AT&T Central Office (pt. I)

I work for AT&T. On Twitter, I often talk about my work and what I am doing. I throw around words like "switch", "T1", "central office" or "CO".

So I have decided to shed some light on what goes on in a CO. What exactly is in there, and what happens when you pick up your phone to call someone. I have difficulty being succinct. Please bear with me, I will try to keep everything simple. When people ask me what I do, I sometimes tell them that my job is like The Matrix: "You can't be told what [it] is, you have to be shown what [it] is."

So here goes:

First there is the conference room:

Okay, I'm kidding. I'm trying to throw in some humor. This is actually the supervisor's office.

Okay, moving on.....

This is the switch. This photo is showing a small portion of the equipment involved in this particular office. It's so big that I could not get it all in one shot. The switch is the heart of the CO. The switch IS the office, so to speak. Thanks to automation, this bad boy does the 24/7 job of over 100 switchboard operators from back in the day. The first automated switch was actually invented by a dentist. He got ticked off over the "criminal" long distance bills and sought to get even with the operators by putting them out of a job. Job well done, I say.

Not counting the area code, the first three digits in a phone number, or "exchange", are specifying a switch. A technician well versed in his region of work can tell what town you live in just by the exchange. However, the advent of the cell phone has made this a little more difficult. With cell phones, the exchanges denote the carrier that you are with. A switch can handle several exchanges, but only one area code. If this office handled two area codes then there would be two switches.

Aside from the Command, Power, and Miscellaneous modules, there are several Switch Modules that have individual channels for each phone line. These channels, or points, must be wired to a cable to get the dial tone to your home or business. This wiring is semi-permanent. Meaning, if you move two streets over and keep the same number, I have to change the wiring to a new cable pair; or if you disconnect your phone, I have to remove the wiring. This wiring is handled on the Main Distribution Frame, or MDF, or just "the frame".

The photo above is the front side to the frame where the technician wires up your phone number to the switch. In the next photo, it looks a little overwhelming, but in our computer systems, every phone number is assigned "coordinates" to direct the technician to the right point. Two wires per line: one blue and one white; Transmit and Receive; Tip and Ring (long story there, but if you wanna know, leave comments).


The next photo is the backside of the frame where the technician wires up the blue/white jumper to the cable pair that will get the dial tone to your home.


Each line has a heat coil that works like a fuse, protecting the switch, or office equipment, from power surges and lightning strikes. The frame allows a technician to test your line when you call to report a trouble with your line. Yes, one can listen in on your phone conversations from here. But because I'm a telephone worker, it's legal for me, and I only do it when it's necessary. Besides, if I divulge anything that I overhear, not only will I quickly lose my job, but I will have committed a federal crime. Not something that I, or my coworkers, take lightly. Your secrets are safe with us. And to assure you further, the line information that I have access to (phone number, switch point, cable pair) DOES NOT include your name or address. This protects you all the more. Again, your secrets are safe with us. So next time you hear a clicking noise on your phone, it may be us working to ensure your satisfaction.

The next two photos, and the last I'm going to do for this post, are of the cables and the cable vault that is in the basement of this building. The smaller gray cables up top are connected to the frame. The large cylinders are the splice casings. And the black cables below the splice casings are going to the basement, two floors down.


Once the cable goes to the cable vault in the basement, it then leaves the building and heads out to your home or business. My line of work stops at the frame. I don't work with cables. I can honestly say "that's not my job."

Notice my hand in the right side of the photo below. I figured I would give you a reference for size. I think these cables are 100 pair cables. There are 200, 24-gauge, individual wires in each casing. It may be 26 gauge. I'm not sure because "that's not my job". I do know that the yellow cables in the top of the photo are fiber optic cables. I'll talk more about those in a later post.
I hope you found this interesting. I have a lot more to show, and I'll show it later. Leave your questions in the comments and I'll try to answer them.