Q: At the Massachusetts Institute of Technology (MIT) Network Group, we are considering upgrading our existing 10Base-5 building backbones with a 10Base-T collapsed-backbone topology. The plan is to standardize on a riser- installation procedure for the entire campus, extending 100-Mbit/sec service, as required, to local networking devices in the riser. My concern is how to provide a solution to support this request.
Currently, all stations on the existing backbones are supported by Asante 72-port unshielded twisted-pair managed repeaters. An RJ-45 interface will support the repeater links after the new backbone is installed. We have collapsed backbones in a few buildings. The riser connections are supported by AT&T 1061 station cable in a point- to-point configuration from the base repeater to various intermediate distribution frame (IDF) repeater locations.
My position is to go with what I know and what I currently require from my contractors; that is, installation of six 4-pair 1061 cables terminated to AT&T specifications.
One suggestion is to install a 25-pair, Category 5 cable between the main distribution frame and each IDF on Category 5 patch panels. Patch cords will connect the base and local repeater locations. I am not certain that 100-Mbit devices can be supported within one 25-pair cable jacket without causing crosstalk.
Can you give me information on the 110 insulation termination and certification procedures for 25-pair, Category 5 cables? Is anyone else using this type of cable in a riser application? What speeds are supported? And does it work?
Stephen Goldman
Transmission Services
Massachusetts Institute of Technology
Cambridge, MA
A: You are not alone in your concern. The TIA/EIA-568A standard, Section 10.3.4.7, cautions designers to separate services with different signal levels, or services that are susceptible to impulse noise, into separate binder groups in a multipair cable. Refer to the specific 100-Mbit/sec application standard to determine if your application is susceptible to impulse noise.
As for "certification," TIA/EIA-568A requires power-sum crosstalk measurement testing for multipair (that is, greater than 4-pair) cables. Currently, no field test instruments are available that can perform power-sum crosstalk measurements on 25-pair cable. The category of the insulation displacement contact connecting hardware--for example, 110 blocks--is unaffected by the pair count of the cable terminated.
If you are crossconnecting the horizontal cable (i.e., cable from the telecommunications closet to the work area) to the backbone (riser) cable to create a channel, you now have extra connecting hardware in the channel. Again, refer to the specific 100-Mbit/sec application standard to determine if your application requires a Category 5 channel (see figure).
I have found that the material cost on six Category 5, 4-pair cables is considerably lower than for Category 5, 25-pair cables, while placement and termination costs are only slightly higher--at least in Austin. (You should use comparable labor figures for your area.)
At the University of Texas, Austin, we see about 90% retrofit to 10% new construction. The older buildings rarely have telecommunications closets on the floors, forcing us either to commandeer space and construct closets, or install what is known as a "collapsed-backbone cabling system." While the 4-pair cables do occupy more space in the pathway, we found it easier and more cost-effective to enlarge the vertical pathways in a building than to construct telecommunications closets. This has proven such a popular solution that the Telecommunications Industry Association recently published a Telecommunications Systems Bulletin, TSB-72, devoted to how to implement centralized cabling systems.