Tài liệu Accelerating the Deployment of VoIP and VoATM pdf

This tutorial will explore the role of next- generation switches which, as they become widely adopted for both access and core networking, must be able to handle voice traffic over both

Accelerating the Deployment of VoIP

and VoATM Overview

The economic advantages of packet voice are driving both the access and core

voice networks away from circuit switching towards packet The industry

continues to debate whether the future of these packet networks will be based on

pure ATM, pure Internet protocol (IP), IP over asynchronous transfer mode

(ATM), IP over multiprotocol label switching (MPLS), or a combination thereof

There are advantages to both ATM and IP and reasons for choosing each This

tutorial will explore the role of next- generation switches which, as they become

widely adopted for both access and core networking, must be able to handle voice

traffic over both IP and ATM networks for future extensibility as the debate

continues and must have the features necessary to interwork with existing public

switched telephone network (PSTN)

Topics

1 Introduction

2 Voice over Packet Architecture

3 Why Voice over IP?

4 Why Voice over ATM?

5 Designer Considerations for Voice over Packet

6 Elements of a Next-Generation Switching Platform

7 Switching Platform/Media Gateway

8 Signalling Gateway

9 The Softswitch/Media Gateway Controller

10 Application Server (AS) and Services

11 Conclusion

Self- Test

Correct Answers

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Glossary

1 Introduction

Carriers are moving voice services to packet networks both to reduce upfront and operational costs and to provide more value-added services in an increasingly competitive environment A recent study by a major carrier found that packet equipment was 70 percent less expensive than traditional voice equipment, and data access lines were 60 percent to 80 percent cheaper than voice lines

Maintenance of packet networks was 50 percent less expensive, while

provisioning was 72 percent lower However, consolidation of voice from the PSTN onto packet networks has, in the past, proven difficult and therefore has happened very slowly International voice-over–IP call volumes, which provide the most compelling business case for packet telephony, are still a drop in the ocean of international telephony traffic but have experienced phenomenal growth since 1998, according to a recent report by Washington, D.C.–based research group TeleGeography According to the "TeleGeography 2001" report, which contains results of an exclusive survey of major voice-over-packet (VoP)

providers in 1999 and 2000, international Internet telephony traffic volumes reached 1.7 billion minutes in 1999—a growth rate of more than 1,000 percent from 1998 IDC projected more than 9 billion minutes of voice traffic to travel over worldwide packet networks in 2000, exceeding 135 billion minutes in 2004 Service revenue is projected at $1.6 billion in 2000 and $18.7 billion in 2004 While it is clear that VoP is growing, there is still considerable debate about whether the underlying network technology will be ATM or IP At the edge of the network the choice, driven primarily by the regional Bell operating companies (RBOCs), is ATM An ATM–dominated access network is clearly in the works because until recently IP did not provide the quality of service (QoS) guarantees that are so important for voice Although QoS protocols such as DiffServ,

resource reservation protocol (RSVP), and MPLS have been implemented, most

of today's IP traffic is actually being carried over ATM However, in the long term with the recent success of MPLS it appears that pure IP over lambda may be the winner And certainly, IP at the application layer and the desktop is a more than just a viable near-term situation

In addition to the challenges in architecting networks with end to end QoS,

service providers must ensure that the rollout of such networks cause no

disruption to their existing voice service revenue, which currently represent about 80 percent of their overall revenue source With more than $650 billion of worldwide revenue generated by traditional voice and fax services and more than

$250 billion installed base of traditional equipment infrastructure in the United States alone, service providers must deploy next-generation packet switches that seamlessly interconnect and competitively function as time division multiplexing

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(TDM)–based PSTN switches as well as support voice over ATM (VoATM) and voice over IP (VoIP)

It appears that most carriers, especially the larger incumbent carriers will start the migration to packet telephony on the trunk side first (Class-4 tandem) and eventually migrate to the access (Class 5) This migration model is similar to the migration from analog switches to digital switches, which started in the late 1970s Carriers first started on the inner network (i.e., tandem) and then moved outwards to the Class 5

The architecture for VoP, the reasons for choosing IP or ATM, and considerations

in next-generation system design need to be understood to accelerate VoP

deployments

2 Voice over Packet Architecture

In principle, two basic technologies are used for building high-capacity networks: circuit switching and packet switching In circuit-switched networks, network resources are reserved all the way from sender to receiver before the start of the transfer, thereby creating a circuit The resources are dedicated to the circuit during the whole transfer Control signaling and payload data transfers are

separated in circuit-switched networks Processing of control information and control signaling such as routing is performed mainly at circuit setup and

termination Consequently, the transfer of payload data within the circuit does not contain any overhead in the form of headers or the like Traditional voice telephone service is an example of circuit switching

Circuit-Switched Networks

Carrier-class next-generation switches need to be high-capacity fault-tolerant TDM and VoP switches They must be designed to significantly enhance the economics of providing traditional TDM–based voice and data services as well as help service providers migrate to a packet-based telecom network (based on VoIP and VoATM) and generate new competitive services Service providers deploying next-generation switches can cap their investment in traditional circuit switches and migrate to a converged switching infrastructure that allows them to reduce the number of overlay network platforms and provide profitable voice and data

services over packet networks See Figure 1

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Figure 1 A High-Capacity TDM Switch Capable of Packet

Switching

Since most of the core packet networks today are ATM–based, but most likely migrating to IP–based, the most future-proof investment is in next generation switches that can be deployed to transport voice on both ATM and IP networks

supporting protocol layers as outlined in Figure 2

Figure 2

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3 Why Voice over IP?

Support for voice communications using IP, which is usually called VoIP, has become especially attractive to consumers given the low-cost, flat-rate pricing of the public Internet

VoIP is the ability to make telephone calls and access service over IP–based data networks with a suitable QoS and superior cost/benefit to PSTN–based calls Today, most of the VoIP implementations are carried over ATM–based transport

as shown in the second column of Figure 2

The benefits of implementing VoIP are mostly consumer-based and can be

divided into the following three categories:

• Cost reduction—IP is everywhere It is on our desktops and it is what

the Internet is based on Many people view the Internet as a "free transport" for data and voice services With the introduction of

Net2Phone and other similar "free" services, many people are now making phone calls over the Internet In addition, businesses and individuals have turned to higher-quality commercial products and services to make voice calls based on IP The prevalence of IP nodes and the abundant supply of better IP–based switches and routers continue to reduce the cost of providing VoIP

• Simplification and consolidation—An integrated infrastructure

that supports all forms of communication could allow more

standardization and could reduce the total equipment complement The differences between the traffic patterns of voice and data offer further opportunities for significant efficiency improvements

Universal use of IP for all applications, voice and data, holds out the promise of both reduced complexity and more flexibility

• Advanced applications—Even though basic telephony and facsimile

are the initial applications for VoIP, the longer-term benefits are

expected to be derived from multimedia and multiservice applications For example, Internet commerce solutions can combine World Wide Web access to information with a voice call button that allows

immediate access to a call center agent from a PC In addition, voice is

an integral part of conferencing systems that could also include shared screens, white boards, etc Combining voice and data features into new applications will provide the greatest returns over the longer term Utilizing an IP–based network for voice traffic can offer advantages to consumers

of reduced costs, simplification, and consolidation due to the proliferation of IP–

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based applications and devices at the desktop These advantages are compelling for consumers and are driving service providers to consider VoIP

implementations In contrast, VoP over the ATM–based network offers distinct advantages directly to service providers and are still much more prevalent today

4 Why Voice over ATM?

ATM, from the start, was designed to be a multimedia, multiservice technology Although ATM has been accepted by service providers for its ability to deliver high-speed data services, until recently its potential for deploying voice services was overlooked With the competitiveness of today's market though, network operators and service providers have been continuously striving to reduce

operating costs and lift network efficiency and have turned to the ATM network

to achieve these goals

With hundreds of millions of dollars of ATM equipment infrastructure in the United States alone, service providers have recognized that significant economies

of scale can be achieved if the data traffic and voice traffic are integrated onto a single network In order to achieve this, service providers have started to use the circuit emulation services (CESs) of ATM switches to carry full or fractional E-1/T-1 circuits between end points These CES mechanisms treat voice as a

constant stream of traffic encoded as a constant bit rate (CBR) stream In

actuality though, voice is a combination of bursts of speech and silence and this increases the complexity of VoP

The ATM Forum and International Telecommunications Union (ITU) came up with several advanced mechanisms to improve the efficiencies of transporting voice traffic, including:

• ATM trunking using AAL–1 for narrowband services

• ATM trunking using AAL–2 for narrowband services

• IP over ATM (AAL–5)

• Loop emulation service using AAL–2

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Table 1 summarizes the benefits of utilizing the different methods for

5 Design Considerations for Voice over Packet

Adding voice to packet networks requires an understanding of how to deal with system level challenges such as interoperability, call control and signaling, voice encoding, delay, echo, reliability, density, and performance of all the elements that make up the next-generation switching platform

6 Elements of a Next-Generation

Switching Platform

The vision for a next-generation switching platform is a distributed architecture

in which media gateway/bearer transport platform, signaling, call control, and application elements are divided into separate logical network components (see

Figure 3), communicating with one another through the use of intraswitch

protocols such as Megaco, media gateway control protocol (MGCP), and

SCTP/M3UA This distributed model allows service providers to scale their

network to support hundreds of thousands of subscriber ports per node In this concept, voice traffic is directed between the traditional voice network and the new packet-based networks by the media gateway The call control is handled by

a softswitch, and the features and services are handled by an application

platform In reality, the softswitch (or call control platform) may support some of the more popular services without requiring a separate application platform An example of this type of service is 7/10 digit routing, which would be handled directly by the call control platform Other examples of where the application

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platform may not be involved are caller name delivery, local number portability (LNP), and E-800 service These services are already implemented in the PSTN using service control points (SCPs) In these cases, the call control platform will send intelligent network (IN)/transactional capabilities application part (TCAP) queries over the signaling system 7 (SS7) network to existing SCPs

Figure 3 Elements of a Next-Generation Switching Platform

Some vendors enable one or more of these logical network elements to be

deployed on the same physical platform There are some inherent advantages to this "integrated" model especially with platforms that support up to 100,000 subscriber ports (DS–0s) per bearer platform/media gateway, and allow efficient execution of the softswitch and signaling gateway software Benefits also include cost savings and deployment and operation simplicity In the "integrated" model, the need for intraswitch protocols such as Megaco and MGCP are not required; however interswitch protocols such as RTP/UDP/IP (for MG to MG) and BICC (for SG to SG) are always required for interoperability with the other ends See

Figure 4 for relevant inter-switch protocols

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Figure 4 Interoperability: Call Control, Signaling, and Bearer

Platforms

7 Switching Platform/Media Gateway

Sometimes referred to as a media gateway, the switching/bearer transport

platform is hardware that sits at the edge of a network and takes in a packet and/or circuit containing voice or data traffic and switches it to a voice or data network Media gateways come in many different flavors depending on the

breadth of definition The most popular consist of Class 4 and Class 5

replacement functionality on a voice over digital subscriber line (VoDSL)

gateway Media gateways are part of the physical transport layer and are

controlled by a call control engine or softswitch (also called a media gateway controller), which provides instructions to direct voice traffic Media gateways are

at the heart of the transformation of the voice network, as they are essential to migrating voice traffic onto a packetized network As part of packetizing voice traffic, a media gateway adapts (by using compression and echo cancellation) the packetized traffic, creates and attaches an IP header and/or ATM header, and sends the packet through the network according to instructions provided by the softswitch

While a media gateway can be physically located almost anywhere within the network, depending on the network architecture and the features it is intended to support, all media gateways share certain features including the following:

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• Scalability—A media gateway needs to be able to scale to support

hundreds of thousands of telephone calls (called DS–0s, running at 64 Kbps per line) to parallel the scalability of the existing PSTN switches

• Support for several types of access networks—Needed support

includes wireless, fiber, cable, and copper In addition to electrical interfaces, a media gateway needs to support a variety of optical

interfaces (including OC–3, OC–12, OC–48, and OC–192 speeds)

• Carrier-class reliability—Also known as five nines (99.999 percent)

reliability (i.e., less than five minutes of downtime per year) and

network equipment building standards (NEBS) certification (the

Telcordia quality rating for meeting environmental stress tests),

reliability is extremely important to service providers because it

enables them to fulfill customer contracts Most carriers cite reliability

as the impetus to transform their current architecture

• Interworking functionality—Media gateways are capable of

supporting multiple voice and data interface protocols and

compatibility between them by converting circuit traffic to packet traffic and vice versa

• Interoperability—Most networks are a compilation of multivendor

solutions, making interoperability essential for success

• Control support—To enable communication between the media

gateway and a softswitch The most common languages (or protocols) emerging for communication between these devices are MGCP and Megaco

• Switching—A media gateway must handle switching and media

processing, based on an ATM, IP, or TDM switching fabric

• Voice transportation—There are 3 transport standards used for

transporting voice traffic: TDM (traditional circuit-switch method), ATM AAL–1/AAL–2, and IP–based RTP/RTCP (over ATM or pure−IP transport)

A packetized approach to transmitting voice faces a number of technical

challenges that spring from the real-time or interactive nature of the voice traffic Some of the challenges that need to be addressed include the following:

• Echo—Echo is a phenomenon where a transmitted voice signal gets

reflected back due to unavoidable impedance mismatch and wire/two-wire conversion between the telephone handset and the communication network Echo can, depending on the severity, disrupt