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
network. 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 more. 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 standards. 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
UMTS or GSM. 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
GSM. 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. “URGH NOT LTE! 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 phone. 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 _enabled_. 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 Number. 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 Channel. 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
channel allows to increase the network capacity, since
two 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
area. 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. So. Time division multiple access. Let’s deconstruct this name. We need multiple phones to access the base
station. To respond. And we do that by dividing time. Every phone gets a timeslot. Vadim suggested I explain that with a cross
road. 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 traffic. 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
TELL ME WHEN I CAN SEND? And the basestation sees that timeslot 3 is
free, and tells you to only send in timeslot 3. 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
Updates. 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
Wireshark. The
phone would forward both Uplink and Downlink packets,
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.

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