AT&T Central Office (pt. II)

Before reading this, if you haven't already, I suggest you read AT&T Central Office (pt. I).

Behold, the old AT&T logo. The Doonesbury comic, back in the 80's, once made a joke about this logo being the Death Star; suggesting that AT&T was the evil galactic empire. Funny idea, but NOT the case. I've got a great job.

As mentioned in my previous post about the Central Office, every line, or phone number, comes from a point in the switch. The majority of those points go to your basic phone line in your home. We refer to those as POTS (Plain Old Telephone System (that's what it really stands for)). Fortunately for our high-tech 21st century world, not every switch point is a POTS line.

Some of these are Special Services circuits. Special Services could be anything from an Integrated Services Digital Network (ISDN) line, Foreign Exchange (FX) line, dedicated 56/64/128 Kb circuit, or a dedicated radio circuit. These circuits go from the switch to a D4 bank where special circuit packs, or channel boards, are seated to carry out these special services and tasks (see photo below). For more details what these types of circuits do, leave your comments and I will respond in kind.


Once the Special Services circuit is wired up to the D4, it is then combined, or multiplexed, into four T1's (we'll discuss T1's in the next post). The T1's are trasported out of the CO to another D4 location somewhere outside the office, near the customer. The T1's are demultiplexed into their individual circuits to the customer's premises. Ta-daaaa! You now have your ISDN line.

And we have aisles and aisles and shelves and shelves of these D4 and SLC thingys!
"What's a SLC?", you say. Keep reading.

Other lines, that are not Special Services go to SLCs, or Subscriber Line Carriers. SLCs are systems that save AT&T money on infrastructure, which in turn saves you money. SLCs also help extend the reach of the Central Office. Distance matters greatly in the telecommunications business. There is electrical voltage and current on your phone line. The farther the copper wire has to go, the more voltage has to be put on the line. This raises the cost of service and could turn phone lines into an electrical or fire hazard.

I don't remember the exact distances but I'll try to make this easy to understand. Customers in "Group A" live within "X" miles of the CO. Their lines go directly from the switch to a cable leaving the CO. Customers in "Group B" live outside the radius of "X" miles and are too far away from the office to get a loud and clear connection on their line. "Group B" get their connections through a SLC.

One SLC system combines 96 customers and transports them over four T1's to another SLC system outside the "X mile" radius. There, the 96 customers are broken down into their individual channels and then to their homes or businesses. A SLC96 system (the oldest system we use) is pictured below. The SLC96 is similar in appearance and function to the D4 system. But if you look closely you can tell they have many differences.


AT&T uses newer types and brands of SLC systems; that put fiber optics to use and better mux/demux technologies. I won't bore you with those details here. We'll do that in another post.

If you have any questions and want to know more, please leave your questions in the comments section and I will get back to you.