| Electronic
Switching Systems were made possible by the invention of the
transistor. They apply the basic concepts of an electronic
data processor, operating under the direction of a stored-program
control, and high-speed switching networks.
The stored-program control (SPC) allows system
designs the necessary flexibility to design new features and
install them easily. The SPC controls the sequencing and call
routing of operations required to establish a call. It can
control a subscriber line or trunk (inter-office) line according
to its application.
The first field trials of an “electronic”
switching trial took place in Morris, IL in
1960.
In 1963, Bell Labs released the 101 ESS PBX
system as an electronic switching system. This led to the
development of a full electronic central office switch, the
#1ESS.
Western Electric’s first field installation
of a fully electronic (computer controlled analog switch)
was the #1 Electronic Switching System (#1ESS) at Succasunna,
NJ in May 1965.
The #1ESS switching system was designed for
areas where large numbers of lines and lines with heavy traffic
are served. It generally serves between 10,000 and 65,000
lines. The memory of the 1ESS is generally read only memory
(ROM) so that neither software or hardware malfunctions can
alter the information content.
The #1ESS was updated in 1976 with the introduction
of the 1A processor. #1ESS switches installed in 1976 and
beyond were known as #1AESS switches. The 1A processor was
designed for local switching applications to be implemented
into a working #1ESS switch. It allowed the switching capacity
to be doubled from the old #1ESS switches also.
The 1A Processor uses both ROM and RAM (Random
Access Memory). Magnetic tape units in the 1A Processor allow
for system reinitialization as well as detailed call billing
functions.
Both the #1ESS and the #1AESS switches use
the same peripheral equipment which allows for easy transition.
Programs in both switches control routine tests, diagnose
troubles, detect and report faults and troubles, and control
emergency actions to ensure satisfactory operation.
The #1 and #1AESS switches were the first
to offer “custom calling features” such as call
waiting, three-way calling and speed calling. They also offer
business features such as Centrex (PBX-like features but using
the regular central office switch).
As of 2006, there are a handful of #1AESS
switches in current use in the public switched network. As
time goes on most of these are being replaced with their fully
digital counterparts.
Western Electric introduced the #2ESS switch
in 1970 as a smaller and cost-effective local central office
switching system, meeting the need for 2,000 to 10,000 line
offices. One of the differences between the #1ESS and the
#2ESS is that in the #2ESS, lines and trunks terminate on
the same side of the network, which is called a folded network.
There is no need for separate line and trunk
link networks as in the #1ESS. Also, the network architecture
was designed to interface with customer lines carrying lighter
traffic, the features were oriented toward residential rather
than business lines, and the processor was smaller and less
expensive.
In 1976, Western Electric developed its first
digital switching system. This time it was not an local central
office switch, but a long-haul tandem switch. This switch,
the #4ESS, was the replacement for the large numbers of #4A
Crossbar tandem switches that were in use at the time.
Western Electric finally introduced its digital
local central office switching system, the #5ESS, in March
1982 with its first installation in Seneca, IL. It is a digital
time-division electronic switching system designed for modular
growth to accommodate local offices ranging from 1,000 to
100,000 lines.
It was designed to be a multi-use and modular
switching platform to perform switching functions for urban,
suburban and rural communities. The switch was also designed
to provide both local office and regional toll tandem functions
within the same switching platform.
The last thing to mention here are Remote
Switching Systems (RSS) and Remote Switching Modules (RSM).
The No. 10A RSS is designed to act as an extension of a #1ESS,
#1AESS, or #2BESS switching equipment host and is controlled
remotely by the host over a pair of dedicated data links.
It shares the processor capabilities of these
nearby ESS switches and uses a microprocessor for certain
control functions under the direction of the host central
processor. The RSS is capable of stand-alone functioning if
the links between it and the host are severed somehow. If
this occurs, though, custom calling, billing, traffic measurements,
etc. are unavailable -- only basic service on intra-RSS calls
is allowed.
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