Tuesday, June 25, 2013

An Equinox LM-48

So I got hit with a bout of appendicitis, so I apologize or not posting, but as promised I am going to talk about the digital telephone multiplexer that I have. Here goes:

A couple years ago I found this quite battered piece of phone equipment being thrown out along with a number of other boards.

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It looked interesting and claimed to be a T-1 multiplexer so I brought it home and started doing some research. As is the case with most of the things I find, there was little to nothing. But I'm also not one to give up on something without a story. It wasn't until I unearthed it just recently that I searched again and found it mentioned in some old telecom magazines that Google has now digitized.

Long story short, it is a part of a digital telephone system. It multiplexes 48 telephone lines for interface to a private branch exchange (PBX). Equinox was a manufacturer of PBX, specifically digital ones for companies. This device would have been used to interface up to 48 RS-232 lines to a single T-1 line that could then be switched and processed.

...and that's about all I've got. The articles that mentioned the device were mostly talking about the viability of Equinox to provide faster data rates for modems through the device, specifically 9600 kbps (it was the late 80s). So the specific way in which it works and the pinout of the 50 pin connectors on the side are kind of lost. Although what use is an old device like this other than parts and history?

What does befuddle me is the fact that it claims that it is a T-1 multiplexer. A T-1 line by definition is supposed to only support 24 phone lines. In order to handle twice the amount of lines the system would need to either double the data rate of the transmission line or limit the number or precision of the samples.
I've come to the conclusion that it's not a T-1 line by definition. It's probably some proprietary standard that equinox used that may be similar, but adjusted to handle only digital data. I'll never stop researching though....

So now with some more information about the device there is only one more thing left to do- crack it open.

When I found the machine it was covered on the inside with a fine black powder that had been sucked into it by the internal fan. I just ran it under a stream of water to clear it off. This shouldn't have too ill an effect on it. But, upon first glimpse there are clearly three main components- a power supply, a control board and a data board(called a driver board).

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Close inspection of the control board shows that it's a standard micro-controller system with a bunch of 7400 series chips to make it talk to the other circuitry. It's connected to the driver board by a ribbon cable.

The driver board is much more interesting. I originally thought that this was for an analog phone system and it wasn't until studying the chips on this board that I realized it was all digital. The bottom half of the board, with the array of chips, contains only three types of chips. First, an RS-232 driver, which tipped me off to it being for digital phones, then pairs of 1-to-8 and 8-to-1 digital multiplexers to drive and sample the serial RS-232 lines, respectively.

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The top portion of the board is all the circuitry to handle the transmission line to the PBX and contains what look like a couple of hybrid networks and line drivers.

As would be assumed, the device is pretty purpose-built. Looking at the circuitry the microprocessor just cycles through the RS-232 inputs, sampling, possibly storing and then transmitting their contents down the "T-1 line." Also, simultaneously reading the "T-1 line" and distributing its data to the RS-232 lines.

Kinda boring.

I would say it could be used to fan out a computer's serial access for terminals, but it would really require a lot of work on the processor's part. Alternatively if it had used UARTs it would be more interesting, but for a phone system board that would be a bit overkill.

Despite it being rare, and because it's not too interesting, I might just scrap this for its RS-232 drivers and other chips. In an attempt to see if the power supply was salvageable I found out why the thing was trashed in the first place. The -12v rail of the power supply was only putting out -10v. So there may be some hope yet for these parts to live on.

Friday, June 14, 2013

The Origins of VOIP

The other day someone asked me how VOIP (voice over internet protocol) worked and it got me thinking about the origins of digital telephony. I have a mild (hah) interest in telephony, loving every automatic switching system Ma Bell ever made. Too many days have been spent scouring the internet for descriptions of SxS, panel, crossbar and ESS switches. One of the things that has also piqued my interests was the various trunk lines that the Bell System used to carry long distance calls, including the very beautiful WECo microwave "funnels." Developments in both of these areas, first trunks then switches, will combine to form the basis for the digital subscriber line that many of us use in our homes and offices.

Our story starts perhaps a bit earlier than you might expect it to, back in the 1960s. In the Wikipedia article for VOIP the earliest recorded event in VOIP history is the development of a "Network Voice Protocol (NVP) developed by Danny Cohen and others to carry real time voice over Arpanet" in 1973. But we had the inklings of digital telephony as early as 1961 when Bell Labs introduced the T-Carrier, commonly known as a T-1 line. T-1 was a trunk multiplexer. It could carry 24 phone calls -digitally- over a pair of copper cables.

Before the introduction of T-1 the only way to carry multiple conversations over a single cable was to treat them almost like radio stations using an L carrier. Each conversation would be filtered and then modulated with a certain carrier to have it fit into a certain bandwidth. For example, you could take four signals, low-pass filter them to 4KHz and modulate them with frequencies that put them in different bands of the transmission line such as 20K, 25K, 30K and 35K (allowing for a little guard band). Then you could apply a similar process and get 16 lines down one pipe in full analog. But this was the 50s and these things were expensive, so pulse code modulation was considered as an alternative. In addition, the call quality improved, so they were all the more better for it.

So now we have all these digital trunk lines going everywhere, but the odd thing is that every time one of them passed through a class 4 switch (those that handle long distance linking) the trunk line was decoded back to analog, switched and then perhaps re-encoded to be sent off to a local office. This odd little practice caused a controversial change to be made in the way in which long distance calls were routed. In 1970 Bell began to develop the #4ESS, a completely digital class 4 Electronic Switching System. T-1 lines would come in, their data would we parsed and then routed through T-1 lines going out. Calls could now be completed long distance as if you were still calling within your own district (at least quality-wise). The first #4ESS was delivered in 1976 in Naperville, Illinois.

Now that we've gotten into the seventies, we need to take a step back to the sixties to start looking at the development of the internet. The idea first started as a government project, in the 1950s as the SAGE air defense network and developed into commercial and private computers being able to use standard phone lines and leased lines to send data to each other. Originally it was just a number of "dumb terminals" dialing in as users on a shared system or universities sharing programs by sending them serially (perhaps simply even tape-to-tape over a phone line). But towards the end of the decade, plagued by bad connections and the desire to have a constant network of shared information, a protocol was developed of sending packets of data so that if any one of them was somehow corrupted, the receiving computer could ask for that one little bit to be re-sent. Soon the military would get involved again and the rest is the history of the ARPANET.

Here's where we catch up with the developments made in 1973, because those individuals took the first development, pulse code modulation, and paired it with the new network that was starting to parallel the phone system. However, it appears that these were simply "voice" over the internet. As much as you could send someone an email or share a program or data, you could communicate with someone over this new internet protocol. However, you could only do that with someone who had access and the proper equipment and was online. You could not really call into the standard phone system. This was essentially a single phone channel out of the T-1 line using the internet as 'wires.' This is where the #4ESS comes back into play.

At the time the #4ESS was developed, the reason the developers decided to do switching completely digitally wasn't entirely because they saw the demodulation-remodulation as silly, but also because they saw the writing on the wall and realized that digital switching was the future. When you encoded things digitally and then sent them down a wire you could do so many things so much better than a standard analog switch. When higher level of T-carriers came out, the data that was coming in on slower T-1 lines from local offices could then be routed to faster T-2 or T-3 lines that could carry many more calls in between long distance offices. It was no longer a one-to-one relationship in transmission speed, which is kind of similar to the packet system, where bits could be sent anywhere in little chunks, or frames, to use T-carrier language.

As much as it was a similarity to packets, it was kind of a downside of T-1 that it was just a 24-channel transmission line. You could only send data down it in a specific way, in frames of bits that designated channels. But, theoretically once you got rid of the multiplexer and used it to interconnect class4 offices, it was just a 1.544Mbit/s data channel. You could send anything down it. Eventually this is what happened and some of these connections were used to form the backbone of the internet.

As a bit of an aside, I feel many people think that the internet was carried on phone lines originally because they used to use modems to dial into it. But I don't think that many people realize that the internet, up until somewhat recently, was completely separate. When you 'dialed in' to the internet, you were dialing into a serial data connection on a computer (similar to what we did in the 60s with terminals) that was connected to the internet- through which you could access data on it. This is part of why the next step almost perhaps seemed seamless when it came out in public use.

It wasn't until the mid 90s that someone realized that they were all using the same data cables to send phone data as computer data and that they already had the technology to send voice through packets and then created what are called "soft switches." Soft switches are essentially class4 and class5 (local office equipment) switches that can utilize the packet system of the internet to carry their voice data. So now for the first time you had a switched telephone network being carried over the internet and ironically carrying calls of people connecting to the internet. But you wouldn't really know if your phone calls were being carried over the internet at this point anyway, so the roundabout-ness of this was not always noticed.

In the present day, with the network of the internet being expanded as it is, where our computers connect to the internet like the computers we used to dial into, the switched aspect of the telephone service can take a completely different shape. Now class4 and 5 switches can be fully implemented as servers run by the phone companies existing as completely integrated bits of the internet. A simple box in your residence can do all of the pulse code modulation of the old T-1 line and send the data as packets out to be processed much like the #4ESS would. Perhaps the transition to this type of system came about in an attempt to not duplicate facilities as the internet was growing. What is clear though is that the idea that we have of encoding everything as data and sending it all down one big pipe to multiple destinations started out as a means of providing a more efficient pipe and a better switch. A phone system is a network. One that carries 'data' that can be encoded. And as demand for internet access grew and more people got better and faster access, enough room for a single 64Kbit/s voice channel was easy enough to provide.

-Always improving. That's the Bell System for ya!-

P.S.- There's one little offshoot in this story. In the mid 80s such a thing as digital telephones became popular in offices. The pulse code modulation was done in the phone and, rather than using an analog line to the switch, were connected via a serial RS-232 line. All switching was still done by a purpose-built private branch exchange, as it had for years prior. However, this is the direct ancestor of phones in offices today that connect via ethernet LAN to access the phone system. These appear to be still and only popular in offices. I have an early multiplexer for one of these systems that I'll be dusting off soon for the cameras - stay tuned!

Sunday, June 9, 2013

Terminal velocity

Last night I stayed up way too late playing around with one of the coolest little toys I have: a Microsys 80386 desktop (named 'tongue').

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(apparently at 4am flashlights take over your camera and make themselves the focal point of all photos)

Specifically, I was trying to get a dial up modem and terminal emulation software installed on it in order to cruise through some of the last remaining BBSs out there, in true retro style. And well, I hit a wall. That wall is made up of the boxes of old computer bits that I have from the late 80s  and into the 90s that have little to no documentation because, well, it was the internet boom and many companies formed, but not all survived. Hence all their documentation which may or may not have existed online is no longer. So when I came across the only non plug-and-play ISA modem I have, of course, nobody knows how it works.

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It's a Zoom Faxmodem pc96/24 which means that it has a max speed of 9600bps and slowest of 2400. it's got a bunch of Rockwell chips on it and from other pictures on the net, it also comes with the option of a rom chip in the empty socket lower middle of the card. Past this, I can't get any of the terminal emulators to see it.

There is some solace however. Some companies were gracious (or cheap) enough to basically provide their manual on the board itself, with well defined jumpers. Keeping this in mind, I took a look at my card and was thrilled and then depressed. The manufacturer (Zoom) had in fact labelled these jumpers, but wouldn't you know it, they weren't great at their placement of text near vias. Once I was able to figure out (just about) how to strap it for which com port you wanted it to be, I tried strapping the IRQ to find the (I've highlighted it) part of the board below.

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I could guess at what these say, but...   I don't even know if I'm missing a jumper, considering that the pins that I think are for the IRQ don't actually have any on them.

One site that I usually rely on for things like this is stason.org which is a really great resource for when you have no idea what you're holding in your hand, but know the serial number and that it's an ISA card. Unfortunately they don't have my modem in there, or it's one of the unidentified ones that I don't feel like sifting through.
I was able to find some for sale, which would give me a brand new one, with documentation and software probably, but that would mean, like, actually paying money for a 9600bps dial up modem that I already have and I just...  I just can't justify that.

I do have another route for playing around with BBSs though. I have a WYSE terminal and an external dial up modem that I have paired. I didn't want to use it with the 386 only because there are open card slots in it and the modem takes up space and a power plug. (I used to do a little party trick with it, using the terminal to tell the modem in Hayes codes to dial a friend's cell phone who would then be amazed that I had called them with a modem.) That setup would probably work with a BBS but I kinda wanted to do more than just view it if I could. I also just wanted to have a modem on my computer that I could use to call it remotely if I wanted to, or setup some kind of BBS of my own. So maybe I'll see if I can find any other options.

(I'll talk a little more about the 386 and the plans I have for it in a later post. It's kind of a lot of dreaming with too many cards and involving other computers and I'll just give that its own post...)