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May 2000

Jeff Lawrence Battle Of The Infrastructures


Go Right To: The Voice-To-Data Shift

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.

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 networks.

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 infrastructures.

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.

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 flow.

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

The Voice-To-Data Shift

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 services.

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.

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