Open Source Telephony

Future of Open Source Telephony with Asterisk and its impact on VOIP industry.

1. Introduction:

History & Background:

The free software movement was launched in 1983. In 1998, a group of individuals advocated that the term free software should be replaced by open source software (OSS) as an expression which is less ambiguous and more comfortable for the corporate world [1]. Software developers may want to publish their software with an open source license, so that anybody may also develop the same software or understand its internal functioning. Open source software generally allows anyone to create modifications of the software, port it to new operating systems and processor architectures, share it with others or market it.

The Open Source Definition, notably, presents an open source philosophy, and further defines the terms of usage, modification and redistribution of open source software. Software licenses grant rights to users which would otherwise be reserved by copyright law to the copyright holder. Several open source software licenses have qualified within the boundaries of the Open Source Definition. The most prominent and popular example is the GNU General Public License (GPL). While open source distribution presents a way to make the source codes of a product publicly accessible, the open source licenses allow the authors to fine tune such access.

The open source label came out of a strategy session held in Palo Alto in reaction to Netscape's January 1998 announcement of a source code release for Navigator (as Mozilla). A group of individuals at the session included Todd Anderson, Larry Augustin, John Hall, Sam Ockman, Christine Peterson and Eric S. Raymond. They used the opportunity before the release of Navigator's source code to clarify a potential confusion caused by the ambiguity of the word "free" in English. The 'open source' movement is generally thought to have begun with this strategy session. Many people, nevertheless, claimed that the birth of the Internet, since 1969, started the open source movement, while others do not distinguish between open source and free software movements. [2]

The Free Software Foundation (FSF), started in 1985, intended the word 'free' to mean freedom to distribute (or "free as in free speech") and not freedom from cost (or "free as in free beer"). Since a great deal of free software already was (and still is) free of charge, such free software became associated with zero cost, which seemed anti-commercial.

The Open Source Initiative (OSI) was formed in February 1998 by Eric S. Raymond and Bruce Perens. With at least 20 years of evidence from case histories of closed software development versus open development already provided by the Internet developer community, the OSI presented the 'open source' case to commercial businesses, like Netscape. The OSI hoped that the usage of the label "open source," a term suggested by Peterson of the Foresight Institute at the strategy session, would eliminate ambiguity, particularly for individuals who perceive "free software" as anti-commercial. They sought to bring a higher profile to the practical benefits of freely available source code, and they wanted to bring major software businesses and other high-tech industries into open source. Perens attempted to register "open source" as a service mark for the OSI, but that attempt was impractical by trademark standards. Meanwhile, due to the presentation of Raymond's paper to the upper management at Netscape—Raymond only discovered when he read the Press Release, and was called by Netscape CEO Jim Barksdale's PA later in the day—Netscape released its Navigator source code as open source, with favorable results.

Open Source Applications Criteria:

Open source doesn't just mean access to the source code. The distribution terms of open-source software must comply with the following criteria:

1. Free Redistribution

The license shall not restrict any party from selling or giving away the software as a component of an aggregate software distribution containing programs from several different sources. The license shall not require a royalty or other fee for such sale.

2. Source Code

The program must include source code, and must allow distribution in source code as well as compiled form. Where some form of a product is not distributed with source code, there must be a well-publicized means of obtaining the source code for no more than a reasonable reproduction cost preferably, downloading via the Internet without charge. The source code must be the preferred form in which a programmer would modify the program. Deliberately obfuscated source code is not allowed. Intermediate forms such as the output of a preprocessor or translator are not allowed.

3. Derived Works

The license must allow modifications and derived works, and must allow them to be distributed under the same terms as the license of the original software.

4. Integrity of The Author's Source Code

The license may restrict source-code from being distributed in modified form only if the license allows the distribution of "patch files" with the source code for the purpose of modifying the program at build time. The license must explicitly permit distribution of software built from modified source code. The license may require derived works to carry a different name or version number from the original software.

5. No Discrimination Against Persons or Groups

The license must not discriminate against any person or group of persons.

6. No Discrimination Against Fields of Endeavor

The license must not restrict anyone from making use of the program in a specific field of endeavor. For example, it may not restrict the program from being used in a business, or from being used for genetic research.

7. Distribution of License

The rights attached to the program must apply to all to whom the program is redistributed without the need for execution of an additional license by those parties.

8. License Must Not Be Specific to a Product

The rights attached to the program must not depend on the program's being part of a particular software distribution. If the program is extracted from that distribution and used or distributed within the terms of the program's license, all parties to whom the program is redistributed should have the same rights as those that are granted in conjunction with the original software distribution.

9. License Must Not Restrict Other Software

The license must not place restrictions on other software that is distributed along with the licensed software. For example, the license must not insist that all other programs distributed on the same medium must be open-source software.

10. License Must Be Technology-Neutral

No provision of the license may be predicated on any individual technology or style of interface.

Open Source Telephony/ IP PBX:

Open source IP PBX is based on the open source software (OSS) model. The software can be downloaded for free, and the source code is available to all users for modification and redistribution. Like other OSS licenses, General Public Licenses (GPLs) enable users to download and deploy the software without paying a license fee to the original developer. Unlike other OSS licenses, GPLs provide contributors with financial incentive that encourage active contribution to ongoing development. Cost savings, coupled with a rich and robust feature set and customization capabilities make open source IP PBX a viable option for your entire organization, especially for your contact center, which handles customer-facing interactions.

Converged TDM and packet-based communications systems are a popular topic in computer

science and telecommunications arenas. This research work discusses a system called Asterisk that is an open source hybrid TDM and packet voice platform.

Asterisk that is an open source hybrid TDM and packet voice platform. Asterisk is designed to interface any piece of telephony hardware or software with any telephony application. This makes Asterisk a powerful component that can be easily used in NGN softswitches, conferencing servers and Private Branch eXchanges (PBX). Asterisk VoIP environment is integrated with H.323 and SIP packet-based networks. Its interworking with our H.323 and SIP networks and the functionality it offers in terms of a converged TDM and packet-based communications system.

Asterisk plays very important role to bridging the gap between traditional and network telephony while Voice over IP (VoIP) is often thought of as little more than a method of obtaining free long-distance calling, the real value of VoIP is that it allows voice to become nothing more than another application in the data network.

Asterisk's architecture is fundamentally simple, but different from most telephony products. Essentially, Asterisk acts as middleware, connecting telephony technologies on the bottom to telephony applications on the top, creating a consistent environment for deploying a mixed telephony environment. Telephony technologies currently supported by Asterisk include VoIP protocols like H.323, SIP, IAX and MGCP (both gateways and phones), as well as more

traditional TDM technologies like ISDN (PRI and BRI) and PSTN. Telephony applications include call bridging, conferencing, voicemail, auto attendant, custom interactive

voice response (IVR) scripting, call parking, etc.

Asterisk is an open source software package. Hundreds, if not thousands, of developers are working every day on Asterisk, extensions of Asterisk, software for Asterisk, and customized installations of Asterisk. A big portion of the product's flexibility comes from the availability of the source code. This means, we can modify the behavior of Asterisk to meet our needs.

The challenge comes from the fact that an industry that has changed very little in the last century shows little interest in starting now.

What is Voice over IP?

Voice over Internet Protocol (VoIP) is the basic method of carrying voice calls over an IP network. This includes digitization and packetization of voice streams. In other words, it is a mechanism of sending voice traffic over a data network.

Voice over Internet Protocol (VoIP) refers to voice communications that are delivered via an IP network, such as packet-switched networks or the Internet. The advent of this technology offers businesses myriad benefits, but many businesses are reluctant to completely overhaul their analog systems. The cost and labour associated with a full-VoIP replacement can be mitigated with the implementation of a Hybrid VoIP system, which offers the functionality of a full VoIP system without the extensive investment.

The Benefits of VoIP

Because VoIP converts an analog voice signal into digital format, telephone conversations essentially represent data transfers via Internet Protocol (IP) packets. These data packets can be sent with considerably less expense than traditional analog signals. Furthermore, the flexibility of data transmission means that VoIP technology offers a wide range of capabilities. Therefore VoIP provides several cost and usability benefits:

* Reduction in communication costs, especially for long-distance and international calls

* Simplified office wiring, with only one set of wires at each workstation, for both data and voice applications

* IP-based technology that allows sales and other personnel to share content, email, and files while conferencing

* Unified messaging, so that voicemail can be accessed from workstation handsets, soft phones, or PC

* Access to the Private Branch Exchange (PBX) from outside the office, for employees who travel or telecommute

* Seamless, update-free employee transfer among offices, since the phone number transfers with the IP phone set

VoIP offers a single dialing and voicemail system that allows free calls among office sites and supports remote workers. These appealing options make VoIP an attractive business communication solution. Yet people remain hesitant to switch from PBX to full VoIP, due to several factors:

* The prospect of replacing equipment that still functions and retraining employees presents considerable expense.

* If the existing LAN network does not have sufficient excess capacity, rewiring will be necessary, to ensure the quality of voice and data transmission.

* Disruption to business during these upgrades can result in lost revenue.

* It is somewhat difficult to quantify and measure the financial benefits of the productivity benefits and cost benefits of VoIP.

What is Asterisk?

Asterisk LogoAsterisk is the world's most popular open source telephony project. Under development since 1999, Asterisk is free, open source software that turns an ordinary computer into a feature-rich voice communications server. Asterisk makes it simple to create and deploy a wide range of telephony applications and services.

Code for Asterisk, originally written by Mark Spencer of Digium, Inc., has been contributed from open source software engineers around the world. Currently boasting over two million users, Asterisk supports a wide range of telephony protocols. It includes rich support for the handling and transmission of voice over traditional telephony interfaces including analog lines, ISDN-BRI lines and digital T1/E1 trunks. Asterisk also features support for a wide range of VoIP protocols including SIP, IAX and H.323 among others. It supports U.S. and European standard signaling types used in business phone systems, allowing it to bridge between next-generation voice-data integrated networks and existing infrastructure.

Asterisk is released as open source under the GNU General Public License (GPL), and it is available for download free of charge. Asterisk® is the leading open source telephony project and the Asterisk community has been ranked as a key factor in the growth of VoIP.

What Does Asterisk Do?

Asterisk is like an erector set or a box of Legos for people who want to create communications applications. That's why we refer to it as a "tool-kit" or "development platform". Asterisk includes all the building blocks needed to create a PBX system, an IVR system or virtually any other kind of communications solution. The "blocks" in the kit include:

* Drivers for various VoIP protocols.

* Drivers for PSTN interface cards and devices.

* Routing and call handling for incoming calls.

* Outbound call generation and routing.

* Media management functions (record, play, generate tone, etc.).

* Call detail recording for accounting and billing.

* Transcoding (conversion from one media format to another).

* Protocol conversion (conversion from one protocol to another).

* Database integration for accessing information on relational databases.

* Web services integration for accessing data using standard internet protocols.

* LDAP integration for accessing corporate directory systems.

* Single and mult-party call bridging.

* Call recording and monitoring functions.

* Integrated "Dialplan" scripting language for call processing.

* External call management in any programming or scripting language through Asterisk Gateway Interface (AGI)

* Event notification and CTI integration via the Asterisk Manager Interface (AMI).

* Speech synthesis (aka "text-to-speech") in various languages and dialects using third party engines.

* Speech recognition in various languages using third party recognition engines.

This combination of components allows an integrator or developer to quickly create voice-enabled applications. The open nature of Asterisk means that there is no fixed limit on what it can be made to do. Asterisk integrators have built everything from very small IP PBX systems to massive carrier media servers.

Asterisk Key Applications:

Asterisk as A PBX

Asterisk can be cond as the core of an IP or hybrid PBX, switching calls, managing routes, enabling features, and connecting callers with the outside world over IP, analog (POTS), and digital (T1/E1) connections.

Asterisk runs on a wide variety of operating systems including Linux, Mac OS X, OpenBSD, FreeBSD and Sun Solaris and provides all of the features you would expect from a PBX including many advanced features that are often associated with high end (and high cost) proprietary PBXs. Asterisk's architecture is designed for maximum flexibility and supports Voice over IP in many protocols, and can interoperate with almost all standards-based telephony equipment using relatively inexpensive hardware.

Asterisk as A Gateway

It can also be built out as the heart of a media gateway, bridging the legacy PSTN to the expanding world of IP telephony. Asterisk's modular architecture allows it to convert between a wide range of communications protocols and media codecs. Asterisk as a feature/media server.

Need an IVR? Asterisk's got you covered. How about a conference bridge? Yep. It's in there. What about an automated attendant? Asterisk does that too. How about a replacement for your aging legacy voicemail system? Can do. Unified messaging? No problem. Need a telephony interface for your web site? Ok.

Asterisk in The Call Center

Asterisk has been adopted by call centers around the world based on its flexibility. Call center and contact center developers have built complete ACD systems based on Asterisk. Asterisk has also added new life to existing call center solutions by adding remote IP agent capabilities, advanced skills-based routing, predictive and bulk dialing, and more.

Asterisk in The Public Network

Internet Telephony Service Providers (ITSPs), competitive local exchange carriers (CLECS) and even first-tier incumbents have discovered the power of open source communications with Asterisk. Feature servers, hosted services clusters, voicemail systems, pre-paid calling solutions, all based on Asterisk have helped reduce costs and enabled flexibility.

Supported platforms

Asterisk® is primarily developed on GNU/Linux for x/86 and runs on GNU/Linux for PPC along with OpenBSD, FreeBSD, and Mac OS X. Other platforms and standards-based UNIX-like operating systems should be reasonably easy to port for anyone with the time and requisite skill to do so.

Asterisk® is available in Debian Stable and is maintained by the Debian VoIP Team

Supported hardware

Asterisk® needs no additional hardware for Voice over IP. For interconnection with digital and analog telephony equipment, Asterisk® supports a number of hardware devices, most notably all of the hardware manufactured by Digium®, the creator of Asterisk®.

Supported protocols

Asterisk® supports a wide range of protocols for the handling and transmission of voice over traditional telephony interfaces including H.323, Session Initiation Protocol (SIP), Media Gateway Control Protocol (MGCP), and Skinny Client Control Protocol (SCCP).

Using the Inter-Asterisk eXchange (IAX™) Voice over IP protocol Asterisk® merges voice and data traffic seamlessly across disparate networks. The use of Packet Voice allows Asterisk® to send data such as URL information and images in-line with voice traffic, allowing advanced integration of information.

Asterisk® provides a central switching core, with four APIs for modular loading of telephony applications, hardware interfaces, file format handling, and codecs. It allows for transparent switching between all supported interfaces, allowing it to tie together a diverse mixture of telephony systems into a single switching network.

Asterisk Features

Asterisk-based telephony solutions offer a rich and flexible feature set. Asterisk® offers both classical PBX functionality and advanced features which interoperates with traditional standards-based telephony systems and Voice over IP systems.

Call Features

ADSI On-Screen Menu System
Alarm Receiver
Append Message
Automated Attendant
Blind Transfer
Call Detail Records
Call Forward on Busy
Call Forward on No Answer
Call Forward Variable
Call Monitoring
Call Parking
Call Queuing
Call Recording
Call Retrieval
Call Routing (DID & ANI)
Call Snooping
Call Transfer
Call Waiting
Caller ID
Caller ID Blocking
Caller ID on Call Waiting
Calling Cards
Conference Bridging
Database Store / Retrieve
Database Integration
Dial by Name
Direct Inward System Access
Distinctive Ring
Distributed Universal Number Discovery (DUNDi™)
Do Not Disturb
Fax Transmit and Receive (3rd Party OSS Package)
Flexible Extension Logic
Interactive Directory Listing
Interactive Voice Response (IVR)
Local and Remote Call Agents
Music On Hold
Music On Transfer:
- Flexible Mp3-based System
- Random or Linear Play
- Volume Control

Call Features

Predictive Dialer
Open Settlement Protocol (OSP)
Overhead Paging
Protocol Conversion
Remote Call Pickup
Remote Office Support
Roaming Extensions
Route by Caller ID
SMS Messaging
Spell / Say
Streaming Media Access
Supervised Transfer
Talk Detection
Text-to-Speech (via Festival)
Three-way Calling
Time and Date
VoIP Gateways
- Visual Indicator for Message Waiting
- Stutter Dialtone for Message Waiting
- Voicemail to email
- Voicemail Groups
- Web Voicemail Interface

Computer-Telephony Integration

AGI (Asterisk Gateway Interface)
Graphical Call Manager
Outbound Call Spooling
Predictive Dialer
TCP/IP Management Interface


TDMoE (Time Division Multiplex over Ethernet)
Allows direct connection of Asterisk PBX
Zero latency
Uses commodity Ethernet hardware
Voice-over IP
Allows for integration of physically separate installations
Uses commonly deployed data connections
Allows a unified dialplan across multiple offices


G.711 (A-Law & μ-Law)
G.723.1 (pass through)

VoIP Protocols

SIP (Session Initiation Protocol)
IAX™ (Inter-Asterisk Exchange)
MGCP (Media Gateway Control Protocol
SCCP (Cisco® Skinny®)

Traditional Telephony Protocols

E&M Wink
Feature Group D
MF and DTMF support
Robbed-bit Signaling (RBS) Types
MFC-R2 (Not supported. However, a patch is available)

PRI Protocols

BRI (ISDN4Linux)
Lucent 5E
National ISDN2

Research Aim and Objectives:

Integrating Asterisk (Open Source Telephony) as VOIP Gateway in Telecom Industry.

Research Questions:

* Why Asterisk is better than Other VOIP Telephony Platforms?

* Why Traditional Telephony is switching towards the Voice over IP (VOIP)/data networks?

· What is the future of Open Source Telephony with Asterisk and its importance in VOIP Industry?



[1]. Raymond, Eric S. (1998-02-08). "Goodbye, "free software"; hello, "open source"". Retrieved 2010-02-07

[2]. Muffatto, Moreno (2006). Open Source: A Multidisciplinary Approach. Imperial College Press. ISBN 1860946658.

[3] Retrieved 2010-02-07 Open Source Initiative

Jim V, Leif M (2004). Asterisk™: The Future of Telephony. USA: O'Reilly. p 01-09

David G (2005). Building Telephony Systems with Asterisk. UK: Packt Publishing. p05-15

Penton, A. (2004). Asterisk: A Converged TDM and Packet-based. Voice over IP. 01 (01), p01-05.

Anonymous . (2009). Asterisk (PBX). Available: Last accessed 11-Dec-2009.

M. Spencer, M. Allison, C. Rhodes, The Asterisk Handbook, 2003,

Asterisk Documentation Team, Available online at

Arsalan H. Abbasi. (2003). Voice over IP. A Discussion of Business and IT Challenges. 1 (1), p04-06.

Kristin Masters. (2009). PBX to VoIP. Available: Last accessed 15 JAN 2010.

Jody, V. (2009). Future Telephony Platforms. Available: Last accessed 18 JAN 2010.

David ,B (2009). Build feature-rich telephony systems with Asterisk. Birmingham: Packt Publishing. p7.

Please be aware that the free essay that you were just reading was not written by us. This essay, and all of the others available to view on the website, were provided to us by students in exchange for services that we offer. This relationship helps our students to get an even better deal while also contributing to the biggest free essay resource in the UK!