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By Justin Junkus

When telephone engineers talk about the great changes to their technology since Bell invented the telephone, the network interface device (NID) typically is not high on the list. That shouldn't be surprising, given the appearance of a NID. It's just a small gray box on the side of the house.

On the other hand, I happen to believe that this device ranks right up there with the digital switch. To appreciate my opinion, you need to do some thinking about how the NID came into being, what it does now, and how some vendors see its future role as the residential multiservices gateway.

The NID began as a hardware demarcation point. Few houses had one before the divestiture of the Bell system, because Mother Bell owned everything connected to the telephone network. Nothing to 'demarc' existed.

With divestiture, however, subscribers became the owners of the wires, station sets and connectors within their property. Because real estate differs substantially, a clear point had to be established where the customer's responsibility ended, and the telephone service provider's began.

A simple cross-connection at a terminal block would do the job, but it had to be accessible to both the customer and the service provider. That ruled out mounting the terminal block in the basement. An exterior weatherproof container for the block was the best solution to the access issue, and the telephony NID was born.

Even in the early days, the NID was more than a simple cross-connection. Both the telephone network and the customer's equipment needed isolation from stray voltages, so a resistive protector block typically was included in the NID housing. The NID also is a convenient place for an RJ-11 jack, where the do-it-yourself subscriber could unplug the house and isolate "no dialtone" situations with a screwdriver and a standard phone.

Although it was the telephone industry that created the NID, it was the cable company that gave it real life. When cable engineers began to think about where to convert signals on a coax cable to a copper pair connection for a standard phone, the NID was the likely candidate. The wire pairs in the residence already were terminated at an NID, and the NID was defined as the demarcation point between the subscriber and the telephony provider. All cable had to do was replace the phone company NID with its own version.

But the cable version became incredibly more sophisticated than the telco version. The network side interface is carrying radio frequency (RF) signals rather than voice frequency, so shielding is an important consideration. To make it more interesting, when data or telephony are provided, the RF carrier is modulated digitally. That has major implications for what has to be done inside the NID.

Dan Paone, Antec Digital Systems vice president, cites the Arris VoicePort as an example of the functionality found in a cable telephony NID. "The NID contains the access network electronic intelligence, as well as the home interface devices. It can pass RF through to cable modems and TVs, while doing digital to analog conversion of the telephony intelligence passed down the RF carrier to the subscriber's telephone equipment. By the way, that now commonly includes not only several makes of phone sets, but also fax machines, caller ID boxes and analog modems. The NID also provides the powering point-of-presence for the telephone equipment. It has to be designed to accept coaxial cable center core powering, composite pair (twisted pair) powering from the network or a local power supply."

So what's the future for the NID? We may get some insight from what vendors have recently announced. The trend is to eventually replace proprietary RF modems in the NID with Data Over Cable Service Interface Specification (DOCSIS)-compliant cable modems. That leap in technology means the NID will be a focal point in cable's ability to offer both multiple levels of data service, and brand new applications tied to the digital home.

Initially, the new applications probably will be relatively simple, such as one that Eric Hartford, ADC market manager, mentioned recently. "As cable competes against satellite and digital subscriber line (DSL), there are new services that may roll into an IP gateway, like home security. It could be possible to remotely unlock doors, for example."

Other possibilities exist, of course. Consider that the major cable telephony vendors have all announced or discussed intentions to include Home Phoneline Networking Alliance (HPNA) circuitry in the IP versions of their NIDs. Embedding this technology in the NID makes it possible to network several data generating devices within the home and share Internet access, using existing phone wiring. HPNA isn't the only possibility for new device interconnections to the NID. At the last Western Show, Arris exhibited a concept "Converged Services Port" that included a wireless interface to multiple home devices.

Several advantages to using these technologies to put new functions in the NID rather than in a set-top exist. Dan Paone did a good job summarizing: "The 'intelligent NID' on the side of the house allows the provider to maintain ownership of the equipment and the services. It keeps network media options open, because it could be coax-fed, fiber-fed or a hybrid."

"On the other hand," Paone noted, "I picture the set-top box as having a maze of wiring that is very difficult for the homeowner to manage, especially in an open room environment. Powering is far more difficult and less secure. With the new National Electrical Code (NEC) 830 rules, network power cannot pass into the home. The unit would have to be locally powered, and in the case of primary line service the provider must meet the rules established for power back-up during commercial power outages. The home access issues come back to haunt the service provider more often than not, causing multiple truck rolls. The NID provides an essential operator-accessible demarcation point."

The bottom line is that the NID may not look like a big item, but it has big potential in the world of future services and applications.

Justin J. Junkus is president of KnowledgeLink, Inc., and applications consultant for Antec. To discuss this topic further, you may reach him at .

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