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\begin{document}

\title{Anatomy of contemporary GSM cellphone hardware}
\author{Harald Welte $<$laforge@gnumonks.org$>$}
\maketitle

\begin{abstract}
%% add citation for the nubmer of gsm users and phones / carriers.
Billions of cell phones are being used every day by an almost equally large
number of users.  The majority of those phones are built according to
the GSM protocol specifications and interoperate with GSM networks of hundreds
of carriers.

Despite being an openly published international standard, the architecture of
GSM networks and its associated protocols are only known to a relatively
small group of R\&D engineers.

Even less public information exists about the hardware architecture of the
actual mobile phones themselves, at least as far as it relates to that part
of the phone implementing the GSM protocols and facilitating access to the
public GSM networks.

This paper is an attempt to serve as an introductory text into the hardware
architecture of contemporary GSM mobile phone hardware anatomy.  It is intended
to widen the technical background on mobile phones within the IT community.
\end{abstract}

%\tableofcontents

\section{Foreword}

This document is the result of my personal research on mobile phone
hardware and system-level software throughout the last six years.

Despite my past work for Openmoko Inc., I have never been professionally
involved in any aspect of the actual GSM related hardware of any phone.
Nevertheless I have the feeling that in the wider information technology
industry, I am part of a very, very small group of people who actually
understand mobile phones down to the lowest layer.

I hope it is useful for any systems level engineer with an interest in
understanding more about how mobile phone hardware actually works.

There are no guarantees for accuracy or correctness of any part of the
document.  I happily receive your feedback and corrections.

\section{Is your phone smart or does it have features?}

Initially, for the first couple of years, GSM cell phones were actual phones
with very little additional functionality.  They provided everything that was
required for voice calls, as well as SIM phone book editing features.  The only
additional non-features were simple improvements like the ability to use them
as an alarm clock.

In the mid-1990s, a certain new type of devices became popular: The PDA
(personal digital assistant).  They pioneered handheld computing by introducing
touch screen user interfaces and a wide range of application programs, ranging
from calendar/scheduling applications, dictionaries, exchange rate and tip
calculators, scientific calculators, accounting / finance software, etc.

While in mobile phones the actual cellphone aspect was becoming more and more
commoditized, at some point the PDA features and functionalities were added to
phones, coining the term {\em smartphone}.  At that point there was a
need to differentiate from those phones that were not-so-smart.  Those
phones were then called {\em feature phones}.

There has never been an industry-wide accepted definition of those terms,
and especially in the late 2000s, feature phones started to inherit a lot
of the functionality that was formerly only present in smartphones.

This document will define the terms (only for the purpose of this document)
along a very clear border in hardware architecture, as will be described in the
following sections:

\subsection{Feature Phone}

A feature phone is a phone that runs the GSM protocol stack (the software
implementing the GSM protocol) as well as the user interface and all
applications on a single processor.  For historic reasons, this
processor is known as the so-called {\em baseband processor} (BP).

The baseband processor often exposes a serial port (or today USB) over which
the phone can be used as a terminal adapter, similar to old wireline modems.
The industry standard protocol for this interface is an AT command set -
extended and modified from how computers interfaced old wireline modems.
The AT-command interface can be connected to a computer.  The computer can then
use the phone to establish data calls, send/receive short messages via SMS,
and generally remote-control the phone.

\subsection{Smartphone}

A smartphone is a phone that has a dedicated processor for the GSM protocol
stack, and another (potentially multi-core) general purpose processor for
the user interface and applications.  This processor is known as the
{\em application processor} (AP).

The first hardware generations of smartphones did nothing else but to put the
feature phone and a PDA into one case.  The keypad and display of the baseband
processor is removed.  What remains of the feature phone is a {\em GSM modem},
controlled by AT commands sent from the AP.

Each processor has its own memory (RAM and Flash), peripherals, clocking, etc.
So this setup is not to be confused with the symmetric multi-processor setups
like those seen in the personal computer industry.

Later generations of smartphones have exchanged the AT command interface by
various proprietary protocols.  Also, the serial line was replaced by a
higher-bandwidth hardware connection such as USB, SPI or a shared memory
interface.

Due to market pressure for ever smaller phones with ever more functions,
the industry has produced highly integrated products, uniting the AP and BP
inside one physical package.  Further pressure on reducing cost and PCB
footprint has led to products where there is no need to have independent
RAM and Flash chips for AP and BP.  Rather, a single RAM and Flash chip
is divided by assigning portions of the RAM and Flash to each of the two
processors.

However, the fundamental separation between the AP and BP, each with their own
memory address space and software, remains present in all smartphones until
today.

\section{GSM modem architecture}

Every GSM phone, feature phone and smartphone alike, has a GSM modem interfacing
with the GSM network.

This GSM modem consists of several parts:
\begin{itemize}
\item RF Frontend, responsible for receiving and transmitting on GSM frequencies
\item Analog Baseband, responsible for modulation and demodulation
\item Digital Baseband, responsible for digital signal processing and the GSM protocol stack
\end{itemize}

\begin{figure}[h]
\centering
\includegraphics[width=160mm]{calypso-block.pdf}
\caption{Block schematics of a TI Calypso/Iota/Rita GSM modem}
\label{reg-form}
\end{figure}

\subsection{The RF Frontend}

The RF Frontend is tasked with the physical receive and transmit
interface with the GSM air interface (sometimes called Um interface).

It minimally consists of an antenna switch, GSM band filters, low-noise
amplifier (LNA) for the receive path, power amplifier for the transmit
path, a local oscillator (LO) and a mixer.

By mixing the LO frequency with the received RF signal, it generates an
analog baseband signal that is passed to the Analog Baseband (ABB) part
of the modem.  By mixing the output of the ABB with the LO frequency, it
generates a RF signal that is to be transmitted in the GSM frequency
band.

As the receive and transmit framing has an offset of 3 TDMA frames,
there is no need for a frequency duplexer.  Instead, an antenna switch
is used.  The switch typically is implemented using a MEMS or diode
switch.  For a quad-band phone, typically a single-pole 6-throw (SP6T)
switch is used: 4 for the four Rx bands (one for each band), and 2 for
Tx (where 850+900 and 1800+1900 each share one PA output, respectively).

\subsubsection{RF Frontend receive path}

The antenna picks up the GSM radio signal as it is sent from a GSM cell tower
(properly called a Base Transceiver Station, or abbreviated as BTS).  The
antenna signal first hits the antenna switch, which connects the antenna with
the Rx path for the GSM band of the to-be-received radio frequency.  It is then
filtered by a bandpass to block out-of-band signals before entering a low-noise
amplifier for increasing signal amplitude.

After passing the LNA, the RF signal is mixed with a frequency generated
by the LO.  Depending on the LO signal, either an intermediate frequency
(IF) or a direct baseband signal is produced.  In modern GSM modems,
zero-IF designs with immediate down-conversion to analog baseband
signals are most common.

The baseband signal is then filtered to remove unwanted images and sent
as analog I/Q signals (representing amplitude and phase) to the ABB.

\subsubsection{RF Frontend transmit path}

The ABB generates analog I/Q signals, which are filtered and passed
into the mixer, where they are mixed with the LO frequency and thus
up-converted to the GSM RF band.  From there, they are sent to the
transmit amplifier (RF PA) for amplification.  After amplification, they
traverse the antenna switch and are transmitted by the antenna.

\subsubsection{Local Oscillator}

The LO of a GSM modem has to be synchronized very closely to that of the
cell (BTS).  To achieve the required precision, a Voltage-Controlled,
Temperature-Compensated Crystal Oscillator (VCTCXO) is used.

Common frequencies for this VCTCXO are 26MHz or 13MHz, as the GSM bit
clock (270,833 Hz) is an integral division (/96 or /48, respectively) of
those frequencies.  The tuning range of the VCTCXO is several kHz to
compensate for temperature drift.

\subsection{The Analog Baseband (ABB)}

The ABB part of a GSM modem is responsible to interface between the
digital domain and the analog domain of the GSM modem.

\subsubsection{ABB Receive path}

The analog baseband I/Q signals are potentially filtered again and
digitized by an Analog-Digital converter (ADC).  The sample clocks used
are typically integral multiples of the GSM bit-clock.  The sample clock
itself is derived by dividing the VCTCXO of the RF frontend.

The digital I/Q samples are passed to the Digital Signal Processor
(DSP) in the Digital Baseband (DBB).  To reduce the number of traces to
be routed on the PCB, the samples are typically sent over some kind of
synchronous serial link.

\subsubsection{ABB Transmit path}

There are multiple architectures found in the ABB transmit path.

The obvious architecture is to do the inverse of the receive operation:
Transmit digital I/Q samples from the DSP to the ABB and convert
them into an analog signal that is then to be sent to the mixer of the
RF Frontend.

However, sending a GSM signal with its GMSK modulation is much simpler
than receiving.  So in order to reduce computational complexity (and
thus cost as well as power consumption) inside the DSP, the modulation
of the bits is often performed in hardware inside the ABB.

In this design, the unmodulated GSM burst bits are sent from the DBB to
a burst buffer inside the ABB.  From there, based on ROM tables and a
Digital-to-Analog converter (DAC), an analog GMSK modulated signal is
generated.

\subsection{The Digital Baseband (DBB)}

The digital baseband implements the actual GSM protocols from Layer1 up
to Layer3 as well as higher layers such as a user interface in the case
of the feature phone.  In a smartphone, the DBB only implements a
machine interface to be used by the AP.

A typical DBB design includes a Digital Signal Processor (DSP) for the
lower half of Layer1, and a general-purpose processor (MCU) for the
upper part of Layer1, as well as anything above.

\subsubsection{Digital Signal Processor}

The choice of DSP architecture largely depends on the DBB chipset
vendor. Often they already have a line of DSP cores in-house and will of
course want to reuse that in their DBB chip designs.  Every major DSP
architecture can be found (TI, Analog Devices, ...).

The DSP performs the primary tasks such as Viterbi equalization,
demodulation, decoding, forward error correction, error detection, burst
(de)interleaving.

Of course, if actual speech data is to be communicated over the GSM
network, the DSP will also have the auxiliary task to perform the
computation of the lossy speech codec used to compress the speech.

Communication between the DSP and MCU happens most commonly by a shared
memory interface.  The shared memory contains both actual data that is
to be processed, as well as control information and parameters
describing what to be done with the respective data.

For the receive side, the MCU will instruct the DSP to perform decoding
for a particular GSM burst type, after which the DSP will receive I/Q
samples from the ABB, perform detection/demodulation/decoding and
report the result of the operation (including any decoded data) back to
the MCU.

For the transmit path, the MCU will present the to-be-transmitted data
and auxiliary information to the DSP, which then takes care of encoding
and sends the corresponding burst bits to the ABB (remember, most ABB
devices take care of the modulation to reduce DSP load).

The detailed programming information (API) of the DSP shared memory
interface is a closely-guarded secret of the baseband chip maker and is
not commonly disclosed even to their customers (the actual phone making
companies).

In doing so, the baseband chip makers create a close dependency between
the GSM Layer1 software (running on the MCU) driving/implementing this
API and the actual baseband chip.  Whoever buys their chip will also
have to license their GSM protocol stack software.

It is thus almost impossible for an independent software vendor to get
access to the DSP API documentation, which the author of this paper
finds extremely anti-competitive.

\subsubsection{DSP Peripherals}

The specifications of the GSM proprietary On-air encryption A5/1 and
A5/2 are only made available to GSM baseband chip makers who declare
their confidentiality.  Implementing the algorithm in software is
apparently considered as breach of that confidentiality.  Thus, the
encryption algorithms are only implemented in hardware - despite them
being reverse-engineered and publicly disclosed by cryptographers as
early as 1996. %% cite

Thus, the DSP in a DBB commonly has a integrated peripheral implementing the A5
encryption. 

Further integrated DSP peripherals may include a viterbi hardware accelerator,
a DMA capable serial interface to the ABB and others.

\subsection{Baseband Processor (MCU)}

The MCU of almost all modern GSM DBBs is a System-on-a-Chip (SoC) utilizing a
32bit ARM7TDMI core.  The only notable exception are low-cost Infineon chips
like PM7870, who still use a version of their 16bit C166 core.

Baseband chips that support 3G cellular networks often use a more powerful
ARM926 or ARM975 core as MCU.

\subsection{MCU peripherals}

The MCU cores have the typical set of peripherals of any ARM7 based
microcontroller, such as RTC, UARTs for RS232 and IRDA, SPI, I2C, SD/MMC card
controller, keypad scan controller, USB device, ...

However, there are some additional peripherals that are very GSM specific:
\begin{itemize}
\item A GPRS crypto unit for the proprietary GEA family of ciphers
\item Extended power management facilities, including a timer that can calibrate the RTC clock based on the synchronized VCTCXO in order to wake-up the MCU ahead of pre-programmed events in the GSM time multiplex
\item GSM TDMA timers that can synchronize to the on-air time frames and generate interrupts to MCU and DSP
\item Software-programmable hardware state machines for sequencing GSM burst Rx or Tx in ABB and RF Frontend
\item An ISO7816 compatible smart card reader interface for the SIM card
\item Audio routing, i.e. selecting how audio is routed in the phone,
considering integrated earpiece, ringtone speaker and microphone, as
well as external analog headset, handsfree or even bluetooth-attached
audio devices.
\end{itemize}

The programming of those peripherals is highly device-specific and there are no
industry standards.  Every DBB architecture of every supplier has its own
custom register set and programming interface.

The register-level documentation for those proprietary peripherals is (like all
documentation on DBB chipsets) closely guarded by NDAs, effectively preventing
the development of Free Software / Open Source drivers for them, unless such
documents are leaked by third parties.

However, as opposed to the DSP API documentation, the register-level
documentation to the MCU peripherals is normally provided to the cellphone
manufacturers.

\section{Digital Baseband Software Architecture}

This section provides an introductory reading in the typical software
architecture as it is found on contemporary GSM digital baseband designs.

The MCU usually runs a very small realtime operating system (RTOS) such
as Nucleus, VxWorks or the L4 microkernel.  In some cases, no operating
system is used at all, in order to save royalties or licensing fees that
would occurr for proprietary RTOS. 

\subsection{GSM Layer 1}

The Layer1 (L1) software is highly device-specific, as it closely interacts
with the DSP using the shared memory DSP API, as well as the proprietary
integrated peripherals controlling the ABB and RF Frontend.

However, there are some general observations that can be made about the L1:

\subsubsection{L1 Synchronous part}

The synchronous part is executed synchronously to the GSM TDMA frame clock.
Both CPU and DSP are interrupted by some hardware GSM timer every TDMA frame.

The L1 synchronous part typically runs inside IRQ or FIQ context of the MCU,
taking care of retrieving data from and providing data to the DSP API.

\subsubsection{L1 Asynchronous part}

The asynchronous part is scheduled as a normal task, potentially with high
or even real-time priority.  It picks up the information provided by the L1
Sync and schedules its next actions.

The L1 async typically communicates via a message queue with the Layer2
above.  Common primitives for L1 control are described (as non-normative parts)
of the GSM specifications.

\subsection{GSM Layer 2}

As opposed to L1, the GSM Layer 2 (L2) is already fully hardware independent.
It implements the LAPDm protocol as specified in GSM TS 04.06.  LAPDm is a
derivative of the ISDN Layer 2 called LAPD, which in turn is a descendent
of the HDLC family of protocols.

LAPDm takes care of providing communication channels for Layer3.  Those
channels are protected from frame loss by the use of sequence numbers and
retransmissions.

The interface to Layer3 is typically implemented by means of a message queue.

Primitives (but no detailed protocol) for use of the Layer2 / Layer3 interface
are provided in the GSM specifications.

\subsection{GSM Layer 3}

GSM Layer 3 (L3) consist of sublayers for Radio Resource (RR), Mobility
Management (MM) and Call Control (CC).

There is sufficient treatment of the GSM L3 and its sublayers in
existing texts, so there is no point in making a futile attempt
repeating that here.

\section{Synchronization and Clocking}

The author of this paper has been quoted saying {\em GSM is a synchronous
TDMA nightmare}.  This is by no means intended as an insult to the
technology itself or to its inventors.  It merely serves as evidence how
hard it is to get into the synchronous TDMA mindset, especially for
engineers who have spent most of their career in the world of packet
switched networks.

GSM is synchronous in multiple ways between cell (BTS) and phone (MS):
\begin{itemize}
\item Synchronization of the carrier clock to tune the receiver and transmitter to the correct frequency
\item Synchronization of the bit clock in order to perform sampling at the most optimal sample intervals
\item Synchronization of the frame clock (and thus timeslots) to know when a TDMA frame and its 8 timeslots start
\item Synchronization of the TDMA multiplex to correctly (de)multiplex the logical channels that are sent over each timeslots
\end{itemize}

As all those clocks are related to each other, they can (and should) all be
derived from the same master clock:  The VCTCXO present in each GSM
phone.

\subsection{How to synchronize the VCTCXO}

Every cell sends frequency correction bursts as part of the Frequency
Correction CHannel (FCCH), which is itself part of the BCCH, which in turn is
constantly transmitted by the BTS.

To acquire initial synchronization to the GSM network, the LO is tuned to the
desired GSM RF channel (ARFCN) frequency.  However, at this point, the LO
frequency is a multiple of the VCTCXO frequency which in turn still has an
undetermined error.  This initial frequency error is as large as that of a
regular crystal oscillator, potentially already with temperature compensation.

The resulting baseband signal thus can be shifted by a fairly large amount in
our baseband spectrum.  A specific DSP code is now using correlation and other
techniques to identify the frequency correction burst.  The DSP can then
further calculate the actual frequency error of the LO by comparing the
received FCCH burst with the FCCH burst as specified.

This computed frequency error can be fed into a (software) frequency control
loop filter.  The loop filter output is applied to an auxiliary DAC, which
generates the control voltage for the VCTCXO.

After a number of FCCH bursts and corresponding frequency control loop
iterations, the VCTCXO generated clock has only a residual error.  Whenever the
phone is receiving, the frequency control loop is continuously exercised in
order to maintain synchronization.

\subsection{How to synchronize the frame clock}

When the DSP performs FCCH burst detection as described above, it identifies
the exact position in the incoming sample stream when the FCCH burst was
happening.  By knowing from the specification that the FCCH burst is
part of the BCCH, and that the BCCH is sent on timeslot 0, the Layer1
software can then synchronize the phone to the TDMA frame start.

Commonly, a hardware timer unit is clocked by a (divided) VCTCXO clock and thus
counts in multiples of the GSM bit clock, wrapping/resetting at the TDMA duration
of 1250 bits.

By scheduling events synchronously to this GSM bit-clock timer, the L1 can now
trigger events (such as asking the DSP to demodulate incoming data) or
instructing the LO to retune synchronously to every TDMA frame.
From this timer the DBB typically also generates interrupts to the DSP and MCU.

\subsection{How to synchronize the GSM TDMA multiplex}

As part of the BCCH, the BTS not only sends the FCCH but also the
Synchronization CHannel (SCH).  The Synchronization channel indicates the
current GSM time / frame number (skipping the 3 least significant bits).
By using this received GSM time and incrementing it every time the GSM bit-clock
timer wraps at the beginning of a new TDMA frame, the GSM time is synchronized.

Understanding the multiple layers of time multiplex such as the 26/51
multiframe, superframe and hyperframe, the L1 can multiplex and demultiplex all
the logical channels of GSM.
%% this is awkwardly worded

\section{Miscellaneous Topics}

\subsection{GPRS}

GPRS was the first packet switched extension to GSM.  In fact, it is much more
its entirely own mobile network, independent of GSM.  The only parts shared are
the GSM modulation scheme (GMSK) and time multiplex, in order to ensure peaceful
coexistence between them.

The L1 and L2 protocols are very different (and much more complex) than GSM.

So while the phone baseband hardware did not need any modifications for a basic
GPRS enabled phone, the software needed to be extended quite a lot.

\subsection{EDGE}

EDGE is a very small incremental set of changes from GPRS.  It reuses all of
the time multiplex and protocol stack, but introduces a new modulation: Offset
8-PSK instead of GMSK to increase the bandwidth that can be transmitted.
Offset 8-PSK is used (as opposed to simple 8-PSK) to avoid
zero-crossings in the modulator output.

So while the software modifications from GPRS to EDGE are minimal, the 8PSK
modulation scheme has a significant impact on the DSP, ABB and even RF
PA design.

\subsection{UMTS}

UMTS (sometimes called WCDMA) is an entirely separate cellular network
technology.  Its physical layer, modulation schemes, encoding, frequency
bands, channel spacing are entirely different, as is the Layer1.

UMTS Layer2 has some resemblance to the GPRS Layer2.

UMTS Layer3 for Mobility Management and Call Control are very similar to GSM.

Given the vast physical layer and L1 differences, a UMTS phone hardware design
significantly differs from what has been described in this document.

Notwithstanding, all known commercial UMTS phone chipsets as of today still
include a full GSM modem in hardware and software to remain
backwards-compatible.

\subsection{Dual-SIM and Triple-SIM phones}

In recent years, a large number of so-called {\em Dual-SIM} or even {\em
Triple-SIM} phones have entered the market, particularly in China and other
parts of East Asia.

Those phones come in various flavours.  Some of them simply have a multiplexer
that allows electrical switching between multiple SIM card slots.  This is
similar to replacing the SIM card in a phone, just without the manual process
of mechanically removing/inserting the card.  As a result, you can only use one
of the two SIMs at any time.

The more sophisticated Dual-SIM phones have two complete phones in one case.  Yes,
that's right!  They contain two full GSM phone chipsets, i.e. 2 antennas, 2 rf
frontends, 2 analog basebands, 2 digital basebands, ...

However, they use the same trick as smartphones:  One of the two basebands does
not have keypad or display and is simply a GSM modem connected via serial line
to the other baseband processor.

So if a smartphone (as defined in this document) is a GSM modem connected to a
PDA in one case, a Dual-SIM phone is a GSM modem connected to a feature phone
in one case.

Triple-SIM phones often combine the two approaches, i.e. they contain two
complete GSM baseband chips, but three SIM slots that can be switched among
the base bands.  Only two SIMs can be active at the same time.

\subsection{Powerful feature phones}

Feature phones are becoming more and more powerful.  However, their
comparatively lower market price cannot afford a full-blown smartphone design
with its two independent processors and the associated design complexity.

Thus, more and more hardware peripherals are added to the only processor left
in the phone: The baseband processor.  Such peripherals include sophisticated
camera interfaces, high-resolution color display controllers, TV output,
touchscreen controllers, audio and video codecs and even interfaces for mobile
TV reception.

However, all of those features are still implemented on a fairly weak ARM7 or
ARM9 CPU core (compared to ARM11 and Cortex-A8 in the smartphone market).  They
also lack a real operating system and still run on top of a real-time
microkernel intended for much less complex systems.  They almost always lack
any form of memory protection or multiple address spaces.  This makes them
more prone to security issues as there is no privilege separation between
the GSM protocol stack and the applications, or between the applications
themselves.

\subsection{GSM baseband security features}

There are several (sometimes conflicting) security requirements that
apply to mobile phones.  Interestingly, the security features are
typically used to protect some industry interest against the interest of
the customer.  There are very few security features in a phone that are
meant to protect the users or their interests.

\subsubsection{IMEI - The hardware serial number}

The International Mobile Equipment Identifier (IMEI) uniquely identifies
a GSM phone.  It is a globally unique serial number which is programmed
into the phone non-volatile memory (Flash or EEPROM) during the
production process.  There are no particular security features used to
store the IMEI.  Once you are able to write to flash and have found it,
it can be changed.

\subsubsection{The SIM Card}

The SIM card is a cryptographic smart card that stores the secret key
used for authenticating the user to the GSM network (Ki).  The Ki is
never released by the card, and as such copying (cloning) of the SIM
is prevented.  Some early implementations of the SIM card had
cryptographic weaknesses that inadvertently permitted cloning until the
late 1990s.

Furthermore, the SIM stores the International Mobile Subscriber Identity
(IMSI).  The IMSI is not encrypted or protected in any way.

There is no security channel on the connection between the SIM card and
the baseband MCU.  Furthermore, there is no way how the MCU can securely
identify/authenticate the SIM itself.  Physical access to the SIM card
slot allows sniffing and/or modification of the communications between
the MCU and the SIM.

\subsubsection{SIM or Operator Locking}

GSM is an international standard.  This ensures interoperability, i.e.
any phone can be used on any network.

However, in some cases, the vendors of a GSM phone want to restrict this
interoperability and lock a phone to one specific network, or even lock
it to a particular SIM.

Those locks are implemented by software only, i.e. the MCU software will
instruct the GSM protocol stack not to register with a network unless
its network operator code is a certain factory-programmed network number.

As such, techniques for circumventing those locks have become
commonplace.  It's somewhat of an ongoing race between the phone makers
and the phone-unlockers.  The industry invents ever more complex methods
of obfuscating their locks in the software, while the phone-unlockers
reverse engineer those bits for each and every phone model after some
time.

\subsubsection{DBB firmware signatures}

In order to protect the operator and phone manufacturers peculiar
business models based on SIM or operator locking, some vendors
extended their baseband software with cryptographic signatures.  Only
if the correct signature is present in a software update, the bootloader
program will accept the new software.

However, there are more or less invasive hardware-related approaches in
circumventing those signature checks, such as hardware debugging
interfaces like JTAG, or replacing the external flash memory components.

More recently, GSM chipset vendors introduced features such as
hardware-assisted software signature checks.  In this case a master key
hash might be present in DBB-internal fuses, together with a
signature-verifying boot loader in DBB-internal mask ROM.  As the root
of the chain of trust is moving deeper into the hardware, it becomes
more difficult for anyone to make software modifications to the DBB.

Especially with tighter integration, where RAM and FLASH are no longer
present as discrete components but part of a multi-chip-package, the
number of options are becoming more limited.

On the other hand, an ever more complex baseband software stack is
opening up many more options for exploiting software vulnerabilities.
Given the lack of a proper/modern operating system with privilege
separation and virtual memory, such exploits immediately give away
full access to all aspects of the respective DBB.

\section{Personal rant on the closedness of the GSM industry}

The GSM industry is one of the most closed areas of computing that I've
encountered so far.  It is very hard to get any hard technical
information out of them.  All they like to spread is high-level
marketing information, but they're very reluctant when it comes down to
hard technical facts on their products.

If you want to build a phone, you need to buy a GSM chipset for your
product.  There are only very few companies that offer such chipsets.
The classic suppliers are Infineon, Texas Instruments, ST/Ericsson, ADI
(now MediaTek) and Freescale.

The GSM handset products they sell are not generally available and
distributed like other electronic components they manufacture.  If you
need a Microcontroller/SoC, a power management IC, a Wifi or Bluetooth
chip, RFID reader ASIC, you simply approach the respective distributors
and order them.  You get your samples directly from Digikey.

This is impossible for GSM (or other cellphone) chipsets.  For some
reason those chips are sold only to hand-picked manufacturers.  If you
want to qualify, you have to subscribe to at least six-digit annual
purchasing quantities.  And in order for them to believe you, you have
to cough up a significant NRE (non-refundable engineering fee).  This
has been reported as high as a seven-digit US\$ amount and is to make
sure that even if you end up buying less chips than you indicate, the
chipset maker will still have your upfront NRE money and keep it.

And if you buy your way into that select club of cellphone makers, what
you get from the chipset maker is typically not all too impressive
either.   The documentation you get is incomplete, i.e. it alone would
not enable you as a cellphone maker to make any use of the hardware,
unless you license the software (drivers, GSM protocol stack, ...) from
the chipset maker, too.

On the software side, most of the technologically interesting bits (like
the protocol stack) are provided as binary-only libraries, you only get
source code to some parts of the systems, i.e. some hardware drivers
that might need modification for your particular phone electrical
design.

That GSM protocol stack was not written by the chipset maker either.
They simply license a stack from one of the estimated 4 or 5
organizations who have ever implemented a commercial GSM protocol stack.

It is not like the GSM protocols were some kind of military secret.
They are a published international standard, freely accessible for
anyone.  So why does everybody in that industry think that there is
a need to be so secretive?

Having spent a significant part of the last 6 years with reverse
engineering of various aspects of mobile phones in order to understand
them better and to write software tools for security analysis, I still
don't understand this secrecy.

All the various vendors do more or less the same.  The fundamental
architecture of a GSM baseband chip is the same, whether you buy it from
TI, Infineon or from MediaTek.  {\em They all cook with water}, like we
Germans tend to say.  The details like the particular DSP vendor or
whether you use a traditional IF, zero-IF or low-IF analog baseband
differ.  But from whom do they want to hide it?  If people like myself
with a personal interest in the technical aspects of mobile phones can
figure it out in a relatively short time, then I'm sure the competition
of those chipset makers can, too.  In much less time, if they actually
care.

This closedness of the cellular industry is one of the reasons why there
has been very little innovation in the baseband firmware throughout the
last decades.  Innovation can only happen by very few players.  Source
code bugs can only be found and fixed by very few developers at even
fewer large corporations.  There is little to no chance for a small start-up to
innovate, like they can in the sphere of the internet.

It is fundamentally also the reason why the traditional phone makers
have been losing market share to newcomers to the mobile sphere like
Apple with its iPhone or Google with its Android platform.

Those innovations really only happened on the application processor on
high-end smartphones.  The closed GSM baseband processor had to be
accompanied by an independent application processor running a real
operating system, with real processes, memory management, shared
libraries, memory protection, virtual memory spaces, user-installable
applications, etc.

They still don't happen on the baseband processor, which is as closed as
it was 15 years ago.

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