Cellular Network Infrastructure

h1. [[OpenBSC:]] GPRS/EDGE Setup page

{{>toc}}

h2. Pre-requisites

* BTS hardware: [[openbsc:sysmoBTS]], USRP, [[UmTRX:]], [[openbsc:nanoBTS]] with GPRS or EDGE support. [[openbsc:BS11]] are not supported.

* A compiled GGSN from [[OpenGGSN:]]

* A working and up-to-date [[OpenBSC:]] (see [[openbsc:Building_OpenBSC]])

*** You will need to (re-)build it after having installed [[OpenGGSN:]] so that the @libgtp@ is detected and the SGSN binary @osmo-sgsn@ built.

* A custom SIM for your network (recommended)

*** Depending on your authorization policy (see [[OsmoSGSN:]] user manual), it may refuse inbound roaming. It is easiest if you have a SIM that matches your custom MCC/MNC network

h2. Setup

First a little picture to illustrate the different elements and their interactions :

!gprs.3.png!

h3. Compiling [[OpenBSC:]] with [E]GPRS

The guide below was tested on Ubuntu 15.10 but should work on Debian as well.

First you need to download all dependencies:

<pre>

apt install libdbi0-dev libdbd-sqlite3 libtool autoconf git-core pkg-config make libortp-dev

</pre>

Please follow instructions provided at [[Build from source]] in order to install these projects:

* openggsn

* libosmocore

* libosmo-abis

* libosmo-netif

* openbsc

* osmo-pcu

h3. [[OpenBSC:]] configuration

The first step is to configure [[OpenBSC:]] for gprs support. Add this to the @network/bts@ node in @openbsc.cfg@:

<pre>

gprs mode gprs

gprs routing area 0

gprs cell bvci 2

gprs nsei 101

gprs nsvc 0 nsvci 101

gprs nsvc 0 local udp port 23000

gprs nsvc 0 remote udp port 23000

gprs nsvc 0 remote ip 192.168.0.128

</pre>

The @gprs nsvc 0 remote@ entries @192.168.0.128:23000@ is the IP/port of the machine running the SGSN as seen from the BTS. It will be sent by [[OpenBSC:]] to the BTS in the configuration phase and the BTS will connect back to the SGSN using that address and port.

The second step is to allocate some timeslots to packet data. For this, just change the 1 or more @network/bts/trx/timeslot@ nodes using :

<pre>

phys_chan_config PDCH

</pre>

h3. [[OsmoSGSN:]] configuration

Here's a sample SGSN configuration file @osmo-sgsn.cfg@ with some explanations :

<pre>

!

! Osmocom SGSN configuration

!

!

line vty

 no login

!

sgsn

 gtp local-ip 192.168.1.128

 ggsn 0 remote-ip 192.168.1.129

 ggsn 0 gtp-version 1

!

ns

 timer tns-block 3

 timer tns-block-retries 3

 timer tns-reset 3

 timer tns-reset-retries 3

 timer tns-test 30

 timer tns-alive 3

 timer tns-alive-retries 10

 encapsulation udp local-ip 192.168.0.128

 encapsulation udp local-port 23000

 encapsulation framerelay-gre enabled 0

!

bssgp

!

</pre>

* The @gtp local-ip@ entry is the local IP the SGSN will bind to.

* The @ggsn 0 remote-ip@ entry if the remote IP of the GGSN. The SGSN will connect to it.

* Those two IPs *must* be different even if you're running both processes on the same machine. A solution for that is to put several IP aliases on the same network interface or use the loopback interface.

* The @encapsulation@ settings must be the same IP/port than you've setup in @openbsc.cfg@. The SGSN will listen to that address and port for connections coming from the BTS.

h3. [[OpenGGSN:]] configuration

The ggsn.conf file is pretty well documented. What is mostly of interest here is :

* The configuration of the GTP link. (Must match the @ggsn 0 remote-ip@ entry in @osmo-sgsn.cfg@)

<pre>

# TAG: listen

# Specifies the local IP address to listen to

listen 192.168.1.129

</pre>

* The configuration given to phones, IP pool & DNS.

<pre>

# TAG: dynip

# Dynamic IP address pool.

# Used for allocation of dynamic IP address when address is not given

# by HLR.

# If this option is not given then the net option is used as a substitute.

# dynip 192.168.254.0/24

# TAG: pcodns1/pcodns2

# Protocol configuration option domain name system server 1 & 2.

pcodns1 208.67.222.222

pcodns2 208.67.220.220

</pre>

h3. [[OsmoPCU:]] configuration

<pre>

pcu

 flow-control-interval 10

 cs 2

 alloc-algorithm dynamic

 alpha 0

 gamma 0

</pre>

h3. Network configuration

You will also need to configure some networking rules to allow connectivity from @tun0@. Look up linux networking/nat howtos on google.

h4. Modern systems

Add following setting to /etc/systemd/network:

wired.network:

<pre>

[Match]

Name=enp2s0

[Network]

DHCP=ipv4

IPMasquerade=yes

</pre>

tun.network:

<pre>

[Match]

Name=tun0

[Network]

Address=192.168.0.1

IPMasquerade=yes

</pre>

The setup above assume that you're using interface *enp2s0* to access the internet from the machine which runs openggsn. Notice the *IPMasquerade* option on both interfaces. This also assume the default 192.168.0.1/24 ip address range used by openggsn. If your situation differs - adjust accordingly using https://www.freedesktop.org/software/systemd/man/systemd.network.html as a reference.

h4. Legacy systems

The basic setup for testing only in a safe environment would be :

<pre>

sh# echo 1 > /proc/sys/net/ipv4/ip_forward

sh# iptables -A POSTROUTING -s 192.168.254.0/24 -t nat -o enp2s0 -j MASQUERADE

</pre>

(replace @enp2s0@ by the interface connected to the gateway which provides your machine connectivity)

h4. DNS issues

In some cases the DNS server might be hardcoded in the phones APN settings. To work around this problem one might choose to enforce the usage of a specific DNS server by redirecting all DNS traffic via iptables:

<pre>

sh# iptables -t nat -I PREROUTING -i tun0 -p udp --dport 53 -j DNAT --to-dest 1.2.3.4

</pre>

(replace @1.2.3.4@ with the ip-address of your DNS-Server)

You can put this into .sh file and reference it from openggsn config using

<pre>

ipup /full/path/to/your.sh

</pre>

h2. Running

Sample startup sequence (adjust logging and configuration files location as you see fit):

<pre>

osmo-nitb -s -c ~/.config/osmocom/open-bsc.cfg -l ~/.config/osmocom/hlr.sqlite3 -P -m -C -T --debug=DSQL:DLSMS:DRLL:DCC:DMM:DRR:DMSC:DHO:DGPRS:DNS:DLLC:DCTRL 2>&1 | tee /tmp/openbsc.log

sudo ggsn -c ~/.config/osmocom/ggsn.conf -f -d

osmo-sgsn -c ~/.config/osmocom/osmo-sgsn.cfg -d DRLL:DCC:DMM:DRR:DNM:DMSC:DHO:DGPRS:DNS:DLLC:DCTRL

cd osmo-trx/Transceiver52M

sudo chrt 20 ./osmo-trx

cd osmo-bts/src/osmo-bts-trx

sudo chrt 15 ./osmobts-trx -c ~/.config/osmocom/osmo-bts.cfg -i 224.0.0.1 -d DRLL:DCC:DMM:DRR:DNM:DMSC:DHO:DGPRS:DNS:DLLC:DCTRL

osmo-pcu/src

sudo ./osmo-pcu -c ~/.config/osmocom/osmo-pcu.cfg

</pre>

Note: [[OsmoTRX:]] is only necessary with USRP/UmTRX transceivers (At the time of writing 201509-fairwaves-rebase branch of [[OsmoBTS:]] is necessary as well for compatibility). 

Once you're done with experimenting and ready for production setup it might be convenient to create systemd units so all the parts are started automatically.

h2. Authorization Policy

Authorization determines whether a particular subscriber can access

your network or not.

The following 4 authorization policy options are available:

** @accept-all@: All IMSIs will be accepted.

** @acl-only@: Accept only IMSIs, which are explicitly white-listed by the Access Control List (ACL), and the rest will be rejected.

** @closed@: Accept only home network subscribers. The combination of MCC and MNC fully identifies subscriber's Home network. The ACL is also heeded.

** @remote@: GSUP protocol is uesed to remotely access a HLR. Only remote subscription data will be used.

Example: Assign or change authorization policy:

<pre>

OsmoSGSN> enable

OsmoSGSN# configure terminal

OsmoSGSN(config)# sgsn

OsmoSGSN(config-sgsn)# auth-policy acl-only 

OsmoSGSN(config-sgsn)# write

Configuration saved to sgsn.cfg

OsmoSGSN(config-sgsn)# exit

OsmoSGSN(config)# exit

OsmoSGSN# disable

OsmoSGSN>

</pre> 

In the example acl-only is selected as authorization policy.

After this we are saving current changes to configuration file using command @write@ to make this policy persistent.

h2. Troubleshooting

* double-check that your phones have APN set to something. "Internet" will do for example. The value of APN is not checked but if it's unset the phones' baseband might not even try to initiate GPRS connection.

You can access vty from 

* [[OsmoNITB:]] on port 4242 See [[osmonitb:osmo-nitb_VTY]]

* [[OsmoSGSN:]] on port 4245. See [[osmosgsn:osmo-sgsn_VTY]]

h2. enable wireshark dissector for GPRS / GMM messages

In wireshark's default configuration, GMM messages show merely as UDP packets without being decoded.

(port 23000 going between the SGSN and PCU, see [[cellular-infrastructure:Port Numbers]])

Persistently:

* _Preferences_

* _Protocols_

* _GPRS-NS_

* Add _,23000_ to _GPRS NS UDP ports_

Quick way but is apparently not saved:

* Right-click on a UDP port 23000 packet to open the context menu

* _Decode as..._

* _GPRS-NS_