GSM Mobile Network Intro – Nokia Network Monitor

GSM Mobile Network Intro – Nokia Network Monitor

So in the first video I introduced SIM cards
and how we can use SIMtrace by osmocom to
trace the communication between the phone
and the SIM card.
So let’s slowly make our way up into the
Before we look at the first phone, I quickly
want to get everybody on track when it comes
to the different telecommunications technology.
So you definitely have heard the terms 2G,
3G, UMTS, GSM, LTE, GPRS, 4G, EDGE and maybe
The Gs stand for generations.
2nd, 3rd and 4th generation, and basically
they are the PR names for GSM, UMTS and LTE.
GPRS and EDGE and loads of other terms are
extensions that added something to those bigger
Phone connections are generally direct connections.
That’s very different from the internet
where we use single packets.
At least historically phone networks are direct
connections to enable realtime communication.
LTE is actually now also packet based.
Packet based communication has gotten so fast
nowadays, that you can transmit basically
realtime communication through packets.
You know this from skype.
So in GSM, things like GPRS and EDGE add packet
based communication to it to the circuit switched
style GSM network.
While now in LTE we need an extension to have
calls, Voice over LTE enables phone calls
over LTE.
The packet based internet has become so important.
And so actually this extension Voice over
LTE is not very widely deployed yet so typically
for calls your modern phone will still use
It’s all a huge mess.
GSM was first deployed in Finland in December
1991 and as of 2014, it has become the global
standard for mobile communications – with
over 90% market share.
At some point we then got UMTS and nowadays
we have LTE.
And as you probably know from the corner on
your phone, you still see all of those things.
It’s not like LTE replaced GSM today.
So you would maybe think we ignore GSM nowadays,
it’s old and we focus on LTE.
But the truth is, GSM is still hugely important.
A lot of things you might not know about use
So for example a lot of point of sale terminals
where you can pay by card have a SIM card
inside, and that’s how it communicates with
the credit card banking network.
And so understanding GSM is still super beneficial,
however it’s difficult to do that.
I mean nobody has to setup their own phone
network at home, right?
But that doesn’t stop people from wanting
to understand it.
And so the project OSMOCOM, which stands for
Open source mobile communications, is trying
to do just that.
And here is a small snippet from Harald Welt,
LaF0rge on twitter, who is the founder of
osmocom describing the state of the development
in 2015 at the 32c3 conference.
By the way they also run a conference GSM
network during the congress.
Let’s start with a little bit of the history
of open source in mobile communication protocols.
You have to remember that we started about
16 years after the proprietary implementations.
The GSM network that we are running here at
the event, or that we started to run 7 years
ago, started 16 years after GSM networks were
run first in the public in europe.
At public operators.
So we are really really late.
And if you like to compare the status of open
source mobiel communications with open source
operating systems, then I would say we are
about where linux was 1994/95.
So i would say capable but not taken seriously.
Sort of the general status.
So I would like to at that maybe the mobile
operators that built the proprietary technologies
might not take you seriously, but I think
we as the wider IT community really appreciates
the work you and all the other contributors
have done.
As you know, I make these videos because Vadim
Yanitskiy who is an osmocom contributor reached
out to me.
Maybe that’s not what you expected.
GSM it’s like old crap.
What do I want with that?”.
But that’s the reality.
These things are so complex and require a
huge amount of work.
So I hope you can really appreciate the work
that has been done here and I hope you see
the incredible value that this still has.
Long story short.
What I wanted to say is.
We look at GSM in these videos.
Last video I promised you to tell you what
is so special about the old motorola and nokia
And maybe you remember the TROOPERs badge
that I have shown in this video.
I also had a interview with one of the creators
of it.
The reason for that was that the Nokia phone
has some really nice capabilities.
So let’s Vadim introduce us to it.
Some old phones from Nokia, like this one,
do have a well researched debug interface
Well, better to say *not disabled* after manufacturing.
For example, this interface can be used to
enable well known Network Monitor.
BTW: this Nokia 1280 was released later than
3310, and it has no Network Monitor
Wikipedia even has an entry for that:
Nokia network monitor or Monitor Mode was
a hidden mode on most Nokia cell phones used
to measure network parameters.
The mode can only be activated over a special
FBus, or MBUS cable.
If you checkout the troopers video you will
hear a bit about the fbus.
But yeah, so here the network monitor is running.
So what can we see here?
Basically, there are many test displays where
one can observe some information or even modify
some parameters.
For example, here we can see which channel
this phone is using at the moment.
On the top right corner we can see the ARFCN,
which stands for Absolute Radio Frequency
Let’s look up the GSM ARFCN number.
So we have 2G and the number was once 681
and once 683, not that important.
And the frequency is 1743 Mhz up and 1839
Mhz downlink.
Just a quick refresher.
GSM is just like any other radio wave.
It has a certain wavelength.
It’s all on the electromagnetic spectrum.
Visible light is just a small part of it.
And so 1700Mhz is I think a bit under 20cm
wavelength, so it would be somewhere here
on the spectrum.
From a nokia network monitor manual we can
also see that if you are transmitting, then
you would have here the transmit strength.
But phones are not always transmitting.
Most of the time they are just listening.
On the top middle part *of this screen* one
can see how strong is the signal from current
base station.
At the moment this phone is listening to a
broadcast channel called CCCH, or Common Control
And this is exactly where the phone expects
to ‘see’ Paging Requests.
Paring Requests are used by the betwork in
order to notify subscribers about incoming
calls or SMS messages.
As soon as Paging Request is received, the
phone needs to establish a dedicated channel
with the network.
So we are currently in CCCH.
That’s a control channel.
If we were in a call, we would switch to a
traffic channel.
There are two types of Traffic channels:
– TCH Full Rate
… and …
– TCH Half Rate
The Full Rate channel provides higher bit
rate and
better quality of speech, while the Half Rate
allows to increase the network capacity, since
subscribers can use a single time-slot at
the same time.
So as vadim said in the control channel we
are waiting for for example so called paging
requests from the network.
If there is for example a call or sms incoming,
the base station would send out a paging request,
asking if this particular phone is in this
It’s a broadcast.
Why that particular basestation thinks the
phone should be in this area we will talk
about some other time.
I mean it’s logical that not all basestations
in the WHOLE WORLD can send out a paging request
asking where the phone is, right?
So in this case a basestation just yells out,
“hey is this phone here nearby?”.
And our phones are constantly listening for
these paging request, and when our phone realizes,
“OH THAT’S ME” it will respond with
a channel request.
And that is done through RACH, the random
access channel.
So this is a channel any phone is allowed
to use and happens on the same frequency that
we looked up with the ARFCN number.
And the basestation is listening for those,
and so if two phones at the same time would
ask for a dedicated channel at the same time,
they would collide, like two people talking
over each other, and they would not get their
own channel.
but let’s say it succeeded, the basestation
received your channel request, now the basestation
looks at all the currently used channels and
will assign you a time slot..
So this is where TDMA comes into play.
Time-division multiple access.
You somehow need to divide up the limited
radio space you have.
You need to make sure multiple phones can
talk to a basestation and the other way around.
Time division multiple access.
Let’s deconstruct this name.
We need multiple phones to access the base
To respond.
And we do that by dividing time.
Every phone gets a timeslot.
Vadim suggested I explain that with a cross
Imagine you have a crossroad with traffic
lights and the lights turn red and green in
a nice pattern.
If all directions, would be allowed to drive
at the same time, that would be a problem.
So the traffic light always assigns a short
time slot where one phone is allowed to drive
Pun intended.
So a traffic light is a good example for a
time divison multiple access system.
If we keep using this traffic example we can
also use it to refer back to the ARFCN numbers
and the frequencies.
Earlier we learned that there are two different
frequencies, up and down.
So that’s like a road with two lanes.
Each frequency can be used to transmit data.
It’s all electromagnetic waves.
And we can build antennas that can be tuned
to only recognize and send a particular frequency.
It’s basically the same way how our eyes
are tuned to only see a band, so a small slice
of a certain visible light color frequency.
Anyway because we have two frequencies we
can send and receive at the same time.
If there were only one frequency only the
basestation or only the phone could send something
and then you would have to figure out how
to organize that.
The same way like a narrow street with two
cars on it.
But luckily we have two frequency, one up
and down.
So we only need to organize multiple phones,
whcih we solve with time division like on
the cross road.
And instead of a traffic light telling us
when we can send, we use the random access
channel, yell at the basestation, CAN YOU
And the basestation sees that timeslot 3 is
free, and tells you to only send in timeslot
By the way, here with RACH messages you also
have the first security issue with GSM.
Here is a denial of service.
Of course you could just jam the radio frequencies
so the phone or basestation couldn’t communicate
anymore, but you can also flood the basestation
with RACH messages.
You request a channel, then the basestation
will allocate a timeslot for you and there
are limited timeslots.
You can’t have infinitely many phones sending
in the limited radio space.
Of course the basestation will free up channels
when they are not used anymore, but if you
keep requesting new ones, other phones will
probably not be able to get a connection going.
Let’s reboot this phone in order to see what
would happen.
As you can see, it remembers the last ARFCN
is was tuned to.
The phone always looks for a channel with
the best signal quality.
So, now it’s on SDCCH (Stand alone Dedicated
Control Channel).
This channel is usually used to perform Location
Location Update is something like “Hey, I
am here, at this particular part of the planet
And this answers the question how the network
knows which basestation to use for the braodcast
when you get a call.
When you turn your phone on, or move around,
your phone will send a location update request
to a nearby basestation, which will update
that information on a central server of the
operator, and when somebody wants to call
you, the operator can look up your last location,
or the last basetstaion, and then use that
particular basestation, or basestations in
the area to send a paging request out.
And hopefully your device responds.
The network monitor has a few different screens
with information.
And one interesting one that is kinda related
to your location are the neighbor basestations.
The first one here is the current serving
cell, the one you communicate with and the
other two are neighbors.
And they obviously use a different ARFCN number,
because they need to communicate on a different
frequency pair, otherwise the two basestations
would get into the way.
Makes sense, right?
This is also super important to know for the
phone, because imagine when you are in a call
and walk.
Or worse drive fast in a car.
Your phone CONSTANTLY has to switch basestations,
always trying to talk to the one, with the
highest signal strength, which is usually
the closest one.
Imagine in the middle of a call two basestations
and your phone have to perform a crazy handover
so you can keep talking.
It’s crazy.
It amazes me that it all works so well.
The network monitor is already pretty cool
and we can learn quite some stuff about GSM
with it.
But it can actually do a bit more when hooked
via the fbus to a computer.
It’s also possible to forward L2 messages
via F-bus interface
and then inspect them, for example, using
phone would forward both Uplink and Downlink
as well as the SIM card related messages.
So basically this debug interface allows you
to see the SIM messages where we last time
needed the external tool simtrace.
And it can also show us GSM messages.
But anyway, this solution is not as powerful
as Calypso
based phones, like this one.
So next time, let’s see what’s up with
the callypse chip in motorola.


  1. So would you be able to have a list of people nearby kust by listening to the diffrent channels?
    So far you havent mentioned encryption at all. So it would be possible to just listen to any phonecall in the area

  2. Very cool, brings back memories from 1999, when I had a nokia 5110 with network monitor and a 100+ pages manual for it. Back in the day, I almost knew it by heart. Cheers for the video!

  3. Hi, Great video but I would like to add that the MSC keeps track of the VLR and it's that which is updated with the Location Update contain the Location Area Codewhich is a group of BTS cells on one or a few BSCs. The VLR may be located logically within the MSC but it does not have to be. This is because some vendors had limitations on how many IMSIs could fit within each VLR database that normaly covered a geographical area.

  4. I haven't really cared about phones and this is really interesting. since the phones talk directly to each other, does this mean you can enumerate a general location of their phone with the replies from the base stations?

  5. Up here in Norway, VoLTE is deployed and available wherever we have LTE. And we have LTE in all Norwegian cities (i.e. more than ~10,000 inhabitants)

  6. where does authentication take place? It seems unlikely to me that you could jam a tower spamming RACH packets. The company should have a way of banning reincident users or something, right? If you need to authenticate before sending anything then problem solved.

  7. 2:35 Wait, that means that when I purchase by card on a POS terminal, there are actually 3 Tiny Computers in this device? That info from the first video still blows my mind.

  8. hey guys can't we make some kind of Whatsapp or telegram group where we can share our resources more and more. Because sometimes it's more confusing topics which are not easy to understand

  9. I discovered the fun of this back in the days when i got a cable of ebay so i could upload ringtones and images to the phone. The network monitor could also be used to lock in on a certain tower or cell. I remember I used to lock on strong cells close to school and then see how long i could keep a connection on it on the bus ride home…

    Good days!

  10. old NOKIA F-bus to send an SMS message and rotary-phone to GSM and Like landline phone in GSM

  11. It was not "secret" per se, engineering mode was used, for example, when testing / accepting a new BTS, a test was timeslot per timeslot hop observation when in tcch, another one was validation of bcch when idle, and so on. Fun times!

  12. Depending on the firmware, you can maybe enable engineering mode on Nokia with *3001#12345#
    If that doesn't work, in the old days you had to ask Nokia to flash the program in your field engineers phones…

  13. What is happening between your phone and the BTS when you call someone and their phone is not available, whether they are using it (having another call) or it is switched off? And why you phone will be available for other calls even if you are having another call at that moment?

  14. I have one of these nokia phones but I need a charger. Any one have one for sale or has seen one for sale on a website or at a flea market?

  15. Hey, this is super cool stuff, even if just GSM. It's like a whole "internet of things" that nobody stops to think about.

  16. Very useful information. Thanks a lot for making this attempt. For letting us to know all these uncommon matters.😊👍

  17. I remember YEARS ago, adding a Vibrate motor to my 3210, and enabling the 'hidden' menu's, The network monitor was fun to watch, but never knew what they all meant, until now xD

    I still have that 3210 – might hook it up and see what happens 😉

  18. Hey Liveoverflow , it would be really helpful if you shared the sample files(.cfile) of the network capture with us. Loving the series btw ! Keep it coming

  19. Thank you for these videos! They are fantastic! You should familiarize yourself with SS7 and the possible hacks on that. It would be great to see your video on it.

  20. So If we can listen to paging request broadcast would be very useful. Think non governmental entity trying to narrow down if a phone is in some area or not. ?

  21. Back in the early 2000s I used to have a modded Nokia 3310 with a custom rom flashed. It had many new functions like DCT3 and DCT4 (and other phones) code calculator that I used a lot to make some money 🙂

  22. Awesome sir.I have a doubt .I have read in some blogs that sim cloning can be done easily in GSM.Is it possible sir??If yes is possible with LTE ??And how sim cloning works??How will it effect people? Because if a sim is cloned anything can be done.

    If any people knows plz answer here??

  23. You're saying that 4G and LTE are the same, yet when I Google "LTE" I'm seeing a ton of videos and blogs about "The difference between 4G and LTE". Can you clarify that 4G and LTE are one and the same thing, or are they different terms?

  24. Ohh, that was the network monitor means, i had a 3315 phone that was reprogrammed its firmware with a custom one. I always enabled it to show more geeky on my phone. Uhh nostalgia… Those days..

  25. If you have some time… Could you answer one question: Can I buy or build a small network antenna for home or office use and have my own local network PABX local phone network?

  26. WOW, how interesting, I didnt know how crazy is all tis stuff, i mean, like you said, if you are walking and in a call the phone is continualy changing basestations all the time, and you don't even notice it, is like… WOW, bc works sooo well, and this is on GSM, i mean, from 20+ years ago, I can't believe it. Keep the good work, I love your videos, everything is so well explained.

  27. So can you, in theory, sniff Paging Messages so see what other phones are in the area? Like a cell network airodump?

  28. Hey, by using time division multiple access we will be sending signal after particular time. So wouldn't it be a packet

  29. This tech is amazing. The FBI just caught the guy sending bombs in Florida easily because the terrorist didn't think that having a cellphone pretty much broadcast their location constantly to whoever is listening.

  30. Hallo LiveOverflow. Die Art und Weise wie du komplizierte Zusammenhänge einfach darstellst ist überragend. Deine beiden Osmocom Videos sind hochinteressant. Hab mir jetzt ein C123 geordert und bin gespannt auf ein weiteres Video zum Themengebiet. Keep on doing this good stuff i really preshade that. Thank u very much!

  31. I like that you drew ASCII emoticons, which exist solely because one was not able to draw anything 😀 also, typing the ASCII emoticon would have been too easy anyways

  32. Great video. You should write an ebook for complete beginners and update it as more information becomes available. If you start a crowd funding campaign you could give the donators free editions of the ebook.

  33. link to the troopers 17 badge ft badgewizard video -> cellmapper ->

  34. As far as I know, the Nokia game "Cavern Crawl" hasn't been dumped or emulated anywhere. Any chance you'd take on a challenge to dump it? 🙂

  35. TDMA was replaced by CDMA some time ago! GSM and CDMA are being phased out and being replaced by LTE. In the US, a few years ago – AT&T and T-Mobile were the original GSM carriers, where Verizon and Sprint were CDMA only and did not use SIM cards (giving users less choice, flexibility). Now that everyone is using LTE, every phone uses a SIM card to access the network, as well as the phone's radio needs to use that carrier's band(s).

  36. They use basic radio trunking alike used by emergency services. Catch here though you didn't note is usually base stations have more than 1 frequency pair obviously the more pairs the more time slots and thus capacity available.

    Also here in Australia 2G is finished, gone, ripped up and binned with all eftpos etc sent to 3G and 4G for the newer ones.

    I have to admit that the cell/tower/base handover is bloody magic for the phone network. A 2way radio is a lot easier you just force the change between a and b talking (silent time) to do the change but a phone requires seamless changeover and does so without most noticing (a little klunky at times out here in the country with larger gaps bbetween towers and thus different lag times).

  37. I still have one of these phones and the cable they sold back then (bought directly from harald welke). Back then I compiled the osmocom software and was looking at the phone screen. Felt like I was doing some important hacking but had no idea what it all meant. Thanks for the video.

  38. BTW about basestations. That's the one of the earliest reasons to turn off your phones while on plane. Imagine hundred of phones on plane constantly changing basestations at the speed higher than 300 kmh.
    At that time it was creating a huge load on basestations from time to time.
    Now load is dispersed thanks to different generations and improved capacity of basestations.
    Correct me if I'm wrong.

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