T1 Connection

[big john]

I've been removing a bunch of obsolete phone equipment lately. I ran into something I didn't understand, which turned out to be a high-speed T1 connection.

Anyone have any information on how these things are wired? The T1 comes out of some sort of networking box, which looks like it's making use two or even three phone lines coming out of the NID. One of the lines has a 180VDC signal going out on it, right into the NID. The networking box itself is running off of a large wall-wart power supply.

Everything I've read about it so far says that most T1s have a direct fiber connection, which is most definitely not the case here, just plain copper twisted pairs.

Anyone have and clues or tutorials? I'm not gonna play with it, but I am very curious how it all ties together. Thanks.

-John

 

[wireguru]

T1 never has a fiber connection, unless you are in a fiber lit building and the carrier has a mux there with T1 ports (or a DS3 port and another mux like a widebank 28 to break T1s out of a channelized DS3)

T1s are delivered over copper. The demarc for a T1 is called a smartjack, it has active electronics and is loop powered. T1s used to have two copper pairs from the CO to the smartjack, but now they use one (HDSL). From the smartjack, you have two pairs (TX and RX) on an 8P8C RJ48 jack. Into this you plug a router. This is probably your device with the wall wart. It will have an ethernet port for the computer network. There are also other devices which will connect to the T1 directly such as PBX equipment.

 

[big john]

Thanks for the fast reply!

I looked up "smart-jack" and the images I was finding are very similar to the "networking device" I was trying to describe. It does have at least two copper pairs coming into it from the NID. I am 99.9% certain it isn't a router (A) because it's Ma Bell's equipment, and (B) because the 8P8C cable that comes out of it is the connection for our server.

The only thing that doesn't jibe is that this thing definitely has it's own power supply, it does not appear to be loop powered. Any idea what the 180VDC on the twisted-pair going from the "smart jack" into the NID would be? EDIT: I'm reading something about "loop-back" monitoring from the smart-jack to the TELCO. Maybe that's what this is...?

-John

 

[wireguru]

Thanks for the fast reply!

I looked up "smart-jack" and the images I was finding are very similar to the "networking device" I was trying to describe. It does have at least two copper pairs coming into it from the NID. I am 99.9% certain it isn't a router (A) because it's Ma Bell's equipment, and (B) because the 8P8C cable that comes out of it is the connection for our server.

The only thing that doesn't jibe is that this thing definitely has it's own power supply, it does not appear to be loop powered. Any idea what the 180VDC on the twisted-pair going from the "smart jack" into the NID would be?

-John

do you have a picture?

t1 loop voltage is 180vdc. This is why you will often see red caps over a couple terminals on a 66 block.

 

[big john]

Unfortunately, I don't have a photo. I'm gonna be digging back into this first thing tomorrow, so I'll take one then.

EDIT: I wish this thing did have red caps. I was tracing out phone lines to the NID when I found the 180V pair by touching them. I was pretty sweaty and it was not a fun surprise.

-John

 

[zbang]

t1 loop voltage is 180vdc. This is why you will often see red caps over a couple terminals on a 66 block.

Actually, the red markers specify "special service protection" (SSP), which means that it isn't a regular, grab it if you need it, POTS line. Could also be a DDS circuit, alarm or broadcast loop. ISDN, etc.

FWIW, a T-1 is the copper-based transmission of a DS1 bit stream (Data Stream 1), just like a T-3 is coax carrying a DS3 stream.

 

[LarryFine]

Any idea what the 180VDC on the twisted-pair going from the "smart jack" into the NID would be?

Maybe they're using your electricity to power their equipment.

 

[big john]

So, here's the skinny:
This is the outside of the suspected "smart jack."
Here's the wiring.

• The black terminal strip on the top right is where the two phone lines are connected from the NID.
• The green terminal strip on the top left, I'm not sure about. That pair goes to somewhere within the plant and might be abandoned.
• The second from the bottom with the blue and blue/white pair attached to it is the 180VDC that goes to the demarc.
• The terminal strip on the very bottom is from the wall-wart power supply. That is 48VDC.

And this final photo is of the wiring diagram within the enclosure.

-John

 

[wireguru]

ok, that chassis holds 4 cards and is powered. it looks pretty old. This will hold up to four T1 smartjacks or other various cards. It has two cards in it. The wide card with the blue test jacks in it is used to terminate 2 wire and 4 wire loops. It provides attenuation, loopback, and a couple other functions. The skinny card looks to be a standard HDSL2 T1 smartjack. Look at the slot numbers on the chassis and figure out which corresponding black RJ48 jack on the rear of the chassis had your T1 equipment plugged into it. Im pretty sure it would be the skinny card. The blue / white pair with the 180vdc on it would be HDSL2 going to the skinny T1 smartjack. I cant speak to what the other red/green/black/yel wires would be. Might be an old alarm circuit or something else wierd.

 

[big john]

Wiriguru,

You're right, slot 4 is where the T1 connection comes from.

I'll keep investigating all the other lines. I got permission to take the T1 out of service in preparation for reworking those red/green/yellow/black lines, but the T1 stayed online even when those were disconnected, so who knows.

Thanks for the help!

-John

 

[wireguru]

Wiriguru,

You're right, slot 4 is where the T1 connection comes from.

I'll keep investigating all the other lines. I got permission to take the T1 out of service in preparation for reworking those red/green/yellow/black lines, but the T1 stayed online even when those were disconnected, so who knows.

Thanks for the help!

-John

yeah those red/green/yellow/black are for something else and go to the other card. Was there anything plugged into the RJ48 for slot 1?

in that chassis the black screw terminals are the telco side and the rj48 is the customer side. The green terminal block is also customer side. Seeing that there is only one pair coming from the green block tells me that there is some other 2 wire circuit there. Try to track down the red/green coming from the green terminal block.

What type of facility is this?

 

[big john]

This is a hydroelectric power plant. We've got all kinds of archaic things installed. They tell me they've TELCO techs come in who have no idea what's going on with this stuff, which is why I got tasked with trying to clean it up.

There is nothing else plugged into the other RJ48 connectors for slots 1, 2, or 3.

It might not be an accurate test, but just for kicks I put a frequency-counter on each of those pairs at the top, and I get a pretty darn solid 1.5kHZ on all of them. In the demarc, the red/green and yellow/black pairs don't have listed phone numbers, instead they say "62F00A2167" and there's a tag that says "Full Data."

I'm thinking these are dry loops for something, but I don't know what at this point.

-John

 

[hurk27]

This is a hydroelectric power plant. We've got all kinds of archaic things installed. They tell me they've TELCO techs come in who have no idea what's going on with this stuff, which is why I got tasked with trying to clean it up.

There is nothing else plugged into the other RJ48 connectors for slots 1, 2, or 3.

It might not be an accurate test, but just for kicks I put a frequency-counter on each of those pairs at the top, and I get a pretty darn solid 1.5kHZ on all of them. In the demarc, the red/green and yellow/black pairs don't have listed phone numbers, instead they say "62F00A2167" and there's a tag that says "Full Data."

I'm thinking these are dry loops for something, but I don't know what at this point.

-John

I have run into this on older lift stations which used older ISDN's as dedicated data lines back to the main sewer plant, It was something called a leased pair?

not somthing in my league

 

[wireguru]

This is a hydroelectric power plant.

ah! I wonder if this other circuit is some sort of leased line for a SCADA system?

 

[zbang]

It might not be an accurate test, but just for kicks I put a frequency-counter on each of those pairs at the top, and I get a pretty darn solid 1.5kHZ on all of them. In the demarc, the red/green and yellow/black pairs don't have listed phone numbers, instead they say "62F00A2167" and there's a tag that says "Full Data."

That's a "circuit ID". It might actually be "62FD.."- FD is a code for a type of 4-wire data line.

 

[Jerry the Phone Guy]

I think you mean 1.5 MHz? (T1 data rate is 1.5 megahertz.)

Hmm... I thought HDSL was a 2-pair service? Because otherwise you'd have problems with crosstalk between send and receive? Also, the high DC voltage (135-180V) on a single pair would cause problems with saturation in the hybrid transformer, which is why they apply it from the center tap of the receive pair's hybrid to the center tap of the transmit pair's hybrid (AKA simplex operation).

What's with those prehistoric protectors, BTW?

 

[wireguru]

I think you mean 1.5 MHz? (T1 data rate is 1.5 megahertz.)

Hmm... I thought HDSL was a 2-pair service? Because otherwise you'd have problems with crosstalk between send and receive? Also, the high DC voltage (135-180V) on a single pair would cause problems with saturation in the hybrid transformer, which is why they apply it from the center tap of the receive pair's hybrid to the center tap of the transmit pair's hybrid (AKA simplex operation).

What's with those prehistoric protectors, BTW?

T1 is 1.5Mb/s (1.544 actually)

HDSL (now HDSL2) operates on one pair. Has -180vdc on it to power the smartjack. Its duplex, different frequencies for tx and rx. I dont think these have a hybrid transformer in the circuit. Isnt that for voice?

What prehisoric protectors?

 

[big john]

I think you mean 1.5 MHz? (T1 data rate is 1.5 megahertz.)

Turns out those pairs aren't associated with the T1. They go the chasis containing the smart-jack where they tie into the the two "data only" lines. Not sure if they're even still in service, but they go to some ancient telemetry equipment associated with a utility SCADA system.

I thought HDSL was a 2-pair service? Because otherwise you'd have problems with crosstalk between send and receive? Also, the high DC voltage (135-180V) on a single pair would cause problems with saturation in the hybrid transformer, which is why they apply it from the center tap of the receive pair's hybrid to the center tap of the transmit pair's hybrid (AKA simplex operation).

Say wha-? That's a bit over my head. All I know about it is what I've written.

What's with those prehistoric protectors, BTW?

Those are the original demarc from when the plant was built. There were probably about a dozen of them, plus a bunch of unused 66 blocks, about 400 unused pairs from different parts of the plant, and about a million RJ-11 phone jacks. That's all the the stuff I was removing. I've gotten it parred down to four 66 blocks, the NID, and that chasis with the the smart-jack in it.

What prehisoric protectors?

In the gallery of photos I posted, there's a picture of some of the original arresters. As an aside, I had problems with a couple of the phone lines not being recognized by our new multi-line phones. They would act like those lines weren't even installed. Being the telecom newby I am, I thought it was my fault. Come to find out "tip" and "ring" are actually reversed on a couple of the terminals in the NID, and the phones didn't like the reversed polarity. So much for professional TELCO installers. :roll:

-John

 

[wireguru]

ah! I wonder if this other circuit is some sort of leased line for a SCADA system?

so i guessed right

 

[big john]

so i guessed right

Spot on. I appreciate the help!

-John