path: root/2017/open_source_mobilfunk-clt2017/running-foss-gsm.adoc

Open Source Cellular Networks
=============================
:author:	Harald Welte <laforge@gnumonks.org>
:copyright:	sysmocom - s.f.m.c. GmbH (License: CC-BY-SA)
:backend:	slidy
:max-width:	45em
//:data-uri:
//:icons:


== What this talk is about

[role="incremental"]
* Implementing GSM/GPRS network elements as FOSS
* Applied Protocol Archeology
* Doing all of that on top of Linux (in userspace)
* If you expeccted kernel stuff, you'll be disappointed


== Running your own Internet-style network

* use off-the-shelf hardware (x86, Ethernet card)
* use any random Linux distribution
* configure Linux kernel TCP/IP network stack
** enjoy fancy features like netfilter/iproute2/tc
* use apache/lighttpd/nginx on the server
* use Firefox/chromium/konqueor/lynx on the client
* do whatever modification/optimization on any part of the stack


== Running your own GSM network

Until 2009 the situation looked like this:

* go to Ericsson/Huawei/ZTE/Nokia/Alcatel/...
* spend lots of time convincing them that you're an eligible customer
* spend a six-digit figure for even the most basic full network
* end up with black boxes you can neither study nor improve

[role="incremental"]
- WTF?
- I've grown up with FOSS and the Internet. I know a better world.


== Why no cellular FOSS?

- both cellular (2G/3G/4G) and TCP/IP/HTTP protocol specs are publicly
  available for decades.  Can you believe it?
- Internet protocol stacks have lots of FOSS implementations
- cellular protocol stacks have no FOSS implementations for the
  first almost 20 years of their existence?
[role="incremental"]
- it's the classic conflict
 * classic circuit-switched telco vs. the BBS community
 * ITU-T/OSI/ISO vs. Arpanet and TCP/IP


== Enter Osmocom

In 2008, some people started to write FOSS for GSM

- to boldly go where no FOSS hacker has gone before
[role="incremental"]
** where protocol stacks are deep
** and acronyms are plentiful
** we went from `bs11-abis` to `bsc_hack` to 'OpenBSC'
** many other related projects were created
** finally leading to the 'Osmocom' umbrella project


== Classic GSM network architecture

image::Gsm_structures.svg[width=850]


== GSM Acronyms, Radio Access Network

MS::
	Mobile Station (your phone)
BTS::
	Base Transceiver Station, consists of 1..n TRX
TRX::
	Transceiver for one radio channel, serves 8 TS
TS::
	Timeslots in the GSM radio interface; each runs a specific combination of logical channels
BSC::
	Base Station Controller


== GSM Acronyms, Core Network

MSC::
	Mobile Switching Center; Terminates MM + CC Sub-layers

HLR::
	Home Location Register; Subscriber Database

SMSC::
	SMS Service Center


== GSM Acronyms, Layer 2 + 3

LAPDm::
	Link Access Protocol, D-Channel. Like LAPD in ISDN
RR::
	Radio Resource (establish/release dedicated channels)
MM::
	Mobility Management (registration, location, authentication)
CC::
	Call Control (voice, circuit switched data, fax)
CM::
	Connection Management


== Osmocom GSM components

image::osmocom-gsm.svg[width=850]


== Classic GSM network as digraph

[graphviz]
----
digraph G {
	rankdir=LR;
	MS0 [label="MS"]
	MS1 [label="MS"]
	MS2 [label="MS"]
	MS3 [label="MS"]
	BTS0 [label="BTS"]
	BTS1 [label="BTS"]
	MSC [label="MSC/VLR"]
	HLR [label="HLR/AUC"]
	MS0->BTS0 [label="Um"]
	MS1->BTS0 [label="Um"]
	MS2->BTS1 [label="Um"]
	MS3->BTS1 [label="Um"]
	BTS0->BSC [label="Abis"]
	BTS1->BSC [label="Abis"]
	BSC->MSC [label="A"]
	MSC->HLR [label="C"]
	MSC->EIR [label="F"]
	MSC->SMSC
}
----

== Simplified OsmoNITB GSM network

[graphviz]
----
digraph G {
	rankdir=LR;
	MS0 [label="MS"]
	MS1 [label="MS"]
	MS2 [label="MS"]
	MS3 [label="MS"]
	BTS0 [label="BTS"]
	BTS1 [label="BTS"]
	MS0->BTS0 [label="Um"]
	MS1->BTS0 [label="Um"]
	MS2->BTS1 [label="Um"]
	MS3->BTS1 [label="Um"]
	BTS0->BSC [label="Abis"]
	BTS1->BSC [label="Abis"]
	subgraph cluster_nitb {
		label = "OsmoNITB";
		BSC
		MSC [label="MSC/VLR"]
		HLR [label="HLR/AUC"]
		BSC->MSC [label="A"]
		MSC->HLR [label="C"]
		MSC->EIR [label="F"]
		MSC->SMSC;
	}
}
----

which further reduces to the following minimal setup:

[graphviz]
----
digraph G {
	rankdir=LR;
	MS0 [label="MS"]
	BTS0 [label="BTS"]
	MS0->BTS0 [label="Um"]
	BTS0->BSC [label="Abis"]
	BSC [label="OsmoNITB"];
}
----

So our minimal setup is a 'Phone', a 'BTS' and 'OsmoNITB'.


== Which BTS to use?

* Proprietary BTS of classic vendor
** Siemens BS-11 is what we started with
** Nokia, Ericsson, and others available 2nd hand
* 'OsmoBTS' software implementation, running with
** Proprietary HW + PHY (DSP): 'sysmoBTS', or
** General purpose SDR (like USRP) + 'OsmoTRX'

We assume a sysmoBTS in the following tutorial


== OsmoBTS Overview

image::osmo-bts.svg[]

* Implementation of GSM BTS
* supports variety of hardware/PHY options
** `osmo-bts-sysmo`: BTS family by sysmocom
** `osmo-bts-trx`: Used with 'OsmoTRX' + general-purpose SDR
** `osmo-bts-octphy`: Octasic OCTBTS hardware / OCTSDR-2G PHY
** `osmo-bts-litecell15`: Nutaq Litecell 1.5 hardware/PHY


== Configuring Osmocom software

* all Osmo* GSM infrastructure programs share common architecture, as
  defined by various libraries 'libosmo{core,gsm,vty,abis,netif,...}'
* part of this is configuration handling
** interactive configuration via command line interface (*vty*), similar
   to Cisco routers
** based on a fork of the VTY code from Zebra/Quagga, now 'libosmovty'
* you can manually edit the config file,
* or use `configure terminal` and interactively change it


== Configuring OsmoBTS

* 'OsmoBTS' in our example scenario runs on the embedded ARM/Linux system
  inside the 'sysmoBTS'
* we access the 'sysmoBTS' via serial console or ssh
* we then edit the configuration file `/etc/osmocom/osmo-bts.cfg` as
  described in the following slide


== Configuring OsmoBTS

----
bts 0
 band DCS1800 <1>
 ipa unit-id 1801 0 <2>
 oml remote-ip 192.168.100.11 <3>
----
<1> the GSM frequency band in which the BTS operates
<2> the unit-id by which this BTS identifies itself to the BSC
<3> the IP address of the BSC (to establish the OML connection towards it)

NOTE: All other configuration is downloaded by the BSC via OML. So most
BTS settings are configured in the BSC/NITB configuration file.


== Configuring OsmoNITB

* 'OsmoNITB' is the `osmo-nitb` executable built from the `openbsc`
  source tree / git repository
* just your usual `git clone && autoreconf -fi && ./configure && make install`
** (in reality, the `libosmo*` dependencies are required first...)
* 'OsmoNITB' runs on any Linux system, like your speakers' laptop
** you can actually also run it on the ARM/Linux of the 'sysmoBTS' itself,
   having a literal 'Network In The Box' with power as only external
   dependency


== Configuring OsmoNITB

----
network
 network country code 1 <1>
 mobile network code 1 <2>
 shot name Osmocom <3>
 long name Osmocom
 auth policy closed <4>
 encryption a5 0 <5>
----
<1> MCC (Country Code) e.g. 262 for Germany; 1 == Test
<2> MNC (Network Code) e.g. mcc=262, mnc=02 == Vodafone; 1 == Test
<3> Operator name to be sent to the phone *after* registration
<4> Only accept subscribers (SIM cards) explicitly authorized in HLR
<5> Use A5/0 (== no encryption)


== Configuring BTS in OsmoNITB (BTS)

----
network
 bts 0
  type sysmobts <1>
  band DCS1800 <2>
  ms max power 33 <3>
  periodic location update 6 <4>
  ip.access unit_id 1801 0 <5>
  codec-support fr hr efr amr <6>
----
<1> type of the BTS that we use (must match BTS)
<2> frequency band of the BTS (must match BTS)
<3> maximum transmit power phones are permitted (33 dBm == 2W)
<4> interval at which phones should send periodic location update (6 minutes)
<5> Unit ID of the BTS (must match BTS)
<6> Voice codecs supported by the BTS


== Configuring BTS in OsmoNITB (TRX)

----
network
 bts 0
  trx 0
   arfcn 871 <1>
   max_power_red 0 <2>
   timeslot 0
    phys_chan_config CCCH+SDCCH4 <3>
   timeslot 1
    phys_chan_config TCH/F <4>
    ...
   timeslot 7
    phys_chan_config PDCH <5>
----
<1> The RF channel number used by this TRX
<2> The maximum power *reduction* in dBm. 0 = no reduction
<3> Every BTS needs need one timeslot with a CCCH
<4> We configure TS1 to TS6 as TCH/F for voice
<5> We configure TS6 as PDCH for GPRS


== What a GSM phone does after power-up

* Check SIM card for last cell before switch-off
** if that cell is found again, use that
** if not, perform a network scan
*** try to find strong carriers, check if they contain BCCH
*** create a list of available cells + networks
*** if one of the networks MCC+MNC matches first digits of 'IMSI', this is
the home network, which has preference over others
* perform 'LOCATION UPDATE' (TYPE=IMSI ATTACH) procedure to network
* when network sends 'LOCATION UPDATE ACCEPT', *camp* on that cell

-> let's check if we can perform 'LOCATION UPDATE' on our own network


== Verifying our network

* look at stderr of 'OsmoBTS' and 'OsmoNITB'
** 'OsmoBTS' will terminate if Abis cannot be set-up
** expected to be re-spawned by init / systemd
* use MS to search for networks, try manual registration
* observe registration attempts `logging level mm info`

-> should show 'LOCATION UPDATE' request / reject / accept

* use the VTY to explore system state (`show *`)
* use the VTY to change subscriber parameters like extension number


== Exploring your GSM networks services

* use `*#100#` from any registered MS to obtain own number
* voice calls from mobile to mobile
* SMS from mobile to mobile
* SMS to/from external applications (via SMPP)
* voice to/from external PBX (via MNCC)
* explore the VTY interfaces of all network elements
** send SMS from the command line
** experiment with 'silent call' feature
** experiment with logging levels
* use wireshark to investigate GSM protocols


== Using the VTY

* The VTY can be used not only to configure, but also to interactively
  explore the system status (`show` commands)
* Every Osmo* program has its own telnet port
|===
|Program|Telnet Port
|OsmoPCU|4240
|OsmoBTS|4241
|OsmoNITB|4242
|OsmoSGSN|4245
|===
* ports are bound to 127.0.0.1 by default
* try tab-completion, `?` and `list` commands

== Using the VTY (continued)

* e.g. `show subsciber` to display data about subscriber:
----
OpenBSC> show subscriber imsi 901700000003804
    ID: 12, Authorized: 1
    Extension: 3804
    LAC: 0/0x0
    IMSI: 901700000003804
    TMSI: F2D4FA0A
    Expiration Time: Mon, 07 Dec 2015 09:45:16 +0100
    Paging: not paging Requests: 0
    Use count: 1
----

* try `show bts`, `show trx`, `show lchan`, `show statistics`, ...


== Extending the network with GPRS

Now that GSM is working, up to the next challenge!

* Classic GSM is circuit-switched only
* Packet switched support introduced first with GPRS
* GPRS adds new network elements (PCU, SGSN, GGSN)
* tunnel for external packet networks like IP/Internet
* tunnel terminates in MS and on GGSN


== Extending the network with GPRS support

[graphviz]
----
digraph G {
	rankdir=LR;
	MS0 [label="MS"]
	MS1 [label="MS"]
	MS2 [label="MS"]
	MS3 [label="MS"]
	BTS0 [label="BTS"]
	BTS1 [label="BTS"]
	MSC [label="MSC/VLR"]
	HLR [label="HLR/AUC"]
	MS0->BTS0 [label="Um"]
	MS1->BTS0 [label="Um"]
	MS2->BTS1 [label="Um"]
	MS3->BTS1 [label="Um"]
	BTS0->BSC [label="Abis"]
	BTS1->BSC [label="Abis"]
	BSC->MSC [label="A"]
	MSC->HLR [label="C"]
	MSC->EIR [label="F"]
	MSC->SMSC

	BTS0->PCU
	subgraph cluster_gprs {
		label = "GPRS Add-On"
		PCU->SGSN [label="Gb"]
		SGSN->GGSN [label="GTP"]
	}
}
----

* 'PCU': Packet Control Unit. Runs RLC+MAC
* 'SGSN': Serving GPRS Support Node (like VLR/MSC)
* 'GGSN': Gateway GPRS Support Node (terminates tunnels)


== GPRS Signalling basics

* GPRS Mobility Management (GMM)
** just like GSM Mobility Management (MM)
*** 'GPRS ATTACH', 'ROUTING AREA UPDATE', 'AUTHENTICATION'
* GPRS Session Management (SM)
** establishment, management and tear-down of packet data tunnels
*** independent from IP, but typically IP(v4) is used
*** 'PDP Context' (Activation | Deactivation | Modification)


== GPRS Protocol Stack

image::gprs_user_stack.svg[width=850]


== GPRS Acronyms, Protocol Stack

* Layer 3
** 'SM': Session Management (PDP contexts)
** 'GMM': GPRS Mobility Management (like MM)
* Layer 2
** 'MAC': Medium Access Control
** 'LLC': Link Layer Control (segmentation, compression, encryption)
** 'RLC': Radio Link Control
** 'SNDCP': Sub-Network Dependent Convergence Protocol

[role="incremental"]
- Scotty to the bridge: 'You have to re-modulate the sub-network dependent convergence protocols!'


== Simplified OsmoNITB network with GPRS

[graphviz]
----
digraph G {
	rankdir=LR;
	MS0 [label="MS"]
	BTS0 [label="OsmoBTS"]
	BSC [label="OsmoNITB"]
	PCU [label="OsmoPCU"]
	SGSN [label="OsmoSGSN"]
	GGSN [label="OpenGGSN"]
	MS0->BTS0 [label="Um"]
	BTS0->BSC [label="Abis"]
	BTS0->PCU
	subgraph cluster_gprs {
		label = "GPRS Add-On"
		PCU->SGSN [label="Gb"]
		SGSN->GGSN [label="GTP"]
	}
}
----

* 'OsmoPCU' is co-located with 'OsmoBTS'
** connects over unix-domain PCU socket to BTS
* 'OsmoSGSN' can run on any Linux machine
* 'OpenGGSN' can run on any Linux machine
** `tun` device is used for tunnel endpoints
* circuit-switched and packet-switched networks are completely separate

We need to configure those additional components to provide GPRS
services.

== Simplified OsmoNITB network with GPRS

image::osmocom-gprs.svg[width=750]

//* show IP addresses at nodes
//* show GSM functional elements, Osmocom programs and hardware


== Configuring OsmoPCU

We assume we have obtained and compiled the `osmo-pcu` from
git://git.osmocom.org/osmo-pcu

* 'OsmoPCU' runs co-located with 'OsmoBTS' to access/share the same PHY + Radio
* 'OsmoPCU' is primarily configured from 'OsmoBTS'
* 'OsmoBTS' receives relevant config via A-bis OML
* 'OsmoNITB' sends those OML messages to OsmoBTS
** we thus need to set the PCU configuration in the NITB config file!


== BTS config for GPRS (in OsmoNITB)

----
 bts 0
  gprs mode gprs <1>
  gprs nsei 1234 <2>
  gprs nsvc 0 nsvci 1234 <3>
  gprs nsvc 0 local udp port 23000 <4>
  gprs nsvc 0 remote ip 192.168.1.11 <5>
  gprs nsvc 0 remote udp port 23000 <6>
----
<1> enable `gprs` or `egprs` mode
<2> NSEI for the NS protocol layer (unique for each PCU in SGSN)
<3> NSVCI for the NS protocol layer (unique for each PCU in SGSN)
<4> UDP port on PCU side of Gb connection
<5> IP address of SGSN side of Gb connection
<6> UDP port on SGSN side of Gb connection


== Configuring OsmoSGSN (Gb and GTP)

----
ns
 encapsulation udp local-ip 192.168.100.11 <1>
 encapsulation udp local-port 23000 <2>
sgsn
 gtp local-ip 127.0.0.2 <3>
 ggsn 0 remote-ip 127.0.0.1 <4>
 ggsn 0 gtp-version 1 <5>
 apn * ggsn 0 <6>
----
<1> SGSN-local IP address for Gb connection from PCUs
<2> SGSN-local UDP port number for Gb connection from PCUs
<3> SGSN-local IP address for GTP connection to GGSN
<4> remote IP address for GTP connection to GGSN
<5> GTP protocol version for this GGSN
<6> route all APN names to GGSN 0


== Configuring OsmoSGSN (subscribers)

'OsmoSGSN' (still) has no access to the 'OsmoNITB' HLR, thus all IMSIs
permitted to use GPRS services need to be explicitly configured.

----
sgsn
 auth-policy closed <1>
 imsi-acl add 262778026147135 <2>
----
<1> only allow explicitly authorized/white-listed subscribers
<2> add given IMSI to the white-list of subscribers


== Setting up OpenGGSN

In `ggsn.cfg` we need to set:

----
listen 172.0.0.1 <1>
net 10.23.24.0/24 <2>
dynip 10.23.42.0/24 <3>
pcodns1 8.8.8.8 <4>
----
<1> IP address to bind GSN to.
<2> network/mask of `tun` device
<3> pool of dynamic IP addresses allocated to PDP contexts
<4> IP address of DNS server (communicated to MS via signalling)


== Testing GPRS

* Check if `osmo-pcu`, `osmo-sgsn`, `openggsn` are running
* Check if NS and BSSGP protocols are UNBLOCKED at SGSN
** If not, check your NS/BSSGP configuration
* Check for GPRS registration using `logging level mm info` in SGSN


== Osmocom beyond GSM/GPRS RAN + NITB

* Smalltalk implementation of SIGTRAN + TCAP/MAP
* Erlang implementation of SIGTRAN + TCAP/MAP
* Lots of special-purpose protocol mangling
** `bsc-nat` to introduce NAT-like functionality on A (BSSAP/BSSMAP)
** `mgw-nat` to transparently re-write MAP/ISUP/SCCP
* GSMTAP pseudo-header for feeding non-IP protocols into wireshark
* SIM card protocol tracer hardware + software
* Lots of non-GSM projects from hardware to protocol stacks (TETRA, GMR, DECT, OP25)
* check http://git.osmocom.org/ for full project list


== So... I heard about OpenBTS?

* OpenBTS is completely unrelated to the Osmocom stack
* was independently developed by David Burgess & Harvind Simra
** Kestrel Signal Processing -> Range Networks
* doesn't follow GSM system architecture at all
** no Abis, BSC, PCU, SGSN, GGSN
* is a bridge of the GSM air interface (Um) to SIP
* Osmocom follows classic GSM interfaces / system architecture
* 'OsmoTRX' forked 'OpenBTS' SDR code to use 'OsmoBTS' with SDR hardware


== FOSS 2.75G (EDGE)

* EDGE extends GPRS with higher data rates
** 8PSK instead of GMSK modulation
** lots of new MAC/RLC features (larger windows, incremental redundancy)
** No changes required in 'OmsoSGSN' and 'OsmoGGSN'
* 'OsmoPCU' is extended with EDGE support
** implementation not as mature as GPRS
** easy to enable, simply use `gprs mode egprs` in BSC config


== FOSS 3G/3.5G (UMTS/WCDMA)

* UMTS very similar to GSM/GPRS in principle
** still, almost every interface and protocol stack has changed
** all elements have been renamed -> more acronyms to learn
* UMTS is ridiculously complex, particular PHY + Layer 2
** however, control plane L3 (MM/CC/CM/SM/GMM) mostly the same
* Implementing all of that from scratch is a long journey
* We've already reached 'Peak 3G'
* Osmocom 3G support strategy
** Implement Iu interface in NITB and SGSN
** Implement HNB-GW to offer Iuh interface (osmo-hnbgw)
** Use existing femtocell / small cell hardware with proprietary PHY, RLC and MAC
** Status: Signaling, SMS and Voice working, not all code in master branch yet


== Outlook on FOSS 4G (LTE)

* LTE has nothing in common with 2G/3G
* various FOSS activities
** 'OpenAirInterface' has some code for a software eNodeB
*** but they switched from GPLv3 to 'non-free' license :(
** 'srsLTE' (main focus on UE side, but large parts usable for eNodeB side)
** 'OpenLTE' is another active FOSS project
* No Osmocom involvement so far
** team is small, project scope of cellular infrastructure is gigantic
** most customer funding currently still on GSM/GPRS/EDGE
** if we'd start, we'd start implementing MME + S-GW and use existing
LTE cells, similar to 3.5G strategy


== The End

* so long, and thanks for all the fish
* I hope you have questions!

[role="incremental"]
* have fun exploring mobile technologies using Osmocom
* interested in working with more acronyms? Come join the project!

* Check out http://openbsc.osmocom.org/ and openbsc@lists.osmocom.org

== Thanks to

* CLT Organizers and Volunteersfor running this event, for so many years!
* the entire Osmocom team for what they have achieved
** notably Dieter Spaar, Holger Freyther, Andreas Eversberg, Sylvain Munaut
* last but not least: CEPT for making the GSM specs English
** (who'd want to read French specs anyway?)