ADC Kentrox And 3Com Aggregate ATM WAN Traffic

By David Willis   ATM is creeping out of the carrier core and onto the premises, where it meets with traditional WAN interfaces--T1, frame relay, HDLC (High-Level Data Link Control) or native ATM devices running at OC-3 and T3 speeds. With ATM, the carrier can easily deliver network services that match our traffic requirements at rates that continue to drop. As customers, we benefit from tight control over the traffic submitted to the WAN and more advanced services. To view the Report card. But if you're going to put all of your WAN eggs in the ATM basket, you'll need access equipment that you can rely on. We found out how well the current generation of products handles the heat by stress-testing ATM access concentrators from ADC Kentrox and 3Com Corp. at the MCI Developer's Lab in Richardson, Texas. ADC's AAC-3 and 3Com's AccessBuilder 9600 rise above the pack for their ability to support a range of interfaces, converting and delivering several traffic streams concurrently. More important, they are the first to offer support for IMA (Inverse Multiplexing over ATM), a technology that is destined to spark renewed interest in ATM WANs. Scaling With IMA For access to the ATM network, you need a device that will convert your non-ATM traffic into cells. When first introduced, ATM access concentrators were simple multiplexers that aggregated traffic to an ATM uplink. This in itself is a pretty good trick, involving converting traffic to the appr opriate ATM Adaptation Layer and assigning priorities to various traffic streams. However, access concentrators now offer a lot more. They feature local switching engines to move traffic between local ports on the box, with more sophisticated traffic management facilities than the previous generation offered. A historic problem with large-scale traffic aggregation is the fact that a T1 pipe is often too small to take all of your traffic, but T3 is too large--and too expensive. The latest round of ATM products for the WAN features IMA, a UNI (User-to-Network Interface) standard recently ratified by the ATM Forum. IMA can be used over T1 circuits to bridge the broad price and performance gap between T1 and T3 services. With it, trunk capacity can be easily added by simply installing more T1 circuits, up to a maximum of eight or so, beyond which T3 service makes sense. IMA moves ATM cells across trunks in a cyclic round-robin fashion, so each link is equally loaded. Thus, IMA circuits can provide a measure of fault tolerance, especially when trunks are diversely routed. For this review, we designed a test to examine this capability, by actually disabling some--but not all--circuits in an IMA bundle (see "How We Tested And What We Discovered" on page 90). Diverse routing helps with fault tolerance but can introduce problems of its own. The enemy of IMA is differential delay, a problem that can occur when T1 trunks aren't routed the same way. IMA must deliver cells in order, so buffers are required to keep traffic moving smoothly. We designed another test to examine how well these boxes can tolerate differential delay. Making the Cut Because a single bad cell can mean death to an entire IP packet, we selected products that support ATM Early Packet Discard, which specifies that cells are not transmitted if they are part of an already trashed packet. Requiring this capability, along with IMA over T1 capabilities, weeded the pack down to just the two products.

For the Side Bar on How We Tested And What We Discovered Other Reviews BMC, PLATINUM and Tivoli Bring Enterorise Database Management Down to Earth By Barry Nance