ISDN modem ERICSSON His NT teardown

[krolu]

Hi,
This is my first post here, so please be gentle . I sometimes got some old electronic stuff from my friends for scrap, because I like to see how professional products are made. As you all know that differs greatly from item to item. Recently I got ISDN modem made by Ericsson in the late 90s. I opened it with a thought on my mind that maybe there will be some cool connectors for scrap and maybe just few ICs that I could use. After opening up I was amazed by the quality of the product. Density of the parts layout is very high , top brands for components, quality caps like SRG or Nichicon, and very interesting transformer. What was pretty strange for me also was the spread in the IC manufacturers. Usually manufacturers tend to stick to just few of the brands. Here just the big ICs are: Atmel, PIC, Motorolla and Ericsson, and the list is even longer when you take smaller ones in the consideration. Even the board seemed to be a little to heavy for its size, when I looked closer I saw that it has more layers than just 2 sided. I just didn't expect all that for such simple equipment like ISDN modem. Just see for yourself it the pictures below and remember that this is late 90s and it's just a modem.

[rsjsouza]

Congrats on your first post! The pictures are very clear and, in my experience, Ericsson products are usually well made.

Quite interesting the PCB transformer...

Out of such boards you can usually re-purpose the connectors, the MOVs, the relays, the larger capacitors and the power resistors. If you are good with the de-soldering tool you can get some of the misc ICs and power parts out.

[T3sl4co1l]

Weird, wouldn't think there's anything special about a planar transformer that couldn't have been solved with a much cheaper and smaller wound part.  Odd seeing one in something this old.

They're small enough to be practical today, because switching frequencies are high.  Perhaps it was used for signal rather than power?  But ISDN isn't very fast at all, and I don't think it uses a high frequency carrier.  And if it did, that would be nothing a commodity Ethernet transformer wouldn't handle, anyway.

Tim

[timb]

I just didn't expect all that for such simple equipment like ISDN modem. Just see for yourself it the pictures below and remember that this is late 90s and it's just a modem.

Well, it's not "just a modem" at all! ISDN is actually quite complex and required a good bit of technology to fully implement.

In addition to acting as a high speed digital modem it also provided two separate voice channels. One of those phone line connectors on the back of the board would provide two phone line outputs (the other was the ISDN input). You see, it was the modem's job to to locally create two analog phone lines; it would then digitize each line and use one of the 64Kbps "B" channels to send the G.711 encoded voice data to and from the central office.

So, you essentially had two 64Kbps channels (called B or bearer channels) which could be used for direct digital data or digitally encoded voice data.  So, let's say you were connected to an ISP over both B channels at 128Kbps; if you picked up the phone (attached to the back of the modem) to place a voice call, your data rate would drop from 128Kbps to 64Kbps, and automatically go back up when you finished the voice call. If you used both voice lines, your data would essentially stop (unless you used something like X.25, in which case you could still have a 16Kbps data connection over the D channel) and would resume when one of the voice lines was hung up.

The idea was, you could provide high speed data access (and 128Kbps was high speed for the time) and two voice channels, all over a single copper pair. (Which would traditionally require 3 or 4 separate pair for the same level of service!)

This was perfect for small offices and stores. In some European locales, it caught on for residential use as well.

The unit you took apart appears to be a design from the early 90's, as by the late 90's, most new ISDN modems were coming with Ethernet interfaces (acting as routers). (That's not to say yours wasn't made in the late 90's; it could very well have been!)

I really miss ISDN... I remember getting it in early 2000 to replace my dedicated 56k dialup connection. I would have gotten it sooner as the service was super cheap ($20/mo to the Telco, just for the line), however there was a catch: The telco charged *per minute* (on *each* B channel) for outgoing calls.

What changed is I made friends with--and started interning for--the owner of a local ISP. You see, I discovered a loophole: While outgoing calls were toll, incoming calls were free! So, I setup an "ISDN Dialback" connection in his system. Basically, my ISDN modem would call his primary dialup, authenticate and then immediately hang up (this happened in under a second, so I wasn't charged). His system would then immediately dial my modem back, which would answer, authenticate and then bond both B channels. This essentially gave me permanent 128Kbps (up and down) internet service, and it was freaking awesome.

I remember the first night we got it working, joining a Quake III match and absolutely dominating. It helps when your ping goes from 500ms to <10ms! (I ended up starting my web hosting business on that ISDN line later that year, which grew into a very successful company!)

Anyway, ISDN was pretty cool. The reason your board appears so complex is because it *is* complex! I'd wager that at least 75% of that complexity is in providing the two analog voice lines. If all it had to do was convert an RS-232 serial interface into ISDN signaling, I suspect it would be a lot less complex. But "emulating" two POTS lines means generating a ring voltage, digitizing the analog voice channels, etc. which can take up a lot of space!

Welcome to the forum, and thanks for the blast from the past!

[madires]

BTW, it's called terminal adapter

[krolu]

Wow, thanks for all the answers.
rsjsouza: Yes I'm trying to get what I can to my parts bin, it's a habit that is almost a hobby on it's own, even now days when everything you can imagine can be bought no problem.
T3sl4co1l: That transformer was the first thing that my eyes catch when I open it. Seen some interesting transformer constructions earlier but just did not expect it here.
timb: Thanks for the explanation. I remember ISDN only on just few specs like that it's been 128KB which for those days was freeking awesome. I had 56K connection back than but connecting on that speed was almost imposible and 33.6 was what I was getting almost all the time. I didn't know that it was so complicated, it's really state of the art in the early 90s.
madires: aaa, what's that?

I got a LOT of stuff to take apart, and wondered if anybody will want to see what is inside or is it only me. Because for all the people I know in my local life "it's just a modem".

[rsjsouza]

rsjsouza: Yes I'm trying to get what I can to my parts bin, it's a habit that is almost a hobby on it's own, even now days when everything you can imagine can be bought no problem.

I do the same, just for fun and when time permits. It also allows me to perfect my de-soldering skills.

[Danielw]

I always been wondering if it would be possible to connect two ISDN modems together and get long distance communication for cheap over ordinary say rj45 cable

[madires]

First some nitpicking There is no thing like an ISDN modem, only an ISDN terminal adapter. Back to your question, the answer is no.

[NiHaoMike]

It would be a lot of effort for a 128k link. Just grab a cheap pair of Homeplugs and mod them instead.