Inside your local Phone Company

Operating at great personal risk, we spared no expense to bring you these images of common Central Office equipment, found deep within the hallowed halls of your local CO. Clicking on an image will show it full size at 1152x794 pixels. Please be patient: There are 24 'small' images to load.

Amazing what you can do with one of those $8 Kodak 'cardboard box' 35mm cameras.. The photo mission was necessarily completed in haste. If you spot any errors on this page, please take a moment to let me know so I can make corrections!

The AT&T 3B20D. Computer or Switch?

Mr. Locke informs us that The 3b20d is used as a detached processor. It is used as a computer, not a switch. If this belongs with the analog ESS, then the switch is no longer called a 1ESS or a 1E ESS, it is a 1A ess. The detached processor makes it so. If the 3b20 is not there, then there are hundreds of memory cards, about 8-1/2 x 11 aluminum cards with little iron dots on them, the actual memory, you can see every "bit" on each card.

Mr. Rehor informs us that "The 3B20 came in two variations -- 3B20S (simplex) and 3B20D (duplex hardware). The 3B20D is used for a variety of purposes, including SSP, SS7, billing systems, the Administrative Module of the 5ESS, an adjunct to the 1AESS and 4ESS. The multiple cabinets of the 3B20D were replaced by a 2-shelf 3B21D software compatible version. In some later systems, including the 5ESS VCDX, the 3B20D software runs on a standard Sun Microsystems computer using a custom software emulator of the 3B20D hardware. Ironically, the software runs faster in emulation mode than on the native hardware because the 3B20D is only a 1 MIP machine."

The AT&T 3B20D 9-track and disks.

The AT&T 3B20D Is a duplex computer which interfaces with the 1AESS and other office purposes. It takes care of more advanced accounting, software upgrading, load-balancing and SS7 implementation. Thanks to Mr. Heisler for this great info!

The AT&T 5ESS.

AT&T 5ESS with door and module drawer open. The little modules are processors and amplifiers for voice or data. Your specific phone line connects to an individual one of these little cards. If you go to ISDN or another digital service, the Telco will swap out the little module for a different type, and reprogram the switch.

Mr. Rehor kindly informs us that The photo shows an Integrated Services Line Unit (ISLU), one of several versions of line units in a 5ESS (this is the earlier version of the ISLU; He worked on them around 1987-88). As you note, the ISLU has a separate card for each line, which can be a T-card for ISDN T-interface, S-card for ISDN S-interface, or Z-card for POTS. Later ISLUs had multiple lines per card to reduce the per-line cost. Earlier and later analog-only line units terminate many POTS lines on a single large card.

AT&T 5ESS control console

AT&T 5ESS rear view

AT&T 5ESS rear view

The Northern Telcomm DMS-100 Switch

Northern Telcomm DMS-100. Notice the 9-track tape drives. These are very common at the CO, even today.

Another DMS-100 rack.

DMS-100 rear view

DMS-100 rear view

The Northern Telcom DMS-100 is shown with a module drawer open. The little modules are processors and amplifiers for voice or data. Your specific phone line connects to an individual one of these little cards. If you go to ISDN or another digital service, the Telco will swap out the little module for a different type, and reprogram the switch.

We thought this was a Western Electric 1AESS Switch, but it is a part of the 5E. It was not until 5ESS that equipment was put in what is called 'k cabinets'. None of the 1A processors were ever in cabinets.

The Western Electric 1AESS Switch. The panels at the close end are filled with punchdown blocks. This is a 1979 model, quite a classic. It handles voice (and slow modems) only and uses in-band signalling.

Mr. Locke informs us that "The panels at the close end are punchdown blocks, but your phone wires don't go there. This is the trunk distributing frame. Phone lines don't make an appearance here, only trunks."

Western Electric 1AESS Switch, rear view The 1AESS is also full of relays, hundreds of relays.
Mr. Locke informs us that "both frames are of the ringing and tone plant, the one on the right is the inverter section, the one on the left is the network section. hundreds of relays doesn't come close. even the smallest 1a has thousands. many many thousands of relays.">

1AESS Supplemental Trunk Test Panel.

Mr Locke informs us that this is used to test trunks, noise, loss, etc.

1AESS Switch rear view. Miles of wire..

1AESS miscellaneous trunk frame, top half. The square silver blocks are plug-in relays. The thin brown objects are plug-in cards with transistors and other components on them. The white-faced cards are similarly made.

1AESS miscellaneous trunk frame, bottom half.

1AESS Ringing and Tone Plant. This is where dial tones, busy signals, etc. are generated. Analog circuitry abounds. The rows of objects above are more relays. The innumerable relays constantly click and clatter. The machine is truly alive with its own thoughts and you can almost feel its presence of mind. Power supply is to the left.

Mr. Locke informs us that the left of the picture is the power distribution frame, or pd for short. Those are filter capacitors on the bottom, fuse blocks above. The filter caps serve 2 purposes, filtering any noise, and helping to clear faults during the time a fuse may blow. This frame will distribute the -48v and +24v to the various parts of the switch."

MCC (Master Control Console) of a 1AESS.
Beautiful, eh?

Mr. Locke informs us that "To the right of the picture is a terminal, used for local access to the switch, and to the right of that is the corner of a printer, called the ROP, read only printer. if turned on, it prints whatever is going on. can use up boxes of paper if one is not careful."

Batteries *are* included at your Telefone company. In case you ever wondered why the phones work when the power is out, this is why.

Mr Locke informs us that "judging from the picture, those batteries are exide ftc-21p batteries, 1680 amp hours at the 8 hour rate. If they are old enough, ,they would be Western Electric KS15544 L508 batteries, same batteries only sold by WECO rather than Exide. Above the far end is the "chandelier", the bus bar section of a power plant manufactured by AT&T called a "Lineage 2000" power plant. If the plant is old enough and was manufactured by Western Electric prior to 1985 it would be called the "Bellmaster." The battery stands are non-seismic, and are most likely J85004B, or ED14014 L10 G62, the very common western electric style battery stand."

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