Towards the Development of LTE Networks:

Implementation of OpenIMSCore in
Asterisk/OpenBTS GSM Network
Raja Anwaar Ali, Anooshah Nooshad Khan, Saba Arshad, and Usman Younis
Department of Electrical Engineering
School of Electrical Engineering and Computer Science
National University of Sciences and Technology, H-12, Islamabad, Pakistan
Email: 09beeanwaar@seecs.edu.pk, 09bicseanoshad@seecs.edu.pk, 09beesarshad@seecs.edu.pk, usman.younis@seecs.edu.pk

AbstractGSM network has been implemented using universal We have employed USRP2 to implement the base transceiver
software radio peripheral. Voice services have been established station (BTS) in our GSM network.
between the registered users in our GSM network. Secondly, to USRP2 is attached with a daughter board/card, which is
demonstrate the smooth transformation of GSM networks to LTE
networks, internet protocol multimedia core network subsystem responsible for controlling TX/RX, and in some cases the
has been integrated in our GSM network, and voice services have modulation scheme. USRP2 has a secure digital (SD) card
been established between the GSM mobile phone and IMS open slot on which the firmware of FPGA is loaded. Similarly, it
source softphone. has multiple-input and multiple-output (MIMO) expansion slot
which has not been used in our case. To communicate with
I. I NTRODUCTION personal computer (PC), USRP2 has Ethernet slot which by
Global system for mobile communications (GSM) with default is configured at IP address 192.168.10.2.
various enhancements is currently being followed in many
B. RFX-900 Daughter Board
countries, including Pakistan. The services included in this
standard are 2G (Digital Voice), 2.5G (Packet Data), and As mentioned previously, USRP2 has a daughter board
2.75G (Intermediate Multimedia). In order to add more into attached with it which is responsible for receiving and trans-
the bandwidth offered using mobile communication networks, mitting signals at different frequencies. Each daughter board
evolutionary standards have been introduced such as 3G (Mul- has its own range of frequency and sometimes modulation
timedia). This gives an opportunity for domestic Telcos to schemes.
increase their bandwidth while ensuring a smooth transition in We have employed RFX-900 daughter board in this work.
their resident networks. This work presents a transformation The RFX-900 is a high-performance transceiver which oper-
of 2.5G networks to 4G networks by demonstrating voice ates in 900 MHz band of the electromagnetic spectrum. This
services using universal software radio peripheral (USRP) kit. enables it to be used as a two-way radio frequency (RF) source
OpenBTS [1] and OpenIMSCore [2], which are open source in GSM networks.
softwares, have been used to demonstrate this transformation.
C. VERT900 Antenna
The core network of GSM is modified to achieve this 4G
evolution while keeping the access network same as that of RFX-900 daughter board has two sub-miniature version
GSM; this implies that user would be seeing it as a GSM A (SMA) connectors on it, one is used as RX, and other
network with added services of long term evolution (LTE) is used as RX/TX. These SMA connectors are responsible
network. As a first step, call interface has been successfully for transmitting and receiving RF signals. To strengthen the
replaced. OpenBTS [3], USRP2, and Asterisk [4] have been signals in air, an antenna is needed. Additionally, the antenna
used to implement the GSM network. Later on, Asterisk has has to be compatible with operating frequency of daughter
been replaced with OpenIMSCore (call interface for LTE), and board, i.e., 900 MHz. Therefore, we have used VERT900
GSM network has been demonstrated up and running. A voice antenna with RFX-900 daughter board. VERT900 is dualband
call from IP multimedia subsystem (IMS) softphone to GSM antenna with frequency range of 824-960 MHz, and a gain of
mobile phone has also been made. 3dBi.

D. Personal Computer (Laptop)

II. H ARDWARE C OMPONENTS
Personal computer is a core component in our network. We
A. USRP2 Kit have used Core i5 in this work along with 4 GB of RAM.
USRP2 is a field-programmable gate array (FPGA) based PC with higher specifications is recommended for use with
multi-purpose software radio developed by Ettus Research [5]. USRP2.
 (a) (b)

Figure 1: Implementation of GSM Network. (a) USRP2 kit working as our BTS and computer system working as our BSC
and MSC. (b) Manual selection from the mobile phone shows our GSM network 001 01, which is highlighted in this figure.

III. S OFTWARE C OMPONENTS to maintain the records and settings. Instead of working
We have used open source softwares to aid our hardware standalone, OpenBTS coordinates with other softwares such
in this work. The reason behind choosing the open source as Smqeue and Asterisk. OpenBTS uses Asterisks subscriber
softwares is their ease to configure USRP2 kit according to database to authenticate users on GSM network.
our needs.
C. Asterisk PBX
A. UHD Drivers
USRP hardware drivers (UHD) are required to establish Asterisk is a software implementation of a telephone private
proper communication between PC and USRP2 kit. Without branch exchange (PBX) [4]. Asterisk allows attached tele-
these drivers Linux, operating system in our case, is not able phones to call each other. It uses SIP to make calls between
to communicate with USRP2 kit. These drivers are freely the registered users, whereas, OpenBTS is responsible to map
available on Ettus Research [5]. We have used stable drivers, mobile phones international mobile subscriber identity (IMSI)
however, master branch is also available for developers use. as SIP client of Asterisk.

B. OpenBTS
D. OpenIMSCore
OpenBTS is an open source software developed by Range
Networks [1]. OpenBTS is a Unix application which works as OpenIMSCore is the open source implementation of IP
a software radio to provide GSM Um interface to handsets, and multimedia core network subsystem [2]. The OpenIMSCore
uses session initiation protocol (SIP) soft switches to connect replaces Asterisk when the cellular network is required to sup-
between calls. port 3G/4G voice services in our implementation. Similar to
OpenBTS is aimed at the development of low cost GSM Asterisk, the users are mapped as SIP clients in OpenIMSCore.
air interface, and with the integration of VoIP backhaul, it
achieves cellular communication in rural and remote areas. E. MySQL
This results in a substantial low cost deployment in the form
of private cellular networks. MySQL is a popular open source relational database man-
The whole software is written in C++ and latest version agement system (RDBMS). MySQL is used by both Asterisk
available is OpenBTS 2.8. We have used latest version in and OpenBTS to maintain transaction records, settings, and
this work. OpenBTS uses extensive use of SQL Database billing.
 IV. I MPLEMENTATION In order to make a call in our implemented GSM network,
A. Setting up GSM Network firstly, IMSI (stored in the SIM card) interacts with USRP2.
Secondly, OpenBTS maps IMSI to a SIP client which Asterisk
GSM network is developed by interfacing USRP2 kit with
can understand, therefore, establishing a call means SIP to SIP
OpenBTS [6]. Once this interfacing is achieved, the USRP2 kit
call in computer. Finally, at the front end call is established as
integrated with RFX-900 daughter board establishes a GSM
a IMSI to IMSI call. Figure 3 shows a voice call established
network, as shown in Fig. 1(a). Asterisk works as virtual
between two registered phones in our GSM network.
mobile switching centre (MSC) in our GSM network, while
OpenBTS works as base station controller (BSC), and USRP2
works as BTS. Due to non-calibrated clock of USRP2, not
every cellphone was able to detect our GSM network. Figure
1(b) shows our GSM network identified as 001 01 while
manually searching for the networks using a mobile phone.
If internal clock of USRP2 is used, clock skew is introduced
in RFX-900 daughter boards operation, and due to this, not
every GSM mobile phone is able to detect our GSM network.
We tested our GSM network with 38 mobile phones, out
of which 26 phones were able to detect our GSM network.
Therefore, it is recommended to use an external clock for
USRP2.
Extension registration has been done before making calls,
for which extensions.conf is the core file to be ammended
(configuration file of Asterisk). We have included IMSI entries
in this file [7]. All the dial plans, i.e., how calls will be routed,
for how long the phone will ring, and what options user have,
are written in specified format in this file. Once the registration
has been done, users can send/receive status messages to our
GSM networks, as shown in Fig. 2. Additionally, users can
send text messages to other users registered on the same
network.

Figure 3: A voice call established between two numbers

registered in our GSM network (the call has been initiated
by 9995125).

B. Integrating OpenIMSCore in GSM Network

In order to demonstrate the transformation of GSM network
to LTE network, we have integrated OpenIMSCore in our
virtual MSC. Firstly, OpenIMSCore has been installed and
dial plans have been made in extensions.conf [8]. Once
OpenIMSCore is in place, it takes over Asterisk and all the
call sessions are processed through proxy-call session control
function (P-CSCF) module of OpenIMSCore. IMS clients by
standard are stored as SIP clients. Therefore, it is easy to
establish calls between two IMS clients which have already
been mapped as SIP clients in our system. Figure 4(a) shows
IMS softphone (an open source client) [9], which has been
used to call the GSM mobile phone using our integrated
network, whereas, the received call from IMS softphone is
also visible in the call logs of Fig. 4(b).
The voice calls established between IMS softphone to
GSM mobile phones have a latency of 12 s. However,
IMS softphone to IMS softphone call has a latency of 4 s.
Figure 2: A mobile phone registered with number 9995125 Total number of call sessions which have been established
can send/receive status messages by sending a blank text to using OpenIMSCore have been six in our case. We attribute
411 in our GSM network. this delay and limited number of established sessions to the
 (a) (b)

Figure 4: Voice call established between IMS softphone (an open source client) and GSM mobile phone. (a) IMSI, which is
registered as SIP client, is used to dial the GSM mobile phone. (b) The received call from IMS softphone is visible in the
received call logs (the registered number for IMS softphone is 2000).

installation of OpenIMSCore on the same system which has R EFERENCES

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ACKNOWLEDGMENTS

We would like to thank Mr. Nasir Mehmood, incharge

of the embedded system designs and automation (ESDA)
laboratory, for providing us with USRP2 kit. We would also
like to thank Mrs. Asma Majeed who is a Lab Engineer
at ESDA laboratory for her cooperation. Additionally, we
would like to express our gratitude for Dr. Junaid Qadir for
providing us with USRP1 kit for testing.