|While few doubt that next-generation network
infrastructures will focus on cell- and packet-based architectures, some
still debate the relative importance of IP and ATM. Will one all but
obviate the other? Or will they coexist? And, if they coexist, how might
they complement each other? To explore these questions, we'll have to take
a close look at IP and ATM technologies. They differ in several respects.
For example, IP is much more widely deployed than ATM, and it has a lower
overhead than ATM. However, ATM offers a fuller set of bandwidth
management and quality of service capabilities.
Regardless of the underlying technology used in the network
infrastructure, all network operators and service providers will have to
focus on identifying ways to increase revenues, reduce costs, and increase
profits. Increasing revenues will be possible over the long term as
service providers identify mechanisms to present, deliver, and bind new
voice, data, and video services to the end users.
In the shorter term, however, network operators and service providers
will be alert to other opportunities, especially opportunities to reduce
costs. A significant portion of the costs associated with building and
running a network are related to the operational aspects (service
provisioning, billing, maintenance, etc.). Other costs, such as for access
and transport, are comparatively small.
Operational burdens, then, deserve special scrutiny. And here we arrive
at a particularly costly operational burden: the maintenance of two
network infrastructures, a circuit-switched infrastructure to support
voice, and a cell- or packet-based infrastructure to support data. To
reduce the costs of maintaining two network infrastructures,
consolidation, or convergence, may be the answer. Voice over IP (VoIP) and
voice over ATM (VoATM) are different architectural approaches to
integrating voice into the next-generation cell- or packet- based
infrastructures. The immediate purpose is to reduce costs; eventually, the
focus will shift to enabling new services.
VOICE OVER IP
The H.323 standards, the MGCP (Media Gateway Control Protocol)
specification, and the SIP (Session Initiation Protocol) specification
pertain to voice over IP (also known as "Internet telephony" and
"IP telephony"). H.323 is an International Telecommunication
Union (ITU) family of standards that specifies the components and
protocols required to provide multimedia communications over packet-based
networks, including (most importantly) Internet Protocol (IP)-based
H.323 supports mechanisms for point-to-point or multi-point
communication for audio, video, or data between terminals, gateways,
gatekeepers, and multi-point control units. Gateways perform the
interworking function between the different signaling protocols and media
streams carried on IP- and circuit-switched networks and allow the
seamless integration of telephony services between these two different
VOICE OVER ATM
The voice trunking over ATM (VTOA) specifications, defined by the ATM
Forum, are used to interconnect PBXs or other end-user equipment across
the ATM network infrastructure. VTOA enables the transport of
circuit-switched voice and data using either the ISDN or ISUP (ISDN user
part) signaling protocols over the ATM network infrastructure.
The ISDN and ISUP protocols have been used for some time to establish
and maintain both basic and supplementary voice and data services through
the public switched telephone network (PSTN). ISDN is used to access the
PSTN, and ISUP is used within the PSTN to manage the circuit-switched
connections between PBXs or end-user equipment.
VTOA transports the ISDN and ISUP signaling messages transparently
across an ATM virtual connection which control the transport of voice or
data circuits through other ATM virtual connections. The voice or data
circuits can be transported over ATM Adaptation Layer (AAL) 1 or 2. AAL1
supports Constant Bit Rate services, and AAL2 supports Variable Bit Rate
services. AAL2 allows several different circuits to be supported over a
single ATM connection with either the same or different quality of service
(QoS) requirements, and AAL2 provides a more efficient utilization of ATM
network resources. VTOA allows telephony services to be easily carried
over an ATM-based network infrastructure.
QUALITY OF SERVICE AND MPLS
Today's existing voice services are feature-rich, high-quality, highly
available, and secure. Regardless of the underlying technology, future
voice services offered over the cell- and packet-based network must
continue to offer this level of service. Cells and packets introduce a
level of uncertainty into what, until recently, had been a fairly
deterministic network. Delay, jitter, and potential information loss now
become serious issues that must be addressed to ensure that the
appropriate QoS is available for a wide range of network users.
Multi Protocol Label Switching (MPLS) is becoming the mechanism of
choice to manage traffic flows in IP-, ATM- and frame relay-based
networks. MPLS is an Internet Engineering Task Force (IETF) specified
protocol that provides for the efficient designation, routing, forwarding,
and switching of traffic flows through the network.
MPLS can manage traffic flows of various granularities and provide a
means of mapping IP addresses to simple fixed-length labels used by
different packet-forwarding and packet-switching technologies. MPLS can
interface to existing routing protocols such as RSVP, OSPF, and also
support the IP, ATM, and frame relay Layer 2 protocols.
The Layer 3 protocol forwards the first few packets of a flow. As the
flow is identified and classified (based on various QoS requirements), a
series of Layer 2 high-speed switching paths are set up between the
routers located along the path between the source and destination of the
High-speed switching is possible because the fixed-length labels (also
known as "tags") are inserted at the very beginning of the
packet or cell to be used by hardware to quickly switch packets between
links. MPLS can be introduced into a network without impacting the
existing operation of other routing, switching, and forwarding protocols
within the network, allowing for the gradual deployment of MPLS without
having to replace the network infrastructure all at once.
IP and ATM are battling it out as the technology of choice for the core
network infrastructure. Both technologies have unique strengths when
applied in the core network infrastructure, and there will be no clear
winner. VoIP will be an important service as the spread of IP technology
continues. Likewise, it is clear that network operators using ATM
technology and the equipment manufacturers supplying them will need to
support VTOA to enable the inevitable integration of voice services over
the new broadband network infrastructure.
VTOA reduces the number of switches and trunks needed to build the
network, reduces the bandwidth needed to support voice and data, and
provides the means to integrate existing telephony services into an
infrastructure that is capable of supporting the enormous anticipated
growth of new packet- and cell-based multimedia services. While VoIP and
VTOA technologies compete for their role in the next-generation network,
complementary technologies, such as MPLS, will be needed regardless of the
underlying infrastructure to ensure the QoS requirements continue to be
met as the network evolves.
Jeff Lawrence is president and CEO of Trillium Digital Systems, a
provider of communications software solutions for computer and
communications equipment manufacturers. Trillium develops, licenses, and
supports standards-based communications software solutions for SS7, ATM,
ISDN, frame relay, V5, IP, and X.25/X.75 technologies. For more
information, visit the company's Web site at trillium.com..
Today's network infrastructure is creaking under the strain of data
traffic, which includes text, graphics, and video traffic, and which
continues to grow apace. And yet, while the demand for data bandwidth is
increasing anywhere between 200 and 400 percent each year, the demand for
voice bandwidth is increasing only 5 to 10 percent each year.
In a few years, the amount of data bandwidth required on the network
will be at least an order of magnitude greater than the required amount of
voice bandwidth. Consequently, future demand for data bandwidth will
easily eclipse demands placed on the network by voice.
Voice will still continue to be a necessary -- but economically less
important -- service offered by network operators and service providers.
Already, this trend is evident with the introduction of aggressively
priced offerings from network operators and service providers. Typically,
these offering bundle standalone voice services and voice-Internet-cable
It won't be long until voice services are given away "free"
as part of other more lucrative bundled service offerings. Voice traffic
will certainly not disappear, and it will always be a fundamental service
that must be offered by the network. However, the growth and the build-out
of the next-generation network infrastructure will be focused on cell- and
packet- based architectures to support data.