Friday, February 19, 2010

So, how does a panel office work ??

The Panel Office

This will describe the basic components of the typical panel central office. In our next installment, we'll trace a call through a panel office, showing exactly what happens in each stage of the process. We'll also describe the 'revertive pulsing', the method by which panel offices communicate with eac other, in some depth in a subsequent article.

I've been fascinated with the panel offices ever since being served by one and discovering some of its quirks. In the 1970s I was actually able to schmooze my way in for a late-night off-the-record tour of one, which only increased my interest.

In the 1970s, the panel switch was the Rodney Dangerfield of telecom. It got no respect! It was considered to be crufty, unreliable, low-tech, old, antiquated, a Rube Goldberg machine, and the like.

For the 1920s, however, the panel switch was high-tech. It was bleeding-edge for its time. In those days the practicality of providing machine-switched telephone service to large metropolitan areas was unproven, and the Panel installation in Omaha, beginning in 1921, was the proof of concept.

The term 'panel' comes from the panels of contacts which make up the frames which comprise a panel switching office. Each 'frame' consists of a panel of contacts, arranged normally in 5 banks of 100 contacts each, or in 10 banks of 40 contacts each for the (newer) linefinder frames. Each frame has 60 selector rods, 30 on the front, 30 on the rear, with each rod having normally 5 contact 'brushes', with the rods on the linefinder frames having 10 contact brushes.

The selector rods are positioned over the correct contacts on the panels by motor-driven cork rollers located toward the bottom of the frames. Clutches for up-drive, down-drive (and slow up-drive on the final frame) control the movement of the selector rods.

The brushes are the movable contacts which connect to the fixed contacts upon the banks of the panel switch.

Let's take a quick overview of what comprises (or comprised, this is all in the past tense here, actually) the typical real-world panel office and discuss the various parts.

In the diagram above we show the various frames of the panel office, plus some of the major ancillary components.

Five types of panel switching frames existed, with four of them appearing in all full-blown panel offices and a fifth type existing in those in the largest metropolitan areas.

  • Linefinder frames

    A complete full-scale panel office will have 25 linefinder frames, each serving a group of 400 subscriber lines, for a total of 10,000 lines. (Researching the proverbial 10,500 line panel offices will be an exercise for the student.)

    The linefinder frame provides the connection to the subscriber's telephone set when the receiver is taken off hook, and routes that connection first to the register-sender and then to a corresponding district frame.

  • District frames

    District selector frames appear in all panel offices. In panel offices in smaller communities (ya know, it's difficult for me to write all of this in the past tense, so if you don't mind, I'll do this in the present tense as if these things still exist today, ok?) the district selector frames provide the outgoing connection from the subscriber, via the linefinder, to either inter-office trunks to other central offices, or intra-office trunks to incoming frames in the same central office.

    Each district frame can connect to 500 trunks, so there is a practical limit, and in larger communities a second stage of outgoing selectors is provided.

  • Office frames

    In the largest communities where there are many central offices, 500 outgoing trunks from a panel office is inadequate, so a second stage of outgoing selection is provided by 'office' frames. These frames only appeared (there I go mixing tenses again) in the largest metropolitan areas, such as New York City, Chicago, etc. It's believed that Omaha's panel offices used only the district frames for the outgoing connections.

    The district and office frames select the particular central office of the call to be completed.

  • Incoming frames

    The incoming frame of a panel office is the first stage of selection of the called station. The incoming frame connects the incoming inter-office or intra-office trunk to the particular final frame which serves the called subscriber.

    The incoming frame consists of five banks, each with 100 contacts. Each bank is served by one brush on each of the selector rods. This allows a maximum of 25 connections between the incoming frame and the final frame. These sets of 25 connections are referred to as a 'group', and there are four groups within each bank, for a total of 20 groups within the incoming frame.

  • Final frames

    Each final frame can serve 500 subscribers, therefore there will be 20 final frames in a complete panel office serving 10,000 subscriber lines. The final frame connects the incoming call to the subscriber's station, and provides the logic to test for line busy, to supply ringing voltage to the called line, to return the ringback tone (or busy tone) to the calling subscriber, and to remove ringdown/ringback and complete the talking path when the called subscriber answers.

Besides the actual switching frames, other important parts of the panel office included:

  • The Register-Sender. (Sometimes called just 'sender'.)

    Think of this as the 'brain' or the 'CPU' of the panel office. It's actually a collection of electromechanical relay-logic circuitry. This is the main 'Common Control' section of the panel office. The register-sender records the digits dialed by the calling subscriber and gives the commands to the various frames, both in the originating office and the terminating office, as to how to position the brushes against the banks of contacts, in order to complete the call.

    The register-sender is only needed at the initial call set-up phase. It is dismissed once the selector rods are all in place and the call is in the ringing or the line-busy state.

    A full-scale panel office would have anywhere between 3 and 6 or so register-senders depending on the volume of calls anticipated.

  • The Translator or Decoder.

    While the positions of the selectors on the incoming and final frames are easily determined by the called subscriber's number, the connections to various intra and inter-office trunks are quite arbitrary and vary considerably from community to community.

    The register-sender calls upon the Translator (earlier panel offices) or Decoder (later panel offices) to determine how to route the call through the district frame and, if used, the office frame.

  • The Sender-Link circuitry.

    Each connection between the linefinder frame and the district frame has a set of multi-contact relays and associated circuitry which allows it to 'seize' an idle register sender when the subscriber goes off hook.

    Note that the register-sender is only needed for a few seconds for each call. It receives the digits, does some lookups via the translator/decoder, sets up the call, and is then available for another call.

    'Slow dial tone' on a panel office is almost always due to the fact that all register-senders are busy at the time.

  • The Tone Plant.

    In the typical panel office, this is a rotary electromechanical device about the size of and in some ways resembling a metal lathe. This contains rotary generators for the low-voltage tones which make up dial tone, busy tone, etc., and the high-voltage (90-100 v 20Hz) ringing signal.

    Interrupters are driven by a step-down gear train. These provide the cadences for such things as ringdown, ringback, busy tone, etc. Some of these would use a rotary brush arrangement, while others used mercury-filled rotating drums to interrupt the signals.

    The various tones and signals which come from the tone plant are:

    • Dial tone. This would be the older non-precise dial tone for panel offices not converted for touch-tone service.

    • Busy tone. This would be the older somewhat rude and raucous 'BAWW BAWW BAWW BAWW' tone, interrupted at 60 impulses per minute, returned to the calling subscriber for a line busy condition. Even panel offices which were converted to touch-tone retained this tone for busy-back.

    • Ringback tone. For most panel offices this would be the non-precise 'metropolitan ring' tone, electromechanically generated and interrupted normally with a [corrected] two seconds on, four seconds off cadence.

    • Reorder tone. Same as the busy tone, but interrupted at 120 impulses per minute. 'Fast busy'. Used for various all trunks/circuits/equipment busy conditions.

    • No such number or 'vacant code' tone. A deprecated intermittent fast busy type tone used to indicate numbers, prefixes, etc., which did not exist. This was phased out as recorded 'The number you have reached is not in service' announcements were phased in.

    • Permanent signal, or 'receiver off hook' tone. No, this was not the very loud and obnoxious 'CLANK CLANK CLANK' receiver off hook sound of today. It was a single high-pitched tone, returned to the calling subscriber when the register sender got tired of waiting for dialed digits and routed the call to be 'parked' on a 'permanent signal trunk' on the district frame.

  • The Intra-Office trunks.

    From the perspective of the register-sender, a call within the local central office is identical to that going to a remote central office, or to another panel office within the same building. Each panel office would have a certain amount of hard-wired connections reserved for intra-office calls, connected between the local office (or district) frames and local incoming frames.

  • The Inter-Office trunks.

    These were the outgoing and incoming connections to and from distant central offices. In the early days of panel offices, these were simply loaded two-wire circuits with a frequency response of DC through 3kHz or so. When setting up a call to a distant panel central office, it was the register-sender of the originating CO which controlled the operation of the incoming and final frames in the terminating CO.

  • The batteries and power plant (not shown).

    Primary central office power is provided by huge lead-acid cells, which are continuously charged by motor-generator sets (MGs) or rotary converters. This is very typical of central offices today, except that solid-state rectifiers now replace the former electromechanical devices. In the heyday of panel offices, commercial power was normally used to run the MGs or converters, supplanted by on-site diesel, gasoline, or LP generators to be used for commercial power outages.

This is a basic overview of a typical panel central office. In the next installment we'll show the progress of a call from receiver off hook to receiver off hook as it works its way through the panel switches.

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