In this column, we will find out why, if studies show the typical length of most existing horizontal cabling runs is 150 feet, the standard-specified maximum channel length is 100 meters.
But first, we’ll need to revisit a little history.
In the early 1980s, a sign that hung in many Bell facilities read as follows:
“There are two giant entities at work in our country, and they both have an amazing influence on our daily lives … one has given us radar, sonar, stereo, teletype, the transistor, hearing aids, artificial larynxes, talking movies, and the telephone. The other has given us the Civil War, the Spanish-American War, the First World War, the Second World War, the Korean War, the Vietnam War, double-digit inflation, double-digit unemployment, the Great Depression, the gasoline crisis, and the Watergate fiasco. Guess which one is now trying to tell the other one how to run its business?”
The Bell System had a monopoly on most things related to telephones, including building cabling, which had dated back to the turn of the century and lasted until January 8, 1982. That’s when, in the final settlement of a 1974 United States Department of Justice antitrust suit, AT&T agreed to divest its local exchange service operating companies.
Effective January 1, 1984, AT&T’s local operations were split into seven independent Regional Bell Operating Companies (RBOCs):
2. Bell Atlantic
3. BellSouth Corporation
5. Pacific Telesis Group
6. Southwestern Bell Corporation
7. US West
There was also Bellcore, which provided joint research and development, standards setting, and centralized government point-of-contact functions for its co-owners, the seven RBOCs.
After divestiture, the building cabling became the responsibility-or problem, depending on your perspective-of the building owners. And there were few “rule books” or operations guides for the care and use of these systems.
Before divestiture, when it came to telephone cabling there was primarily one way of doing things, according to The Bell System Practices.
Designs were pretty simple back then. There were two types of closets: apparatus and satellite.
An apparatus closet could serve up to 10,000 square feet of floor space and could also feed up to two satellite closets, each serving up to 5,000 square feet of floor space.
Notice that this was based on square feet served, not channel length.
Post-divestiture, a damage-control effort was undertaken to ensure that The Bell System Practices would still be supported … and so it was.
Bellcore representatives were present not only at the early formulating meetings of the TIA standards, but also for the drafting of the first BICSI Telecommunications Distribution Methods Manual (TDMM).
The Bell System Practices’ use of a service area of 10,000 square feet and IEEE 10Base-T use of 100-meter channel length of “telephone cable” weighted heavily into the negotiations.
The Bell System model called for a 10,000 square-foot service area (100 x 100-foot) with a telephone closet in the middle. This setup results in most cable runs being less than 150 feet.
So, let’s draw a 100 x 100-foot service area and put a telephone closet (TC) in the center of the area served. This was The Bell System model, which resulted in most of the cables being less than 150 feet long-in much the same way that electrical branch circuits terminated either on another wall of the same room or in a (usually larger) adjacent room.
Now, let’s draw the same 100 x 100-foot service area and put a TC in one of the corners. This became the worst-case discussion model (that would still support the then-emerging 10Base-T standard), which was used during the formulation of the original TIA-569 and the BICSI TDMM. After all, this new family of design documents was to support all forms of telecommunications, not just the telephone.
Bellcore representatives were happy because their 10,000 square-foot model was used. And while not a hard requirement (it appeared as a “should” statement in the document), TIA-569 suggested one telecommunications closet per 10,000 square feet. The network equipment manufacturers were happy because their cabling needs were being addressed. And the cabling component manufacturers were delighted because everyone was going to need more and better cabling … many generations of it in the foreseeable future.
That’s why the categories started with Category 3 and went higher, rather than the best cable being number one!
In 1985, the original BICSI TDMM was published, primarily focusing on building cabling designs. And in October 1990, TR-41.8 published the original ANSI/TIA/EIA-569-1990 Commercial Building Standard for Telecommunications Pathways and Spaces. But it wasn’t until July 1991, when TR-41.8 published the original ANSI/TIA/EIA-568-1991 Commercial Building Telecommunications Wiring Standard that the 90-meter horizontal cable was set in stone.
Now, let’s do the math:
200 feet: Worst-case ceiling pathway from TC to work area;
45 feet: Drop from ceiling in TC to work area;
33 feet: Work area and equipment cords;
10 feet: Slack in the TC;
10 feet: Slack in the work area;
30 feet: De-rating for transition points to undercarpet cabling.
Yes, 328 feet was the worst case, and it remains so today.
Whether you think the largest, and likely most-famous, corporate divestiture in history was a good thing or a bad thing is irrelevant. It was a catalyst for broad acceptance of 100 meters as “the channel” and 90 meters as “the link” that today we call structured cabling.
After all, who but the United States Department of Justice was going to take on Ma Bell?
Following is an excerpt from Donna Ballast’s October 2001 column.It was insightful then and remains particularly poignant today.
I still remember the mantra “Level 0 ... if we build it, the applications will follow.” But recently, it seems like structured cabling standards are chasing rather than leading the applications.
In 1985, the structured cabling concept was pretty simple: main cross-connect (MC) to intermediate cross-connect (IC) to horizontal cross-connect (HC) to work area (WA).
In 1996, multi-user telecommunications outlet assemblies (MUTOAs) and consolidation points (CPs) were added to the list. Then there were basically three options:
1. MC to IC to HC to WA;
2. MC to IC to HC to MUTOA;
3. MC to IC to HC to CP to WA.
Fast-forward to 2001, and we are seeing the introduction of horizontal connection points for building automation systems and point-to-point cabling for industrial environments. The complexion has changed so much that many are beginning to question, “What happened to the structure in structured cabling?”
In 1985, the standards focused on commercial office buildings. Today, we see a broadening of where voice, data, video, control, and security applications are installed-from the boardroom to the bedroom to the factory floor. The standards are merely trying to keep pace.
DONNA BALLAST is BICSI's standards representative, and a BICSI registered communications distribution designer (RCDD). Send your question to Donna at: email@example.com