Thursday, May 13, 2010

The Great 531 Overlay of 2011!

After years of speculation, decades actually, and several false alarms, Nebraska will be getting a third area code for general telephone service. (Isn't this exciting? Yeah, right!)

The number of available prefixes in 402 is really-truly approaching exhaustion, even with a judicious degree of sharing of many prefixes between providers. (Yes, you can now get an 'Omaha' cell phone with a 'Springfield' prefix of 402-253!)

NPA (Area Code) 531 will go live in early 2011, overlaying the area currently served by Nebraska's original area code, 402, which now serves Omaha, Lincoln, and several 'nearby' communities such as Hastings, Norfolk, and Valentine.

This time it's really gonna happen, and conditioning of the Teeming Millions for the impending transition is scheduled to begin Real Soon Now.

The Powers That Be decided some time ago that it is For Our Own Good that 531 should be overlaid on 402 rather than split geographically, thus saving half of us the trauma of remembering a new area code.

This will 'require' (explaining why this is really not the case will be an exercise for the student) that local calls within the 402 area must be dialed using ten digits: the area code, the prefix, and the line number. (10 - Count 'Em - 10 NPA-NXX-XXXX)

To phase in the new area code, 'permissive' dialing, which means that you are allowed to dial with ten digits if you really insist, will begin next month in June. Mandatory 10 digit dialing will begin in February, 2011, clearing the way for NPA 531 deployment in March.

Fortunately, one vestige of sanity remains. The Powers That Be have decided that we will not be required to dial '1' to prefix all calls, keeping with the notion (sort of) that '1 means toll.'

The explosion in growth of cellular phones in the 1990s brought this on, and even though it was put off a number of times, it looks like we're now stuck with it.

This was not the first time, however, that AC 402 was broken up.

What we know as Area Codes, actually Numbering Plan Areas (NPAs), were assigned in the 1940s as Ma Bell migrated from manual long-distance service to operator-assisted machine-switched long-distance service, facilitated by the barely-pre-war XBT (Crossbar Tandem) and the post-war #4 Toll Crossbar switch.

Nebraska was originally assigned NPA 402, serving all of Nebraska at the time. Many states, some far more populous than Nebraska was back then, were served by a single area code. In those days, true NPA-NNX-XXXX compliant phone numbers were far from universal. Many larger cities still used 6-digit 2-4 numbering plans. Omaha had such a plan until September 10, 1960. Council Bluffs had a combined 4-5 digit plan, as did many other communities with two or three COs.

As communities adopted the then NPA-NNX-XXXX standard (NPA-NXX was yet to come) in the 1950s, a necessary prerequisite for inbound Direct Distance Dialing, available prefixes were quickly exhausted and several area code 'splits' occurred in the early-mid 1950s. A severely gerrymandered line was drawn arbitrarily across Nebraska, leaving communities such as Rulo, in the southeast corner and Valentine, in northwest Nebraska, as well as Omaha and Lincoln in 402, with everything else in 308.

The phase-in of NXX prefixes in the late 1960s delayed several area code splits, and the sharing of NXXs among providers held things off even further, but countless splits and overlays occurred in the 1990s and 2000s, and we will be one of them soon.

Aren't we excited ??

Oh, one last thing. Let's be sure to notice how our local Talking Heads screw up the details of this as the date approaches. Ted Brockman, are you listening in? ;-)

Saturday, May 1, 2010

Mayday! Mayday! Mayday!

It's Mayday! Saturday, May 1, and I and a friend set off in the direction of Lyons, NE to visit the Lyons Regional Antique Telephone Show, jointly sponsored by the JKL Museum Of Telephony and the Telephone Collectors International Society.

It's an easy drive from Omaha. We stopped and had breakfast in Fremont and arrived at the site before noon.

The show site is the former AT&T Long Lines 'Hardened' Lyons Communication Center, built to withstand a nuclear blast and then some. The site is very easy to find. Just drive in the direction of Lyons, and look for the tower. ;-)

As a reformed phr^H^H^Htelephone enthusiast, I thought my days of 'underground' phone business were over. WRONG!

The show was, literally, three stories down in a blast-proof bay.

The huge blast door was open for the occasion.

The facility is, literally, a cold-war vintage communication center.

Most of the equipment has obviously been removed, but the remnants of what appear to be a MDF remain.

As well as a few items on display.

But anyway, back to the show ...

On my personal 'wish list' were a few specific items:

  • A working or easily restorable 'candlestick' desk set, with a working dial, preferably original WECO, but maybe Kellogg or others. No repros or bastardized sets.

  • A working or easily restorable 202 series desk set, with working dial.

  • A VG or better 302 desk set.

  • A clear-case 500, 1500, or 2500 set.

I must admit that most of what I saw at the show was 'good junk', as opposed to 'junk junk', as most of those showing/selling stuff appeared to be sincere collectors and not obvious rip-off artists.

I was able to score a restorable 202 with a good condition dial and E series handset, and a very good, but obviously repainted 302. Two out of four ain't bad. ;-)

Let's take a look at a few things I wanted, but did not buy.

There were a few nice original WECO candlesticks, with dial, and cosmetically and mechanically good or better.

However, the price points were just far more than I was willing to pay. :-(

There were some very nice clear sets, this one being an AE Monophone,

and this one being a WECO Princess, but unfortunately these were for show only and not for sale.

On the back of a truck outside the building was an old WECO cord board. I'm kind of assuming that this was out here because of the difficulty in getting it down the steps.

Those from Omaha will immediately recognize the name of the business from which it came. ;-)

For some reason, they are saying that this will be the last show of this type at the Lyons facility. I'm not sure why, but I do wish they would reconsider this decision. :=(

Anyway, it was time well spent !!

Thursday, March 25, 2010

The Ad Man Cometh

Let's take a break from the techie stuph, and reflect on another aspect of Ma Bell's omnipresence during her heyday, some vintage Ma Bell ads, compiled from various sources.

Yellow Pages<tm>.

I guess this one was before you let your fingers do the walking.

'Hey Joe, where ya goin' with that phone in your hand?'

But seriously, in these days we take for granted that circuits, whether subscriber loops or international long-haul trunks, are plentiful and always at your beck and call. In the WWII era, party lines, 4 and 8 party in Ma Bell land, up to 12 party in Indie land, were commonplace. Long Distance trunks were in many cases scarce and in high demand.

Ads such as this were common, and rightly so, encouraging subscribers to keep the lines open for servicemembers during the peak evening hours.

Many of Ma Bell's vintage ads feature anthropomorphic and larger-than-life phones.

This one has to be Ma Bell's answer to Reddy Kilowatt. ;-)

They were bragging about a 'short' two-minute time to set up a LD call. Remember, this was back in the days when Long Distance was totally manual. The calls had to be manually worked through Ma Bell's network, often through one or more intermediary offices, and through an 'inward' Long Distance operator at the far end.

These two ads (above and below) feature similar sets of what I would call an unusual configuration, 200 series sets with F series handsets, no dial.

These had to be from the era when manual service was still standard and presumed to be what the majority of the viewers of the ads were accustomed to. I never experienced manual service first hand, but I do know that it was commonplace, even in some parts of major metropolitan areas, into the 1950s. I would guess the vintage of the ad immediately above to be 1930s, showing the 'high tech' F handset, but not rubbing it in to the noses of the viewers that dial service was not available in many areas.

This is also the earliest ad I've seen where the '2368' number was used.

Although the number tag is not entirely shown, it's obvious from the spacing that it's intended to be EX-2368 and not something like EX3-2368. The 2-4 numbering pattern was very common in the 1930s in areas served by both manual offices and newer 'machine switching' offices.

Hmmmm .... I wonder if, during the heyday of Ma Bell's manual service, they had offices such as QUigley or QUeensbridge or ... ;-)

The family at EX-2368 gets a new 302 set, and my guess is that this ad is late 1940s, with 2-4 dialing still commonplace, but before the 500 series was common.

The number morphs to Main 0-2368 in this ad featuring the 302.

And in this featuring the 500.

Phone 'enthusiasts' will be quick to point out that zero, as the third digit of the office code (NNX) was not allowed until the late 1960s.

As I said ...

... the quality of the voice in Ma Bell's glory days was second to none! As critical as I've been of Ma over the years, this is the one thing I admit was superior during the days of one system, one solution.

Likewise ...

The quality of the sets was second to none as well! The 500 was built like a tank. It could take any kind of abuse that an ornery kid, an angry parent, or a curious phr^H^H^Henthusiast could ever deliver. I think the MTBF (mean time between failures) of a late 60s vintage 500 was measured in centuries!

(We now return to our regular program.)

Tell the kids that 'Uncle Jim' is coming over (but he's not really your uncle).

'Hi baby! He's gone and won't be back until tomorrow night. C'mon over. The kids are, uh, occupied!' ;-)

My Aunt had a Princess<tm> phone in her bedroom, early model, sky-blue pink IIRC, rotary dial, maybe 1961 or so.

This one did not have a ringer in the set. It was on the baseboard, color-coordinated with the set, almost like a throwback to the 202 days. It had that lighted dial, powered by the 'brick on a string' transformer plugged into the wall socket.

Then about 1967 Mom got a Princess phone. This one was again rotary dial, but had the ringer in the set. She refused to let the installer staple a line for the light across the baseboard to a 'brick on a string' at the nearest wall socket, which was way across the room, so at the time of installation, this Princess was dark.

It's always handy having a son who was a budding phr^H^H^Hphone enthusiast! I found the correct run of quad down in the cellar, carefully dissected it with the X-acto knife, pieced out the yellow/black pair and connected it to the building's 24 volt doorbell transformer, but with a 'one goes out they all go out' series Christmas lamp in each leg as a ballast and to protect from any shorts. Hey, we don't want a meltdown of 100' of quad in a wall, do we? ;-)

Let there be light!

It worked just fine and I was her hero -- for maybe a day or so. ;-)

It was there and working until they moved out some years later. I never de-installed it. Hey, after they moved it was no longer my problem, right? ;-) I'm sure that TPC would have had a cow if they ever came upon it.

Phoning is fun !!

Just pick up Mommy's phone, dial 1-2-3-4-5-6-7-8 and listen to the recording, rinse, repeat. ;-)

'{sigh} Was it as good for you as it was for me?' ;-)

This is a rather unusual set for the bedroom.

The baby blue 500 is appropriate for the boudoir I admit. So is the rarely-seen dial light (available for a semi-reasonable monthly charge), but notice the two-line turn-key.

Maybe it's so she can listen in on the Teenline<tm> after they are supposed to be asleep! ;-)

Bedroom phones were promoted heavily during the 1950s and 1960s.

As well as extension phones anywhere.

'Congratulations again, Mrs. Bratter, and may there be many extensions!' ;-)

Phones in color!

Notice the straight, non-coiled handset cables. Coiled cables 'spring cords' were optional, with a recurring monthly charge, into the 1960s.

Speaking of which ...

Where does the handset cable terminate?

Your phone away from home.

In this day and age, we think of the pay phone as being in the same category as the dodo bird and the Oldsmobile. They are becoming few and far between.

'Mommy, what's a phone booth?'

If the pay phone is an endangered species, the phone booth is essentially extinct in the wild! You just don't see these anymore.

Back in the days, Ma Bell did promote the comfort and convenience of the pay phone, in the privacy of your own phone booth.

And a local call was only a dime! Cheaper if you had a straight pin and knew how to use it! ;-)

Hey! Curb your dog, Missy!

How handy ...

... a 500 on the patio by the lake!

Wasn't this the thing that Dr. Brown was wearing on his head in the first BTTF?

The Fone Man cometh!

'Hi, my name's Krusty, and I'm from The Fone Company. Let's go into the bedroom so I can {wink-wink} install my cable.' ;-)


Standard issue from around WWII until the mid 1950s.

Is it my imagination, or does that box appear too small for the set and the handset?

I hope you enjoyed today's little digression.

Monday, March 22, 2010

Transmission Zones

As critical as I am of Ma Bell, the one thing I must admit is that the quality of the voice signal during Ma Bell's heyday is the best it's ever been during the age of telephony.

The clarity of the voice transmitted by the Bell System between the 1920s and the 1980s was in most cases far superior to that of today's typical phone call.

Back then, from the era of the 202 with the E series handset, to the phase-out of the 500/2500 and (real) Trimline, we didn't need MOS scores! 5.0 was expected and with rare exception, delivered.

If you had an issue, they would cheerfully fix it, with none of this "If the trouble is in your inside wire, a $2500.00 service charge may apply ..." nonsense. (You did need to remember, of course, to unplug and hide that stray 302, you know, the one with the ringer disconnected, before TPC showed up.) ;-)

The 'cheapie chirper' phone of the early 1990s could never reproduce the voice as clearly as the 500, or even the 302. Even the 'cheapie chirpers' put today's cell phones to shame as far as voice quality is concerned. Of course, toward the bottom of the call clarity food chain are such things as Magic Jack and Skype.

Bell: Wats n, cm hr, I wnt o.
Watson: What? Huh? Say again? Eh? Pardon?

The sets were built like tanks as well. You could literally heave a 500 into a brick wall with only cosmetic damage. Try that with your Mallard Duck phone!

The quality of the voice was paramount to Ma Bell. Many guidelines for provisioning and maintenance were set up to assure voice quality.

In today's world, no matter where you live, no matter how far from the CO, SLC, or ONU pedestal, you don't really care. You buy (or otherwise acquire) a set, plug it in, and talk, or at least attempt to do so.

Back in the heyday of The System, the distance of the wire from the subscriber to the CO fell into one of a number of 'Transmission Zones', each with its own particular needs and characteristics. Originally (or so they tell me) there were several zones: 1, 2, 3, 4, and 5. By the early 1960s or so, they settled into three more broadly-defined zones: Zone 2, Zone 5, and Zone L.

Zone 2 was the closest to the CO, and was the set of subscribers within a 10,000 foot loop length (or so) the the CO. A TPC employee with whom I was acquainted told me to the effect of "we never use these anymore, but back then we estimated Zone 2 as about two miles, as the crow flies, from the CO." Zone 2 was where the loop was short, relatively free of electrical interference and distance-specific transmission loss, and the loop current was relatively high.

Zone 5 was farther out, and in the old days often estimated as "between two and five miles or so from the CO, as the crow flies, but who would want to fly with an old crow!"

If you draw circles on a map, you'll see that a very significant percentage of subscribers fell into Zone 2. Metropolitan CO boundaries typically overlapped in Zone 2 or Zone 5.

Zone L was roughly defined as anything more than about five miles out or so, or alternatively "where you get far out enough that you start having trouble", out to the limit of where things just would not work, even with range boosters, CO long-line equipment, and in-set amplifiers.

The following chart, which appeared in some similar versions in various Ma Bell internal publications at the time, illustrates the zones and recommended station equipment for each.

(This chart also illustrates what's sometimes called the 'Shirley Temple Principle', you can never be too cute!) ;-)

I would estimate this version to be ca. 1963, after the announcement of the Trimline, but before the 302 was out of use.

Note that this chart recommends that 302 sets be confined to Zone 2, short loops only. For many years, the 302 was standard issue in what the chart calls Zone 5 and even in Zone L. From 1949 or so, the newer network in the 500 series sets did give better performance on medium-long to very long loops.

However, notice that the 500, restrained by the ball and chain, is not encouraged in Zone 2.

Historically, that's because the earliest 500 sets, those equipped with the model 425A network, did not perform very well on short loops. They often had very loud sidetone and distorted received audio.

The work-around was to install a type 311A Equalizer on early 500 sets used on short loops. This was shoehorned under the dial toward the left of the base, as viewed from the front.

The 'equalizer' was an add-on to the early 500 series sets with the older internal networks, those lacking the varistor compensation which compensated for short loops and high loop current. The 311A Equalizer was said by those in the know to be an incandescent lamp, used as a makeshift varistor, to improve the performance on short loops.

The newer 425x networks included varistor compensation and worked equally well on loops from a few feet to several miles in length.

That's just peachy for short loops, but what about long loops?

The chart above makes reference to a '238A Amplifier', and that was one of a handful of similar devices.

The 238A was a one-transistor device, constructed around a G series transmitter mounting cup, which was designed to boost the outgoing transmitted audio and compensate for the long loop.

The 238A (and successors) were commonplace on long loops (Zone L) into the 1980s.

The 276A was an improved and somewhat beefier version of the 238A.

For those who truly speak geek, the schematic of the 276A Amplifier is shown below.

The transmitter in series with the inductor is capacitively coupled to the base of the transistor, and the emitter-collector of the transistor replaces the transmitter with respect to the network.

The 238A was very similar.

Other variations existed. The 277, for example, incorporated a polarity guard in the form of a bridge rectifier. This was to be used with switching equipment which reversed battery to the calling set upon far-end supervision.

Friday, March 12, 2010

101 ESS

The 101 ESS was a model of telephone switch which played a very important part in Ma Bell's emerging technology, but it only appears in fleeting references in historical documents, eclipsed by the 1 ESS and others.

The 101 was Ma Bell's first! Yes, predating the 1E!

Some will tell you that the 101 ESS is essentially the red-headed stepchild of the Bell System. It was quirky, had a short operational life, and had a bad reputation in the field.

For the time, the 101 was ahead of its time, literally. It had many innovations which are reflected in the mature technology of today. It was the first 'stored program control' switching system used in commercial service. (More about that term below.) It was also the first switch to use TDM (Time Division Multiplexing) to switch voice paths. It was Ma Bell's first commercially-deployed switch which supported Touch-Tone.

The 101 began life in the 1950s as the EPBX project, Ma Bell's initiative to create a state-of-the-art all-electronic small telephone switch which could be deployed at a customer's premises in lieu of the very bulky, noisy, and space-consuming Step equipment, and the somewhat bulky Crossbar PBX systems.

It was renamed the 101 ESS after the term ESS was settled upon to be the service mark of Ma Bell's electronic switches. The number 101 was chosen since full-scale CO switches were normally assigned single-digit numbers, and PBX systems were normally assigned three-digit numbers. 101 was, of course, the lowest number available under those criteria.

The numbers vary as far as the line capacity of the 101. Some literature will cite limits in the hundreds of lines, others refer to low thousands of lines being available from later 101 systems.

The strategy was to place a large central control unit in the CO, acting as the equivalent of the line link section of an otherwise normal 5 Crossbar office, and a 'smart' remote unit at each customer's premises, connected by an appropriate number of signal and control trunks to the CO. To be commercially viable, each CO supporting 101 service had to serve a certain number of remote customers, 10 or so, depending on size and density.

The 101 service was sold under the service mark of 'Centrex-CU', the CU meaning that (some of) the switching equipment resided at the customer's premises. Centrex-CU included normal features such as direct inward dialing to four-digit extensions, direct outward 'dial 9' service with ANI (automatic calling number identification) with detailed reports and variable billing options, and such 'Custom Calling Features' such as speed dialing, call transfer, camp-on, remote pick-up of a ringing extension, call forwarding, and user-initiated 3-way and 4-way conference calling.

The 101 was also used for some stand-alone, non-DID PBX applications, but these were few and far between.

The technology used at the time for the 101 was discrete-component diode-transistor logic (DTL), making up the bulk of the electronics.

The switch fabric used an innovative but unusual form of analog time division multiplexing. Instead of multiple physical audio paths through the switch matrix, there was one path which was time-shared by all active calls, each being switched in and out at the rate of 12,000 samples per second. The actual 'switching' of audio paths involved assigning the appropriate timeslot to a particular call.

The audio signals were smoothed and integrated by capacitors, giving full response in the telephony band of roughly 300 to 3000 Hz. The 12kHz sampling signal was well above the (intended) bandwidth of the system, but some claimed to hear it, or its effects. Most users, however, heard only the usual crystal-clear analog telephone signals which were expected and typical of telephone systems of that age.

The first production roll-out (that I can find) was at Mayo Clinic in 1963.

It would seem to some that the 101 might be well-suited to CDO (rural Community Dial Office) operation, in a similar architecture to the host-remote scheme commonly seen today. This, unfortunately, was not practical with the 101. If it were, I'm sure that the 101 would have seen a longer and far more visible life. The design of the analog section of the 101 precluded very long loops between the switch and the set. Although off-premises extensions from 101 systems did exist and were somewhat common, the loop length was limited to 10,000 feet or so, and in rural offices, a good percentage of customer loops exceed this threshold. (There's also no evidence that the 101 ever supported party lines of any type, which were still required by the lack of adequate outside plant during the life of the 101.)

There's not that much literature in the public that talks about the nuts and bolts of the 101. The items that are widely available are full of arcane terms such as Stored Program Control, Program Store, Call Store, etc., terms that are not commonly used even by today's hardware-savvy technoids.

Let's talk about some of these.

Stored Program Control means computer control. The system is controlled by a 'stored program engine' which most of us would call a CPU. This CPU was built of discrete components, no microprocessor or VLSI, not even any SSI in the original 101. Computer architects might even call this a very early RISC processor, since the instruction set was small and primitive. However, it had a few very specialized instructions which were well-adapted to the control of the switch.

Program Store is what we would call the executable code, stored in a 1960s era version of what we might call ROM, or Read Only Memory. The ROM memory cards were aluminum-base cards with each bit represented by a magnetized or non-magnetized spot on the card which could be read by the CPU.

Likewise we can think of Call Store, used to store the call information and other items making up the state of the switch, as RAM.

The RAM at the time was ferrite core memory, with each bit being represented by a ferrite bead. The beads were magnetized by a 'write' wire, and the state of each bead was read by a matrix of 'scan' and 'sense' wires.

The 101 could not be re-programmed on the fly in the field. New code sets ('generics') had to be delivered on mag card sets. There were no on-line mass-storage units such as disk drives. These did not come to switching technology until the advent of the 1A ESS many years later.

Many of the features of the 'CPU' of the 101 were adapted to the 1 ESS, but the 1E did not use the TDM switch fabric, instead employing 'fereed' crosspoint switches in crossbar-like configuration.

In Omaha, the 101 Centrex-CU service was promoted heavily beginning in the late 1960s and (at least) two #5 Crossbar offices were adapted to support 101 ESS customers.

The Omaha 101 installations which I can find evidence of are:


Internorth, 348
Clarkson Hospital, 348
Omaha Police, 348
Omaha National Bank, 348
Norwest Bank (now Wells Fargo), 536
Lutheran Hospital, 536
Creighton University, 536
OPPD, 536
UNMC, 536, expanded and cut to 541
Lozier Corporation, 541

This, of course, was the critical mass necessary to make the service practical and profitable to Ma Bell.

Such was not the case in west Omaha.

90th and Western (originally hosted at 135th):

Bergan Mercy Hospital, 398
IBM Corporation, internal PBX, not Centrex, no DID.

Even with the vast westward expansion of Omaha's business community between the 1970s and 1990s, the 101 ESS never saw widespread deployment in the 'burbs', the service being mostly pushed out of the way by 'Centrex CO' services hosted out of 1E and 1A switches.

One of my former employers was 'served' by one of the above 101 installations. My desk phone, for the most part, worked perfectly well as far as regular POTS features, but the custom features such as forwarding, transfer, etc., were sporadic at best.

I know that it worked for 103 type 300bps data calls, not so sure about 1200 and above.

The one quirk which I remember most about the switch was the cadence of the ringdown and ringback. The 101 tone generator used the newer 'precise' sounds for such things as dial tone, ring tone, and busy tone. However, when a call was placed, the first ring was often several times the normal length, then often stuttering before settling in to the normal 1-on, 2-off cadence typical of PBX installations.

I have never received any explanation of why this happened.

I do know that in a later 101 installation in NYC, this quirk was not present, and that 101 had the normal ringback cadence found in other systems.

All 101 installations appeared to have vanished by the mid 1990s or so, mostly replaced by Centrex-CO and Centrex-CU on various Dimension, Definity, and systems of other manufacturers.

An interesting sidebar to the history of the 101 is a legend on the etymology of the term 'octothorpe' to refer to the pound/hash/sharp/whatever sign. It involves the tale of the name being coined on the fly during the commissioning of the Mayo Clinic's Centrex. Researching this will be an exercise for the student. ;-) Hint: Check the archives of Telecom Digest. ;-)

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.

Saturday, January 30, 2010


Thanks to those who sent in updates, additional information, and corrections.

These can be sent in either via e-mail or as comments.

Two updates of some significance have been reported.

  • A recently-retired long-time TPC employee has stated that in his recollection the Elkhorn and Waterloo CDOs were separate, discrete, and each located in their respective towns prior to 289 and 779 going ESS in the mid 1970s. I do recall that at one time prior to the cut, the same numbers could be dialed using either prefix. We may have to agree to disagree on this factoid until something in writing can be located.

  • A person 'in the know' reports that the Internorth Centrex on 633 was Centrex-CU using a local WECO Definity, as opposed to a Centrex-CO hosted off of the main switch as I had erroneously reported.

  • Late update: Another former TPCer reports that Council Bluffs did not use the Omaha time announcement service on 844-8111 and the 844 prefix was not dialable from Council Bluffs. Instead they had their own similar time announcement machine on 328-8116. This time announcement machine could also be reached from Glenwood, which at the time was not within the Council Bluffs calling area.

Thanks, gang, and keep those cards and letters coming.