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osmo-pcu

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.

osmo-pcu is the Osmocom implementation of this PCU functionality.

It implements a custom L1 interface that can be used with OsmoBTS (specifically its
sysmoBTS back-end), and also with
OsmoTRX.

The interface between OsmoPCU and OsmoBTS runs over UNIX socket of SOCK_SEQPACKET type. One of the ways to debug it would be via socat:

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

One might even forward it over network:
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

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.

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.

osmo-pcu is now considered stable and already performing useful service for a number of users, but as one
of the more recent additions to the Osmocom family, there still is plenty of area for improvement.

Position in a typical Osmocom network

Manuals

Source Code

The source code is available from git://git.osmocom.org/osmo-pcu.git

You can browse it online via http://cgit.osmocom.org/cgit/osmo-pcu/

Test Suite

We have a TTCN-3 test suite as part of our Titan_TTCN3_Testsuites

Mailing List

There is a mailing list for (development) discussion regarding the PCU:
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 MailingListRules before posting.

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)

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

Authors / Credits

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

Today it is maintaned by Sysmocom.

Commercial Support

Commercial support as well as development, system integration and training services for this project are available from sysmocom

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