.

This consideration has become much more important to carriers as the volume Heavy Traffic of IP traffic has increased. ser- vice providers (ISPs), local car- riers, and even large corporate users have

Technology News Technology thus begun sending IP directly over Sonet Drives Networks as a fast, cost-effective, reliable, fault-tol- erant, and more easily configurable alter- native. “Companies that have it will be able to IP over Sonet to compete more successfully in the face of deregulation in the new markets,” said Robert Rosenberg, president of The David Clark Insight Research Corp., a telecommuni- cations market research firm.

ABOUT SONET In the face of emerging proprietary ith Internet use continuing optical transmission protocols, Sonet was to explode, with an in- conceived by MCI and developed by creasing number of users Bellcore in the mid-1980s to create an switching to IP-based net- open standard for synchronous data W works, and with data traf- transmission on optical media. The stan- fic about to surpass voice traffic, network dard was approved in 1988 by Comité service providers have begun looking for Consultatif Internationale de Télé- a faster, more efficient, and less expensive phonique et Télégraphique (Consultative transport technology to handle the heavy Committee on International Telephone volumes of traffic they are experiencing. and Telegraphy), the predecessor to With this in mind, many providers Network service today’s International have decided to carry IP traffic directly providers are looking Union, and in 1989 by the American over Sonet (synchronous optical net- for a faster, more National Standards Institute. work), rather than via frame relay, ATM efficient, and less Sonet is widely deployed in the US backbones, or leased lines. Sonet and its expensive transport (composing almost all of Sprint Com- nearly identical European equivalent, munications’ and AT&T’s networks) and SDH (synchronous digital hierarchy), are technology. is quickly being adopted in Japan. the very fast and reliable physical-layer Adoption of SDH in Europe is about a transports used in most fiber-optic tele- year behind the US, according to Michael phone networks. Shariff, Cisco Systems’ packet-over-Sonet The technology was developed in the ets. To make full use of the bandwidth, product manager. mid-1980s and has been used in a num- users mapped IP traffic into their higher ber of networks volume of ATM traffic before trans- How Sonet works since the early 1990s. However, before porting it via Sonet. Sonet works by dividing a fiber trans- the huge growth of Internet traffic, there However, many data-network experts mission path into multiple logical chan- simply wasn’t enough IP traffic to make contend that ATM is not the most effi- nels called tributaries. A tributary’s basic efficient use of Sonet’s huge bandwidth. cient way to transport IP traffic and can- unit of transmission is an STS-1 (syn- This was a particular problem because not meet future demand for bandwidth chronous transport signal, level 1) or IP over Sonet uses the entire link because it has too much overhead. OC-1 (optical carrier, level 1) signal. STS between routers, so no other traffic Users may prefer to use ATM when describes a transmission’s signals when could be multiplexed with the IP pack- sending audio, video, voice, or other IP they’re in an electrical state, while OC data types that require quality of service describes the same traffic after it’s been (QoS). However, IP directly over Sonet converted into optical signals. STS-1 and can and does carry all types of traffic and OC-1 both operate at 51.84 Mbps. (OC- Editor: Lee Garber, Computer, 10662 Los is ideal for data-only networks because 2 is twice as fast, OC-3 is three times as Vaqueros Circle, PO Box 3014, Los Alamitos, it does not use ATM and thus eliminates fast, and so on.) CA 90720-1314; [email protected] a layer of the network stack and has Sonet is based on direct synchronous lower overhead. multiplexing, where separate signals are

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6

5 Packet and private WAN data 4 Public switched telephone network data 3 Tbytes/year Packet and private WAN voice 2

1 Public switched telephone network voice

1997 1998 1999 2000 2001 2002 2003 2004 2005

The amount of data traffic on global networks is increasing considerably faster than the amount of voice traffic, based on a study conducted by Probe Research for AT&T. According to the study, data accounts for only 25 percent of network traffic today but will account for about 50 percent by 2005. In the graph, each layer represents the total volume for a particular type of traffic.

multiplexed directly into higher speed bandwidth and already run on fiber. Meanwhile, as IP becomes the domi- signals. For example, an OC-12 circuit IP over Sonet uses nant internetworking protocol, IP over might carry traffic from four OC-3 links. Sonet will become even more significant. Sonet line rates are currently defined up • HDLC (high-level data link control) And since many carriers already have to OC-192 (9.953 Gbps), with OC-768 framing, which manages the flow at Sonet in their network infrastructures, it (39.813-Gbps) capacity expected next which data is sent and verifies that would be cost-effective for them to adopt year. This is considerably faster than frames have been received; and IP over Sonet. ATM’s maximum current rate of 622 • the Point-to-Point Protocol’s Link Mbps or Gigabit Ethernet’s 1 Gbps. Control Protocol, which manages DEBATE OVER ATM communication functions. There is a debate as to whether it’s Automatic protection switching more effective to use ATM to carry IP One of Sonet’s key features is APS DRIVING THE DEMAND traffic over Sonet. ATM proponents say (automatic protection switching), which The changing nature of data being that because the technology is circuit provides reliability by rerouting traffic if transported over networks will help drive switched and offers quality of service, it there is a fiber cut or other link-layer fail- the demand for IP over Sonet. As shown can more reliably deliver voice, video, ure. Sonet networks with APS are often in the figure on this page, market ana- and other time-sensitive packets. designed in a ring architecture (Sonet can lysts predict that data traffic will eclipse also be designed in a point-to-point archi- voice traffic on public networks shortly Efficiency tecture), using redundant strands of fiber. after the year 2000. To handle the However, proponents of directly map- If one fiber breaks, traffic shifts to another increasing volume of data, network car- ping IP onto Sonet say ATM is inefficient and transmission resumes in less than 50 riers are looking for the efficiency and because its overhead is high. Processing milliseconds, said Neil Grenfell, Sprint’s speed that IP over Sonet offers. ATM header information imposes a “cell vice president of engineering. An APS vari- And in the near future, as the figure tax” of about 60 Mbps on an OC-12 cir- ant called line switching offers even more above shows, text and e-mail traffic will cuit’s 622-Mbps bandwidth. Total band- redundancy by routing around failed be surpassed by image and video traffic, width is reduced further—up to a total Sonet equipment, not just failed fiber. which consume high bandwidth. In fact, of 30 percent—by the network-interface predicts Insight Research, overall overhead needed to map IP into ATM IP OVER SONET Internet use will grow 250 percent per frames. IP over Sonet, also sometimes called year through 2002, and telecommunica- An IP-over-Sonet circuit, on the other packet over Sonet, is predominantly a tions networks will demand bandwidths hand, uses almost all available band- technology for metropolitan or wide area of up to 25 Tbps by 2002. This will also width. According to Trillium Digital networks, many of which demand high drive demand for IP over Sonet. Systems, a communications

18 Computer .

vendor, a test of transmission efficiency showed IP directly over Sonet achieved IP into ATM over Sonet 95.4 percent of available line rate but IP over ATM on Sonet achieved only 79.6 Percent of Percent of percent. This is illustrated in more detail Available total overhead in the figure on this page. Protocol bandwidth line rate added by Moreover, proponents say Sonet layer (Mbps) achieved each layer would be particularly useful because most IP traffic consists of data, which the Sonet 155.520 100.0 3.70 technology handles particularly well, ATM 149.460 96.6 9.43 rather than voice or video. “We think IP-mapping to the Sonet AAL 135.362 87.5 6.41 payload is the most efficient way of trans- porting data,” said Cisco’s Shariff. LLC/SNAP 126.937 80.7 1.37 Networks using this technology would IP 125.918 79.6 0 thus be more cost-effective, he said, which would help small ISPs and local telephone carriers compete with large service and IP over Sonet telecommunications providers. Percent of Percent of Complexity Available total overhead Using ATM to transport IP over Sonet Protocol bandwidth line rate added by adds layers of equipment and protocol layer (Mbps) achieved each layer and thus gets more complex and expen- sive as transmission speed increases, Sonet 155.520 100.0 3.70 according to Bharat Doshi, head of Bell PPP 149.460 96.6 1.54 Lab’s Performance Analysis in Advanced Technologies Department. This is IP 147.150 95.4 0 because ATM has to segment and reassemble (SAR) packet traffic. The SAR function requires expensive chipsets Running IP directly over Sonet uses considerably less overhead than mapping IP into ATM and in ATM switches, which limits scalability. then running it over Sonet. IP over Sonet is thus more efficient. The figures show the level of Now that there’s enough IP traffic to overhead introduced when using both techniques while running an IP packet size of 576 bytes efficiently use the whole Sonet link, over a Sonet STS-3c link, which has a line rate of 155.52 Mbps. When running IP into ATM over Doshi said, users can avoid ATM’s Sonet, 20.4 percent of overhead is taken up by the various layers of the protocol stack: Sonet, “fancy routing.” ATM, AAL (ATM Adaptation Layer), LLC/SNAP (Logical Link Control/Subnetwork Access Protocol) Murat Azizoglu, an associate profes- encapsulation, and iP. When running IP over Sonet, only 4.6 percent of overhead is taken up by sor of electrical engineering at the Sonet, PPP, and IP. (Source: Trillium Digital Systems) University of Washington, added, “If you can bypass ATM, why not get the effi- cations, such as AppleTalk and IBM’s make the highly scalable IP over Sonet ciency and reduction in cost?” Systems Network Architecture. an even more effective technology. WDM Meanwhile, the Internet Engineering increases the capability of optical fiber Continuing to use ATM Task Force (IETF) and a number of ven- by dividing a transmission channel into ATM will still be used under certain dors are working on a Multiprotocol different wavelengths, each representing circumstances. For example, Doshi Label Switching (MPLS) standard that a separate transmission channel. A cable noted, many Sonet customers and service would, among other things, let ATM users could carry large amounts of data simul- providers already have considerable bypass the SAR process. The IETF expects taneously over each channel. ATM infrastructure in place, so there are to complete the MPLS standard—which Research labs that are working on places where using ATM still makes could also be used with technologies other WDM, such as those at Bell Labs and sense. For example, he said, organiza- than ATM—in the near future. Nortel Networks, have already divided tions will continue to use ATM at the transmissions into 200 different 10-Gbps edge of the network, where the LAN THE FUTURE OF IP OVER SONET channels, yielding an aggregate rate of 2 meets the WAN, to integrate multiple Some experts, such as the University Tbps. Azizoglu predicted that vendors data types onto a Sonet backbone. ATM of Washington’s Azizoglu, say that will release products delivering such permits the easy sharing of WAN band- advances in wavelength-division multi- speeds in a year or two. For Sonet to take width between IP and proprietary appli- plexing (WDM) technology will soon full advantage of this, researchers would

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have to develop a new definition for the technology that is faster and well suited Backbone cluster to WDM’s parallel capabilities. Some WDM researchers are even look- ing at ways to eliminate the Sonet layer ADM altogether, Doshi said. Nonetheless, said Ed Szurkowski, director of Bell Labs’ Information Systems Research Center, Sonet won’t be replaced any time soon. Backbone Backbone cluster ADM ADM cluster eployment of IP directly over Sonet or of IP into ATM over Sonet D depends on the type of network a user has, said Sprint’s Grenfell. ADM For example, he said, ISP backbones will probably use IP over Sonet when possible, because of its speed, scalability, and efficiency. They will frequently use the technology as shown in the figure on Backbone Backbone router router this page. In corporate intranets, IP over Sonet will offer cost advantages unless the organization already has an ATM infrastructure in place or if it must merge Access Access Access router router router

Customer premises equipment

When IP over Sonet is deployed in a typical ISP , the ADMs (add-drop multi- plexers) permit users to add and/or drop transmission channels to a Sonet ring at any optical amplifier site without interrupting other channels. The ADM also offers some management func- tions, such as performance monitoring, provisioning, and security management. In a typical ISP backbone network, the ADMs can be set up in rings for survivability and to provide bidirectional or unidirectional traffic. The Sonet ring connects to customer premises via backbone routers and access routers. (Source: Trillium Digital Systems)

a combination of IP and legacy propri- connections to the home, residential etary protocols into the backbone. users will continue to work with tradi- Some campus backbones may be less tional phone-network technology. likely to use Sonet because they don’t However, Rosenberg said, thanks in need the technology’s benefits, and/or part to fiber connections and the effi- much of their network infrastructure ciency and effectiveness of Sonet and runs over copper wire, rather than fiber. WDM technologies, businesses will be Finally, Grenfell said, telecommunica- able to merge voice and data onto IP net- tions-carrier networks already use Sonet works. He said this is one reason why IP in their backbones and, in many cases, use over Sonet represents the next evolu- ATM for voice traffic, so it may be easier tionary step in telecommunications for them to offer IP into ATM over Sonet. transmission technology. ❖ In the long run, said Insight Research’s Rosenberg, the issue is how users can David Clark is a freelance technology inexpensively link to voice and data net- writer based in Torrance, California. works. Because there is a lack of fiber Contact him at [email protected].