed via A-bis OML by
means of configuring the 'CELL', 'NSVC' (NS Virtual Connection and 'NSE'
(NS Entity).

There is one 'CELL' node and one 'NSE' node, but there are two 'NSVC'
nodes. At the time of this writing, only the NSVC 0 is supported by
OsmoBTS, while both NSVC are supported by the ip.access nanoBTS.

The respective VTY configuration parameters are described below. They
all exist beneath each BTS VTY config node.

But let's first start with a small example

.Example configuration of GPRS PCU parameters at VTY BTS node
----
OsmoBSC(config-net-bts)# gprs mode gprs
OsmoBSC(config-net-bts)# gprs routing area 1
OsmoBSC(config-net-bts)# gprs cell bvci 1234
OsmoBSC(config-net-bts)# gprs nsei 1234
OsmoBSC(config-net-bts)# gprs nsvc 0 nsvci 1234
OsmoBSC(config-net-bts)# gprs nsvc 0 local udp port 23000
OsmoBSC(config-net-bts)# gprs nsvc 0 remote udp port 23000
OsmoBSC(config-net-bts)# gprs nsvc 0 remote ip 192.168.100.239
----


=== More explanation about the PCU config parameters

//FIXME: should this go into VTY additions?

==== `gprs mode (none|gprs|egprs)`

This command determines if GPRS (or EGPRS) services are to be enabled in
this cell at all.


==== `gprs cell bvci <2-65535>`

Configures the 'BSSGP Virtual Circuit Identifier'. It must be unique
between all BGSGP connections to one SGSN.

NOTE: It is up to the system administrator to ensure all PCUs are
allocated an unique bvci. OsmoBSC will not ensure this policy.


==== `gprs nsei <0-65535>`

Configures the 'NS Entity Identifier'. It must be unique between all NS
connections to one SGSN.

NOTE: It is up to the system administrator to ensure all PCUs are
allocated an unique bvci. OsmoBSC will not ensure this policy.


==== `gprs nsvc <0-1> nsvci <0-65535>`

Configures the 'NS Virtual Connection Identifier'. It must be unique
between all NS virtual connections to one SGSN.

NOTE: It is up to the system administrator to ensure all PCUs are
allocated an unique nsvci. OsmoBSC will not ensure this policy.


==== `gprs nsvc <0-1> local udp port <0-65535>`

Configures the local (PCU side) UDP port for the NS-over-UDP link.


==== `gprs nsvc <0-1> remote udp port <0-65535>`

Configures the remote (SGSN side) UDP port for the NS-over-UDP link.


==== `gprs nsvc <0-1> remote ip A.B.C.D`

Configures the remote (SGSN side) UDP port for the NS-over-UDP link.


==== `gprs ns timer (tns-block|tns-block-retries|tns-reset|tns-reset-retries|tns-test|tns-alive|tns-alive-retries)` <0-255>

Configures the various GPRS NS related timers. Please check the GPRS NS
specification for the detailed meaning of those timers.


=== Dynamic Timeslot Configuration (TCH / PDCH)

A dynamic timeslot is in principle a voice timeslot (TCH) that is used to serve
GPRS data (PDCH) when no voice call is active on it. This enhances GPRS
bandwidth while no voice calls are active, which is dynamically scaled down as
voice calls need to be served. This is a tremendous improvement in service over
statically assigning a fixed number of timeslots for voice and data.

The causality is as follows: to establish a voice call, the
MSC requests a logical channel of a given TCH kind from the BSC. The BSC
assigns such a channel from a BTS' TRX's timeslot of its choice. The knowledge
that a given timeslot is dynamic exists only on the BSC level. When the MSC
asks for a logical channel, the BSC may switch off PDCH on a dynamic timeslot
and then assign a logical TCH channel on it. Hence, though compatibility with
the BTS needs to be ensured, any MSC is compatible with dynamic timeslots by
definition.

OsmoBSC supports two kinds of dynamic timeslot handling, configured via the
`network` / `bts` / `trx` / `timeslot` / `phys_chan_config` configuration. Not
all BTS models support dynamic channels.

[[dyn_ts_compat]]
.Dynamic timeslot support by various BTS models
[cols="50%,25%,25%"]
|===
|                    |`TCH/F_TCH/H_PDCH` |`TCH/F_PDCH`
|ip.access nanoBTS   |-                  |supported
|Ericsson RBS        |supported          |-
|sysmoBTS using _osmo-bts-sysmo_ |supported |supported
|various SDR platforms using _osmo-bts-trx_ |supported |supported
|Nutaq Litecell 1.5 using _osmo-bts-litecell15_ |supported |supported
|Octasic OctBTS using _osmo-bts-octphy_ | supported  | supported
|===

The _OsmoBTS Abis Protocol Specification_ <<osmobts-abis-spec>> describes the
non-standard RSL messages used for these timeslot kinds.

NOTE: Same as for dedicated PDCH timeslots, you need to enable GPRS and operate
a PCU, SGSN and GGSN to provide the actual data service.

==== Osmocom Style Dynamic Timeslots (TCH/F_TCH/H_PDCH)

Timeslots of the `TCH/F_TCH/H_PDCH` type dynamically switch between TCH/F,
TCH/H and PDCH, depending on the channel kind requested by the MSC. The RSL
messaging for `TCH/F_TCH/H_PDCH` timeslots is compatible with Ericsson RBS.

BTS models supporting this timeslot kind are shown in <<dyn_ts_compat>>.

In the lack of transcoding capabilities, this timeslot type may cause
mismatching codecs to be selected for two parties of the same call, which would
cause call routing to fail ("`Cannot patch through call with different channel
types: local = TCH_F, remote = TCH_H`"). A workaround is to disable TCH/F on
this timeslot type, i.e. to allow only TCH/H. To disable TCH/F on Osmocom
style dynamic timeslots, use a configuration of

----
network
 dyn_ts_allow_tch_f 0
----

In OsmoNITB, disabling TCH/F on Osmocom dynamic timeslots is the default. In
OsmoBSC, the default is to allow both.

==== ip.access Style Dynamic Timeslots (TCH/F_PDCH)

Timeslots of the `TCH/F_PDCH` type dynamically switch between TCH/F and PDCH.
The RSL messaging for `TCH/F_PDCH` timeslots is compatible with ip.access
nanoBTS.

BTS models supporting this timeslot kind are shown in <<dyn_ts_compat>>.

==== Avoid PDCH Exhaustion

To avoid disrupting GPRS, configure at least one timeslot as dedicated PDCH.
With only dynamic timeslots, a given number of voice calls would convert all
timeslots to TCH, and no PDCH timeslots would be left for GPRS service.

==== Dynamic Timeslot Configuration Examples

This is an extract of an `osmo-bsc`` config file. A timeslot configuration with
five Osmocom style dynamic timeslots and one dedicated PDCH may look like this:

----
network
 bts 0
  trx 0
   timeslot 0
    phys_chan_config CCCH+SDCCH4
   timeslot 1
    phys_chan_config SDCCH8
   timeslot 2
    phys_chan_config TCH/F_TCH/H_PDCH
   timeslot 3
    phys_chan_config TCH/F_TCH/H_PDCH
   timeslot 4
    phys_chan_config TCH/F_TCH/H_PDCH
   timeslot 5
    phys_chan_config TCH/F_TCH/H_PDCH
   timeslot 6
    phys_chan_config TCH/F_TCH/H_PDCH
   timeslot 7
    phys_chan_config PDCH
----

With the ip.access nanoBTS, only `TCH/F_PDCH` dynamic timeslots are supported,
and hence a nanoBTS configuration may look like this:

----
network
 bts 0
  trx 0
   timeslot 0
    phys_chan_config CCCH+SDCCH4
   timeslot 1
    phys_chan_config SDCCH8
   timeslot 2
    phys_chan_config TCH/F_PDCH
   timeslot 3
    phys_chan_config TCH/F_PDCH
   timeslot 4
    phys_chan_config TCH/F_PDCH
   timeslot 5
    phys_chan_config TCH/F_PDCH
   timeslot 6
    phys_chan_config TCH/F_PDCH
   timeslot 7
    phys_chan_config PDCH
----

=== Tuning Access to the BTS

OsmoBSC offers several configuration options to fine-tune access to the BTS.
It can allow only a portion of the subscribers access to the network.
This can also be used to ramp up access to the network on startup by slowly
letting in more and more subscribers. This is especially useful for isolated
cells with a huge number of subscribers.

Other options control the behaviour of the MS when it needs to access the
random access channel before a dedicated channel is established.

If the BTS is connected to the BSC via a high-latency connection the MS should
wait longer for an answer to a RACH request. If it does not the network will
have to deal with an increased load due to duplicate RACH requests. However,
in order to minimize the delay when a RACH request or response gets lost the
MS should not wait too long before retransmitting.

==== Load Management

Every SIM card is member of one of the ten regular ACCs (0-9). Access to the
BTS can be restricted to SIMs that are members of certain ACCs.

Since the ACCs are distributed uniformly across all SIMs allowing only ACCs
0-4 to connect to the BTS should reduce its load by 50%.

The default is to allow all ACCs to connect.

.Example: Restrict access to the BTS by ACC
----
network
 bts 0
  rach access-control-class 1 barred <1>
  rach access-control-class 9 allowed <2>
----
<1> Disallow SIMs with access-class 1 from connecting to the BTS
<2> Permit SIMs with access-class 9 to connect to the BTS.


Smaller cells with lots of subscribers can be overwhelmed with traffic after
the network is turned on. This is especially true in areas with little to no
reception from other networks. To manage the load OsmoBSC has an option to
enable one Access Class at a time so initial access to the network is
distributed across a longer time.

.Example: Ramp up access to the BTS after startup
----
network
 bts 0
  access-control-class-ramping <1>
  access-control-class-ramping-step-interval 30 <2>
  access-control-class-ramping-step-size 1 <3>
----
<1> Turn on access-control-class ramping
<2> Enable more ACCs every 30 seconds
<3> At each step enable one more ACC


==== RACH Parameter Configuration

The following parameters allow control over how the MS can access the random
access channel (RACH). It is possible to set a minimum receive level under
which the MS will not even attempt to access the network.

The RACH is a shared channel which means multiple MS can choose to send a
request at the same time. To minimize the risk of a collision each MS will
choose a random number of RACH slots to wait before trying to send a RACH
request.

On very busy networks the range this number is chosen from should be
high to avoid collisions, but a lower range reduces the overall delay when
trying to establish a channel.

The option `rach tx integer N` controls the range from which this number X
is chosen. It is `0 <= X < max(8,N)`.

After sending a RACH request the MS will wait a random amount of slots before
retransmitting its RACH request. The range it will wait is also determined by
the option `rach tx integer N`, but calculating it is not so straightforward.
It is defined as `S <= X < S+N` where `S` is determined from a table.

In particular `S` is lowest when `N` is one of 3, 8, 14 or 50 and highest when
`N` is 7, 12 or 32.

For more information see _3GPP TA 44.018_ <<3gpp-ts-44-018>> Ch. 3.3.1.1.2 and
Table 3.3.1.1.2.1 in particular.

The amount of times the MS attempts to retransmit RACH requests can also be
changed. A higher number means more load on the RACH while a lower number can
cause channel establishment to fail due to collisions or bad reception.

.Example: Configure RACH Access Parameters
----
network
 bts 0
  rxlev access min 20 <1>
  rach tx integer 50<2>
  rach max transmission <3>
----
<1> Allow access to the network if the MS receives the BCCH of the cell at
-90dBm or better (20dB above -110dBm).
<2> This number affects how long the MS waits before (re-)transmitting RACH
requests.
<3> How often to retransmit the RACH request.