Tag Archives: GSM

OsmoHLR - Update Location Request

GSM with Osmocom Part 7: The HLR – Home Location Register (and Friends)

The Home Location Register serves the AAA functions in a GSM / UMTS (2G/3G) network as well as locating which Mobile Switching Center (MSC) a subscriber is being served by.

The HLR is the equivalent of the Home Subscriber Server in LTE (I’ve written a fair bit about the role of the HSS in LTE networks, and I’ve published my own open-source HSS software.)

Authentication

One obvious need is to authenticate our subscribers so the network can verify their identity,

The IMSI (International Mobile Subscriber Identity) is used to identifier the user from all the other mobile subscribers worldwide. The IMSI is exposed to the user, but transmitting the IMSI in the clear is typically something that’s avoided where possible on the air interface.

GSM uses a single shared secret between the SIM and the network (the K key) for authentication. This shared secret is not exposed to the user and is never transmitted over the air.

When a user wants to authenticate, the HSS network takes a Random key (RAND) and mixes it with the secret key (K) to generate a Signed Response called SRES. The network sends the RAND key to the subscriber, and their SIM takes the secret key (K) and mixes it with the RAND value from the network, before sending their signed response (SRES) back to the network.
If the SRES sent by the subscriber matches the SRES generated by the HSS, then the user is authenticated. The set of keys used for one authentication session is referred to as an Authentication Vector or Authentication Tuple.

In Osmocom the generation of Authentication Tuples is requested in the GSUP “SendAuthInfo” request, and responded to by the “SendAuthInfoResponse” sent to the HLR by the MSC.

Side note about GSM Security

In a GSM setting the network only authenticates the subscribers, the subscribers don’t authenticate the network. In practice, this means there’s no way to verify in GSM if the network you’re connected to is the network it’s claiming to be.

Due to this shortfall and the cryptographic weakness in A5/x algorithm, 3GPP specified the AKA algorithm for mutual network authentication in 3G/UMTS networks.

I’ve written a fair bit about the role of SIMs for authentication in LTE which is the same scheme used in 3G/UMTS if you’d like to learn more.

LTE USIM Authentication - Mutual Authentication of the Network and Subscriber

Technically the generation of Authentication Vectors is handled by an Authentication Center (AuC) however OsomoHLR has an internal AuC that handles this internally.

Location Tracking

After a user has authenticated, the MSC sends an UpdateLocationRequest via GSUP to the HLR to let it know the current location of the subscriber is served by that MSC.

The Update Location Request is sent at the start of the session, periodic Update Location Requests can be sent based on the timers configured, and a Cancel Location Request can be sent when the subscriber disconnects from the MSC.

Subscriber Data Information

When the Update Location Request has been sent by the MSC, the HLR sends the MSC the subscriber’s info, and the MSC copies it to it’s own internal HLR called a Visitor Location Register (VLR). The VLR means the MSC doesn’t need to keep querying user data from the HLR.

This is again requseted by the MSC to the HLR via a GSUP request InsertSubscriberData Request which contains:

  • Subscriber’s IMSI
  • Subscriber’s MSISDN (Phone number)
  • Allowed Domains (CS/PS)

Note: In production GSM networks TCAP/MAP is used for communication between the HLR and the MSC. Osmocom uses GSUP for carrying this data instead.

Equipment Identity Register

Because mobiles are expensive they’ve historically been a target for theft.

To try and mitigate this GSMA encourages carriers to implement an Equipment Identity Register (EIR).

The EIR is essentially a database containing IMEIs (The Identifiers of Mobiles / Terminals) and permitting / denying access to the network based on the IMEI.

The idea being if a mobile device / terminal is stolen, it’s IMEI is blacklisted in the EIR and regardless of what SIM is put into it, it’s not permitted to access the network.

When a device connects to the network if configured the MSC will query the EIR (On the HLR in our case) with a Check IMEI Request, and will get a Check IMEI Result either permitting or denying access to the network.

Unfortunately, there is no global stolen IMEI database, meaning if a device is stolen and blocked on MNO X’s network, it may still work on MNO Y’s network if they don’t share stolen IMEI data.

Starting & Configuring OsmoHLR

We actually installed OmsoHLR in the post on Base Station Controllers, so we’ll just need to start the daemon / service:

systemctl start osmo-hlr

I’m going to enable the EIR functionality of the HSS by changing the config of the HLR, this is optional but it’s useful to use the EIR functionality.

Like with our other network elements we’ll use Tenet to interactively configure this one,

root@gsm-bts:/home/nick# telnet localhost 4258
Welcome to the OsmoHLR VTY interface
OsmoHLR> enable
OsmoHLR# configure terminal
OsmoHLR(config)# hlr
OsmoHLR(config-hlr)# store-imei
OsmoHLR(config-hlr)# exit
OsmoHLR(config)# exit
OsmoHLR# copy running-config startup-config

Adding Subscribers to OsmoHLR

But before we go adding subscribers, let’s talk about SIMs.

Okay, I’ve written a lot about SIMs before, but there’s still more to talk about!

There’s really only one peice of information from your SIM we require to add the subscriber to the HLR, and that’s the IMSI – The unique identifier of the subscriber on the SIM. You can typically view the IMSI from your mobile device / terminal.

If you want to authenticate subscribers properly (confirm their identity) and enable encryption on the air interface, you’ll need to know the K key of the SIM, for that you’ll need a programmable SIM card like the Sysmocom programmable SIM cards, (By buying from Sysmocom you’re supporting the Osmocom project too).

So now we’ve got that out of the way, let’s add a subscriber:

We’ll connect to OsmoHLR via Telnet, the port it listens on is 4258:

root@gsm-bts:/home/nick# telnet localhost 4258
Welcome to the OsmoHLR VTY interface
OsmoHLR> enable
OsmoHLR# subscriber imsi 001010000000004 create
OsmoHLR# subscriber imsi 001010000000004 update msisdn 61412341234
OsmoHLR# subscriber imsi 001010000000004 update aud2g comp128v3 ki 465B5CE8B199B49FAA5F0A2EE238A6BC

So I’ve created a subscriber with IMSI 001010000000004 in the HSS and assigned an MSISDN (phone number).

Optionally, if you’re using SIM cards you can program you can set the Ki / K key for authentication using the update aud2g function, if not you can skip that step.

And with that we’ve added our first subscriber, lather rinse repeat with any additional subscribers / SIMs you want to provision.

By default subscribers created using this method have access to both Circuit Switched (Voice and SMS) and Packet Switched (Data) networks. (We haven’t configured Packet Switched services yet)

If you’d like to restrict access to one, both or none of the above options, you can do that by using the subscriber update command to set the services available to those subscribers.

OsmoHLR# subscriber id 3 update network-access-mode cs+ps
OsmoHLR# subscriber id 3 update network-access-mode cs
OsmoHLR# subscriber id 3 update network-access-mode ps
OsmoHLR# subscriber id 3 update network-access-mode none

Creating Subscribers Programmatically

In reality if you’re trying to operate a network it’s not feasible to manually add each subscriber as needed.

If you’re buying SIMs in bulk preconfigured you’ll get sent a file containing the IMSI and Crypto values of each card, and you’d ingest that into your HLR.

We’ve used the Osmocom VTY / Telnet interface in quite a few posts now (hopefully you’re getting comfortable with it) but there’s another interface most Osmocom software has – the Osmocom Control Interface – aimed at providing a uniform way to interface external scripts / programs with Osmocom.

If you’d like to learn more about the Control Interface have a read of the OsmoHLR manual and take a look at this example in Python.

Creating Subscribers on Demand (Optional)

For most scenarios you would pre-provision each SIM in the HLR, if the SIM’s IMSI isn’t in the HLR then it’s access is rejected. However there are some scenarios where you may want to allow anyone to access the network, in this scenario Osmo-HLR features a “Create Subscribers on Demand” function.

This may be useful if you’re setting up a network where you don’t control the SIMs for example.

Let’s say we want to automatically create users with access to voice & data services and assign a 10 digit MSISDN for that subscriber, we can do that with:

OsmoHLR> enable
OsmoHLR# configure terminal
OsmoHLR(config)# hlr
OsmoHLR(config-hlr)# subscriber-create-on-demand 10 cs+ps

Alternatley you may wish to simply add the subscriber to the HLR but not provide any services:

OsmoHLR> enable
OsmoHLR# configure terminal
OsmoHLR(config)# hlr
OsmoHLR(config-hlr)# subscriber-create-on-demand no-msisdn none

Then if you wish to grant access to these users you can use the subscriber update network-access-mode method we talked about earlier to allow services for that user.

Packet Capture

To give some context I’ve attached a packet capture of the connection from the MSC to the HLR for some attach procedures on my lab network.

GSM with Osmocom Part 6: Integrating our LimeSDR BTS with OsmoBSC

In the last post we covered the config required to provision a new BTS in our BSC.

We’re going to do roughly the same thing this time around, as we connect our SDR based BTS to our BSC.

Before we fire up the BTS side of things make sure you’ve stopped the virtual BTS and disabled it.

systemctl stop osmo-bts-virtual
systemctl disable osmo-bts-virtual

Configure Osmo-BTS-TRX

Next up we’ll edit the config of osmo-bts-trx.

vi /etc/osmocom/osmo-bts-trx.cfg

We’ll edit the oml remote-ip to the IP of the server running your BSC, if you’re running on on the same machine you can leave it as localhost (127.0.0.1).

Next up we’ll set the Unit-ID of the BTS, this identifies the BTS inside the BSC,

I’ll set it to unit-id 1234 by changing ipa unit-id 1234 0

Finally we’ll change the logging config to show everything by changing it to:

log stderr
 logging filter all 1
!

Next up we’ll configure the BTS on the BSC

BSC Provisioning

This is essentially a copy of the provisioning process we followed in the last post, the only difference is we’ll use BTS 2 (as BTS 1 is setup for our Virtual BTS) in the config, and set the few different identifier such as the ipa unit id for the SDR based BTS.

telnet localhost 4242
OsmoBSC> enable
OsmoBSC# configure terminal
OsmoBSC(config)# network
OsmoBSC(config-net)# bts 2
OsmoBSC(config-net-bts)# type sysmobts
OsmoBSC(config-net-bts)# description "LimeSDR Based BTS"
OsmoBSC(config-net-bts)# ipa unit-id 1234 0
OsmoBSC(config-net-bts)# band DCS1800
OsmoBSC(config-net-bts)# codec-support fr hr efr amr
OsmoBSC(config-net-bts)# cell_identity 1234
OsmoBSC(config-net-bts)# location_area_code 1234
OsmoBSC(config-net-bts)# base_station_id_code 1234
OsmoBSC(config-net-bts)# base_station_id_code 12
OsmoBSC(config-net-bts)# ms max power 40
OsmoBSC(config-net-bts)# trx 0
OsmoBSC(config-net-bts-trx)# max_power_red 20
OsmoBSC(config-net-bts-trx)# arfcn 876
OsmoBSC(config-net-bts-trx)# timeslot 0
OsmoBSC(config-net-bts-trx-ts)# phys_chan_config CCCH+SDCCH4
OsmoBSC(config-net-bts-trx-ts)# exit
OsmoBSC(config-net-bts-trx)# timeslot 1
OsmoBSC(config-net-bts-trx-ts)# phys_chan_config TCH/F
OsmoBSC(config-net-bts-trx-ts)# exit
OsmoBSC(config-net-bts-trx)# timeslot 2
OsmoBSC(config-net-bts-trx-ts)# phys_chan_config TCH/F
OsmoBSC(config-net-bts-trx-ts)# exit
OsmoBSC(config-net-bts-trx)# timeslot 3
OsmoBSC(config-net-bts-trx-ts)# phys_chan_config TCH/F
OsmoBSC(config-net-bts-trx-ts)# exit
OsmoBSC(config-net-bts-trx)# timeslot 4
OsmoBSC(config-net-bts-trx-ts)# phys_chan_config TCH/F
OsmoBSC(config-net-bts-trx-ts)# exit
OsmoBSC(config-net-bts-trx)# timeslot 5
OsmoBSC(config-net-bts-trx-ts)# phys_chan_config TCH/F
OsmoBSC(config-net-bts-trx-ts)# exit
OsmoBSC(config-net-bts-trx)# timeslot 6
OsmoBSC(config-net-bts-trx-ts)# phys_chan_config TCH/F
OsmoBSC(config-net-bts-trx-ts)# exit
OsmoBSC(config-net-bts-trx)# timeslot 7
OsmoBSC(config-net-bts-trx-ts)# phys_chan_config TCH/F
OsmoBSC(config-net-bts-trx-ts)# exit 
OsmoBSC(config-net-bts-trx)# exit
OsmoBSC(config-net-bts)# exit
OsmoBSC(config-net)# exit
OsmoBSC(config)# exit 
OsmoBSC# copy running-config startup-config 

Starting the SDR based BTS

Before we start the SDR based BTS it’s probably best to have 3 terminals open,

One logged into Osmo-BSC with logging enabled (see the last post for info on how to do that).

We’ll start another terminal for running the TRX modem / Layer 1 interface:

osmo-trx-lms -C /etc/osmocom/osmo-trx-lms.cfg

And in another new terminal we’ll start the BTS side;

osmo-bts-trx -c /etc/osmocom/osmo-bts-trx.cfg

All going well our terminal with Osmo-BSC should report the connection:

OsmoBSC#
<0016> input/ipa.c:287 0.0.0.0:3003 accept()ed new link from 10.0.1.252:39595
<0003> osmo_bsc_main.c:291 bootstrapping RSL for BTS/TRX (2/0) on ARFCN 875 using MCC-MNC 001-01 LAC=1234 CID=1234 BSIC=12

And the osmo-trx-lms and osmo-bts-trx windows should have data flying by at a rate of knots.

Verifying Cell Operation

If all is going to plan, our SDR is connected to our machine via osmo-trx-lms which is acting as a modem for osmo-bts-trx which is now connected to the BSC. Lot to go through, but it gets easier from here.

Let’s run a scan of the networks on our phone. I found putting mine on GSM only before scanning for networks meant it popped up a heck of a lot faster.

And lo, there it is.

Our cell is online and broadcasting it’s info. You won’t be able to connect to it at this stage as we’ve still got a few more steps to go.

In the next post we’ll introduce the Home Location Register and then the MSC.

GSM with Osmocom Part 5: Software BTS with LimeSDR & osmo-bts-trx

Osmo-BSC accepts Abis over IP connections from a number of different sources,

There’s a list of supported BTS hardware that can talk out of the box to the Osmo-BSC, such as the Ericsson RBS series, ip.access nanoBTS, Nokia and Siemens units and even a virtual BTS so you can simulate the connections.

If you’re using any of these premade BTS hardware options, or osmo-bts-virtual, you probably just need to setup the basics on your BTS and point it to your BSC, end of story.

The below post will touch on using common SDR hardware to act as our BTS. If you’re not using SDR hardware you can just skip ahead to the next post on BSCs.

But, if you’re in the same boat as me, without any commercial BTS / RAN hardware, we’ll be setting it up by using an SDR platforms (In my case LimeSDR) and that’s what this tutorial will focus on.

Osmo-TRX

In order to bring in a large array of SDR hardware, Osmocom have introduced Osmo-TRX, which handles the Layer 1 physical layer of the BTS, and connects to Osmo-BTS which serves as the BTS and talks Abis over IP to the MSC.

Certain hardware can talk directly to Osmo-BTS, but we’re going to rely on Osmo-TRX to act as the middleman between our SDR hardware and the BTS.

The above diagram from the Osmocom wiki shows how this fits together with generic SDR platforms, here’s how it fits together for us:

osmo-trx-lms will take care of the SDR side of the equation, pretty much serving as a modem and sending everything it gets on the Uu interface to osmo-bts-trx over UDP, and everything it receives from osmo-bts-trx over UDP it sends out the Uu interface.

osmo-bts-trx will then setup an Abis over IP connection to our BSC.

The LimeSDR

My ever growing collection of SDR hardware now includes a LimeSDR which I’ll be using for this series.

Before we can get too far we’ve got to setup the prerequisites for the LimeSDR to be able to interface with Osmo-TRX.

Osmocom now provide a binary for interfacing with LimeSDR boards directly, instead of having to use the UHD abstraction. This is a much cleaner way of interfacing with the boards and the path I’ll be taking.

Software Install

For this tutorial series I’ll be using Ubuntu 18.04 and trying where possible to use packages from Repos instead of compiling from source.

LimeSuite provides the drives and utilities for interfacing with the LimeSDR.

add-apt-repository -y ppa:myriadrf/drivers
apt-get update
apt-get install limesuite limesuite-udev

Next we’ll connect up the LimeSDR to a USB3 port, confirm it’s there and upgrade it’s firmware:

 LimeUtil --find

Assuming your LimeSDR is hooked up and everything installed you should see an output similar to this:

In which case we can upgrade the LimeSDR firmware with:

LimeUtil --update  

Next we’ll start installing Osmocom Sources;

wget https://download.opensuse.org/repositories/network:/osmocom:/latest/Debian_10/Release.key  
apt-key add Release.key && rm Release.key
echo "deb https://download.opensuse.org/repositories/network:/osmocom:/latest/xUbuntu_18.04/ ./" > /etc/apt/sources.list.d/osmocom-latest.list 
apt-get update

Now that we’ve got the Osmocom Debian repos added we can install the packages we need,

We’re going to install Osmo-BTS-TRX for talking to the BSC over Abis, and install Osmo-TRX-LMS for talking to the SDR.

apt-get install osmo-bts-trx osmo-trx-lms

After you’ve installed the packages, Osmo-BTS-TRX will run as a daemon, we’ll stop it for now and bring it up manually in the foreground.

systemctl disable osmo-bts-trx
systemctl disable osmo-trx-bts

Software Config

So now we’ve got two pieces of the puzzle, it’s time to connect the SDR to Osmo-TRX-LMS and connect Osmo-TRX-LMS to Osmo-BTS-TRX.

We’ll begin by running Osmo-TRX-LMS to connect to the LimeSDR and encapsulate the Uu data into UDP packets we send to Osmo-BTS-TRX.

Config files for Osmocom are installed in /etc/osmocom/ so we’ll run everything from that directory.

osmo-trx-lms -C osmo-trx-lms.cfg

If all was successful you’ll see something similar to what I’ve got below, showing Osmo-TRX-LMS has connected to the SDR and is ready to go.

But if you go scanning the airwaves now, you won’t see any data coming out of the SDR’s transmitter.

That’s because Osmo-TRX-LMS needs to connect to Osmo-BTS-TRX,

We’ll leave Osmo-TRX-LMS running, so let’s open up another session and start Osmo-BTS-TRX.

osmo-bts-trx -c osmo-bts-trx.cfg

You’ll see for starters that it’s Opened our transceiver (hooray),

You’ll see this reflected in the Osmo-TRX-LMS stdout, but it’ll show the poweroff command has been sent to it, so what gives?

Well, the answer becomes clear if you leave Osmo-BTS-TRX running for a minute or two,

Eventually the process stops, reporting:

<000d> abis.c:142 Signalling link down
<0001> bts.c:292 Shutting down BTS 0, Reason Abis close

So what’s going on? In the same way we saw our Virtual BTS shut itself down, without a connection to the BSC (Via the Abis interface) the BTS will shut itself down, as it’s not able to run on it’s own.

This took me a shamefully long time to work out that’s why it was stopping…

In our next post we’ll introduce our BSC and provision a BTS on it.

Further Reading:

OsmoTRX – Osmocom Wiki

OsmoBTS – OsmocomWiki

LimeSDR – Osmocom Wiki

GSM with Osmocom Part 4: The Base Station Controller (BSC)

So in our last post we finished setting up a Base Transceiver Station (BTS) but it’s no use unless it can home itself to a Base Station Controller (BSC).

So what does a BSC do?

The BSC acts as a central controller for one or more BTS.

In practice this means the BSC configures most of the parameters on the BTS and brings each one up onto the air when they’re ready.

The BSC monitors measurements from users to work out when to hand off from one BTS to neighboring BTS,

The BSC also handles the allocation of radio channels and radio resources across the BTSs it manages.

In short, it does pretty much everything radio related for the BTS except transmitting and receiving data over the Air (Um) interface.

As well as managing the BTS under it, the other other equally important role of the BSC is to provide connectivity to the rest of the GSM network, by connecting to a Mobile Switching Center (MSC) which handles calls to and from our mobile subscribers and authenticating them.

By acting as a funnel of sorts, the MSC only needs a connection to each BSC instead of to each BTS (Which would be an impractically large number of connections)

Osmo BSC Install

Osmocom have their own BSC – Aptly called Osmo-BSC.

Installation is pretty straightforward, assuage you’ve got the Osmocom repo in your sources list:

apt-get install osmo-bsc 
systemctl stop osmo-bsc

In order to serve the BTSs it controls, Osmo-BSC relies on connectivity to a Mobile Switching Center (MSC), which in turn connects to a HLR (Home Location Register). The BSC and MSC communicate via SS7, and the routing is done by a Signal Transfer Points (STP).

We’ll go into each of these elements in more detail, but in order to bring our BSC up in a useful way, we’ll need to install and start these applications.

We’ll talk about the MSC, the basics of SS7 / Sigtran and the HLR later in the series, but for now we’ll blindly install and start them:

apt-get install osmo-stp osmo-msc osmo-hlr
systemctl start osmo-stp
systemctl start osmo-msc
systemctl start osmo-hlr

We’ll come back and cover each of these elements in more detail in due course.

Osmo Config – Telnet Interactive Terminal

So now we’ve got the BSC installed we’ve pointed our BTS at the IP of the BSC, we’ll need to get osmo-bsc running and add the config for our new BTS.

Instead of working with the text file we’ll start the service and work on it through Telnet, like we would for many common network devices.

Osmo-BSC listens on port 4242, so we’ll start Osmo-BSC and connect to it via Telnet:

systemctl start osmo-bsc
telnet localhost 4242

The interface should come pretty naturally to anyone who’s spent much time setting up other network devices, a lot of the commands are similar and yes – tab completion is a thing!

We’ll start by enabling logging so we can get an idea of what’s going on:

OsmoBSC> enable
OsmoBSC# logging enable
OsmoBSC# logging filter all 1
OsmoBSC# logging color 1

Next up in a new terminal / SSH session, we’ll run our virtual BTS again;

 osmo-bts-virtual -c osmo-bts-virtual.cfg 

This time we’ll get a different output from the BTS when we try to start it:

root@gsm-bts:/etc/osmocom# osmo-bts-virtual -c osmo-bts-virtual.cfg
 ((*))
   |
  / \ OsmoBTS
<0010> telnet_interface.c:104 Available via telnet 127.0.0.1 4241
<0012> input/ipaccess.c:901 enabling ipaccess BTS mode, OML connecting to 127.0.0.1:3002
<0012> input/ipa.c:128 127.0.0.1:3002 connection done
<0012> input/ipaccess.c:724 received ID_GET for unit ID 4242/0/0
<0012> input/ipa.c:63 127.0.0.1:3002 lost connection with server
<000d> abis.c:142 Signalling link down
<000d> abis.c:156 OML link was closed early within 0 seconds. If this situation persists, please check your BTS and BSC configuration files for errors. A common error is a mismatch between unit_id configuration parameters of BTS and BSC.

root@gsm-bts:/etc/osmocom#

We’ll also see errors in the terminal on the BSC too:

<0016> input/ipa.c:287 0.0.0.0:3002 accept()ed new link from 10.0.1.252:39383
<0016> bts_ipaccess_nanobts.c:480 Unable to find BTS configuration for 4242/0/0, disconnecting

So what’s happening here?

Well our virtual BTS is trying to connect to our BSC, and this time it’s able to, but our BSC doesn’t have any config in place for that BTS, so the BSC has rejected the connection.

So now we’ve got to configure a the BSC to recognise our BTS.

Provisioning a new BTS in the BSC

So as to keep this tutorial generic enough for anyone to follow along, we’re first going to configure a virtual BTS in our BSC to begin with. I wrote about installing Osmo-BTS-Virtual in this post.

We can get the information about the rejected BTS connection attempt from the BSC terminal:

OsmoBSC# show rejected-bts
 Date                Site ID BTS ID IP
2020-03-29 01:32:37    4242      0      10.0.1.252

So we know the Site-ID is 4242 (we set it earlier) and the BTS ID for that site is 0, so let’s create a BTS in the BSC;

OsmoBSC> enable
OsmoBSC# configure terminal
OsmoBSC(config)# network
OsmoBSC(config-net)# bts 1
OsmoBSC(config-net-bts)# type sysmobts
OsmoBSC(config-net-bts)# description "Virtual BTS"
OsmoBSC(config-net-bts)# ipa unit-id 4242 0
OsmoBSC(config-net-bts)# band DCS1800 
OsmoBSC(config-net-bts)# codec-support fr hr efr amr
OsmoBSC(config-net-bts)# cell_identity 4242
OsmoBSC(config-net-bts)# location_area_code 4242
OsmoBSC(config-net-bts)# base_station_id_code 4242
OsmoBSC(config-net-bts)# base_station_id_code 42
OsmoBSC(config-net-bts)# ms max power 40
OsmoBSC(config-net-bts)# trx 0
OsmoBSC(config-net-bts-trx)# max_power_red 20
OsmoBSC(config-net-bts-trx)# arfcn 875
OsmoBSC(config-net-bts-trx)# timeslot 0
OsmoBSC(config-net-bts-trx-ts)# phys_chan_config CCCH+SDCCH4
OsmoBSC(config-net-bts-trx-ts)# exit
OsmoBSC(config-net-bts-trx)# timeslot 1
OsmoBSC(config-net-bts-trx-ts)# phys_chan_config TCH/F
OsmoBSC(config-net-bts-trx-ts)# exit
OsmoBSC(config-net-bts-trx)# timeslot 2
OsmoBSC(config-net-bts-trx-ts)# phys_chan_config TCH/F
OsmoBSC(config-net-bts-trx-ts)# exit
OsmoBSC(config-net-bts-trx)# timeslot 3
OsmoBSC(config-net-bts-trx-ts)# phys_chan_config TCH/F
OsmoBSC(config-net-bts-trx-ts)# exit
OsmoBSC(config-net-bts-trx)# timeslot 4
OsmoBSC(config-net-bts-trx-ts)# phys_chan_config TCH/F
OsmoBSC(config-net-bts-trx-ts)# exit
OsmoBSC(config-net-bts-trx)# timeslot 5
OsmoBSC(config-net-bts-trx-ts)# phys_chan_config TCH/F
OsmoBSC(config-net-bts-trx-ts)# exit
OsmoBSC(config-net-bts-trx)# timeslot 6
OsmoBSC(config-net-bts-trx-ts)# phys_chan_config TCH/F
OsmoBSC(config-net-bts-trx-ts)# exit
OsmoBSC(config-net-bts-trx)# timeslot 7
OsmoBSC(config-net-bts-trx-ts)# phys_chan_config TCH/F
OsmoBSC(config-net-bts-trx-ts)# exit 
OsmoBSC(config-net-bts-trx)# exit
OsmoBSC(config-net-bts)# exit
OsmoBSC(config-net)# exit
OsmoBSC(config)# exit 
OsmoBSC# copy running-config startup-config

Phew,

So what did we actually do here?

Well as we’re getting the majority of the smarts for the BTS from the BSC, we’ve got to tell the BSC all about how we want the BTS setup. (Believe it or not this is the most abridged setup I could muster.)

The type, IPA Unit ID, band and Cell Identity make up some of the parameters we need to identify the BTS (IPA Unit ID) and give it it’s basic identity parameters.

Next up in the trx 0 section we set the contents of the 8 GSM timeslots. Our first time slot we configure as CCCH+SDCCH4 meaning the first timeslot will contain the Common Control Channel and 4 Standalone dedicated control channels, used for signalling, while the reamining 7 timeslots will be used with traffic channels for full-rate speech (TCH/F).

It’s important that what we tell the BSC the capabilities of the BTS are match the actual capabilities of the BTS. For example there’s no point configuring GPRS or EDGE support on the BSC if the BTS doesn’t support it.

If you’ve got logging enabled when the BTS connects to the BSC you’ll see errors listing the features mismatch between the two.

As you can imagine there’s better options than this for adding BTS in bulk – Osmocom Control Interface exposes these functions in an API like way, but before you start on network orchestration it’s good to know the basics.

Connecting the BTS to the BSC

So let’s go ahead and connect our BTS to the BSC.

If you’ve closed the BSC terminal since we enabled logging you’ll need to enable it again:

OsmoBSC> logging enable
OsmoBSC> logging filter all 1
OsmoBSC> logging color 1

And next up we’ll try and start the BTS again:

root@gsm-bts:/etc/osmocom# osmo-bts-virtual -c osmo-bts-virtual.cfg
 ((*))
   |
  / \ OsmoBTS
<0010> telnet_interface.c:104 Available via telnet 127.0.0.1 4241
<0012> input/ipaccess.c:901 enabling ipaccess BTS mode, OML connecting to 127.0.0.1:3002
<0012> input/ipa.c:128 127.0.0.1:3002 connection done
<0012> input/ipaccess.c:724 received ID_GET for unit ID 4242/0/0

And on the BSC you’ll see roughly the same thing:

OsmoBSC# 
<0016> input/ipa.c:287 0.0.0.0:3002 accept()ed new link from 127.0.0.1:40193
<0004> abis_nm.c:490 BTS1 reported variant: omso-bts-virtual
<0004> abis_nm.c:578 OC=BASEBAND-TRANSCEIVER(04) INST=(00,00,ff): BTS1: ARI reported sw[0/1]: TRX_PHY_VERSION is Unknown
<0016> input/ipa.c:287 0.0.0.0:3003 accept()ed new link from 127.0.0.1:44053
<0003> osmo_bsc_main.c:291 bootstrapping RSL for BTS/TRX (1/0) on ARFCN 0 using MCC-MNC 001-01 LAC=4242 CID=4242 BSIC=42

If you’ve made it this far, congratulations. Our virtual BTS is now connected to our BSC – If it wasn’t virtual we’d be on the air!

So in the next post we’ll setup our SDR hardware as a BTS, then provision it on the BSC, and then our cell will be on the air.

Basic GSM Architecture

GSM with Osmocom Part 3: Introduction to Osmo Software & Virtual BTS

So this series of posts will focus on using Osmocom software to create a GSM network, so let’s get some Osmocom software installed, and talk about how we run and configure each network element / node.

Osmocom Packages

For this tutorial series I’ll be using Ubuntu 18.04 and trying where possible to use packages from Repos instead of compiling from source.

This will get the Osmocom key added to your package manager and the Osmocom sources in apt ready for us to install.

wget https://download.opensuse.org/repositories/network:/osmocom:/latest/Debian_10/Release.key
apt-key add Release.key && rm Release.key
echo "deb https://download.opensuse.org/repositories/network:/osmocom:/latest/xUbuntu_18.04/ ./" > /etc/apt/sources.list.d/osmocom-latest.list
apt-get update

Osmo-BTS-Virtual

To get started we’ll install a virtual BTS. This virtual BTS won’t simulate the Um (air) interface, but it will simulate the Abis interface towards the BSC so we can configure this virtual BTS in our BSC.

Installation is pretty straightforward:

apt-get install osmo-bts-virtual

By default Osmocom software runs as a daemon in systemctl, we’ll disable and stop this behaviour for now so we can better understand it running in the foreground:

systemctl stop osmo-bts-virtual
systemctl disable osmo-bts-virtual

Osmo Config – Text Files

If you have a look in /etc/osmocom/ you’ll see .cfg files that contain our config in text files.

But that’s not the only way (or even the recommended way) that we’ll put together the config for Osmocom software, but we’ll get started by editing the config file manually.

We’ll start by setting a Unit ID of the BTS and setting the IP of the BSC.

cd /etc/osmocom/
vi osmo-bts-virtual.cfg

We’ll edit the oml remote-ip to point to the IP of the server that will run our BSC, if you’re planning on running the BTS and BSC on the same machine you can leave it as localhost (127.0.0.1).

Next up we’ll set the Unit-ID of the BTS, this identifies the BTS inside the BSC,

I’ll set it to unit-id 4242 by changing ipa unit-id 4242 0

Finally we’ll change the logging config to show everything by changing it to:

log stderr
 logging filter all 1
!

So that’s it in terms of config for our virtual BTS through text files, so we’ll save the file and try starting up osmo-bts-virtual.

osmo-bts-virtual -c osmo-bts-virtual.cfg

You should get a result similar to this:

root@gsm-bts:/etc/osmocom# osmo-bts-virtual -c osmo-bts-virtual.cfg
 ((*))
   |
  / \ OsmoBTS
<0010> telnet_interface.c:104 Available via telnet 127.0.0.1 4241
<0012> input/ipaccess.c:901 enabling ipaccess BTS mode, OML connecting to 127.0.0.1:3002
<000d> abis.c:142 Signalling link down
<0001> bts.c:292 Shutting down BTS 0, Reason Abis close
Shutdown timer expired

root@gsm-bts:/etc/osmocom#

So what are we seeing here?

Well Osmo-BTS-Virtual is trying to bring up it’s Abis interface but it’s not getting a connection to the the BSC (We haven’t set one up yet). No connection to a BSC means the BTS won’t go on the air as it doesn’t have any processing for itself, so it eventually times out and shuts down.

In the next post we’ll move from using osmo-bts-virtual to using a SDR to run Osmo-BTS. If you’re using commercial RAN hardware, or just playing along without any RAN, skip straight to the post on Base Station Controllers where we’ll pick up again adding our Virtual BTS to the BSC.

GSM base Station

GSM with Osmocom Part 2: BTS Basics

By far the most visable part of any mobile network (apart from your phone!) is the Base (Transciver) Stations.

Dotted around the countryside, on masts, towers and monopoles, whether you notice them or not, base stations are everywhere.

The Architecture

The RF side of LTE has an eNodeB, which is a smart device. – You connect it to a TCP/IP network, it establishes a connection with your MME(s) and away you go.

A GSM BTS (Base Transceiver Station) isn’t all that clever…

The BTS is a similar to the WiFi access points that talk to a centralised controller for all their thinking.
A BTS gets most of its brains from elsewhere and essentially just handles the TX/RX of baseband data.

That elsewhere is the BSC – Base Station Controller. Each BTS connects to a BSC, and a BSC would typically control a number of BTS.

We’ll explore the BSC and it’s connections in depth, but I’ve put together a basic diagram of how everything fits together below.

Basic GSM Access Architecture

Um Interface

The Um interface is the Air Interface of GSM. It’s what takes the data and sends it out “over the air”.

There’s a lot to know about air interfaces, and I know very little. What I do know is I need to set the Um interface to use a frequency band my mobile phone supports (so I can see and connect to the network).

The Abis Interface

The fact that GSM was first deployed in 1991, explains why the Abis interface used ISDN E1/T1 TDM links to connect the Base Transceiver Stations (BTSs) to the Base Station Controller (BSC).

While now looking back you may ask why TCP/IP wasn’t used for the Abis interface, keep in mind that Windows 95 was the first version to include TCP/IP support, and that gives you an idea of the state of play. ISDN is very reliable and was well known in the telco space at that time.

I no longer have any ISDN hardware, so for me this is all going to be built using packet switched networks working as circuit switched.

Osmocom does have support for E1/T1 interfaces, so if you’ve got BTS hardware that only communicates over TDM links, that’s an option too.

GSMA never wrote a standard for taking Abis over IP, so as such each vendor has implemented it differently.

Osmocom have a flavour of Abis over IP protocol they’ve developed based on traces from a commercial implementation which we’ll be using. You can find the full protocol spec for Osmocom’s Abis over IP interface here.

OML Interface

With all the brains for the BTS residing in the BSC, there’s a need to control the BTS from the BSC. The Operation and Maintenance Link (OML) is a protocol for changing certain parameters of the BTS from the BSC.

A prime example of use of the OML would be the BSC turning the BTS off/on.

We’ll see a tiny bit of OML usage in the next post, just for turning the BTS off and on.

So let’s put this into practice and setup a virtual BTS with Osmocom.

Osmocom Logo

GSM with Osmocom Part 1: Intro

Meta

Like most people at the moment because of the lockdown I’ve got a bit more time at home than normal.

Because of this I thought I’d finally dive into GSM/UMTS and all that circuit switched tech you skip out on when getting started with LTE.

Please excuse the loving tone I use when describing some older tech, it’s a result of being a telephony tragic who gets all reminiscent thinking about the first phones they interacted with & wondered about how it all worked…

So why learn GSM?

My best friend is a translator of technical documents. In University, what’s the first language they study? Latin. Because it’s the root of so many languages.

While a lot of carriers have already switched off their GSM networks (There are no public GSM networks in Australia), the core of GSM is essentially shared with that of UMTS / 3G, which is still going to be around for the foreseeable future.

Circuit Switched Fallback (CSFB) is still common today for voice calls for a great many LTE handsets without VoLTE support. GSM powers GSM-R, the rail specific standard of GSM used across Europe. The uplink power of GSM can be up to 8 Watts (while in LTE it’s 20 dBm – 0.1W) which means it’s effective service area could be larger than 3G and 4G air interfaces.

GSM isn’t as dead as it might seem, so let’s have a Weekend at Bernie’s!

Disclaimer: Please let me know if I’ve got anything wrong! These posts will focus on the Omsocom network elements which do handle a few things the “non standard” way.

Platforms

I wrote last week about using YateBTS to get a functional GSM / GPRS network online,

It’s great that it works from a Um interface perspective; your UE / terminal can see and connect to the network. But it’s sort of an all-in-one solution; there’s no Mobile Switching Center, Base Station Controller, Sigtran, HLR or Media Gateway; Yate ties all this up into a single easy to use package.

This gets you on the air in no time, but unfortunately you don’t get exposed to how GSM / UMTS works in the real world – real networks don’t look like YateBTS NITPC.

Enter Osmocom

Osmocom (and the Sysmocom team driving many of the projects) have done a phenomenal job of building each of the network elements I just talked about pretty much “by the book”, meaning most should interop with commercial equipment and comply to the standards.

Over the next few weeks I’ll cover setting up each of the network elements, talking about what they do and how they work, and use them to create a functional GSM network (2G / Circuit Switched) using the software from Osmocom.

Once we’ve got our network functional we’ll be adding SMS, Data (GPRS / EDGE), USSD codes and even inter-RAT handover with LTE.

For this series of posts I’ll be using a mix of hardware. At the start I’ll be using a Software Defined Radio (LimeSDR) to do the RF side of the network (BTS). Osmocom has support for a lot of common SDR hardware, so hopefully for anyone wanting to follow along at home you’ll have access to a LimeSDR or USRP.

Osmocom supports many commercial BTS vendors products, as well as Sysmocom’s hardware, hardware from Range Networks. Osmocom also supports the NanoBTS range from ip.access – which are available second hand quite cheaply, which I’ll be adding to our network as well.

So buy some cheap programmable SIM cards, grab your SDR hardware or order cheap second hand GSM BTS online, and let’s get building a GSM network!

Topics Covered

I’ll update the links in here as I publish these posts, I may forget so if there’s something missing search the site or drop me a line.

My Osmocom Config Files on GitHub

Radio Access Network

BTS Basics

BTS In Practice with LimeSDR & osmo-bts-trx

The Base Station Controller (BSC)

Integrating our LimeSDR BTS with OsmoBSC

Integrating an ipaccess NanoBTS with OsmoBSC

Channel Types

Switching & Signaling

Home Location Register (HLR) (with EIR & AuC)

The SS7 Signaling Transfer Point

The Mobile Switching Center

Basic calls & SMS

Call Routing

SMS Routing

Distributed GSM

Cell Broadcast Center

External USSD Interfaces

Data Services

GPRS & Packet Data Basics

Serving Gateway Support Node

Gateway GPRS Support Node

Call Flows & Handovers

Basic GSM Calls

Basic GSM SMS

Basic GSM Packet Bearers

Inter BSC

Inter MSC

Inter-RAT

Inter RAT between LTE and GSM

GSM CSFB with OmsmoMSC and Open5GS LTE

SGs Interface for SMS over LTE

Magic SIM Card Art

16 in 1 Magic SIM Card Revisited

I found a “16-in-1 Super SIM X-SIM” in my SIM card drawer, I think I ordered these when I was first playing with GSM and never used it.

I was kind of curious about how these actually worked, so after some online sleuthing I found a very suspicious looking rar file, which I ended up running in a VM and mapping the Card Reader to the VM.

What a treat I was in for in terms of UI.

The concept is quite simple, you program a series of IMSI and K key values onto the SIM card, and then using a SIM Toolkit application, you’re able to select which IMSI / K key combination you want to use.

A neat trick, I’d love a LTE version of this for changing values on the fly, but it’d be a pretty niche item considering no operator is going to give our their K and OPc keys,

But come to think of it, no GSM operator would give out K keys, so how do you get the K key from your commercial operator?

I noticed the grayed out “Crack” icon on the menu.

After rifling through my SIM drawer I found a few really old 2G SIMs, stuck one in, reconnected and clicked “Crack” and then start.

I left it running in the background after the manual suggested it could take up to 24 hours to run through all the codes.

To my surprise after 2 minutes the software was requesting I save the exported data, which I did.

Then I put the 16 in 1 back in, selected Magic and then imported the cracked SIM data (IMSI, ICCID, Ki & SMSp).

By the looks of it the software is just running a brute force attack on the SIM card, and the keyspace is only so large meaning it can be reversed in.

I did a bit of research to find out if this is exploiting any clever vulnerabilities in UCCID cards, but after running some USB Pcap traces it looks like it’s just plain old brute force, which could be easily defended against by putting a pause between auth attempts on the SIM.

I’ve no idea if that’s the actual K value I extracted from the SIM – The operator that issued the SIM doesn’t even exist anymore, but I’ll add the details to the HLR of my Osmocom GSM lab and see if it matches up.

Out of curiosity I also connected some of my development USIM/ISIM/SIM cards that I can program, the software is amazing in it’s response:

Configuring YateBTS for Software Defined GSM/GPRS

I did a post yesterday on setting up YateBTS, I thought I’d cover the basic setup I had to do to get everything humming;

Subscribers

In order to actually accept subscribers on the network you’ll need to set a Regex pattern to match the prefix of the IMSI of the subscribers you want to connect to the network,

In my case I’m using programmable SIMs with MCC / MNC 00101 so I’ve put the regex pattern matching starting with 00101.

BTS Configuration

Next up you need to set the operating frequency (radio band), MNC and MCC of the network. I’m using GSM850,

Next up we’ll need to set the device we’re going to use for the TX/RX, I’m using a BladeRF Software Defined Radio, so I’ve selected that from the path.

Optional Steps

I’ve connected Yate to a SIP trunk so I can make and receive calls,

I’ve also put a tap on the GSM signaling, so I can see what’s going on, to access it just spin up Wireshark and filter for GSMMAP

Compiling YateBTS NIPC for Software Defined GSM / GPRS

A lot of the Yate tutorials are a few years old, so I thought I’d put together the steps I used on Ubuntu 18.04:

Installing Yate

apt-get install subversion autoconf build-essential
cd /usr/src svn checkout http://voip.null.ro/svn/yate/trunk yate 
cd yate
./autogen.sh
./configure
make
make install-noapi

Installing YateBTS

cd /usr/src 
svn checkout http://voip.null.ro/svn/yatebts/trunk yatebts
cd yatebts/ 
./autogen.sh 
./configure
make install

Defining location of libyate

On Ubuntu I found I had to add the library location in ldconf:

echo "include /usr/local/lib/" > /etc/ld.so.conf
ldconfig

Installing Web Interface for NIB / NIPC

apt-get install apache2 php
cd /usr/src/yatebts/nipc
make install
cd /var/www/html
ln -s /usr/local/share/yate/nipc_web nipc
chmod a+rw /usr/local/etc/yate/