Cellular Network Infrastructure » Core Network (CN) » OsmoHNBGW

h1. OsmoHNBGW

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We have implemented 3GPP Iuh support in the Osmocom stack, mostly carried out by employees of sysmocom GmbH[1], with highly appreciated (yet undisclosed) external backing.

Iu support in Osmocom allows using a femto-cell aka hNodeB as 3G base transceiver station, thus enabling UMTS voice (IuCS) and data (IuPS) connectivity using FOSS software from the core network right up to the femto-cell's ethernet jack.

Here is an overview of HNBGW operation:

{{graphviz_link()

digraph G {

  rankdir = LR;

  UE [label="UE\n(3G phone)"]

  PBX [label="PBX\nAsterisk, FreeSwitch,\nKamailio, Yate, ..."]

  subgraph cluster_msc_mgw {

    style=dotted

    MSC

    MGW1 [label="MGW"]

    MSC -> MGW1 [label="MGCP",constraint=false]

  }

  subgraph cluster_hnbgw_mgw_upf {

    style=dotted

    MGW3 [label="MGW"]

    UPF

    HNBGW [label=OsmoHNBGW,style=bold]

    HNBGW -> MGW3 [label="MGCP",constraint=false]

    HNBGW -> UPF [label="PFCP",constraint=false]

  }

  hNodeB [shape="box",label="hNodeB\n(3G femto cell)"]

  MSC -> HLR [label="\nGSUP",style=bold]

  SGSN -> HLR [label="GSUP",style="dashed,bold"]

  UE -> hNodeB [label="Uu",style=bold]

  UE -> hNodeB [style="dashed,bold"]

  hNodeB -> HNBGW [label="Iuh",style="bold"]

  STP2 [label="STP\n(SCCP/M3UA)"]

  HNBGW -> STP2 -> SGSN [label="IuPS",style="dashed,bold"]

  HNBGW -> STP2 -> MSC [label="IuCS",style="bold"]

  SGSN -> GGSN [label="GTP-C",style="dashed,bold"]

  hNodeB -> UPF -> GGSN [label="GTP-U(3G)",style="dashed"]

  GGSN -> internet [label="tun",style="dashed"]

  hNodeB -> MGW3 [label="IuUP/RTP",constraint=false]

  MGW3 -> MGW1 [label="IuUP/RTP"]

  MSC -> SIPConnector [label="MNCC socket",style=bold]

  SIPConnector -> PBX [label="SIP",style=bold]

  MGW1 -> PBX [label="RTP"]

  A, B, C, D [style="invisible"]

  A -> B [label="data (PS)",style="dashed"]

  C -> D [label="voice/SMS/USSD (CS)"]

}

}}

<pre>

                      Iuh                         IuCS/IuPS

NAS                   +----+----+                 +----+----+

Non-Access Stratum    | CC | MM |                 | CC | MM |

- - - - - - - - - - - +----+----+-------+         +----+----+

                      | RANAP   |       |    H    | RANAP   |

Access Stratum        +---------+ HNBAP |    N    +---------+ - - SCCP USER SAP

                      | RUA     |       |    B    | M3UA    |  \

                      +---------+-------+    -    +---------+  |

                      |        SCTP     |    G    | SCTP    |  } SIGTRAN

                      +-----------------+    W    +---------+  |

                      |        IP       |         | IP      |  /

                      +-----------------+         +---------+

</pre>

{{include(cellular-infrastructure:MacroBinaryPackages)}}

h2. HNB-GW

The HNB-GW, i.e. the HomeNodeB Gateway, negotiates HNBAP to register hNodeB connections, and then splits Iuh (RUA) to IuCS and IuPS (RANAP). IuCS/RANAP is delivered to the MSC, IuPS/RANAP to the SGSN.

The IuUP/RTP and GTP user plane data is optionally proxied via local MGW and UPF instances, or sent directly to the MSC's MGW and the GGSN. OsmoHNBGW controls its local MGW via the MGCP protocol, and the UPF via the PFCP protocol.

h2. Manuals

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

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

h2. Source code

The source code is available from @gitea.osmocom.org@ (module @osmo-hnbgw@).

Public read-only access is available via

<pre>

git clone https://gitea.osmocom.org/cellular-infrastructure/osmo-hnbgw.git

</pre>

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

Contributions are welcome via [[cellular-infrastructure:Gerrit]].

h2. Test Suite

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

* Test results: https://jenkins.osmocom.org/jenkins/job/ttcn3-hnbgw-test/

h2. Some context

See Harald's 3G talk at the 32c3[2]

h3. ASN1 support

RANAP, RUA and HNBAP, which make up the Iuh interface, are ASN1 encoded. Fair enough, but their ASN1 encoding uses APER, and heavily employs Information Object Classes (which basically means it wraps ASN1 encoded binary data in ASN1 IEs, with several levels of depth). In consequence, the libre asn1c compiler as-is unfortunately is not capable of generating de-/encoders for UMTS. The proprietary ffasn1c is capable of that, and we could publish the ffasn1c generated code without licensing problems, but we'd highly prefer to empower the FOSS community with the ability to modify and fix the ASN1 de-/encoders independently of proprietary software.

The great news is that Eurecom[4] has worked on supporting both APER and the nested ASN1 structures ("Information Object Classes") in asn1c, and we are able to use their solutions in a FOSS way. With some fixes added, we have both their APER support and their pythonic solution for nested ASN1 available in Osmocom's libasn1c and asn1c git repositories (5).

Another problem with the Iuh ASN1 is that various type names are identical across RANAP, RUA and HNBAP, while their encodings differ. This causes type name collisions in the code generated by asn1c, hence we have added prefixing support to our version of asn1c. This simply means that each RANAP-related type name or function begins with "ranap_", and RUA names begin with "rua_", thus avoiding any and all name collisions between those protocols. See osmo-iuh/include/osmocom/ranap/ and ../rua/.

It could be more beautiful, but the bottom line is that we now have fully free/libre support for Iuh ASN1 encodings. Cheers

See also: [[generate sources from ASN1 files]]

h2. External References

fn1. ​http://www.sysmocom.de 

fn2. ​https://media.ccc.de/v/32c3-7412-running_your_own_3g_3_5g_network

fn3. ​http://git.osmocom.org/erlang/osmo_ss7

fn4. ​http://www.eurecom.fr 

fn5. See ​http://git.osmocom.org/libasn1c/ and ​https://gitea.osmocom.org/osmocom/asn1c, osmo-iuh/master branch

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