March 10, 2008

Biz Tel: Real-World Wiring

By Justin J. Junkus

As more operators offer business telephony in addition to a residential offering, operators are finding that supporting business communications differs from supporting the residential triple play.

Just as important as the service dimensions of this support is the physical side of support, where customer premises equipment and premises wiring is similar, but very different from its residential counterpart.

Wiring and the 66 block

Business wiring in particular is formally defined by TIA/EIA standards 568 and 569. A premises wiring system consists of seven components: backbone or riser cabling, horizontal cabling, workstation area, telecommunications closets, equipment room, entrance facility and administration. As a start, let's cover connecting blocks in the wiring closet. Mundane as it sounds, this is an area that can spell big trouble if you don't know what you're doing.

There are two major types of connecting block in popular use in multi-tenant dwellings: the 66 block and the 110 block. The 66 block is the older version and was the typical demarc interface between house wiring and a voice network service provider. It consists of four columns by 50 rows of insulation-displacing metal contacts mounted on a plastic block.

In the typical configuration, called a split block, the left two contacts in each row are electrically connected to each other, as are the right two contacts. An impact punch tool is used to connect house wiring pairs to one set of two contacts in a row and network pairs to the remaining set of two contacts in the same row. Alternatively, 24 pairs may be pre-wired to either side, and connection to external equipment is via a separate RJ-21 connector. Typically, a maximum of 24 pairs are connected, leaving the bottom two rows unused. Connection between house and network wiring is via a bridge clip that is placed across the two centermost terminals in a row.

Cable re-route

Changing service providers at a split 66 block is a simple cable re-route. The network side going to the existing service provider is removed using the same type of impact tool that attached the wires, and a new set of wires from a cable to the new service provider is reattached to the same terminals.

The challenge of 66 blocks is that not all 66 blocks are in the split configuration. Some installations may use a block with all four terminals permanently wired to each other as a way to interconnect multiple line appearances. The actual demarc may then be at another 66 block. In this case, there is no network side to the 66 block, and this is not the point where service changes are made. The installer needs to recognize the demarc block. Looking for bridge clips and verifying continuity between terminals in a row are two good indicators.

- Jay Junkus, Contributing Editor, Communications Technology, and President, KnowledgeLink