Computer Networks 53 (2009) 2050–2090 Contents lists available at ScienceDirect Computer Networks journal homepage: www.elsevier.com/locate/comnet VoIP: A comprehensive survey on a promising technology Stylianos Karapantazis *, Fotini-Niovi Pavlidou Department of Electrical and Computer Engineering, Aristotle University of Thessaloniki, Panepistimioupoli, 54124 Thessaloniki, Greece article info abstract Article history: The Internet has burgeoned into a worldwide information superhighway during the past Received 23 February 2008 few years, giving rise to a host of new applications and services. Among them, Voice over Received in revised form 9 November 2008 IP (VoIP) is the most prominent one. Beginning more as a frolic among computer enthusi- Accepted 17 March 2009 asts, VoIP has set off a feeding frenzy in both the industrial and scientific communities and Available online 28 March 2009 has the potential to radically change telephone communications. In this article, we survey Responsible Editor: K. Kant all these aspects that have the greatest impact on the quality of voice communications over IP networks. The survey begins with the merits and demerits of VoIP, followed by the Qual- ity of Service (QoS) requirements that voice imposes and a description of test methods for Keywords: VoIP the assessment of speech quality. We then proceed with a delineation of the issues related IP Telephony to the conversion of analog voice to packets, namely we spell out the details of the most Voice quality well-known voice codecs, while light is also thrown on voice activity detection and voice Voice codecs packetization. Header compression schemes receive intense scrutiny as well. We also pro- Signaling protocols vide an overview of the signaling protocols that are tailored to the needs of VoIP, and we Call admission control continue with the comparison of the call admission schemes that are geared towards the Security QoS constraints of VoIP. The pivotal issue of security is then discussed, pointing out poten- tial threats as well as approaches for tackling them. Finally, the survey concludes with a discussion on the feasibility of providing VoIP over challenging satellite links. Ó 2009 Elsevier B.V. All rights reserved. 1. Introduction joys widespread popularity as an alternative to traditional telephony in homes and enterprises. The Internet has burgeoned into a worldwide informa- VoIP has the potential to revolutionize telephone com- tion superhighway during the past few years and wrought munications. The trend toward voice communications over significant changes in the telecommunications arena. This the Internet is mainly fuelled by the salient advantages unprecedented growth motivated the development of Internet telephony offers. VoIP opens up exciting possibil- innovative applications with high bandwidth and low ities for users. In particular, it paved the way for monetary end-to-end delay requirements. One of the applications savings. It is cheaper for end-users to make an Internet that thrived is Voice over IP (VoIP). VoIP, also known as telephone call than a circuit-switched call since most VoIP IP or Internet telephony, is the technology that enables providers offer affordable long distance and international people to use the Internet as the transmission medium calling. Moreover, a significant amount of money can be for voice communications. Beginning as a frolic among saved on the monthly phone service considering that IP computer enthusiasts, VoIP has set off a feeding frenzy in telephony service is less expensive than the traditional both the industrial and scientific communities. Since its phone service. In addition, VoIP offers service flexibility inception, huge strides have been made and now VoIP en- since there are no dependencies between the application and the underlying network. VoIP users can already enjoy a variety of features, which they previously had to pay * Corresponding author. Tel.: +30 2310 996380. E-mail addresses: [email protected] (S. Karapantazis), [email protected] for, for free. Some of these features are voicemail, caller (F.-N. Pavlidou). ID, call conferencing, call waiting and call forwarding. This 1389-1286/$ - see front matter Ó 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.comnet.2009.03.010 S. Karapantazis, F.-N. Pavlidou / Computer Networks 53 (2009) 2050–2090 2051 kind of users can also enjoy some new features like e- of a VoIP system, we also spell out several test methods mailed voicemail and the easy management of contacts. that have been devised to gage voice quality. Then, a pleth- Moreover, new services will commence to flourish as the ora of voice codecs are delineated and their performance market matures that will break the shackles of the Public disparities are accentuated, along with all the aspects that Switched Telephone Network (PSTN) for voice services. should be contemplated before opting for a codec. Closely One service that is expected to catch on is voice e-mail. An- related to the selection of voice codecs are also the issues other key asset of IP telephony is flexibility. A user can of silence suppression and voice packetization. Thus, we move the IP Phone wherever he/she needs and still keep touch upon them as well. Another issue that receives in- the same phone number. In addition, this technology offers tense scrutiny in our study is header compression tech- a big step forward in available features and functions and niques. Header compression techniques are the stepping allows end-users to control different media and different stone to efficient bandwidth utilization. All the compres- types of terminals from their Web browsers. Users will sion schemes that have been proposed thus far are com- be able to set up conference calls from their homes, check pared against each other in order to come up with the the state of their accounts at any time and so on. most efficacious one. Several compelling reasons also impelled service pro- Next we turn our attention to signaling protocols. Sig- viders to consider providing voice communications over naling protocols lay the foundations for high-quality voice the Internet. First and foremost, VoIP promises them new communications since they are responsible for establishing revenue sources. Further, new operators can capitalize and tearing down calls. We describe the salient features of upon this technology since it gives them an easy and all the well-known protocols with a view to listing their cost-efficient way to compete with incumbent operators pros and cons. Moreover, we touch on the way Skype by undercutting their pricing regimes, while avoiding the works, albeit information about Skype’s signaling protocol regulatory hindrance to standard voice provision. From is hard to procure. We also cast light to the way VoIP sys- an engineering viewpoint, VoIP also holds considerable ap- tems interface with PSTN. Next, we lay particular emphasis peal because it gives carriers the ability to manage a single on an aspect that has often been considered as the down- network that supports both voice and data. side of VoIP, that is, end-to-end QoS. Development of Notwithstanding the aforementioned advantages, sev- end-to-end QoS presents numerous challenges and is con- eral issues must be accounted for while designing a VoIP tingent upon the ability of the underlying network to pro- system. VoIP is sometimes likened to mobile telephony be- vide differentiated QoS to various applications and users. cause they both present some clear advantages over voice Our attention is drawn towards call admission control communications over PSTN. Mobile networks provide (CAC) techniques. CAC and QoS are inextricably interwo- mobility and flexibility, whereas VoIP has an edge in cost ven. A plurality of CAC techniques can be found in the lit- savings. Moreover, VoIP performance is on a par with the erature. We describe all the schemes that are geared performance of mobile cellular systems. On this account, towards VoIP and some important conclusions are drawn. there are some circumstances where the quality is consid- The next issue that we treat in this survey is security of ered acceptable. However, several strides remain to be VoIP networks. It is beyond the realms of possibility that made in order for VoIP to match the performance of the a system will not experience malicious attacks that will well-engineered PSTN. A laudable aim is to provide the aim to impair its performance. Contrary to the PSTN, VoIP same level of quality as PSTN does. systems are exposed to a number of threats. We first ana- In this paper, rather than glossing over this promising lyze the potential threats and then discuss several ap- technology, we aim to shed light to all these aspects that proaches on how to protect a VoIP network. We also have the greatest bearing on voice quality (Fig. 1). We be- dwell on the impact of security measures on QoS. Last gin our survey with a reference to the Quality of Service but not least, we conclude this survey with a discussion (QoS) requirements that VoIP imposes, followed by a on the feasibility of providing VoIP over satellite links. Next description of how certain performance metrics affect generation telecommunications systems can be viewed as speech quality. Remedies are discussed as well. Before an ultimate amalgamation of all existing and emerging elaborating upon the factors that affect the performance fixed and mobile systems. In this context, satellite systems Voice Data Signaling Data Security Wireless Link Signaling Protocol QoS Mechanisms VoIP Terminal IP Phone Header Compression Voice codec Voice codec Header Compression Fig. 1. General VoIP architecture. 2052 S. Karapantazis, F.-N. Pavlidou / Computer Networks 53 (2009) 2050–2090 will be called to support Internet telephony and we aim to being transmitted and therefore, are subject to queuing cast light on the main problems that arise in this case. delay. Delay has two acute effects on voice performance.