Only connect

Communications News, Feb, 1998 by George Fragos

The saga often starts with what would seem to be a simple need: to share data. If you follow the high-tech press and prognostications, you encounter buzzwords like ATM, ADSL, channel bonding, DSL "lite", HDSL, ISDN, xDSL, and more. But you need solutions that work now. Before buying a box to connect your LAN to a lengthy wire that ultimately connects you to the outside world, you may have to address some fundamental connection issues beyond cost.

What communications technology is available for connection to the LEC (local exchange carrier) at my location ? For example, does my LEC have ISDN BRI capability, and can I be connected to it?

DSL technologies are touted as being able to connect at great distance over normal telephone wiring. In fact, perhaps as few as half of normal telephone wiring circuits will support any DSL service.

Do I wish to connect multiple devices to the network? If yes, all DSL "flavors" except ADSL are eliminated. ADSL can handle a single voice channel in addition to the data circuit. The other DSL technologies are single channel only. Most likely, T1 is the only answer.

How much bandwidth do I need to make available to individual LAN users? LAN users must share bandwidth to the outside world. ISDN BRI with 128 Kbps may sound like a lot when compared to a 28.8 Kbps modem connection, but even a small LAN may yield sub-modem performance to the individual user. This is particularly true for Internet access.

And what happens when you add LAN users and the bandwidth requirements increase? T1 is a real winner here. A single telephone call can increase your bandwidth from 256 Kbps to 384, 512, 768 Kbps or even 1.536 Mbps. DSL technologies can operate in the lab at up to 8 Mbps, but in live environments they may only provide half the bandwidth of a T1.

Do service providers know how to install the service I select? A major issue with ISDN has been installation. You may have difficulty even ordering it. T1 and DDS are mature technologies that are well understood by the carriers.

Odds are, the best answer to these problems is T1 attached to a T1 CSU/DSU. Not too long ago, these T1 devices were used only for private networks. Since T1 services are priced by distance, monthly costs precluded their use by small businesses.

Today is a different story. We can connect to anyone in the world over a T1 connection at a much lower price. With switched networks like the Internet, frame relay, and the public telephone network, we need only pay a monthly T1 tariff for the short distance to a LEC.

T1s are channelized into 24 64 Kbps channels called DSOS. DSI is the group of 24 DSOs in a T1. A CSU/DSU converts each data port's serial data stream into a number of DSOs. It also takes a configured number of DSOs from a D&I port and multiplexes them with the DSOs from the data ports onto the outbound T1.

In today's wide area "transmission" network, the largest common denominator is a 64 Kbps DSO. For the most part, everything gets packaged into one or more DSOs. A voice, modem, or fax call from an analog phone interface is digitized into a DSO at the central office. A 56 Kbps DDS (Digital Data Service) connection becomes a single DSO. Each B channel in an ISDN call becomes a DSO. This is how a T1 can transport what would appear to be very different transmissions at the same time.

The DSO provides the way for many seemingly different devices to communicate with each other. That is why an analog phone can place a voice call to an ISDN digital phone. Perhaps more interesting is the fact that a data connection from a DDS CSU/DSU can be made to a data port on a T1 CSU/DSU.

Not all CSU/DSUs are created equal. Make sure the box you get supports FT1 (Fractional T1) and ESF (Extended Super Frame). Most equipment now includes these features, but some older equipment and some "loss leaders" don't. If you see a CSU/DSU that is really cheap, it is probably for DDS and not T1.

The beauty of FT1 is that you can get lower bandwidth at reduced cost and upgrade later to a higher bandwidth without replacing your equipment. The most popular models provide a single data connection and a single T1 network connection. Some models have up to eight data ports and D&I (Drop and Insert).

D&I is usually a T1 DSU without CSU, so it can't be connected to the public network. It lacks lightning protection and supports only up to 300 feet of cable inside a building. The D&I port can be connected to other T1 equipment like a PBX or channel bank.

Most CSU/DSUs come with fixed port configurations in rack-mount and desktop packaging.