Cellular Network Infrastructure

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h1. Getting Started with 3G

_This reflects the state of Osmocom 3G development on 2017-02-08_

If you have your 3G femto cell, this is how to get started:

The physical setup is:

<pre>

  _____________              _____________

  |           |              |           |

  | femto     |<--Ethernet-->| Laptop/PC |

  |___________|              |___________|

</pre>

On the protocol layer, the femto cell is the hNodeB and talks to various core

network elements over the ethernet cable, typically all located on your

computer, and talks to the phone (UE) over the air:

<pre>

                                 +--------+

                             ,-->| Osmo   |

                            /    | MGCPGW |

                            |    |        |<--MGCP

                            |    +--------+       \

                            /                     |

        +------------+<--RTP     +--------+       `->+----------+

 UE <-->| hNodeB     |           | Osmo   |          | OsmoCSCN |        +------+

 UE <-->|            |<--Iuh---->| HNB-GW |<--IuCS-->|          |        | Osmo |

        |            |           |        |          |     (VLR)|<-GSUP->| HLR  |

        |            |           |        |          +----------+  GSUP->+------+

        +------------+<--GTP-U   |        |                       /

                              \  |        |          +------+<---'       +------+

                              |  |        |<--IuPS-->| Osmo |<--GTP-C--->| Open |

                              |  +--------+          | SGSN |   GTP-U--->| GGSN |

                              |                      +------+  /         +------+

                              \_______________________________/

</pre>

h1. Core Network

To get your 3G core network up and running, you need to manually build specific

branches from various the Osmocom projects:

* libosmocore: master

* libosmo-abis: master

* libosmo-netif: sysmocom/sctp

* libosmo-sccp: sysmocom/iu

* libsmpp34: master

* asn1c: aper-prefix-onto-upstream

* libasn1c: master

* osmo-iuh: master

* openggsn: master

* openbsc: sysmocom/iu -- *DISCLAIMER!* This branch is not yet capable of talking to OsmoHLR and thus no true UMTS (3G) Authentication is possible yet; should be coming soon.

* osmo-hlr: master

See [[Build from Source]] on build instructions; the git command to get onto a

branch other than 'master' is:

<pre>

git checkout the_branch_name

</pre>

Once the CN stack is built, set up the configuration and launch the core

network components. Find further below example configuration files and a run

script that may help to get you started. Here are some details explained:

Tell the osmo-hnbgw which local IP address to use to listen for Iuh

connections. This needs to be on an interface reachable by the hNodeB. The IuCS

and IuPS links towards the osmo-cscn and osmo-sgsn default to 127.0.0.1 and

127.0.0.2, respectively, i.e. it's easiest to run osmo-cscn and osmo-sgsn on

the same machine as the osmo-hnbgw. These will listen on the proper port

(14001) without further configuration.

Also tell the MGCPGW (osmo-bsc_mgcp) which local IP address to bind to, which

has to be reachable both by the hNodeB as well as the osmo-cscn process. The

osmo-cscn.cfg is then told where to reach the MGCPGW.

A notable detail for 3G data is that the GGSN has to be reachable by the

hNodeB. Since the GTP standard defines fixed port numbers which both SGSN and

GGSN have to to use, the SGSN may not bind on the same IP address as the GGSN!

Typically this requires to add another IP address to your ethernet interface.

On linux this is achieved, for example, by:

<pre>

sudo ip addr add 10.9.1.13/32 dev eth0

</pre>

The above adds the address 10.9.1.13 to eth0, which works e.g. when your

machine is in a 10.9.1.0/24 net and the address 10.9.1.13 is not yet taken by

anyone else. (You probably want to use something like 192.168.0.222 instead.)

Also, IP forwarding and masquerading must be enabled for your GGSN to be able

to connect to your mobile phones to the internet uplink. On linux, that can be

done for eth0 by:

<pre>

sudo sh -c "echo 1 > /proc/sys/net/ipv4/ip_forward"

sudo iptables -t nat -A POSTROUTING -o eth0 -j MASQUERADE

</pre>

You may need a different interface, possibly a wildcard like 'eth*'.

Once you have configured the IP addresses, start up your core network: launch

osmo-cscn, osmo-bsc_mgcp, osmo-sgsn, ggsn and osmo-hnbgw. You should see log

messages indicating established IuCS and IuPS links (HNBGW, CSCN and SGSN).

h2. Core Network Configuration Example

Here is a specific 3G core network configuration example, including a highly

specialized launcher script that configures the network (on a linux system) and

runs all the programs needed for a 3G core network -- but be aware:

*config:*

* This was run in a 192.168.0.1/24 subnet, yours may vary.

* Your femto cell will get itself an IP address from your DHCP server, which

  will almost certainly differ from this example. You (so far still) need to

  set this address in the mgcp.cfg.

* The ggsn.conf 'listen' address here is the additional interface (ala 'ip addr

  add' above) and needs to match whichever address you picked there.

* Make sure all the other IP addresses in the config files match yours.

* Make sure the MCC and MNC match the ones configured in the femto cell.

*run.sh*:

* It assumes the Osmocom libs and programs to be installed in /usr/local/*.

* It launches each component in a separate X terminal.

* It uses rxvt-unicode, you may want to change the @urxvt@ call to your favorite xterm instead.

* Make sure the ethernet interface makes sense.

* Make sure the IP address makes sense.

* Run it in a directory where it will find the various configuration files.

* It runs most of the processes in a gdb for easier debugging.

* Once you hit Enter in the original terminal where you started, all the other

  terminals will be torn down in one fell swoop (hopefully).

* *TODO* it doesn't launch osmo-hlr *TODO* (add once the 3G branch is rebased onto the VLR branch)

Now that you've taken notice of the points above, here are the config and

script: attachment:3G-config-example.tar

h1. Femto Config

With your CN up and running, configure the hNodeB to contact osmo-hnbgw via

Iuh. Also make sure the PLMN ID matches the MCC and MNC in the osmo-cscn.cfg --

otherwise the hNodeB may reject all attach requests.

Depending on your femto cell, the steps to configure it will be radically

different:

* [[Configuring the ip.access nano3G]]

* ...

h1. Subscriber Configuration

For your phone loaded with your USIM to be able to subscribe to your network,

the IMSI needs to be authorized to enter and the authentication keys need to be

added to the HLR.

*TODO* currently, the sysmocom/iu branch has a hardcoded Ki of

000102030405060708090a0b0c0d0e0f, and you need a SIM card that offers only

non-UMTS pre-R99 authentication. We need to rebase the sysmocom/iu branch onto

the VLR development (neels/vlr) in order to enable full UMTS authentication and

so that sysmoUSIMs added to our OsmoHLR work with our 3G stack.

h1. Further Links

The "3G Voice Works" blog post from 2016 has some explanations:

https://osmocom.org/news/59