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First Fork in the Road

The most important question to ask yourself is the ubiquitous, "Where do I want to go tomorrow?" The answer will influence your path. If a need for huge bandwidth is not seen for many years down the line or not at all, then the best approach is to use multiplexed DS-1 circuits for more bandwidth. If you think you will need higher bandwidth sooner, then starting with a fractional DS-3 might be an option. The downside is that no matter which road you take, the circuits you currently have installed will have to be replaced.

An NxDS-1 is likely to be the least expensive solution. Eight DS-1 circuits can be multiplexed to form one frame relay connection providing up to 12 Mbps of bandwidth. Not only can your corporate site use NxDS-1 circuits, but even remote sites that may need more than a single DS-1 can take advantage of multiplexed circuits. Although you achieve extra bandwidth by using more DS-1 circuits, you will not be able to use the DS-1 you already have installed. Carriers will terminate NxDS-1 circuits to different equipment and cannot use the circuits that are terminated by single DS-1 service. Carriers also require you to use a particular CSU/DSU, as it is more specific than units used for single DS-1 connections and needs to be matched at both ends. By using equipment like Quick Eagle Networks' DL-3800, carriers or users can scale their needs from 1.5 Mbps up to 12 Mbps. These units are adjustable at both ends, and single DS-1 circuits can be added or taken offline.

NxDS-1 frame relay circuits work by "bonding" single DS-1s. The first DS-1 acts as management for the entire group and sets timing for the others. If this timing DS-1 goes down, data transmission will stop. If you start out small with only two DS-1s--providing total bandwidth of 3 Mbps--additional bandwidth can be added at a later date. This requires only that additional DS-1s be terminated at both CSUs and then activated. The CSU will immediately begin passing data down all the DS-1s connected to it. Latency issues can arise if you have multiple circuits that are connected at different times as you expand your bandwidth. Because not all circuits are installed at the same time, some may take different paths back to the carrier, creating latency differences in the lines. The multiplexer usually compensates for this automatically, but extreme differences may cause problems.

The V.35 interface you have been using on your router may not be able to handle the extra bandwidth. Cisco Systems' implementation, in particular, won't go above 3 Mbps, and other vendor equipment will top out at around 10 Mbps. If your V.35 port won't handle the increase in bandwidth, a HSSI (high-speed serial interface) port is in your router's future.

The Leap to DS-3

Before you install up to eight DS-1s in your office, you might consider jumping to a DS-3. By purchasing a channelized DS-3, not only can you pull in those DS-1 circuits for data, you can mix voice channels in your corporate network as well. If you have several DS-1s carrying voice traffic or other data services, it will ease wire-management problems by dropping only a single DS-3 at your office instead of multiple DS-1s. The DS-3 would be terminated at an M13 (a device that multiplexes a DS-3 into 28 DS-1 circuits) within your telecom closet and then broken out into the 28 individual DS-1s that would terminate at your telephone or data equipment.

Because the DS-3 would be channelized, you'll pay for only the bandwidth used and not the entire DS-3. As your bandwidth needs grow, you can merely arrange additional services provided via the DS-3 already installed. You can still pull off individual DS-1s and pass them through a multiplexer to achieve up to 12 Mbps.

After you've grown out of 12 Mbps, it's time to take the next step up the bandwidth ladder to a DS-3. If you are already using a DS-3 to bring in NxDS-1 circuits, the solution can be as simple as changing out the M13 and multiplexer. Replacing them with an Adtran Atlas 800 or similar unit will let you peel off DS-1 circuits for voice and distribute the rest to your router via a HSSI connection. Fractional DS-3 service can be ordered from your frame relay provider, usually in 3-Mb increments from 12 Mb to 18 Mb on up, to fit your needs.