Cellular Network Infrastructure » Radio Access Network » OsmoPCU

h1. osmo-pcu

{{>toc}}

A PCU (Packet Control Unit) is one of the two GPRS elements in the BSS. It implements the RLC and MAC layers of the GPRS

Um (radio) interface on the MS-facing side, as well as the Gb Interface (NS,BSSGP) on the SGSN-facing side.

OsmoPCU is the Osmocom implementation of this PCU functionality.

Towards the SGSN, the BSSGP and NS protocols are implemented over UDP/IP, sometimes called "NS over IP". 

It has been tested with [[OsmoSGSN:]] as well as an Ericsson SGSN so far.

h2. OsmoPCU in co-location with OsmoBTS

OsmoPCU is usually used in co-location with [[OsmoBTS:]]. In this scenario all OsmoBTS variants are supported, including

[[OsmoTRX:]] and PHY based variants, such as "sysmoBTS":https://sysmocom.de/products/bts/.

{{graphviz_link()

digraph G {

    rankdir = LR;

    OsmoBTS -> OsmoBSC [label="Abis/IP"];

    OsmoBSC -> OsmoMSC [label="3GPP AoIP"];

    OsmoBTS -> OsmoPCU [label="pcu_sock"];

    OsmoPCU -> OsmoSGSN [label="Gb/IP"];

    OsmoMSC -> OsmoHLR [label="GSUP"];

    OsmoSGSN -> OsmoHLR [label="GSUP"];

    OsmoSGSN -> OsmoGGSN [label="Gp"];

    OsmoGGSN -> Internet [label="Gi"];

    OsmoPCU [color=red];

    { rank=same OsmoBTS OsmoPCU }

    { rank=same OsmoMSC OsmoSGSN }    

}

}}

h2. OsmoPCU in co-location with OsmoBSC

OsmoPCU can also be used in co-location with [[OsmoBSC:]]. This scenario is required to support third party BTSs that

do not include an integrated PCU. This is mainly the case for classic E1 based BTS, such as the [[ericsson-rbs-6xxx:]]

series.

{{graphviz_link()

digraph G {

    rankdir = LR;

    BTS -> OsmoBSC [label="Abis/IP"];

    BTS -> OsmoPCU [label="GPRS/TRAU"];    

    OsmoBSC -> OsmoMSC [label="3GPP AoIP"];

    OsmoBSC -> OsmoPCU [label="pcu_sock"];

    OsmoPCU -> OsmoSGSN [label="Gb/IP"];

    OsmoMSC -> OsmoHLR [label="GSUP"];

    OsmoSGSN -> OsmoHLR [label="GSUP"];

    OsmoSGSN -> OsmoGGSN [label="Gp"];

    OsmoGGSN -> Internet [label="Gi"];

    OsmoPCU [color=red];

    { rank=same OsmoBSC OsmoPCU }

}

}}

The support for BSC co-location in OsmoPCU and OsmoBSC, as well as the hardware specific extensions needed to support

Ericsson RBS2000/RBS6000 base stations have been made possible through funding by the

"NLnet foundation":https://nlnet.nl/ (see also: https://nlnet.nl/project/Osmocom-RBS/).

h2. Interface between OsmoPCU and OsmoBTS/OsmoBSC

The interface between OsmoPCU and OsmoBTS/OsmoBSC runs over UNIX socket of SOCK_SEQPACKET type. It is used to configure

the network parameters inside the PCU and to exchange signalling information. It also features a custom L1 interface

that allows PDCH MAC blocks. However, for enhanced performance, OsmoPCU also implements a direct PHY acces for some BTS

models.

h3. Documentation

The primitives, which are exchanged between OsmoPCU and OsmoBTS/OsmoBSC are defined in pcuif_proto.h

* pcuif_proto.h: https://gitea.osmocom.org/cellular-infrastructure/osmo-pcu/src/branch/master/include/osmocom/pcu/pcuif_proto.h

Message flow charts that explain PDCH activation and deactivation can be found in the OsmoBTS Abis Protocol Specification

* OsmoBTS Abis Protocol Specification: https://ftp.osmocom.org/docs/osmo-bts/master/osmobts-abis.pdf

h3. Debugging

One of the ways of debugging the PCU socket interface would be via socat:

<pre>

socat -t30 -x -v UNIX-LISTEN:/tmp/pcu_bts.socat,mode=755,type=5 UNIX-CONNECT:/tmp/pcu_bts,type=5

</pre>

One might even forward it over network:

<pre>

socat -t30 -x -v UDP-LISTEN:1666,reuseaddr,fork UNIX-CONNECT:/tmp/pcu_bts,type=5

socat -t30 -x -v UNIX-LISTEN:/tmp/pcu_bts.socat,mode=755,type=5 UDP-CONNECT:127.0.0.1:1666

</pre>

Note: in the examples above OsmoBTS (listening side) uses default @/tmp/pcu_bts@ socket while OsmoPCU (connecting side)

is configured to use @/tmp/pcu_bts.socat@ via @pcu-socket /tmp/pcu_bts.socat@ config option. It's not recommeded to use

outside of TTCN-3 test environment due to amount of traffic over the socket.

h2. Manuals

* User Manual: https://ftp.osmocom.org/docs/latest/osmopcu-usermanual.pdf

* VTY Reference: https://ftp.osmocom.org/docs/latest/osmopcu-vty-reference.pdf

* Gb/IP Manual: https://ftp.osmocom.org/docs/latest/osmopcu-gb.pdf

h2. Source Code

The source code is available from https://gitea.osmocom.org/cellular-infrastructure/osmo-pcu.git

You can browse it online via https://gitea.osmocom.org/cellular-infrastructure/osmo-pcu

h2. Test Suite

We have a TTCN-3 test suite as part of our [[cellular-infrastructure:Titan_TTCN3_Testsuites]]

* source code: https://gitea.osmocom.org/ttcn3/osmo-ttcn3-hacks/src/branch/master/pcu

* results: https://jenkins.osmocom.org/jenkins/view/TTCN3/job/ttcn3-pcu-test/

h2. Mailing List / Forum

There is a mailing list for (development) discussion regarding the PCU: osmocom-net-gprs@lists.osmocom.org

You can subscribe to it (or manage your subscription) at http://lists.osmocom.org/cgi-bin/mailman/listinfo/osmocom-net-gprs

Please make sure to read our [[cellular-infrastructure:MailingListRules]] before posting.

There's now also a "2G RAN category in our discourse forum":https://discourse.osmocom.org/c/cni/geran

h2. Features 

osmo-pcu is considered stable and is performing useful service for a number of users, but due to the complexity of GPRS

there is still plenty of area for improvement.

h3. Supported

* GPRS CS1 to CS4 with dynamic link/rate adaption

* EDGE MCS1-MCS9 with dynamic link/rate adaption

* Single-slot and multi-slot operation on downlink

* Single-slot operation on uplink

* Independent TFI/USF per PDCH in single-slot allocation mode

* Automatic or fixed PDCH allocation algorithm selection

* Delayed release of downlink TBF

* Active queue management for downlink TBF (!CoDel)

* Initial setting of timing advance based on RACH

* BSSGP BVC flow control support

* Using pchan types of @TCH/F_TCH/H_PDCH@ or @TCH/F_PDCH@, OsmoBTS supports

  dynamic PDCH/TCH switching (implemented entirely in osmo-bts)

* Ericsson RBS2000/RBS6000 support (BSC co-located)

h3. Shortcomings / Limitations / Restrictions

_Note: this list may be outdated_

* No GPRS/EGPRS mixed mode (yet)

* No support for hand-over of any sort yet

* No support of processing measurement reports

* No timing advance loop, i.e. radial movement is limited

* No support for power control

* No Packet Access Reject message on congestion, access is just ignored

* No PCCCH support

* Only "Dynamic allocation RLC data block transfer" is supported

* No network controlled measurement/cell reselection order supported

* No PTCCH support

* No support for Packet Flow Contexts (PFC)

* No support for MS or PFC flow control

* No support for RLC unacknowledged mode operation

* No support for MBMS

h2. Authors / Credits

osmo-pcu was originally developed by Ivan Kluchnikov and Andreas Everberg.

Today it is maintaned by Sysmocom.

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