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Industry Insight
April 2000

Jim Machi IP Versus ATM: It's No Debate 


At least once every week someone asks my opinion on when IP will replace ATM. Since I'm getting a bit tired of the question, let's set this straight. While ATM and IP do offer some competing methods to packetize voice, they co-exist in most networks -- and will continue to for some time. 

The real debate is this: Which is the better solution? Packets over SONET (POS)? Or IP over ATM? Today this debate is mainly among network engineers working for service providers. But it will become an increasingly important issue as more voice traffic migrates to IP.

As in most debates, both sides have some advantages and there is no one best answer. The right solution will depend on the operating environment and business model where the technology will be used.

Getting Down To Basics
To understand some of the points being argued, it's essential to understand some basic technical issues. SONET (Synchronous Optical NETwork) is the underlining transport mechanism of both POS and IP over ATM. SONET is a standard for transporting and multiplexing data over optical media between two points. This technology has been deployed in all carriers' networks.

A scalable technology, SONET has a hierarchy of optical signals defined from an OC-3 (155 Mbps) to OC-192 (9.6 Gbps). Each optical signal maps into the next highest layer. SONET also has a low overhead rate of about 3.7 percent, making it a very efficient transport technology.

POS is technically simple. SONET provides a point-to-point connection between two routers. IP packets can be mapped directly into SONET using a point-to-point link protocol (PPP). At each node, the IP packets are wrapped into PPP frames. When they get to the next node, they are unwrapped. The destination address is examined, routed to the correct path, and re-wrapped as new PPP frames. This is a very efficient way to transport IP. But it leaves the network management and control to the routers.

IP over ATM is more complex. It is important to have a basic understanding of ATM. This standards-based multiplexing and switching technology is built on a fixed-cell length of 53 bytes. ATM is a connection-oriented technology. For two end points to communicate, a virtual circuit must be established between them. The virtual circuit concept is crucial to ATM. Each virtual circuit is assigned a traffic or service class, which provides the quality of service (QoS).

There are several ways to place IP directly over ATM. These include classical IP over ATM and LAN emulation. The most effective method is multiple protocols over ATM (MPOA), which basically provides for a distributed virtual router. In simple terms, it allows an increase in efficiency and higher throughput compared to other IP over ATM methods.

While the technical issues are always interesting to debate, their impact can be seen in some of the network attributes including cost, performance, and management.

Bandwidth Issues
POS is a very efficient way to transport IP. IP can achieve up to 95 percent of the available line rate when running over PPP compared to 80 percent of the available line rate when running IP over ATM. The difference is the overhead required to support ATM.

IP over ATM has the advantage on bandwidth management. The virtual circuits allow you to assign flexible bandwidth to each virtual circuit (VC). This allows differential service levels among multiple users. A VC carrying VoIP can be assigned a high priority compared to one that is strictly data, so higher quality of services levels can be easily achieved.

PPP has no mechanisms providing bandwidth management, which is the responsibility of the IP layer. The IP layer must schedule its packet transmissions to ensure each flow receives its fair share of link bandwidth. MPLS (MultiProtocol Label Switching) will provide some bandwidth management feature; however, in the near term it will not match IP over ATM.

Quality Of Service
QoS is related to packet loss, throughput, end-to-end packet delay, and jitter. ATM traffic/service classes and management functions can help mitigate the first three problems. Since ATM is connection-oriented, jitter is less of an issue. However, IP over ATM can only offer these advantages between the ATM switches. If the quality of service is crucial and most of the problems occur in the WAN, IP over ATM is a good choice. Otherwise, neither technology has an advantage.

It's The Environment That Matters
While POS and IP over ATM offer different features, the operating environment and business model are essential to determining the best solution.

Traditional carrier networks usually own the fiber, making bandwidth cost less of an issue. However, bandwidth management and QoS are crucial. These companies need to provide high-quality voice services to customers paying 10 cents per minute, deal with legacy services that might not fit into a pure IP network, and provide Internet service. IP over ATM is a good fit for these customers. While IP may be the network layer of choice, ATM allows them to integrate legacy services and to offer differentiated services.

ISPs and emerging carriers are not encumbered by legacy services. Data-based services generate a large portion of their revenues compared to traditional carriers. Many ISPs and emerging carriers lease the fiber, making bandwidth cost an issue. They are usually more concerned with routing large numbers of packets between their backbone routers than in offering differentiated voice services. POS is a good choice for them.

Enterprises have a blend of the challenges facing ISPs and traditional carriers. They must lease bandwidth from the carriers. They need to provide different level of service to intranet traffic and intra-company voice traffic, and to deal with legacy data communications traffic. Many of these companies are migrating from frame relay to ATM-based services, allowing them to minimize their cost while providing the level of services end users require.

MPLS may solve many of the today's QoS and bandwidth management problems. Newer technologies like IP directly over DWDM, or DTM, might replace POS. However, in the future, please don't ask me about the great ATM versus IP debate. There just isn't a clear answer. Ultimately, the winner depends on the operating environment and business model of whoever's asking the question. c

Jim Machi is director of product marketing, Internet Telephony, for Dialogic Corporation (an Intel company). Dialogic is a leading manufacturer of high-performance, standards-based computer telephony components. Dialogic products are used in fax, data, voice recognition, speech synthesis, and call center management CT applications. For more information, visit the Dialogic Web site at www.dialogic.com.

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