T1 Facility Summary
This next section describes the elements in a typical T1 backhaul facility. Based on this information, the next section shows typical fault conditions and what procedures are typically followed to restore service to the T1 span.
With almost no exception all circuits leased from the TELCO are routed through at least one central office facility. If this were not the case, planning and managing the repeaters needed for T1 transmission would be an onerous task. Repeater facilities are need to be spaced at a distance of 6000 feet (about 1.8 km) and generally contain slots for multiple repeater units. See the Measurement Guide for a typical circuit layout.
The following definitions apply:
Central Office: This is usually located at the geographic center of a particular service area. It is normally a building that contains the switching equipment and cable cross-connect facilities. Although this diagram shows a single central office, each end is often served by a separate central office. Facilities exist at the central office to accommodate remote testing and monitoring of T1 facilities.
Field Repeater: Repeaters are necessary when the customer is located beyond 6000 feet (about 1.8 km) from the central office. Multiple repeaters are necessary when the span exceeds 12000 feet. Most commercial customers are located within 12000 feet (about 3.6 km) of the central office. The repeaters are powered from the central office. Typically 90 volts is provided between the center-taps of the transmit and receive transformers. A particular repeater can be looped back by applying a specific loop-back tone. Often a label affixed to the cabinet denotes the loop-back tone. This may look like "FLTR A" or "FLTR B".
NIU: A device provided by the TELCO that contains the necessary equipment to provide the expected electrical levels to the customer. This device may be active and the serves as the customer demarcation (demark) point. When this device is active it can be looped back by applying the proper loop-back code. The customer is responsible for equipment beyond this point.
CSU: A customer provided unit that is required by the TELCO as part of the tariffed service agreement. This device may be looped back applying a specific loop-back code.