This document is badly organized, and some of the
information is wrong. All will be fixed eventually.
Someday this will be a clickable image map. Until then, you can
just pretend. Full-size
picture. Other pictures: The Teletype
Controllers (TTYCs), a close-up view of the connectors for the Serial Periphal
Interface.
At the Museum of
Communications I often work to maintain and restore the No. 3
Electronic Switching System. This document will be rife with three
letter acronyms. The Bell System had tens of thousands of TLAs, with
numerous collisions (there are only 17,576 possible three-letter
acronyms). If they could understand it, so can you.
The 3ESS is one of the least written-about of the Western Electric
switches. Of course, this is partially due to the fact that there
were only a few hundred of them in service for a few years each.
Other switches, such as the No. 5 Crossbar,
which served more large metropolitan areas, have received
correspondingly more coverage. I intend to remedy this. Because I
interact with a 3ESS on a semi-regular basis, I will (eventually)
include samples of interaction with the system, audio recordings, et
ceteraa.
History
The 3ESS was created in the early 1970s. It came after the 1ESS and the 2ESS.
The 3ESS has a switching capacity of between 1 and 4500 telephone
lines, and was intended for the low-volume rural and semiurban
markets. Switches that fill this niche are called community dial
offices
(CDOs).
In the early 1980s, before the Bell System was broken up, the
Federal Communications Commission required Bell to allow telephone
subscribers equal access to any long-distance carrier they wished to
use. This required extensive changes to the switch generics. The
cost of modifying the 3ESS's generic (base program) to support equal
access turned out to be greater than the cost of replacing all the
3ESS offices then in use. Thus the effective service life of the 3ESS
was less than ten years in most cases. (In the Bell System, the
service life of a telephone switch is usually between twenty and sixty
years.)
Physical Attributes
The 3ESS is quite a small switch. Even including the distributing
frame, it can easily operate inside a semi-truck trailer.
The 3ESS's processor is the 3A CC, which stands for 3A Central
Control
.
Most parts of the switch are duplicated for reliability. This
includes the CC (one is running, the other idle; they switch when
there is a problem, or just if someone feels like it) and associated
Main Store, the standby CC is made active and the active CC is made
standby. When there is a problem (or when the idle CC is being
maintained) it is taken Out Of Service. Practically any part of the
switch can be made OOS. Before a piece of equipment can be put back
in service (INS), it must pass each part of a full diagnostic routine.
Sometimes it is possible to force equipment INS, though it isn't the
best of ideas.
Each 3A CC has its own main store (MAS). Each MAS memory word is
stored with two parity bits, one each for the high and low words.
Addresses are 20 bits, allowing for a full megabyte of address space
(not necessarily memory), while integers are 16 bits. I have no clue
how this works out in memory. Perhaps the machine is basically
nibble-based, with four nibbles for integers and five for pointers.
(But I'm pretty sure each parity bit is calculated on exactly half of
the word.)
Logical Attributes
The software is a custom event-loop, written in EPL, which stands
for ESS Programming Language
. It's basically a complex macro
package for assembly programming. The system documentation includes
commented listings of the compiled software. It operates at a normal
rate of XXX:SOME_RATE full cycles per second. If the average cycle
time drops below XXX:SOME_RATE, something happens. Therefore, all
tasks that will take more than a short amount of time (tens of
microseconds?) have to be split up and executed piecewise.
At one time, Dennis Ritchie considered
porting Unix to the 3ESS, but then decided it wouldn't work out. If
that had been done, it is likely (in my opinion) that it
could have been more easily upgraded to operate in the modern
telephone network environment, and thus the 3ESS would likely still be
in service. (The 5ESS has always used Unix, and it is one of the more
popular switches in service today.)
