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What this talk is about

+ +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 + +
- + +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? + +
+ +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 + +
- + +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

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

Classic GSM network as digraph

Simplified OsmoNITB GSM network

which further reduces to the following minimal setup:

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

+ +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>

+
+the GSM frequency band in which the BTS operates +
+
+
+the unit-id by which this BTS identifies itself to the BSC +
+
+
+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>

+
+MCC (Country Code) e.g. 262 for Germany; 1 == Test +
+
+
+MNC (Network Code) e.g. mcc=262, mnc=02 == Vodafone; 1 == Test +
+
+
+Operator name to be sent to the phone after registration +
+
+
+Only accept subscribers (SIM cards) explicitly authorized in HLR +
+
+
+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>

+
+type of the BTS that we use (must match BTS) +
+
+
+frequency band of the BTS (must match BTS) +
+
+
+maximum transmit power phones are permitted (33 dBm == 2W) +
+
+
+interval at which phones should send periodic location update (6 minutes) +
+
+
+Unit ID of the BTS (must match BTS) +
+
+
+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>

+
+The RF channel number used by this TRX +
+
+
+The maximum power reduction in dBm. 0 = no reduction +
+
+
+Every BTS needs need one timeslot with a CCCH +
+
+
+We configure TS1 to TS6 as TCH/F for voice +
+
+
+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

+ +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

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 + +
  - + +Scotty to the bridge: You have to re-modulate the sub-network dependent convergence protocols! + +
  +
+

Simplified OsmoNITB network with GPRS

+ +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

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>

+
+enable gprs or egprs mode +
+
+
+NSEI for the NS protocol layer (unique for each PCU in SGSN) +
+
+
+NSVCI for the NS protocol layer (unique for each PCU in SGSN) +
+
+
+UDP port on PCU side of Gb connection +
+
+
+IP address of SGSN side of Gb connection +
+
+
+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>

+
+SGSN-local IP address for Gb connection from PCUs +
+
+
+SGSN-local UDP port number for Gb connection from PCUs +
+
+
+SGSN-local IP address for GTP connection to GGSN +
+
+
+remote IP address for GTP connection to GGSN +
+
+
+GTP protocol version for this GGSN +
+
+
+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>

+
+only allow explicitly authorized/white-listed subscribers +
+
+
+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>

+
+IP address to bind GSN to. +
+
+
+network/mask of tun device +
+
+
+pool of dynamic IP addresses allocated to PDP contexts +
+
+
+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! + +
+ +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?) + +
+

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