:gfdl-enabled:

OsmoPCU Gb Protocol Specification
=================================
Harald Welte <hwelte@sysmocom.de>

== Introduction

This document describes the Gb interface of *OsmoPCU*. Based on 3GPP TS
48.016 and 48.018, this document indicates which of the 3GPP specified Gb
messages and IEs are implemented according to 3GPP specifications, which of
these are not or not fully implemented, as well as OsmoPCU-specific extensions
to the Gb interface not specified by 3GPP.

Extensions to the Gb interface specific to OsmoPCU are detailed in this
document. For details on the messages and IEs that comply with above-mentioned
3GPP specifications, please refer to those documents.

.3GPP document versions referred to by this document
[cols="20%,80%"]
|===
|3GPP TS 08.56 | version 8.0.1 Release 1999
|3GPP TS 08.58 | version 8.6.0 Release 1999
|3GPP TS 08.60 | version 8.2.1 Release 1999
|3GPP TS 12.21 | version 8.0.0 Release 1999
|3GPP TS 48.016 | version 15.0.0 Release 15
|3GPP TS 48.018 | version 15.0.0 Release 15
|===

.IETF documents referred to by his document
[cols="20%,80%"]
|===
|IETF RFC 768 | User Datagram Protocol
|IETF RFC 791 | Internet Protocol
|===

== Overview

The OsmoPCU Gb interface consists of the NS (Network Services) and
BSSGP (Base Station Subsystem Gateway Protocol), encapsulated in UDP
(User Datagram Protocol) and IP (Internet Protocol) version 4.
Use of other underlying protocols (e. g. Frame Relay) is not supported.

.UDP port numbers used by OsmoPCU Gb/IP
[options="header",width="50%",cols="35%,65%"]
|===
|TCP Port Number|Usage
|23000|NS over UDP (default port)
|===

The NS-over-UDP link is established in the PCU -> SGSN direction, i.e.
the PCU is running as client while the SGSN is running as server.

Establishment of the NS-over-UDP link is only possible after OsmoPCU
has been configured via the *PCU socket* interface from OsmoBTS.

OsmoBTS in turn receives relevant configuration parameters from
OsmoBSC or the BSC component inside OsmoNITB.

.Overview of Gb link establishment
["mscgen"]
----
include::{srcdir}/gb/gb-startup.msc[]
----

[[ident]]
=== Identities

The Gb interface identities of the PCU are configured via BSC ->
OsmoBTS -> PCU Socket. They consist of

NSEI:: NS Equipment Identifier
NSVCI:: NS Virtual Connection Identifier
BVCI:: BSSGP Virtual Connection Identifier

For an explanation of those identifiers and their use in the NS and
BSSGP protocols, please see the relevant 3GPP specifications for NS (TS 48.016)
and BSSGP (TS 48.018).

In most cases, all above identities belong to different namespaces and
must be unique within their respective namespace and within the SGSN
they connect to.

This means that typically each OsmoPCU has one unique set of NSEI,
NSVCI and BVCI in your network.

include::{srcdir}/gb/ns.adoc[]

include::{srcdir}/gb/bssgp.adoc[]

include::./common/chapters/port_numbers.adoc[]

include::./common/chapters/glossary.adoc[]

include::./common/chapters/gfdl.adoc[]