ATM rides high atop Sonet
- By Jane Morrissey
- Oct 31, 1997
Although sometimes pitted against each other in the race to deliver ever-higher network bandwidth Asynchronous Transfer Mode (ATM) and Synchronous Optical Network (Sonet) technology are more frequently teamed.
ATM is fast becoming the network switching technology of choice to ride atop high-speed Sonet infrastructures. The pair federal agencies have found are inherently suited to tackle the nascent trend to consolidate data voice and video traffic onto one mammoth wide-area network. Each technology brings its own strengths to the mix.
ATM sports the flexibility and intelligence to support a mix of traffic types permits users to assign different bandwidth guarantees for each type and leverages high-speed Sonet at the underlying physical layer. Aside from data rates starting at OC-1 or 51 megabits/sec and scaling to OC-192 or 9.9 gigabits/sec Sonet brings its self-healing ring architecture which provides unparalleled survivability in the event of a fiber cut or node failure. If such a break occurs Sonet is able to automatically reroute data in the other direction around the ring.
"You really can't do ATM without a Sonet infrastructure " said Diana Gowen director of MCI America a unit of MCI Communications Corp. McLean Va. MCI runs a nationwide ATM network at OC-12 (622 megabits/sec) Sonet speeds and sells ATM and Sonet services. "You can have DS3-based [45 megabits/sec] ATM networks but there's a lot of overhead with ATM and the efficiency of those networks is not as good as the higher-speed Sonet " Gowen said.
ATM and Sonet's proven combination of speed reliability and flexibility also has enabled early adopters within the government to tackle a host of applications that were stymied by the bandwidth limitations of previous network setups. Those applications typically rely on multimedia capabilities ranging from compute-intensive imaging applications to distance learning videoconferencing and even ultra-high-speed Internet access.
Early Adopters ATM is regarded by some as a still-maturing technology. Nonetheless it is moving rapidly from experimental to production networks within the government. With its multibillion-dollar Defense Information Systems Network project the Defense Department is building the world's largest private Sonet network and is exploiting ATM to integrate existing data video and voice networks.
"Sonet is big bandwidth scalable has very low restoration times high reliability and was a perfect match for our requirements " said Col. Robert Reynolds deputy chief of the Network Division of the Directorate of Operations in the Defense Information Systems Agency. ATM was not part of DISA's initial design three years ago but the technology has since matured enough to become an indispensable component Reynolds said.
"We have a fairly complex network we're putting down and I don't see how we could do it without ATM over the long haul " he said. "We are going to get to a true multimedia network. It will be possible from your desktop to have [global] video teleconferencing around the Defense Department to make a phone call if you like from your computer and to use that same computer for traditional e-mail database and other applications."
DOD also will gain dynamic bandwidth allocation and end-to-end network visibility - control features lacking under its previous arrangement with AT&T Corp. for turnkey services under the Defense Commercial Telecommunications Network (DCTN) Reynolds said. DISN will offer OC-3 and OC-12 links initially scaling up to some OC-48 (2.4 gigabits/sec) links within the next two years Reynolds said.
NASA has been running ATM over Sonet in its WAN test beds for two years. Comfortable with its stability last month the agency began deploying ATM over Sonet for its operational networks. The biggest driver is price said Christine Falsetti project manager for the NASA Research and Education Network and Next Generation Internet at NASA's Ames Research Center Moffett Field Calif.
"If you have all these dedicated lines you have to run ATM is much better in terms of price/performance " Falsetti said.
NASA contracted with Sprint Corp. which operates a nationwide Sonet network with a four-fiber architecture - as opposed to the typical two-fiber architecture - for extra redundancy.
NASA runs OC-3 links among five sites and expects to bring OC-12 online in the next few months. Sharing Bandwidth Driven by a need to marry multimedia data sets from different labs across the country NASA exploits ATM to share network bandwidth between experiments and to guarantee class of service.
Over Sonet performance has been sensational the agency said. During the Mars Pathfinder mission for example Ames and the Jet Propulsion Laboratory experimented with supporting a huge amount of World Wide Web traffic for Mars images. "JPL was saturated running at 6 megabits/sec and people weren't getting access to the information " Falsetti said.
"Then we cut it over to our experimental network and the traffic immediately shot up fourfold. All of a sudden people had access to the data they didn't have before that's the kind of dramatic performance improvement we're seeing."
Falsetti is now confronting one issue that puts ATM and Sonet head-to-head: Internet Protocol. With the surge of the Internet IP has become a dominant network protocol and heavy IP users are interested in transporting IP directly over Sonet rather than IP over ATM over Sonet.
The benefit comes from cutting out the ATM overhead - the so-called cell tax which slows down performance by 10 to 30 percent. But IP over Sonet is relatively new and Falsetti questions the real-world performance benefit and wonders how much flexibility she will have to compromise.
Furthermore some ATM users have found that the cell tax has been overestimated.The value of IP over Sonet is hotly debated. However network experts agree on some guidelines. For IP data networks IP over Sonet is probably an efficient option. But IP over Sonet may not suit environments with legacy systems or those that have already deployed IP over ATM over Sonet.
"If you believe that IP is not the only game in town then you clearly want to go with ATM because it will support legacy data - voice video multimedia applications - going forward " said Bruce Tanzi product manager for AT&T's high-speed packet services.
"You're not going to get that with IP on Sonet unless you take that voice traffic packetize it and stick it inside an IP frame. You also lose out on ATM's class-of-service advantages " Tanzi said. "With IP on Sonet you may have some of the same issues you have on the Internet: Where is your actual path what other routers are between you and your destination?" he added. "So there's predictability reliability and management issues that you don't have with IP on ATM. And the only thing that would negate that argument is if someone was going to a private Sonet IP network."
MCI which is testing Cisco Systems Inc.'s new Gigabit Switch Router to deploy IP over Sonet also has reservations. "The problem right now is that that device doesn't allow you to do IP over ATM at all " said MCI executive manager Charles Lee. "Where people have gone out and built high-speed IP over ATM networks if you bought this new gigarouter you'd have to have a separate network and the two couldn't talk to each other." However Cisco is working on interfaces that will allow the two technologies to interact.
Another issue: It's new. "No one has loaded it down to see how it will perform " Lee added. "Right now it's very premature to put all your bets on that one horse." Meanwhile Sprint has already deployed IP over Sonet services using the Cisco product which supports OC-12 speeds. But Sprint is consolidating all its networks with ATM for consistency - one platform to handle Internet long distance and other traffic said spokesman Charles Fleckenstein.
According to Cisco the solution ultimately depends on the services required. "If you're doing mostly IP or pure IP then it would probably be more cost-effective and cheaper to put IP directly over Sonet " said Richard Palmer director of marketing for Cisco's service provider line of business San Jose Calif. "But if you are doing multiple types of services then it's going to be to your benefit to use an ATM infrastructure."
Certainly there are caveats when moving to ATM and/or Sonet networks vendors said. On the ATM front installation can be complex and the level of network management has yet to catch up with the features according to vendors. "It's not exactly woefully lacking but it's certainly not at the maturity level that other services are " AT&T's Tanzi said.
"Right now the significant impedi-ment for people going whole hog to ATM is that the technology is new so there's not a lot of experience out there and experienced folks are expensive " Tanzi said. "And the cost of an ATM interface component is many times more than a standard frame-relay or private-line interface."
ATM prices are falling at a rate of 40 percent per year however so price will become less of an issue he said. Sonet's benefits also come at a significant expense vendors said. "Scaling it up is a very expensive process " said Steve White manager of technical field marketing for Fore Systems Inc. Bethesda Md.
"In an ATM infrastructure I can put as much capacity on any individual link as I want and I can scale up individual links without having to scale up the whole network." Fore is building Sonet-like restoral into its ATM switches via the Private Network to Network Interface (PNNI) a standard to dynamically reroute traffic and via Sonet Automatic Port Switching. "Basically ATM with a good PNNI implementation will recover in about the same time frame as Sonet " White said.
Susan Barbiere Lucent Technologies Inc.'s director of market development for data networking contends that Sonet is a good buy whenever substantial traffic is involved. "If you think about the amount of traffic you're sending and the flexibility of it you really do get a lot for your money."
Network traffic analysis is critical if users want to avoid roadblocks and avoid paying for what they don't need said Steve Gossage senior member of the technical staff at Sandia National Laboratories Albuquerque N.M. "It's really easy to overbuild in places " Gossage said. "If you've got a great big pipe but you can't get to it because you are congested locally then you've done the wrong thing at the wrong time. You've got to measure measure measure."
Knowing traffic patterns also will help agencies to work out the best price because rates are all over the map depending on volume and architecture vendors said. For example "DOD and the Department of Agriculture's needs are very different " MCI's Gowen said. "If the Department of Agriculture were to seek Sonet prices with very low volume and the DOD were to do the same thing there might be an order of magnitude difference in the price they would get." And ATM won't bring that much to the table if a user's needs really approach a dedicated private line she said.
"Once you upped the quality of service the bandwidth the sustained cell rate the peak cell rate you wanted you start approaching the price of a private line. But as long as your application really matches a virtual product and it's bursty then you should enjoy about a 4-to-1 or maybe even more benefit with ATM " Gowen said.
Ultimately flexibility may be the most important aspect of an ATM network said Kathryn Korostoff president of Sage Research Inc. Natick Mass. "What I'm hearing from telecom managers is that they want a service that's going to be very flexible because there's enormous uncertainty about what the traffic pattern is going to be and how the total traffic is going to increase " Korostoff said. "One of the challenges is figuring out what combination of local access they're going to have so they can afford to enjoy all the benefits of ATM over the wide area."
Technology Advances A plethora of advancements in ATM and Sonet technologies promise to extend their performance reliability and efficiency even further. With Sonet the hot button is Wave Division Multiplexing (WDM) which permits transmitting multiple wavelengths over a single fiber. Sprint has implemented 16 wavelengths on a fiber each one capable of carrying a Sonet channel. "With the 16 windows and OC-48 Sonet we have now we can handle half a million phone calls on one fiber at one time " Fleckenstein said.
Lightwave Switching technology promises to enhance WDM's ability to recover from broken links. Just as Sonet can redirect data over another ring if a link is lost Lightwave Switching technology will make it possible to redirect the individual wavelengths created by WDM into different directions. And Optical Switching technology will improve reliability and costs associated with Sonet rings by eliminating the need to reduce light into an electrical signal and back into light again.
ATM customers will benefit from three enhancements to the ATM standard: MultiProtocol Over ATM (MPOA) Voice Telephony Over ATM (VTOA) and Inverse Multiplexing over ATM (IMA). MPOA provides "shortcut" routing by looking at the first packet in a flow and switching the rest for faster throughput.
DISA which is beta testing MPOA is eager to get the technology into the field to gain efficiency at less expense Reynolds said. "For one thing we'll be able to continue with IP without being burdened by the Tandem router that is normally found in large IP networks " he said. Voice over ATM is considered immature and inefficient in its current form.
Sometime next year VTOA is expected to bring more sophistication to the task with features such as dynamic bandwidth allocation for voice calls and silence suppression. Silence suppression makes it possible to take advantage of gaps in conversation to send other streams of data packets.
IMA meanwhile is a new development that enables so-called "NbyT1" ATM service. This service gives users incremental bandwidth above T-1 which has a 1.54 megabits/sec transmission rate instead of forcing them to jump to a full T-3 connection which supports 45 megabits/sec rates.
Morrissey is a free-lance writer based in Denver. She can be reached at email@example.com.
* * * * * Sandia rethinks ATM performance
For Sandia National Laboratories which is an Energy Department lab the ATM cell-tax issue is overblown.
In 1992 Sandia opted for ATM over Sonet to consolidate its networks and span the 1 100-mile distance between its labs in Albuquerque N.M. and Livermore Calif.
"Yes you pay a tax but we're actually delivering more bits than we can deliver any other way at longer distances so we'll pay the price " said Steve Gossage senior member of the technical staff in Albuquerque. "It's not really onerous when you can move 125 megabits/sec out of a machine and do it reliably."
With its OC-3 OC-12 and OC-48 connections Sandia has achieved more throughput and reliability than the lab set out to get Gossage said. He added that he was especially impressed with Sonet's Automatic Port Switching which provides link restoration in less than 50 milliseconds.
"That's freed us up from a lot of those physical plant problems that were part of our day-to-day world " Gossage said. Aside from tackling heavy-duty scientific modeling applications Sandia is deploying ATM to the desktop for typical productivity uses.
"It turns out that a machine you can buy for $2 500 is capable just in Web access of bursting data that swamps an Ethernet " Gossage said. Because ATM is a workstation LAN and WAN standard everything meshes together easily. "If you understand how ATM switching works at one data rate you understand how it works at another so you have a real nice building block and a real ability to leverage a small number of technologies across a wide range of problems."
Sandia which had linked its sites over two DS-3s installed Lucent Technologies Inc.'s Globeview-2000 which is a 20-gigabit ATM core switch. "Budget pressure said we had to change what we were doing. By the way we manipulated the ATM switch we consolidated down to one link " Gossage said. "That consolidation of our external links saved us pretty close to our total investment in the switch - about $1 million."