Cellular Network Infrastructure

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

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

*** Currently [[OsmoSGSN:]] refuses all roaming, so you need a SIM that matches your custom MCC/MNC network

h2. Setup

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

!gprs.2.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 configration phase and the BTS will connect back to the SGSN.

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@

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.

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

<pre>

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

bash# iptables -A POSTROUTING -s 192.168.254.0/24 -t nat -o eth0 -j MASQUERADE

</pre>

(replace @eth0@ by the interface providing your machine connectivity)

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>

bash# 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)

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. 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.

* check that NAT and packet forwarding works properly. Something like this:

<pre>

[Match]

Name=tun*

[Network]

Description=Expose GGSN's connected mobiles to Internet

IPForward=ipv4

IPMasquerade=yes

Address=192.168.0.1

</pre>

might be necessary for systemd-networkd.

You can access vty from 

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

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