Richard E. McGrail
America Cable Systems
There is a new buzzword in the telecommunications cabling industry today--zone distribution. This new cabling distribution method can result in significant cost savings over conventional structured cabling schemes. However, several questions need to be answered about this method: What is zone distribution? What are the savings? What are the tradeoffs? Will it meet standards?
First of all, zone distribution is not really new. Before there were cabling standards, zone cabling was used to provide telephone service to part of an open office area, typically using a 25-pair cable as feeder to interconnect 3- or 4-pair drop cables to individual telephones. With the growth of standards, zone cabling has been used to distribute under-carpet cabling while avoiding overlapped cable runs or the convergence of large numbers of cable runs at a wiring closet.
Today`s version of zone distribution is an update of a concept that has already been proven. The approach still uses feeder cables to a central distribution point in the zone, where they interconnect to individual cable runs that fan out to each workstation outlet. The zone can be any size, but a typical one would be the area between four building columns--which in an office environment might contain 8 to 12 workstations. In a multifloor installation, a zone is generally limited to an area on one floor.
Star-wiring outlets
What distinguishes this approach from accepted structured cabling schemes is that the EIA/TIA-568 standard for telecommunications cabling in commercial offices calls for star-wiring all workstation outlets on a floor directly to a single wiring closet by using uninterrupted cable runs. The basic difference in zone cabling is the additional interconnection.
Although zone distribution is usually associated with an open office, it could also be used in a manufacturing area; the method has also been used for power distribution for many years. With the introduction of modular power-for-convenience cabling almost five years ago, some zone distribution systems now provide telecommunications and power cabling together.
If star-wiring all workstations directly to the closet is the accepted approach, and it works, then why change? The basic problem is that the guidelines in the EIA/TIA-568 standard were written for permanently installed cabling. In fact, one of the primary goals of the standard has been to define a scheme in which the horizontal portion of the cabling will not have to be touched again after the cabling has been installed. Star wiring provides this permanency by letting the installer or network manager change equipment and troubleshoot the cabling plant without disturbing the installed cabling or disrupting office employees.
Unfortunately, even in offices that strictly follow the structured approach defined in the standard, cabling still changes. In an open office environment, for example, cabling changes because the office layout changes. As departments reorganize, offices are shifted around and regrouped. As offices move, outlets are relocated. And as outlets relocate, cables are rerouted--which is where the problems start.
In star-wired cable runs, unless the new outlet location is near enough to the old one or enough excess cable was included in the original installation to reach the new location, new cable may have to be pulled--the problem that the cabling standard was meant to avoid.
Zone distribution provides a way to keep most of the cabling permanent and undisturbed while making it easier to move the rest. Outlets can be relocated within a zone to accommodate a new office layout, but cables to those outlets need only be rerouted from the zone distribution point. The feeder cabling from the wiring closet to the zone distribution point remains the same.
Where are the savings?
Although this seems to be a more flexible wiring solution, zone distribution involves additional connecting hardware that adds to the initial costs of the installation. So where are the savings?
Moving outlets and rerouting cables in a standard structured cabling configuration may still be more economical if a move only has to be made once every five years, but unfortunately, that is not usually the case. Today, frequent changes in an organization`s management and structure necessitate corresponding changes in office layout. With corporations shifting to project-team or workgroup structures, more flexibility is required. Facilities or information systems managers in major corporations or other organizations suggest that up to 40% or 50% of their employees move each year.
The frequency of moves is only one factor favoring zone distribution, however. In a communications-intensive office, generic cabling must support multiple services to each desk. A typical service mix may include a telephone, one or several local area networks, a modem connection to the Internet and a fax hookup. Many organizations now plan their structured cabling systems to support three to six services per workstation.
High-performance Category 5 unshielded twisted-pair (UTP) cabling has become the accepted medium for generic cabling from the wiring closet to the desk. To minimize crosstalk or noise coupling, the accepted rule has been to restrict each service to a separate UTP cable. A generic cabling system that supports three to six services to each workstation could, therefore, require the same number of separate UTP cables to each outlet.
Proper termination is critical
Moving outlets and rerouting cables usually means that the cables must be disconnected and reterminated. In a Category 5 cabling system, proper termination is critical to the overall performance of the system`s ability to support high-speed services. Removing too much jacket or untwisting pairs too far can degrade performance. Exceeding the bend radius where cables enter the outlet or clamping or tie-wrapping the cables too tightly can also affect performance. Proper termination, then, requires trained and experienced technicians.
Because of the concern that installations may vary in quality because of differences in workmanship, it has become standard practice to verify the performance of a Category 5 system after installation. Testing requires sophisticated field instruments used by trained technicians. If cables are moved and reterminated, testing must be repeated.
Today`s open office environment involves frequent moves of an extensive and expensive cabling plant. These moves require sophisticated labor and testing, and office productivity is lost during the move. As the lifecycle cost of the cabling plant has become a major concern of facilities and information systems managers, zone distribution has received increasing attention because it addresses these factors.
The actual savings in lifecycle cost will depend on the size of the zone, the number of workstations, the life of the system, the number of moves and the actual method of implementation. However, zone distribution methods can result in an installed cost that is 40% less than that of conventional cabling methods.
However, even if zone distribution is justified by lifecycle cost, will the additional connections degrade high-performance cabling? Specifically, will a Category 5 UTP zone distribution system meet standards and support high-speed local area networks?
The Electronic Industries Association/Telecommunications Industry Association (EIA/TIA) now recognizes the problems of open office cabling and is reinterpreting some of its original guidelines to provide more flexible solutions. In the original EIA/TIA-568 standard, a transition point was permitted between the wiring closet and outlet to accommodate the connection of round feeder cables to flat under-carpet cables. The revised TIA/EIA-568A standard incorporates the performance effects of such a transition connection.
A new EIA/TIA Telecommunications Systems Bulletin, titled "Additional Horizontal Cabling Practices for Open Offices," officially expands the definition of the transition point to zone distribution point for any type of cabling. The actual terminology is "consolidation point" or "multiuser outlet," but the approach is the same. The open office bulletin should be available in the second half of 1996.
Zone distribution, then, has become an accepted option for structured communications cabling, and the performance of the zone distribution point is built into accepted cabling performance criteria.
In addition, two other developments could affect how a zone distribution system is implemented and how it performs. The first is renewed work on 25-pair or multipair cabling. (Anything with more than four pairs is considered to be multipair.) Once dismissed because of concern about potential crosstalk between multiple signal transmissions, multipair cabling is making a comeback. Cable manufacturers have developed 24- and 25-pair cable configurations that can handle multiple transmissions with overall crosstalk that is no worse than that of a Category 5 4-pair cable carrying only one transmission.
TIA/EIA-568A recognizes these developments with a new set of performance specifications for Category 5 multipair cables. AMP Inc. (Harrisburg, PA) has introduced a 50-pin connector to match the cable performance, making it possible to use plug-and-play multipair Category 5 cable assemblies as concentrated zone distribution feeders. Other vendors plan to introduce similar products.
The second development addresses the problem of noise coupling to UTP cabling from nearby power cabling. With today`s telecommunications systems, high-frequency power-line transients from switches, solenoids or motors can affect nearby low-voltage circuits. While Category 5 UTP cabling is less susceptible to this coupling than is telephone-grade cabling, it is not totally immune. The problem is under study by an EIA/TIA committee, and updated guidelines will soon be published.
Richard E. McGrail is vice president of America Cable Systems, New Bedford, MA.