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To determine how today's ATM IADs perform with voice and data, we tested Accelerated Networks' Accelerator AN-30, Lucent Technologies' PacketStar PSAX 50 Multiservice Media Gateway, Mariposa Technology's ATX150 and Memotec Communications' CX950 Access Switch.

Voice for All

Voice capability is the most-hyped attribute of today's access devices. Although customer acceptance has been slow, the devices' ability to packetize voice calls and carry them over the wide area network is the Holy Grail that vendors have been touting for years. The main hindrance to widespread adoption has not been a lack of hardware, but more the economics involved: It's cheaper for large companies with hundreds of employees using the telephone to pay a few cents per minute for long distance than to foot the bill for the increased bandwidth necessary to accommodate those calls over the network. SVCs (switched virtual circuits), which should have solved this problem, have not been widely offered by carriers, and most customers can't afford a private ATM network to do with as they please.

Voice-equipped access devices can play a part in multioffice, small-to-midsize businesses. For these offices, the bandwidth required to handle one or two calls can more easily be accommodated by their WAN connections, especially with voice compression making the calls use even less bandwidth than normal TDM (time-division multiplexing) would. Offices that need to communicate with each other only occasionally can avoid long-distance charges simply by placing their calls through an IAD--and the savings can add up quickly. But with each call requiring 8 to 32 Kbps, depending on the compression used, customers will need to determine the number of simultaneous calls for which they can afford to use bandwidth. This is where the voice-over-WAN revolution will begin. We designed our tests to see which vendors are ready to lead the charge.

Voice compression is the key to lower bandwidth requirements in an office-to-office setting. Although circuit emulation (CE) has enabled the addition of voice capabilities to ATM equipment for years, using a full 64 Kbps of bandwidth for calls is too costly. Until the past year, most vendors were still delivering voice compression via proprietary algorithms. Thus, customers easily became locked into one vendor's product, and therefore often faced a problem of scale. Vendors rarely provided complete solutions that would accommodate both smaller and larger offices. Instead, they usually settled on a niche package in the middle--appropriate for some, but too large or too small for others. With such products, not only did customers need to find a compression quality they liked, they also had to match that compression quality to a vendor whose products scaled to their needs. Now that compression standards exist, a customer can place one vendor's equipment in its smaller offices and another's in its corporate office, and the entire setup will work together. All the products we tested support G.711 PCM voice encoding and G.726 voice compression. Three of the four products (the exception being Accelerated Networks' AN-30) also support G.729A compression. (See "Voice Compression: Hear the Difference?")

Frame It

Frame relay has been the WAN technology used by small-to-midsize businesses for years, and there's no need to get rid of it yet. IAD vendors understand that frame relay exists in most environments, so they haven't left it out of their products. It can coexist with ATM in many situations, since ATM doesn't scale below DS-1, but frame relay does. For example, in a nationwide corporate environment, regional offices can connect to the corporate headquarters via a higher bandwidth ATM DS-1 link, while connecting to smaller sales offices via frame relay at lower bandwidths.

For frame relay to travel over ATM, the IAD needs to support FRF.8 service interworking or FRF.5 network interworking. In most cases, the carrier, not the customer, performs this function, though the feature adds to an IAD's robustness. FRF.8 acts as a gateway, stripping off the frame header and then placing the packet into ATM cells by performing a SAR (segmentation-and-reassembly) function. Cells are translated into frames by reversing the actions.

Less common in most enterprise networks, FRF.5 acts as a frame relay tunnel through an ATM network. FRF.5 differs from FRF.8 in that the frame relay header is never stripped off. A SAR function is performed on the frame with headers intact. At the other end, the ATM headers are stripped, the frame is reassembled, and the frame continues on another frame relay network, using ATM only as a transport in the core. All our tested products support FRF.5 network interworking and FRF.8 service interworking.

The Test

We wanted to see how our four products could handle data and voice calls over an ATM network, using standard voice-compression technologies, and we aimed to get the most consistent data and voice throughput. Because all our participants use standards-based compression techniques, it was less crucial to compare the voice quality produced by each, as it should be similar if not the same across all four. And without an actual ATM network between the units, jitter due to latency was not an issue.

We transmitted data through each unit's Ethernet port from an Adtech AX/4000, and digital voice calls generated by a Hammer Technologies Hammer IT (see "How We Tested ATM IADs". Voice prevalence--that is, the ability of each IAD to give priority to voice calls before surrendering bandwidth to data--was most important to us, and all four products came through with shining colors in this area. Almost as important was the units' ability to keep a consistent data rate, regardless of packet size, when mixed with voice calls; only the Memotec CX950 and Accelerated AN-30 were able to maintain fairly even data throughput. We also graded on the number of calls that could be completed during each of our tests. Lucent's, Mariposa's and Memotec's IADs all had respectable scores here, but Accelerated's unit took almost 25 percent longer to complete each call, thereby allowing fewer total calls to go through. Last, we graded the products on their expandability. Memotec's CX950 had the most slots and plenty of options to go with them. At the other end of the scale, Accelerated's AN-30 was the least expandable, with the only options being digital or analog voice ports and DSL or ATM on the WAN side.

None of the units was plug-and-play, but setting each one up wasn't particularly difficult. They can all be configured either from their built-in "craft" serial interfaces or inline via their Ethernet ports, and they all support CLI (command-line interface) configuration. The Memotec CX950 and Accelerated AN-30 also offer a graphical interface via separate software packages, while the Lucent PSAX 50 and Mariposa ATX150 have a built-in Web server for access via a standard browser. Once configured, the units can be managed using any standard SNMP toolset.

Taking all this into account, we singled out the Memotec CX950 as our Editor's Choice award winner because it could pass the most calls through the network while maintaining the most consistent data rates. We were also impressed with the CX950's expansion capabilities. The Mariposa ATX150, our previous winner, dropped to second place this time around--albeit a close second--primarily because its data rates lacked consistency.