The carrier shelves in a multicarrier cabinet connect to a common backplane at the rear of the cabinet. At the rear of each printed circuit board card slot is a connector that attaches to the backplane. The backplane wiring, which uses wire-wrapped connections or printed circuit board technology, contains the system bus structure for intra- and intercarrier processor control and communications signaling transmission. The processor control signals are multiplexed onto the backplane bus structure via time slot assignments. The time slot assignments to the port circuit card slots are usually arranged to allow maximum flexibility in the placement of different device boards in the carrier, although some systems are designed with a fixed number of time slot access connections per port card slot. Physical backplane connectors, such as 25-pair connectors, are used to interface the port circuit card packs. There are also connectors for intercabinet or intercarrier connections. The shelf backplane of a control carrier may have multiple RS-232C connectors to support management and maintenance terminals or call detail recording equipment. Another connector is used for connecting a tip-and-ring pair and maintenance leads to a crossconnect field.
PBX cabinets and/or distributed carriers are usually connected with optical fiber cabling links. Optical fiber links provide very high transmission bandwidth in support of switch network traffic requirements and call processor signaling. Optical fiber distance limitations between cabinets vary from system to system, but the typical loop length is about 1,000 feet without repeaters or special interface cards.
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