. This consideration has become much more important to carriers as the volume Heavy Traffic of IP traffic has increased. Internet ser- vice providers (ISPs), local telephone 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 Telecommunication 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 telecommunications 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 December 1998 17 . Technology News 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 software 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.