Revertive Dialing – History

by Colin Berkshire

We all know about pulse dialing and DTMF (Touch-Tone) dialing. Both of these are used between Central Offices to place calls. In addition, inter-Central Office calls would use SF and MF signaling. MF and SF dominated from the 1970s until SS7 took over in the last 1980s.

Most don’t remember the dark sheep of inter-office signaling: Revertive dialing. It was weird. I mean it was really, really, really weird. It the the MOST common form of signaling in major metropolitan areas from the 1920s until perhaps the 1960s.

To understand Revertive signaling think first about pulse dialing. With pulse dialing you go off hook and then pulse the number you want (5=5 pulses) with a pause of ½ second between digits. (Pulses are 1/10 second or 10PPS.) As a pulse dealer you are always in control. You choose when to send pulses and the other side had better be ready and counting them.

Revertive dialing is inside-out pulse dialing.

You go off hook and get battery current, just like with any phone line. But then the battery starts pulsing at you. The far end sends you pulses. Now this is weird because you know the phone number to be dialed, the far end does not. So how does it know how many pulses to send you? Answer: it can’t know how many to send because only you know the number to be called.

So the pulses come streaming into you with Revertive dialing.

When you get enough pulses you hang up. So if the number you want is 5 you then hang up after receiving 5 pulses. The other side now knows that 5 was enough and is the first digit you were thinking of.

You go back off hook ¼ second later and again pulses are sent to you. You hang up when you have enough pulses for the second digit. And so on and so on until all of the digits are received. This means that both sides need to be counting pulses with the far end counting until you slam down the line and you counting so you can quickly hang up when you have enough.

Oh, and the pulses come flying at you at a rate of 20 per second, or double normal pulse dialing. So you need to be really fast at hanging-up to stop them from coming at you. (This would prove to be a problem for inter-city dialing since the speed of light would start to become significant and by the time you got your count and hung up the far end would be onto the next pulse.)

This this is weird? Well, it gets weirder.

The number being sent would not be the phone number. It would be the Line Equipment Number in the distant Central Office. I don’t remember the terminology anymore, but it would be something like the brush, rod and frame. Since on Panel Type offices there was a 1:1 relationship between Line Equipment Numbers you could work backwards to get the phone number, but it took mental gymnastics since the number was’t sent in decimal. One of the digits would be sent Base 4 and another a different number base its a spectacular demonstration of Rube Goldberg complexity.

Revertive signaling was used primarily to and between Panel Offices (Panel was a type of office just like Crossbar or Step-by-step.) Panel Offices were monsters of switches, usually deployed in large cities like New York, Boston, and Washington DC. Seattle has a couple, of them, but as they were exceedingly expensive they weren’t so common on the west coast.

Eventually the #1 ESS and #4 ESS and #5 ESS could talk Revertive Signaling. This was done mostly for compatibility but sometimes would be used on lines for no good reason at all.

If you are 45 or older you may remember hearing Revertive Dialing. Remember placing a call and then hearing this thin sounding super fast clicking with the pauses between digits being too short. It sounded like somebody wheezing into the line just before the call would start ringing.

I’m told that Revertive signaling and Panel Offices were designed to avoid the patent problems that were associated with Strowger type Step-by-Step switches.

Seattle had its last Panel Type Central Office phased out In about 1975. New York City has a lot of them and it was the early 1980s before they were gone. (They were never adapted for carrier equal access.)