== Configuring OsmoPCU

Contrary to other network elements (like OsmoBSC, OsmoNITB), the
OsmoPCU has a relatively simple minimum configuration.

This is primarily because most of the PCU configuration happens
indirectly from the BSC, who passes the configuation over A-bis OML via
OsmoBTS and its PCU socket into OsmoPCU.

A minimal OsmoPCU configuration file is provided below for your reference:

.Example: Minimal OsmoPCU configuration file (`osmo-pcu.cfg`)
----
pcu
 flow-control-interval 10 <1>
 cs 2 <2>
 alloc-algorithm dynamic <3>
 alpha 0 <4>
 gamma 0
----
<1> send a BSSGP flow-control PDU every 10 seconds
<2> start a TBF with the initial coding scheme 2
<3> dynamically chose between single-slot or multi-slot TBF allocations
    depending on system load
<4> disable MS power control loop

However, there are plenty of tuning parameters for people interested to
optimize PCU throughput or latency according to their requirements.

=== Configuring the Coding Schemes and Rate Adaption

The BSC includes a bit-mask of permitted [E]GPRS coding schemes as part
of the A-bis OML configuration.  This is passed from the BTS via the PCU
socket into OsmoPCU.

Some additional parameters can be set as described below.

==== Initial Coding Scheme

You can use the `cs <1-4> [<1-4>]` command at the `pcu` VTY config node
to set the initial GPRS coding scheme to be used.  The optional second
value allows to specify a different initial coding scheme for uplink.

==== Maximum Coding Scheme

You can use the `cs max <1-4> [<1-4>]` command at the `pcu` VTY config
node to set the maximum coding scheme that should be used as part of the
rate adaption.

==== Rate Adaption Error Thresholds

You can use the `cs threshold <0-100> <0-100>` command at the `pcu` VTY
config node to determine the upper and lower limit for the error rate
percentage to use in the rate adaption.  If the upper threshold is
reached, a lower coding sheme is chosen, and if the lower threshold is
reached, a higher coding scheme is chosen.

==== Rate Adation Link Quality Thresholds

You can use the `cs link-quality-ranges cs1 <0-35> cs2 <0-35> <0-35> cs3
<0-35> <0-35> cs4 <0-35>` command at the `pcu` VTY config node to tune
the link quality ranges for the respective coding schemes.

==== Data Size based CS downgrade Threshold

You can use the `cs downgrade-threshold <1-10000>` command at the `pcu`
VTY config node to ask the PCU to down-grade the coding scheme if less
than the specified number of octets are left to be transmitted.

=== Miscellaneous Configuration / Tuning Parameters

==== Downlink TBF idle time

After a down-link TBF is idle (all data in the current LLC downlink
queue for the MS has been transmitted), we can keep the TBF established
for a configurable time.  This avoids having to go through a new one or
two phase TBF establishment once the next data for downlink arrives.

You can use the `dl-tbf-idle-time <1-5000>` to specify that time in
units of milli-seconds.  The default is 2 seconds.

==== MS idle time

Using the `ms-idle-time <1-7200>` command at the `pcu` VTY config node
you can configure the number of seconds for which the PCU should keep
the MS data structure alive before releasing it if there are no active
TBF for this MS.

The OsmoPCU default value is 60 seconds, which is slightly more than
what 3GPP TS 24.008 recommends for T3314 (44s).

The MS data structure only consumes memory in the PCU and does not
require any resources of the air interface.

==== Forcing two-phase access

If the MS is using a single-phase access, you can still force it to
use a two-phase access using the `two-phase-access` VTY configuration
command at the `pcu` VTY config node.

=== Configuring BSSGP flow control

BSSGP between SGSN and PCU contains a two-level nested flow control
mechanism:

. one global flow control instance for the overall (downlink) traffic
  from the SGSN to this PCU
. a per-MS flow control instance for each individual MS served by this
  PCU

Each of the flow control instance is implemented as a TBF (token bucket
filter).

==== Normal BSSGP Flow Control Tuning parameters

You can use the following commands at the `pcu` VTY config node to tune
the BSSGP flow control parameters:

`flow-control-interval <1-10>`::
	configure the interval (in seconds) between subsequent flow
	control PDUs from PCU to SGSN
`flow-control bucket-time <1-65534>`::
	set the target downlink maximum queueing time in centi-seconds.
	The PCU will attempt to adjust the advertised bucket size to match this
	target.

==== Extended BSSGP Flow Control Tuning parameters

There are some extended flow control related parameters at the `pcu` VTY
config node that override the automatic flow control as specified in the
BSSGP specification.  Use them with care!

`flow-control force-bvc-bucket-size <1-6553500>`::
	force the BVC (global) bucket size to the given number of octets
`flow-control force-bvc-leak-rate <1-6553500>`::
	force the BVC (global) bucket leak rate to the given number of bits/s
`flow-control force-ms-bucket-size <1-6553500>`::
	force the per-MS bucket size to the given number of octets
`flow-control force-ms-leak-rate <1-6553500>`::
	force the per-MS bucket leak rate to the given number of bits/s


=== Configuring LLC queue

The downlink LLC queue in the PCU towards the MS can be tuned with a
variety of parameters at the `pcu` VTY config node, depending on your
needs.

`queue lifetime <1-65534>`::
	Each downlink LLC PDU is assigned a lifetime by the SGSN, which
	is respected by the PDU *unless* you use this command to
	override the PDU lifetime with a larger value (in centi-seconds)
`queue lifetime infinite`::
	Never drop LLC PDUs, i.e. give them an unlimited lifetime.
`queue hysteresis <1-65535>`::
	When the downlink LLC queue is full, the PCU starts dropping
	packets.  Using this parameter, we can set the lifetime
	hysteresis in centi-seconds, i.e. it will continue discarding
	until "lifetime - hysteresis" is reached.
`queue codel`::
	Use the 'CoDel' (Controlled Delay) scheduling algorithm, which
	is designed to overcome buffer bloat.  It will use a default
	interval of 4 seconds.
`queue codel interval <1-1000>`::
	Use the 'CoDel' (Controlled Delay) scheduling algorithm, which
	is designed to overcome buffer bloat.  Use the specified
	interval in centi-seconds.
`queue idle-ack-delay <1-65535>`::
	Delay the request for an ACK after the last downlink LLC frame
	by the specified amount of centi-seconds.


=== Configuring MS power control

GPRS MS power control works completely different than the close MS power
control loop in circuit-switched GSM.

Rather than instructing the MS constantly about which transmit power to
use, some parameters are provided to the MS by which the MS-based power
control algorithm is tuned.

See 3GPP TS 05.08 for further information on the algorithm and the
parameters.

You can set those parameters at the `pcu` VTY config node as follows:

`alpha <0-10>`::
	Alpha parameter for MS power control in units of 0.1.
	Make sure to set the alpha value at System Information 13 (in
	the BSC), too!
`gamma <0-62>`::
	Set the gamma parameter for MS power control in units of dB.


=== Enabling EGPRS

If you would like to test the currently (experimental) EGPRS support of
OsmoPCU, you can enable it using the `egprs` command at the `pcu` VTY
config  node.

WARNING: EPGRS functionality is highly experimental at the time of this
writing.  Please only use if you actively would like to participate in
the OsmoPCU EGPRS development and/or testing.  You will also need an
EGPRS capable OsmoBTS+PHY, which means `osmo-bts-sysmo` or
`osmo-bts-litecell15` with their associated PHY.