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

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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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.
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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.

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

No No Yes No
ATM
trunking
using AAL−1
No No Yes Yes
VoIP over
ATM
Yes Yes No No
AAL−2
Yes Yes Yes Yes
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