An Embedded Asterisk Platform Instructional Design to Teach Voice

over IP in Information Technology Undergraduate Courses

Using Raspberry PI and Asterisk to Build an Embedded Portable Didactic Tool

M. C. Dias

1,2

, L. C. Bezerra

, D. Morais

, C. F. Gabi

and A. Perkusich

Electrical Engineering Coordination, Laboratory of Telephony and Convergent Networks – LTCON,

Federal Institute of Education, Science and Technology of Paraíba – IFPB, João Pessoa, Brazil

Post-graduate Program in Electrical Engineering – PpgEE – COPELE, Electrical Engineering Department, Federal

University of Campina Grande – UFCG, Campina Grande, Brazil

Keywords: Learning, Embedded Platform, Asterisk, Voice over IP - VoIP.

Abstract: This paper shows the building of an embedded Asterisk platform as a key technique for providing Information

Technology classes with an extra laboratorial environment for learning Voice over Internet Protocol. Besides

building the platform, the students are asked to mount a functional telephony system with it that meets the

criteria of low cost, low building complexity and high adaptability to convergent networks. The Raspberry PI

version B was the board used to provide portability for the students to work outside the laboratory

environment. Asterisk was presented as a useful tool on which the students could create a flexible system that

could be easily deployed on convergent networks. This work was developed in the Telephony subject for the

Electrical Engineering and the Telecommunications Systems undergraduate courses from the Federal Institute

of Education, Science and Technology of Paraíba – IFPB, and presented good results in the students´ learning

achievements.

1 INTRODUCTION

Nowadays, part of the theory in technical education is

narrowed by what is practiced in the laboratory,

mediated by the infrastructure that is offered. Among

these theories, one that demands extensive practice is

IP Telephony Technology, which is essential to have

an entire comprehension in Converged Networks

structure. The lab training about these technologies is

necessary to meet the need for qualified workforce

(Eady and Lockyer, 2013).

Nonetheless, the networking laboratorial

infrastructure has an increased complexity and a high

cost, due to the purchase of Gateways, IP phones and

other several devices, what makes the individual

acquisition impractical (Spencer, 2003); (Rowe,

2007).

However, according to Lamar (2012) and Rowe,

(2007) students would not be so restricted at the

school labs if they could make their own experiments

anywhere, by carrying all the resources compatible

with the academic laboratories

The use of free Asterisk Platform in VoIP

education has become a very recurrent methodology

lately, especially when combined with the

experiments on Converging Data Networks. A recent

study on this matter was proposed by Dias et al

(2013), who presented a successful experience using

this software. Another study, based on the

configuration of an Embedded PBX Platform, driven

by Abid et al (2012), discussed the elaboration of a

compact system for VoIP aplications and the details

about how to get a better performance.

Our work shows how the building of a portable

platform can improve the learning of students from

Theory Information classes that work with IP

Telephony and Converged Networks. In addition to

developing the theoretical content with practice

during the construction of the portable platform,

students no longer have to hold themselves to the

laboratory environment, because they have the

possibility to develop experiments in extra-class

environments.

In addition to this, the construction of the

prototype shown in our work also considered factors

such as low cost, complexity of construction and

adaptation to converged networks. The main goal is

to provide the students with a didactic tool that

enables them to reproduce situations and scenarios

461

Dias M., Bezerra L., Morais D., F. Gabi C. and Perkusich A..

An Embedded Asterisk Platform Instructional Design to Teach Voice over IP in Information Technology Undergraduate Courses - Using Raspberry PI

and Asterisk to Build an Embedded Portable Didactic Tool.

DOI: 10.5220/0005489504610466

In Proceedings of the 7th International Conference on Computer Supported Education (CSEDU-2015), pages 461-466

ISBN: 978-989-758-107-6

 2015 SCITEPRESS (Science and Technology Publications, Lda.)

only available previously in the laboratory

environment.

The paper is organized in sections, as follows:

section 2 will focus on a bibliographical review on the

foremost concepts concerning VoIP and a

presentation about portable platforms, involving the

Asterisk concepts; section 3 will describe the

materials and methods applied in the design of the

proposed device; in section 4, the results obtained

from the construction and usability test will be

presented; in section 5, conclusions are discussed.

2 VOICE OVER IP (VOIP) AND

ASTERISK CONCEPTS

2.1 Voice over IP (VoIP)

Voice over IP is a set of networking protocols that

transport the voice over a TCP/IP data network. The

analog waves of the human voice are quantified,

digitalized and sent in packets from the origin call

point to their destination

(Meggelen; Smith and

Madsen, 2007); (Hartpence, 2013).

To make this possible, a lot of resources were

used: specifics sessions protocols like SIP or H323;

codecs to transcode and optimize the sound; and

quality of service (QoS) to maintain the integrity on

limited connections.

2.2 Embedded Platforms

An Embedded Platform is a micro-processing system

completely encapsulated and dedicated to the device,

or system, controlled by him. Differently from a

computer, whose architecture is diversified on

countless ends, an embedded platform has a structure

particularly designed to perform specific and

predefined operations.

These characteristics result in a considerable

economy on resources and components, generally,

reducing their material costs, labour costs and,

mainly, the physical size of the devices that embrace

this kind of system (Ganssle and Barr, 2003).

Embedded platform system devices examples:

 Automotive navigating systems;

 Smartphones and PDAs;

 Biometric access control systems;

 Air-conditioning temperature control;

 MP3 players;

 Printers;

 Networking devices;

 Portable measuring devices;

 Medical monitoring systems.

There are a number of boards available in the

market that make possible to implement embedded

systems, like the Beaglebone Black (Richardson,

2013), the Cubie Board (Schinagl, 2014), The Intell

Galileo (Ramon, 2014), and the Raspberry PI (Upton

and Halfacree, 2014). For this work, we chose the

Raspberry PI because it’s a low cost and well known

board that has plenty of working material available.

Therefore, the next session shows the main features

of the Raspberry PI.

2.2.1 The Raspberry PI

Raspberry PI is a development board, with small

dimensions, manufactured since 2012 by Raspberry

PI Foundation (UK). The primary goal from this

design is to stimulate the computing science learning

on children (Upton and Halfacree, 2014).

The board, in its B version (Figure 1), comes with

System on Chip (SoC) Broadcom CM2835, which

includes:

 ARM1176JZF-S of 700 MHz Processing;

 GPU Video Core IV;

 512 MB RAM (SoC);

 SD card entry;

 Two USB ports 2.0 ;

 HDMI video and VCA video outputs;

 Ethernet 10/100 (RJ45);

 Connecting port with 26 pins, which 17 are

GPIO;

 Audio output via 3,5 mm jack connector.

Figure 1: Raspberry PI board, version B.

To reach a larger audience, the Raspberry PI

average price needed to be as low as possible. So, the

project’s creators convinced the North American

Broadcom to sell the processors to be used in the

boards at very low prices. (Cooper and Knight, 2014).

In fact, this device has been used by children

worldwide, and it has been helping them on

computing learning. Also, Raspberry PI has been

CSEDU2015-7thInternationalConferenceonComputerSupportedEducation

462

applied in other several projects in many different

fields of education, like Automation and Electronics,

Programming languages like Python, Web services

and others. (Upton and Halfacree, 2014).

2.3 Linux and Asterisk

The Unix is an Operational System created by Ken

Thompson in the late 60s, to fulfil the necessity of

many researchers whom, in order to proceed with

their works, required a multiuser and multitask

system, which also could be easily converted to

different hardware platforms (Machtelt, 2007);

(Hartpence, 2013).

Asterisk is an open source platform, distributed by

Digium under a General Public License (GPL),

initially developed for Linux Systems, combining all

the characteristics of a Public Switched Telephony

Network (PSTN) with an integrated set of

customizable applications (Meggelen; Smith and

Madsen, 2007).

All Asterisk architecture (Figure 2) was

elaborated to be highly flexible, intending to provide

support on several applications and many signalling

profiles originated from public networking. As so, it

supplies interfaces to any kind of hardware or

software without limitations (Martin, 2009);

(Spencer, 2003).

Figure 2: Asterisk Architecture. (Asterisk Project, 2014).

3 MATERIALS AND METHODS

To show the device effectiveness and its applicability,

an experiment was mounted under a simple topology,

composed of three elements, placing the Embedded

Asterisk Platform connected to a LAN and two access

peers as presented in Figure 3. The described attempt

about the equipment to communicate was monitored

by a Network Sniffer, which revealed the data traffic,

signalling and connection.

The experiment was divided into three steps:

1. Hardware mount;

2. Embedded systems installing;

3. Base scenario execution and results.

As learning goals, the following aspects were

defined: a) Features on embedded systems and free

software; b) VoIP definition and its architecture; c)

Asterisk server configuration and functioning; d)

Protocols operations, as SIP (Session Initiation

Protocol) (Hartpence, 2013) and RTP (Real-time

Transport Protocol, and its signalling (Meggelen;

Smith and Madsen, 2007).

On the first step, scenario preparing has as choice

of criteria a versatile topology, easily implemented,

taking into consideration the resources availability,

without limiting the range of experiments that can be

tested. If the scope of applications on this stage was

limited, it would lower the amount of knowledge to

be put as proof by the students.

The proposal of this scenario (Figure 3) is that the

student can, at first, implement it in any place where

he/she wants to practice, based on its high portability

feature. Also, step 3 is bounded to the understanding

of the functioning of the Asterisk server, and how it

is possible, through the configuration files, to make

and receive calls among IP terminals, whichever they

are - telephones or softphones - in a regular computer.

Figure 3: Functionality Test Scenario: proposed topology

for the connection between the Embedded Server and the

Peers.

Another important concern is related to

acknowledge how the SIP and RTP protocols act, its

signalling messages and media flow to determinate

the connectivity between the peers, the call quality

and which error messages are likely to appear on a

SIP signalling session.

On the experiment, it was used an LG A550

notebook with I5 2,8 Ghz Processor, 4 GB RAM

AnEmbeddedAsteriskPlatformInstructionalDesigntoTeachVoiceoverIPinInformationTechnologyUndergraduate

Courses-UsingRaspberryPIandAsterisktoBuildanEmbeddedPortableDidacticTool

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memory, and a softphone Zoiper to authenticate 3

testing extensions, 3001 and 3002 (Zoiper.com,

2015); (Gonçalves, 2007).

On step 3, we proceeded installing the systems to

test the proposed scenario, under the Raspberry PI B

version hardware model, provided on IFPB’s

Telephony and Convergent Networks Laboratory.

According to the equipment instruction manual, we

started on the peripherals connections. To the LAN

connection, we used the RJ 45 ports; the USB port to

the keypad; HDMI port to link the monitor, RCA to

connect the sound and the standard port to receive the

power supply (Figure 4).

Figure 4: Peripherals connections in the Raspberry PI

board.

Once it was connected, we initiated the Operating

System install, as shown in Figure 5. Following the

recommended version, it was used the RaspbianOS

distribution, which is based on the Linux Debian

Distribution.

Figure 5: Operating System installing details.

As the OS installation was concluded, the Asterisk

software has been installed and configured to make

possible the creation of the SIP channels and

extensions recognition, as demonstrated in the code

below:

[general]

udpbindaddr=0.0.0.0

context=default

disallow=all

allow=alaw,ulaw,gsm

nat=yes

canreinvite=no

language=pt_BR

[common](!)

type=friend

secret=rstc2015

dtmfmode=rfc2833

host=dynamic

[3001](common)

callerid=<3001>"RstcVoip1"

[3002](common)

callerid=<3002>"RstcVoip2"

In order to the IP phones communicate with each

other, the Embedded Asterisk dialling plan must also

be created. The extensions.conf file is responsible for

this feature in the server. The code presented below is

part of a dialling plan made to the proposed scenario.

[default]

exten => 3001,1,Dial(SIP/3001,20,tT)

exten => 3001,2,Hangup()

exten => 3002,1,Dial(SIP/3002,20,tT)

exten => 3002,2,Hangup()

At last, after configuring the extensions and the

dialling plan, the SIP accounts shall be configured on

the IP phones to send the request for registration to

the Asterisk Embedded server, using the credentials

set up in sip.conf file. Figure 6 shows how this

configuration is made in the Zoiper Softphone.

Figure 6: Zoiper Softphone set up.

For monitoring, the Asterisk platform has a

console prompt that can exhibit the program current

version, the dialling plan, latency among the

registered extensions, used ports and on-going

connections, as seen in Figure 7.

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Figure 7: Asterisk platform monitoring console prompt.

4 RESULTS

As proposed in the primary scenario (Figure 2), we

observed that the Embedded Asterisk Server does the

intermediating role in communications among the

devices linked to the LAN. In order to verify the

system communication integrity, we used the

Wireshark software to check the local networking,

capturing the packets that flow between the server

and the peers. Figure 8 shows the kinds of packets

captured by the tool (Wireshark, 2011).

Figure 8: Packets captured with Wireshark Software.

On detailed mode, it was also possible to observe

the protocols used to establish the VoIP connection:

the SIP protocol was used for peer-to-peer signalling;

the Codec (G711) applied to audio codification and

RTP protocol for the media flow. The trial sequence

and connection request among terminals, ringing

signalling, connection ACK, media flow,

disconnection request and the final confirmation

between the end points are shown in Figure 9.

By using this low cost embedded platform, the

student will be able to examine the behaviour of the

connection to be tested, accessing detailed

information about the integrity of the peer-to-peer

link, passing through signalling pattern sequel,

codecs usability or even media flow interruption.

Knowing about this is useful for problem resolutions

proposed by the teacher with scenarios that are much

more complex, where the apprentice must develop

his/her own solution. Even though a complex

scenario cannot be unveiled during class, the portable

solution will be able to be carried anywhere inside

the student’s backpack or pocket thanks to its small

dimensions, as demonstrated in Figure 10.

By using this low cost embedded platform, the

student will be able to examine the behaviour of the

connection to be tested, accessing detailed

information about the integrity of the peer-to-peer

link, passing through signalling pattern sequel,

codecs usability or even media flow interruption.

Knowing about this is useful for problem resolutions

proposed by the teacher with scenarios that are much

more complex, where the apprentice must develop

his/her own solution.

Figure 9: Details about trial connection status, extracted

from the Wireshark program on running.

Even though a complex scenario cannot be

unveiled during class, the portable solution will be

able to be carried anywhere inside the student’s

backpack or pocket thanks to its small dimensions.

The assessment by the teacher showed that all

students correctly answered the questions and that

100% of students were satisfied or very satisfied with

AnEmbeddedAsteriskPlatformInstructionalDesigntoTeachVoiceoverIPinInformationTechnologyUndergraduate

Courses-UsingRaspberryPIandAsterisktoBuildanEmbeddedPortableDidacticTool

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the practice. And all of them said that the most

attractive platform was mobility.

5 CONCLUSIONS

The results confirm that a portable Embedded Server,

besides being completely functional, it is most likely

a very important and viable resource in the

educational field, particularly in Technology

Courses.

Besides the possibility of customization, likewise

the Free Software, including Asterisk, the program

used to build this tool is the Open Source and it is

freely distributed, appearing to be a low cost solution

for the educational institutions.

In addition to its computational versatility and

portability, Raspberry PI provides us a new

perspective in regard to educational material to apply

inside and outside classes, amplifying the range of

testing scenarios and problem-solving situations to be

presented to the students, enriching their academic

degree.

In future works, more experiments that use this

platform will be developed. Furthermore, this tool

will be used during other academic terms so as to

consolidate and extend its validation.

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