Carrier Grade VOIP.Pdf

Source: Carrier Grade Voice Over IP CHAPTER 11 Introduction Downloaded from Digital Engineering Library @ McGraw-Hill (www.digitalengineeringlibrary.com) Copyright © 2004 The McGraw-Hill Companies. All rights reserved. Any use is subject to the Terms of Use as given at the website. Introduction 2 Chapter 1 The two terms that make up the title of this book, “carrier-grade” and “Voice over IP (VoIP),” mean many things to many people, and each term may have several meanings depending on the context. Furthermore, many people might consider the two terms to be mutually exclusive, making the phase carrier-grade VoIP an oxymoron. That used to be the case, but those days are over. The excitement that VoIP has generated and the resources that have been applied to developing technical solutions for VoIP have allowed it to become a serious alternative for voice communications. That is, VoIP is a serious alternative to the circuit-switched telephony that we have known for decades. Not only can VoIP technology now offer straightforward telephony services, but it can offer a lot more besides, and it can do it with the same quality of service (QoS) that we have become familiar with in traditional telephony. The overall objectives of this book are to examine the technical solutions that make VoIP a reality and to illustrate how today’s VoIP solutions can offer everything that traditional circuit switching provides, plus a lot more. This first chapter introduces the reader to VoIP in general and looks at some of the reasons why it is such a hot topic. Thereafter, a brief overview of technical challenges and solutions is presented. This chapter can be considered a glimpse of the topics that are covered in more depth in later chap- ters. Firstly, however, we need to understand what exactly is meant when we talk about terms such as VoIP and carrier-grade. What Is Meant by Carrier-Grade? Perhaps the easiest way to describe the term carrier-grade is to think of the last time you picked up your residential phone and failed to get a dial tone. With the exception of extraordinary circumstances, such as an earthquake, most people cannot remember such a time. Basically, the phone always works. Carrier-grade means extremely high availability. In fact, the require- ment for availability in commercial telephony networks is 99.999 percent. In other words, a carrier-grade network should be operational at least 99.999 percent of the time. This corresponds to a down time of no more than about five minutes per year and is known as five nines reliability. Carrier-grade means high capacity,the capability to support hundreds of thousands, perhaps millions, of subscribers and similar numbers of simul- taneous calls. For example, at the end of 2001, the Verizon network sup- Downloaded from Digital Engineering Library @ McGraw-Hill (www.digitalengineeringlibrary.com) Copyright © 2004 The McGraw-Hill Companies. All rights reserved. Any use is subject to the Terms of Use as given at the website. Introduction Introduction 3 ported over 70 million voice access lines.1 Meanwhile, the AT&T network carries about 300 million voice calls a day.2 Carrier-grade means that when you dial a number, you get through to the number you dialed. It means that when you finish dialing, the phone at the other end starts ringing within two to three seconds. It means that when someone answers and conversation takes place, the speech quality is very high, without any perceptible echo, noticeable delay, or annoying noise on the line. These are tough standards to meet. In terms of the network, these requirements translate to systems that are fully redundant, in some cases self-healing, highly scalable, and manageable. They mean compliance with numerous technical specifications to ensure interoperability with other networks. They also imply a highly skilled network maintenance organization, on duty around the clock, and ready to respond to a network problem in an instant. What Is Meant by VoIP? VoIP is simply the transport of voice traffic using the Internet Protocol (IP), hardly a surprising definition. However, it is important to note that VoIP does not automatically imply voice over the Internet. The Internet is a collection of interconnected networks, all using IP. The connections between these networks are used by anyone and everyone for a wide range of applications, from e-mail to file transfer to electronic commerce (e-commerce). As we shall see later, one of the greatest challenges to VoIP is voice quality and one of the keys to acceptable voice quality is bandwidth. If we are to ensure that sufficient bandwidth is available to enable high-quality voice, then we need to control and prioritize access to the available bandwidth. Currently, that does not happen on the Internet. In fact, each user of the Internet is at the mercy of all other users. One person transferring a huge file may cause other users’ transactions to proceed more slowly. As a result, voice quality over the Internet today may vary from acceptable to atrocious. 1Verizon Annual Report, 2001. 2www.att.com/network. Downloaded from Digital Engineering Library @ McGraw-Hill (www.digitalengineeringlibrary.com) Copyright © 2004 The McGraw-Hill Companies. All rights reserved. Any use is subject to the Terms of Use as given at the website. Introduction 4 Chapter 1 All is not lost, however. The Internet is changing rapidly in terms of its size, the number of users, and the technology that it uses. As technology changes and as more and more bandwidth is made available, it is possible that high-quality voice over the Internet may become the norm rather than the exception. However, that day is still not at hand. Consequently, while the term carrier-grade VoIP may not be an oxymoron, the term carrier- grade voice over the Internet may well be. Internet telephony may be considered a subset of, or a special case of, VoIP (aka IP telephony). However, within this book, when we refer to VoIP, we generally exclude Internet telephony.We shall see that high voice quality in IP networks requires the use of managed networks, QoS solutions, and service-level agreements (SLAs) between providers. Although the Internet might include all those things in the future, today’s Internet does not. In the meantime, a significant opportunity is available to deploy VoIP in networks where access and bandwidth are better managed. One such envi- ronment is that of next-generation Telcos, those companies building tele- phone networks using VoIP from the outset and posing a challenge to traditional carriers. A Little about IP IP is a packet-based protocol. This means that traffic is broken into small packets that are sent individually to their destination. In the absence of special technical solutions, the route that each packet takes to its destination is determined independently at each network node on a packet-by- packet basis. IP is not the only packet-based protocol in existence. However, it is by far the most successful. The explosive growth of the Internet proves this. IP itself provides no guarantees. For example, it is possible for different packets to take different routes from the origination point to the destination point, leading to the possibility of packets arriving at a destination in a different order than originally sent. Not only can packets arrive out of sequence, but some packets might not arrive at all or might be severely delayed. To combat these shortcomings, other protocols have been developed to operate in conjunction with IP to ensure that packets are delivered to their ultimate destination in the correct sequence and without loss. The most notable such protocol is the Transmission Control Protocol (TCP). TCP includes functions for the retransmission of packets that may have been Downloaded from Digital Engineering Library @ McGraw-Hill (www.digitalengineeringlibrary.com) Copyright © 2004 The McGraw-Hill Companies. All rights reserved. Any use is subject to the Terms of Use as given at the website. Introduction Introduction 5 lost or delayed and the assembly of packets in the correct order at the destination end. TCP is so widespread that the term TCP/IP is almost as com- mon as IP and is sometimes used synonymously with IP. Although the mechanisms used by TCP are appropriate and successful for data transfer (such as file transfers and e-mail), they are not appropriate for the delivery of voice traffic. Most data traffic is asynchronous and extremely error sensitive. For example, it hardly matters if an e-mail message takes 10 seconds or 30 seconds to reach its destination, but it is critical that every bit is received cor- rectly. On the other hand, voice traffic is synchronous in nature and a little more tolerant of errors. In speech, when someone speaks, the listener should hear it practically immediately, although it is not as critical that every millisecond of speech is heard. Given that IP provides no guarantees regarding the efficient transport of data packets, one wonders why IP would even be considered as a means for transporting voice, particularly with the stringent delay requirements that voice imposes. One also wonders how VoIP can be made to match the quality, reliability, and scalability of traditional networks. After all, if VoIP is to be a successful competitor to traditional telephony technology, then it must meet all the requirements met by traditional telephony, it must offer new and attractive capabilities beyond traditional telephony, and it must do so at a lower cost. Why VoIP? It is probably best to break this question into two parts.

Load more