A Problem-based Learning Case Study for Teaching Voice over

Internet Protocol - VoIP

Using Asterisk as a Tool for Teaching VoIP for Information Technology Classes

M. C. Dias

1,2

, C. F. Gabi

, E. P. Rodrigues

, V. R. Souza

and A. Perkusich

Coordination of Electrical Engineering, 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: Problem-based Learning - PBL, Asterisk, Voice over IP - VoIP.

Abstract: This paper shows the use of PBL (Problem-Based Learning) technique as a key to learning VoIP in courses

like Electrical Engineering and Computer Networks in conjunction with open source and the public domain

software called Asterisk which was used to create the scenario of the experiment and the problems

presented to the students. In order to make the validation, the experiment was applied to students of

Bachelor in Electrical Engineering and in Communication Technology System, undergraduate courses at the

Federal Institute of Education, Science and Technology of Paraíba – IFPB, in the Telephony subject, with

promising results. The Asterisk software was presented as a useful and flexible tool for constructing

scenarios and problems for the teaching of VoIP technologies and the used approach resulted as effective

for improving the attainment of the defined learning objectives.

1 INTRODUCTION

In the context of the technological evolution of

communications networks, the reality that the

telephone networks and data switching networks are

converging to an infrastructure, that will allow both

voice and data to be transmitted over the same

network, is becoming more feasible for the

technology and communication professionals on a

daily basis. This fact affects professionals from the

most diverse backgrounds, who work in the

convergent network area, from courses in

information technology up to the electrical and

telecommunication engineering fields.

The convergence of data networks with

telephone networks makes information technology

professionals face the challenge of working with

scenarios involving IP networks as well as with the

existent infrastructure of the traditional telephone

networks. Increasingly, the solution to this challenge

is related to the use of "Voice over IP" (VoIP)

technology in which the phone calls travel through a

broadband connection instead of traveling by

conventional telephone networks (Keller 2009).

In contrast to such development, the formation of

most of these professionals is still based on

traditional methodologies in which the teacher is the

holder of knowledge and the mass production of

labor force is prioritized.

However, methods have been changed and the

universities´ great challenge is to provide training

courses with the purpose to frame the content

according to the student so that he/she can become a

technically qualified professional in addition to

being able to adapt to frequent changes and demands

of the labor market (Silva & Viana, 2013).

With the change of teaching methods, an

approach is proposed in which learning is based on

practical problems solution (PBL). Such problems

are applied in scenarios that simulate a company´s

environment and allow students to develop and

clarify, in practice, the theoretical concepts that were

learned in the classroom, but that were formed in an

obscure and abstract way (Fernandes, 2013).

According to Ali and Samaka (2013), problem-

based learning is a student-centered, self-directed,

inherently collaborative pedagogy where students

learn by working in groups through solving

197

C. Dias M., F. Gabi C., P. Rodrigues E., R. Souza V. and Perkusich A..

A Problem-based Learning Case Study for Teaching Voice over Internet Protocol - VoIP - Using Asterisk as a Tool for Teaching VoIP for Information

Technology Classes.

DOI: 10.5220/0004848701970204

In Proceedings of the 6th International Conference on Computer Supported Education (CSEDU-2014), pages 197-204

ISBN: 978-989-758-021-5

 2014 SCITEPRESS (Science and Technology Publications, Lda.)

problems and reflecting their experiences. These

students are supervised by a tutor or supervisor.

The literature suggests that strategies, in which

learning is based on problem solving, are effective in

teaching in summarily practical areas, such as

information technology. According to Cavalcante

and Embiruçu (2013), it is possible to realize how

this practice has been established around the world

and how it can be applied in engineering courses.

The use of VoIP technology has also been

established. Furthermore, its importance in the

communication systems, that are about to come, is

highlighted in Goode (2002). Recently, Dias (et al.

2013) showed that the use of practical experiments

can aid in teaching practical concepts of traditional

telephony and IP telephony.

This paper will present a case study on the

implementation of the Asterisk software as an aid

tool related to the practical teaching by using the

PBL approach to teach concepts of VoIP technology

and its interaction with the traditional telephone

system. This methodology was employed in courses

in Electrical Engineering and Telecommunication

System Technology, but it could easily be applied to

any course which had subjects with VoIP

technologies in their course programs.

The PBL technique was chosen so as the concept

and the motivation could be introduced to the

students at the beginning of the subject. This

method consists of groups´ choice and of the

fulfillment of practical problems which will be

solved through the use of software, laboratory

practices, theoretical content and the professor´s

support. Such method aids the undergraduate

students to solve practical questions by themselves

(Lamar et. al. 2012).

According to Ribaud and Saliou (2013), PBL can

help students to learn with the complexity and

perceive that there are no simple responses for

problem scenarios, nevertheless learning and life do

occur in contexts which can alter the available and

possible type of solution.

The problem-based learning method was initially

carried out in the Telephony subject with emphasis

on the telecommunication area. The students were

evaluated in accordance with their theoretical and

practical performance, being their critical sense

analyzed in relation to the proposed problem.

This paper is organized as follows: section 2 will

focus on a bibliographical review on the concepts of

VoIP and a presentation of the Asterisk tool; section

3 will describe the materials and methods used in the

design of the proposed experiment; in section 4, the

results obtained from the experiment will be

presented and in section 5, we present the

conclusions.

2 VOICE OVER IP AND

ASTERISK CONCEPTS

2.1 Voice over IP (VoIP)

Voice over IP is a set of networking protocols that

have the function to normalize and regulate the

sending of the voice from a source to a destination

by using TCP/IP data networks (Keller, 2009). That

is, an analog voice signal is converted into a set of

digital signals, which is then sent through an internet

connection in the form of packets with IP

addressing.

The main difference between VoIP and

traditional telephony is related to the way the voice

is transported. This difference suggests that the only

requirement to use VoIP technology is concerned

with the use of a TCP / IP connection between two

points with end to end delay less than 150 ms. This

requirement creates some unique advantages to

VoIP, such as (Keller 2009; Goode 2002):

 Cost reduction: expenditure decrease with

traditional telecom operators and the use of

softphones instead of conventional phones;

 Unique infrastructure: the convergence of voice

and data networks will also make the physical

network unique;

 Mobility: the branch line must be in a position

where you can connect to the Internet;

 The telephone system control: reduces the users´

dependence from the telephone exchange

maintenance company; and

 New features: some of them which are not

available in the traditional telephony become

possible, such as voice encryption during calls.

In VoIP technology, signaling protocols are

responsible for determining a standard that specifies

the data format and the rules to be followed by data

traffic. Moreover, these signaling protocols are used

to establish connections, determine the destination

and also for issues related to signs such as: ring,

caller ID, disconnection, among others. Currently

the major signaling protocols for VoIP are (Silva,

2010):

 Session Initiation Protocol (SIP);

 Media Gateway Control Protocol (MGCP);

 Jingle;

 H.248/Megaco; and

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 Inter-Asterisk eXchange (IAX).

Taking into account what has been mentioned, it

is observed that information technology

professionals, who work with communication

networks, need practical tools that help them to learn

the concepts of IP telephony during their training.

The Asterisk, the software already mentioned,

however, is presented as an alternative to building

practical experiments of similar complexity and low

cost for the reality of the job market.

2.2 The Asterisk Platform

The Asterisk software is able to perform the function

of a private telephone exchange, which has as one of

its primary functions the management of audio

transmitted in digital communication channels

(Madsen, Meggelen & Bryant, 2011). Asterisk can

be used as an extremely powerful and flexible tool

designed for the learning of VoIP technologies and

protocols once it allows reproducing in laboratories

situations and problems only seen in real public or

private telephone networks.

Moreover, one of the advantages of working with

Asterisk in the classroom is the fact that Asterisk is

free software distributed by Digium® that is based

on the GPL (General Public License) (Martín, 2009).

The free version of Asterisk eliminates the need for

a conventional private telephone exchange because

its version has no limits of application. Additionally,

Asterisk receives users´ contributions from all over

the world, making this software always updated

(Madsen, Meggelen & Bryant, 2011).

Asterisk's architecture was designed with great

care so that there was as much flexibility as possible

with regard to the operation of different types of

hardware and software (Martín, 2009). Figure 1

shows the Asterisk internal architecture which is

formed by a core and specific APIs (Application

Programming Interfaces) which support the

switching of internal information to PBX.

Information processing in the Asterisk core

occurs in such a way that the specific protocols,

codecs and hardware interfaces are abstracted from

the information. This allows Asterisk to be able to

connect to any hardware technology available (either

current or future) to perform its essential functions

(Silva, 2010).

The functioning and operation of Asterisk are

based on the use of modules that the programmer

can choose to use or not, depending on the

application that he/she is working with. Table 1

describes the main modules for the correct

functioning of an Asterisk server (Asterisk, 2010).

Figure 1: Arquitetura Interna do Asterisk (Asterisk, 2011).

Table 1: Asterisk modules.

Module Description

Channel

Drivers

The channel drivers make the

communication with devices

outside Asterisk possible by

translating the signaling, or

protocol, to the core.

Dialplan

Applications

This module provides call

functionality to the system.

Dialplan

Functions

This module is used to set and

retrieve parameters of

configurations on a call.

Resources

Used to provide resources to

Asterisk, like music on hold

and call parking.

CODECs

This module is used to encode

and decode audio or video so it

takes less bandwidth.

File Format

Drivers

Used to save media to disk in

specific file formats and

convert files back to media

streams on the network.

Call Detail

Record

(CDR)

Drivers

Used to write call logs to a disk

or to a database.

Call Event

Log (CEL)

Drivers

Similar to CDR, but with

details of what happened inside

Asterisk during a particular call.

Bridge

Drivers

Used by bridging architecture

in Asterisk to provide various

methods of bridging call media

between participants in a call.

When installing Asterisk, the student will be

automatically deploying the use of VoIP (Keller,

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2009). The software use provides the broadening of

learning so that the student can study from the

creation of the used VoIP extension lines up to the

monitoring of packets sent and the signaling

exchange between the terminals. With the purpose

of carrying out this investigation, an additional

program called Wireshark will be used (Wireshark,

2011).

3 MATERIALS AND METHODS

The experiment was carried out in five phases:

1. Definition of the learning objectives;

2. Construction of a scenario to be set up in a

laboratory;

3. Definition of the problems to be presented for the

students to solve them;

4. Validation in an undergraduate subject with 60

credit hours, in the minimum, and that had VoIP

in its course description; and

5. Evaluation of the obtained results.

The following learning objectives were defined

in the first phase:

 Define what VoIP is;

 Differentiate VoIP technologies from the other

ones used for establishing telephone calls both in

public and private telephony;

 Understand the functioning of SIP, RTP and

SRTP protocols; and

 Set up the Asterisk software for establishing

telephone calls using VoIP technologies.

In the second phase, the scenario was defined

and constructed to be used in the phase related

specifically to PBL. The chosen scenario

encompasses studies on VoIP with analog terminals

and IPs telephones, as shown in Figure 2.

Figure 2: Scenario of the proposed solution.

The objective of this scenario is to provide an

environment where students can understand the

basic operation of a server set up with Asterisk and

that they can, through the Asterisk configuration,

make calls between IP terminals that are connected

to the server. Subsequently, to understand how the

signaling and voice traffic occur on the TCP/IP

network, a network monitoring is carried out, in

which calls and SIP protocols are captured and

RTP/SRTP are analyzed.

When assembling the scenario, the following

pieces of equipment available in the IFPB

Telephony and Convergent Networks Laboratory

were used: Computer with 4GB RAM, processor

Intel QUAD CORE, plate FXS / FXO, E1 board,

Impacta 68 Intelbras hybrid telephone exchange and

Grandstream GXP-1200 VoIP phones.

After defining the scenario, the Linux operating

system is installed on a computer which serves as a

platform for the use of Asterisk. Figure 3 shows the

Asterisk console after it is installed on Linux. It is

through the console that the main information

regarding Asterisk operation is accessed as well as it

is possible to give operation commands to the

software.

Figure 3: Initiation of Asterisk Server.

Then, it is necessary to create the SIP

communication channels so that the Asterisk server

can identify each extension. Creating SIP channels

with Asterisk occurs by editing the sip.conf

configuration file. The code segment below shows

how to create four SIP extensions.

[general]

Bindport = 5060

Bindaddr = 0.0.0.0

disallow = all

allow = alaw

language = en_US

[commom_to_branches](!)

type = friend

context = branches

host = dynamic

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[2000](commom_to_branches)

secret = 1234

mailbox = 2000

[2001](commom_to_branches)

secret = 1234

mailbox = 2001

[3000](commom_to_branches)

secret = 1234

mailbox = 3000

[3001](commom_to_branches)

secret = 1234

mailbox = 3001

In order to have communication among the

created extensions, it is necessary to set the Asterisk

server dial plan. The dial plan is created by editing

the Asterisk extensions.conf. The following code

segment shows how the dial plan setup is made for

the scenario in Figure 2.

[branches]

; Impacta 68 branches

exten => 2000,1,Dial(SIP/2000,30)

exten => 2001,1,Dial(SIP/2001,30)

; IP phones branches

exten => 3000,1,Dial(SIP/3000,30)

exten => 3001,1,Dial(SIP/3001,30)

The use of analog terminals is allowed by setting

the central Impacta 68 using specific software, as

shown in Figure 4. It is interesting to note that the

Impacta 68 central exchange recognizes the Asterisk

server as a registration server on the network.

Without this setting, it would not be possible to

make Impacta 68 extensions to connect with the

GXP-1200 IP terminals.

Figure 4: Configuração do Servidor de Registro da

Impacta 68.

An IP terminal configuration can be performed on

the phone itself or via the web server with the

terminal IP address. Figure 5 shows the web server

configuration for GXP-1200.

Figure 5: Programming Interface for GXP-1200.

After configuring the extension lines, the

terminals send a registration request to the Asterisk

server and connect to the server. Figure 6 shows the

Figure 6: Registration of SIP Channels in Asterisk Server.

In addition to the default implementation, it is

possible to activate SRTP protocol on the devices

that will provide greater security in the sent packets.

Encrypted data, even with packets being captured by

network monitoring software, will not give access to

the content within them. Therefore, the devices

become secure on the network.

In the showed scenario (Figure 2), it is possible

to observe how the Asterisk server is able to

establish communication with the IP telephony

devices and the Impacta 68 Central Telephone

through the Internet. Such scenario has great

importance to students´ training in the Telephony

and Convergent Networks areas, given that its

market performance will involve working from

assembly to the understanding of the concepts that

were applied in practice, in the classroom.

In the next phase, the problems were made up so

as to be presented to the students. After researching

in workplaces of several telecommunication

AProblem-basedLearningCaseStudyforTeachingVoiceoverInternetProtocol-VoIP-UsingAsteriskasaToolfor

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companies and interviews with their professionals,

the problems were defined to be presented to the

students. The main objective was to focus on

problems that reflected situations commonly found

in professional environments involving VoIP.

For the validation phase, the Telephony subject

of the Electrical Engineering and

Telecommunication System Technology courses

was chosen. Such subject presents all the necessary

requirements: class load higher than 60 hours and

has VoIP in the course description. In the PBL, the

students were evaluated by the professor, by using

continuous evaluation in several sessions that the

presented problem demanded it, taking into account

the learning objectives. At the end, grades were

expressed on a scale of 0-100 and they were

attributed to each one of the students.

In the evaluation phase, the students answered to

a questionnaire of satisfaction and the professor of

the mentioned subject carried out a subjective

evaluation on the experience. This feedback allowed

outlining new actions for improving the used

approach. Furthermore, a comparison regarding the

repetition and final general mean of the group´s

grades in relation the previous semester was

accomplished.

4 RESULTS

The mentioned scenario was established several

times by students of Telephony discipline, of the

Bachelor in Electrical Engineering and the Higher

Course in Telecommunication System Technology

from the Federal Institute of Education, Science and

Technology of Paraíba (IFPB), which covers

detailed study of VoIP. This enabled students to

carry out the implementation of all practice and

successfully absorb the theoretical content

previously seen in the classroom.

Using Wireshark to monitor the network, it was

possible to capture packets related to the exchange

of SIP protocol signaling sent from the terminals to

the Asterisk server and vice versa. It was also

possible to capture the data stream transmitted

among the terminals which uses the RTP protocol.

Figure 7 shows an example of the exchange of SIP

signaling captured during the experiment.

At the end of the experiment, the students were

able to further increase the tab because two IP

telephone protocols, RTP and SRTP, were used for

security check in sending packets on the network. It

was realized that when the SRTP mode is chosen in

the terminals, the packets travel on the network in

Figure 7: SIP Protocol Signaling Exchanged Between Two

Terminals.

the safest way and it will not be possible to hear the

content of the calls made, even with the capture of

the packets. However for the RTP protocol, it was

possible to examine the content of the calls when

fulfilling the capture of the packets on the network.

The experiment accomplishment in the

classroom allows the understanding of the

theoretical content on VoIP technology because the

use of Asterisk develops concepts related to both the

traditional telephony and IP telephony as well as

convergent networks. Furthermore, the use of

Asterisk adds to the training of information

technology professional a powerful and low cost

solution to problems related to voice traffic in

computer networks.

After validation, results were encouraging. While

comparing with the numbers of the same subject, in

the previous semester, the repetition rate was

reduced to zero and the final general mean of the

group´s grades also increased meaningfully from

61,7 to 81 – an increase of 31,35%. Students were

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questioned in relation to the PBL satisfaction level

to detriment of the classical approach (in which the

professor only transmits knowledge and the student

has a secondary role in the learning process) as well

as in regard to the general satisfaction with the

subject. These results are presented in Figures 8 and

9. In both questionings, the students should mark

only one of the following alternatives: very satisfied,

satisfied, fairly satisfied, little satisfied or not

satisfied.

Figure 8: Satisfaction with PBL compared with the

classical approach.

Figure 9: General Satisfaction with the Telephony Subject.

The subjective evaluation of the professor

pointed out four important aspects:

1. The proposition of non-trivial problems and with

no unique solution increased the challenge level

for the students and it acted as stimulus to the

participation in the activities of the subject.

2. Students faced difficulty in coping with the

diversity of pieces of equipment that make up the

scenario of the experiment, though they have had

the professor´s explanation about the functioning

and operation of such devices at the beginning of

the class.

3. There was a noteworthy evolution in regard to

the attainment of the general learning objectives

in the group from the previous semester.

4. In relation to the PBL, the professor managed to

improve the students´ performance, but he/she

highlighted the need for teacher´s assistant in

case of groups larger than 24 students.

5 CONCLUSIONS

The experience with the PBL using Asterisk as a

tool for making up scenarios and problems for VoIP

technology teaching was extremely positive both for

the professor and the students. In the experimental

phase of the mentioned subject, the students´

satisfaction level with the PBL reached 87,5% (

satisfied and very satisfied as shown in Figure 8).

Furthermore, the evaluation by means of grade and

the professor´s subjective perception indicated

progress regarding the attainment of the learning

objectives on the students´ behalf.

With Asterisk, it was possible to create a

reasonable complexity scenario which is present in

several telecommunication companies that offer

VoIP services; all of this with low cost when

compared with the proprietary solutions available in

the market. Therefore, providing the students

problems that are close to the professional reality,

which they will face in the job market, constituted a

feasible activity.

As consequence of the analysis of the results,

before presenting the problems to be solved for the

students´ appreciation, three simple laboratory

practices, with the aim to make the students familiar

with the equipment and software to be used in the

proposed scenario, were introduced. Hence, it is

expected to mitigate the problem pointed out by the

professor regarding the students´ difficulty in

dealing with such devices and the associated

software during the resolution phase of the presented

problems, in addition to allowing an enhanced focus

on the work itself.

Currently, this approach is being used in the

same subject and the results will be evaluated in a

continuous improvement process. The objective is to

consolidate and improve the model so as it can be

used for the VoIP technology teaching in the

Engineering, Computing and Technology courses.

12,5

SatisfactionwiththePBL(%)

VerySatisfied

Satisfied

FairlySatisfied

LittleSatisfied

NotSatisfied

62,5

12,5

GeneralSatisfactionwith

TelephonySubject(%)

VerySatisfied

Satisfied

FairlySatisfied

LittleSatisfied

notsatisfied

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