Running FOSS Cellular Networks on Linux ======================================= :author: Harald Welte :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 Archaeology * Doing all of that on top of Linux (in userspace) == 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 See separate talk about BTS hardware options later today. == Configuring Osmocom software * all _native_ 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...) * nightly packages for Debian 8, Ubuntu 16.04 and 16.10 available * '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. == 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 https://osmocom.org/ and openbsc@lists.osmocom.org