in a central location SO that each cable run will be equidistant from the MDF. For more efficient cable distribution in larger installa- tions, IDFs (Intermediate Distribution Frames) are sometimes used. On multi-floor installations there may be one or more IDFs per floor. The IDF is connected to the MDF with a feeder cable containing enough pairs of wires to accommodate all of the cable pulls back to the IDF, plus additional pairs of wires for growth. The IDFs and MDFs are laid out to accommodate growth and to enable records to be kept on the originating and terminat- ing end of each cable. If good records are not kept, you may find it costs less to run new cable than to figure out the old. The IDF and MDF are made up of connecting blocks which are devices made of plastic and metal designed for terminating cable. Although M66 blocks are still in place in most installa- tions, newer installations use 110 blocks developed by AT&T (Figure 12.3). These are higher in density, enabling more pairs of wires to be terminated in a smaller space. These blocks can be mounted on plywood backboards or can be installed on free-stand- ing racks. Sometimes there are cable trays above the racks to neatly hold and support the cable. Some telephone systems are installed using modular patch panels at the MDF. This appears to make things easier since any- one can make changes without knowing how to punch down (connect the wires) cable on the block. However, if there is a lot of moving and changing, the modular panel can become a tangle of cables and problematic to manage. 252 The Telecom Handbook CHAPTER 12 Figure 12.3 AT&T Developed 110 Block A 600 Pair Block for Main Distribution Frame The Telecom Handbook - 253 CABLE Another possibility for the MDF which is costly and used primarily for data installations is an electronic digital patch bay in which the cables remain in the same place. The con- nections are changed using software. This method may be used for voice communications in the future. The problem with it is that it creates