Listen Up! Cisco Now Does Voice

By David Willis  Already an indisputable leader in the data routing and switching arena, Cisco Systems is voicing its strengths with the MC3810 Multiservice Access Concentrator. This device consolidates voice, fax, video and LAN traffic over a frame relaybased, ATM-based or TDM (Time-Division Multiplexing) circuit-based WAN. And it does all this at a price competitive with many voice FRADs (frame relay access devices), yet packs more features. The MC3810 starts with a multiprotocol routing engine using an extended version of Cisco's IOS (Internetwork Operating System) for configuration. Base hardware includes an Ethernet jack and a pair of Cisco Universal I/O inter faces that support TIA-530/V.35, TIA-232/449 and X.21. You can add six analog voice ports with the "personality" of your choice--FXO (Foreign Exchange Office), FXS (Foreign Exchange Station) or E&M in any combination--for interconnecting analog phones, PBXes and central office links. In lieu of analog ports, the unit supports a single T1/E1 digital voice access port, allowing up to 24 compressed voice channels for T1 or 30 channels for E1. I tested the Beta 2 release of the MC3810 at the MCI Developer's Lab, running concurrent data and voice streams over MCI Hyperstream frame relay circuits between Portland, Ore., and West Orange, N.J., POPs (points of presence). I used separate PVCs (Permanent Virtual Circuits) for voice and data traffic, passing voice over Hyperstream's Priority PVCs, while using the MC3810's priority queuing services between a pair of MC3810s. Although our tests involved only frame relay, you can also connect the unit to an ATM backbone, running voice, fax and data traffic over A AL5/VBR (Variable Bit Rate) service to minimize bandwidth utilization. Voice Sings, Data Stutters In the lab, the unit provided fair compressed voice quality, though it suffered considerable data loss when sending data above 750 Kbps. As data service approached 1 Mbps, packet loss was about 20 percent. Like other voice over frame/packet products on the market today, data performance takes a hit because large packets are chopped up to avoid intolerable voice delays--whether or not voice traffic is present. Despite the data loss, voice services worked well. After configuring a simple three-digit dialing plan, I dialed from one analog port to another, holding three calls open over a PVC configured with a 48-Kbps CIR (Committed Information Rate). These calls stayed up without a hiccup for more than 30 minutes, while our Wandel & Goltermann DA-30-based RTBench tests blasted IP traffic across the network. What Box Does This Box Fit In? The MC3810 has incredible flexibility, supporting a variety of multiservice applications. It's obviously marketed as an access concentrator, but it's a lot more than just a traffic multiplexer. This unit provides voice over frame relay, but it's much more than a voice FRAD. These services alone would justify the price tag. Because it's also possible to place calls between ports on the same unit, it can serve as a small PBX. You can even use the MC3810 as a small digital cross connect, mapping inbound channels on the digital access port to outbound channels on a trunk. Over the WAN, the unit compresses voice traffic using the ITU G.729 and G.729a CS-ACELP algorithms, letting an active call consume as little as 8 Kbps. While the range of algorithms may be more limited than some competing products, 8 Kbps is generally accepted as the minimal amount of bandwidth required for voice approaching toll-quality. Other vendors may offer compression at under 5 Kbps, but when it comes to installation, they'll tell you to use the 8-Kbps or even 16-Kbps algorithms. These algorithms are also part of the Frame Relay Forum's FRF.11 standard for voice over frame relay interoperability. The MC3810 features many other voice-optimization capabilities, such as echo cancellation up to 32 milliseconds (16 ms in our beta unit). More importantly, it uses voice activity detection--substituting ambient noise samples for pauses in conversation--a technique that often reduces typical voice call bandwidth by 30 percent to 60 percent. Instead of wasting a full 64 KB channel for fax delivery, the MC3810 demodulates input fax tones, transfers digital data at 2,400 to 9,600 bps over the backbone and remodulates the data back to an analog signal at the remote end. One caveat: It is not possible to place analog modem calls over a compressed channel, as the voice compression algorithms don't operate on individual tones. Although the MC3810 does not include video codecs, it transports video in a variety of ways: over a TDM T1/E1 trunk, allocating time slots to video; over ATM CBR links; or by using video over IP or frame relay services. I discovered a few bugs in the beta version--for example, issuing the "show voice calls" command would restart the unit. Frame Relay Forum FRF.9 data compression, traffic shaping and ATM ILMI (Interim Local Management Interface) were not yet available in this Beta 2 release. But when this product is completed, you'll want to check it out. David Willis can be reached at dwillis@nwc.com.

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