page_title: Number One Crossbar Switching System
section_title: Maintenance Facilities
hierarchical_section_title: Number One Crossbar Switching System / Maintenance Facilities
caption_attribution_description: English: Maintenance center for the VErmont No. 1 Crossbar office at Connections Museum, Seattle. This switch was installed in 1942 and served the neighborhoods of northeast Seattle. From left to right, Incoming Trunk Test (ITT), Terminating Sender Test (TST), Terminating Trouble Indicator (TTI), Originating Trouble Indicator (OTI) Outgoing Trunk Test (OGTT), OGT Jack Bay, Sender Make Busy frame (SMB frame), Originating Sender Test (OST), District Junctor Test (DJT)
context_page_description: The Number One Crossbar Switching System, was the primary technology for designing urban telephone exchanges in the Bell System in the mid-20th century. Its switch fabric used the new electromechanical crossbar switch to implement the topology of the panel switching system of the 1920s. The first 1XB system was installed in the PResident-2 office at Troy Avenue in Brooklyn, New York which became operational in February 1938.
context_section_description: The No. 1 Crossbar system consolidated maintenance and testing activities into a single area of the central office, sometimes known as the MTC or "Maintenance Test Center". Whereas earlier systems, such as the panel switch had maintenance facilities spread throughout the office, it was found desirable to centralize these processes into a single space so that technicians could work more effectively. This decision was also driven by the use of markers, which carried out their individual tasks quickly, and interfaced with several other circuits while doing so. Because of the nature of their rapid operation, and the importance of consistency and correctness, originating and terminating trouble indicators were employed, which illuminated lamps to indicate the source of and nature of trouble when it occurred. This allowed technicians to quickly identify and resolve issues with the equipment without taking it out of service to perform lengthy troubleshooting routines. Because it was often necessary to run tests on other equipment while locating the source of trouble, other test frames were located near the trouble indicators.
  Since maintenance centers were standardized in No. 1 Crossbar offices, most MTCs contained one or more of the following frames:
  Incoming Trunk Test (ITT) - Tested incoming trunks connected to this office's outgoing trunks.
  Terminating Sender Test (TST) - Tested Terminating Senders of various types (FS, MF, "B")
  Terminating Trouble Indicator (TTI) - Used both to indicate trouble as it occurred, and to test the operation of the terminating marker.
  Originating Trouble Indicator (OTI) - Used to indicate trouble as it occurred, and to test the operation of the originating marker.
  Outgoing Trunk Test & Jack Bay (OGTT) - Tested outgoing trunks to distant offices. Facilities were provided for testing voltage, continuity and polarity, and also for making calls over the trunks to test transmission quality.
  Sender Make Busy Frame (SMB) - Lit lamps toindicate senders that were "stuck", such as when encountering an unrecoverable error, and allowed senders to be made-busy to service calls, thereby taking them out of service.
  Originating Sender Test (OST) - Ran a range of tests on originating senders, including tests which exceeded the severity of conditions they were expected to function with on actual service calls.
  District Junctor Test - Tested District Junctors, including flat-rate, coin, and message rate.
caption: Maintenance center for the VErmont No. 1 Crossbar office at Connections Museum, Seattle. This switch was installed in 1942 and served the neighborhoods of northeast Seattle.
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