Chapter 7. Center Office Cabling

Office wiring typically goes from the cable vault directly to the office repeater shelf, from the office repeater shelf to the Automatic Protection Switch (if equipped), and from there to the channel bank, radio mux, or other terminal equipment. Along the way, there may be one or more DSX-1 cross connect panels. Western Electric ABAM cable is a traditional selection to connect among all of these elements. Use a loss figure of approximately 0.4 dB per 100 feet of office cabling, assuming 22 AWG. Remember that 14 to 16 pF per foot is the normal cable capacitance. Cable with higher capacitance will give you problems on the longer cable runs. CAT 5 LAN cable was not intended for T1, but it has low capacitance, so it makes a good substitute.

In some rural service areas, the incidence of lightning strikes on aerial cable is so frequent that one extra measure is applied at the cable vault. The outside cable might be 22 gauge and the tip wiring to the line terminating shelf might be 22 gauge, but one short section of higher gauge (smaller diameter) cable is added at the cable vault. This 25 to 50 foot length of 24 or 26 gauge cable is called fuse cable. As its name implies, it acts as a fuse element that will open when huge currents from lightning appear. This technique is effective in keeping lightning surges out of the central office, but it has side effects. The 24 or 26 gauge insert may be short, but it interjects one extra attenuation factor in cable loss estimation. Further, when the fuse cable opens up, that pair must be abandoned. That is a lot of work to correct.