The Number One
Electronic Switching System (1ESS) was the first
large-scale stored program control (SPC) telephone exchange or
electronic switching system in the Bell System. It was manufactured by
Western Electric and first placed into service in Succasunna, New
Jersey, in May 1965. The switching fabric was composed of a reed
relay matrix controlled by wire spring relays which in turn were
controlled by a central processing unit (CPU).
The 1AESS central office switch was a plug compatible, higher capacity
upgrade from 1ESS with a faster 1A processor that incorporated the
existing instruction set for programming compatibility, and used
smaller remreed switches, fewer relays, and featured disk storage.
It was in service from 1976 to 2017.
View of 1AESS frames.
1 Switching fabric
1.1 Line and trunk networks
1.2 Fabric error
3 Scan and distribute
4 1ESS computer
6 See also
8 External links
The voice switching fabric plan was similar to that of the earlier 5XB
switch in being bidirectional and in using the call-back principle.
The largest full access matrix switches in the system, however, were
8x8 rather than 10x10 or 20x16. Thus they required eight stages rather
than four to achieve large enough junctor groups in a large office.
Crosspoints being more expensive in the new system but switches
cheaper, system cost was minimized with fewer crosspoints organized
into more switches. The fabric was divided into Line Networks and
Trunk Networks of four stages, and partially folded to allow
connecting line-to-line or trunk-to-trunk without exceeding eight
stages of switching.
For a switch with 1000 input customers and 1000 output customers, a
full connection would require a matrix of 1000x1000, or 1 million,
physical switches for full interconnection possibility. When one
considers that a large telephone system can have many more than 1000 x
1000 customers, the hardware to establish a full interconnection can
grow rapidly and exceed practical implementations. Agner Krarup Erlang
first theorized a compromise which is based upon the concept that not
all telephones lines are connected at the same time. From statistical
theory, it is possible to design hardware that can connect most of the
calls, in the sense of a high percentage, and block others as
exceeding the design capacity. These are commonly referred to as
blocking switches and are the most common in modern telephone
exchanges. They are generally implemented as smaller switch fabrics in
cascade. In many, a randomizer is used to select the start of a path
through the multistage fabric so that the statistical properties
predicted by the theory can be gained.
Line and trunk networks
Each four stage Line Network (LN) or Trunk Network (TN) was divided
Junctor Switch Frames (JSF) and either Line Switch Frames (LSF)
in the case of a Line Network, or Trunk Switch Frames (TSF) in the
case of a Trunk Network. Links were designated A, B, C, and J for
Junctor. A Links were internal to the LSF or TSF; B Links connected
LSF or TSF to JSF, C were internal to JSF, and J links or Junctors
connected to another net in the exchange.
All JSFs had a unity concentration ratio, that is the number of B
links within the network equalled the number of junctors to other
networks. Most LSFs had a 4:1 Line Concentration Ratio (LCR); that is
the lines were four times as numerous as the B links. In some urban
areas 2:1 LSF were used. The B links were often multipled to make a
higher LCR, such as 3:1 or (especially in suburban 1ESS) 5:1. Line
Networks always had 1024 Junctors, arranged in 16 grids that each
switched 64 junctors to 64 B links. Four grids were grouped for
control purposes in each of four LJFs.
TSF had a unity concentration, but a TN could have more TSFs than
JSFs. Thus their B links were usually multipled to make a Trunk
Concentration Ratio (TCR) of 1.25:1 or 1.5:1, the latter being
especially common in 1A offices. TSFs and JSFs were identical except
for their position in the fabric and the presence of a ninth test
access level or no-test level in the JSF. Each JSF or TSF was divided
into 4 two-stage grids.
Early TNs had four JSF, for a total of 16 grids, 1024 J links and the
same number of B links, with four B links from each Trunk
to each Trunk Switch grid. Starting in the mid-1970s, larger offices
had their B links wired differently, with only two B links from each
Junctor Grid to each Trunk Switch Grid. This allowed a larger
TN, with 8 JSF containing 32 grids, connecting 2048 junctors and 2048
B links. Thus the junctor groups could be larger and more efficient.
These TN had eight TSF, giving the TN a unity trunk concentration
Within each LN or TN, the A, B, C and J links were counted from the
outer termination to the inner. That is, for a trunk, the trunk Stage
0 switch could connect each trunk to any of eight A links, which in
turn were wired to Stage 1 switches to connect them to B links. Trunk
Junctor grids also had Stage 0 and Stage 1 switches, the former to
connect B links to C links, and the latter to connect C to J links
also called Junctors. Junctors were gathered into cables, 16 twisted
pairs per cable constituting a
Junctor Subgroup, running to the
Junctor Grouping Frame where they were plugged into cables to other
networks. Each network had 64 or 128 subgroups, and was connected to
each other network by one or (usually) several subgroups.
The original 1ESS
Ferreed switching fabric was packaged as separate
8x8 switches or other sizes, tied into the rest of the speech fabric
and control circuitry by wire wrap connections. The
transmit/receive path of the analog voice signal is through a series
of magnetic-latching reed switches (very similar to latching
The much smaller
Remreed crosspoints, introduced at about the same
time as 1AESS, were packaged as grid boxes of four principal types.
Junctor Grids and 11A Trunk Grids were a box about 16x16x5
inches (40x40x12 cm) with sixteen 8x8 switches inside. Type 12A Line
Grids with 2:1 LCR were only about 5 inches (12 cm) wide,
with eight 4x4 Stage 0 line switches with ferrods and cutoff contacts
for 32 lines, connected internally to four 4x8 Stage 1 switches
connecting to B-links. Type 14A Line Grids with 4:1 LCR were about
16x12x5 inches (40x30x12 cm) with 64 lines, 32 A-links and 16 B-links.
The boxes were connected to the rest of the fabric and control
circuitry by slide-in connectors. Thus the worker had to handle a much
bigger, heavier piece of equipment, but didn't have to unwrap and
rewrap dozens of wires.
The two controllers in each
Junctor Frame had no-test access to their
Junctors via their F-switch, a ninth level in the Stage 1 switches
which could be opened or closed independently of the crosspoints in
the grid. When setting up each call through the fabric, but before
connecting the fabric to the line and/or trunk, the controller could
connect a test scan point to the talk wires in order to detect
potentials. Current flowing through the scan point would be reported
to the maintenance software, resulting in a "False Cross and Ground"
(FCG) teleprinter message listing the path. Then the maintenance
software would tell the call completion software to try again with a
With a clean FCG test, the call completion software told the "A" relay
in the trunk circuit to operate, connecting its transmission and test
hardware to the switching fabric and thus to the line. Then, for an
outgoing call, the trunk's scan point would scan for the presence of
an off hook line. If the short was not detected, the software would
command the printing of a "Supervision Failure" (SUPF) and try again
with a different junctor. A similar supervision check was performed
when an incoming call was answered. Any of these tests could alert for
the presence of a bad crosspoint.
Staff could study a mass of printouts to find which links and
crosspoints (out of, in some offices, a million crosspoints) were
causing calls to fail on first tries. In the late 1970s, teleprinter
channels were gathered together in Switching Control Centers (SCC),
later Switching Control Center System, each serving a dozen or more
1ESS exchanges and using their own computers to analyze these and
other kinds of failure reports. They generated a so-called histogram
(actually a scatterplot) of parts of the fabric where failures were
particularly numerous, usually pointing to a particular bad
crosspoint, even if it failed sporadically rather than consistently.
Local workers could then busy out the appropriate switch or grid and
When a test access crosspoint itself was stuck closed, it would cause
sporadic FCG failures all over both grids that were tested by that
controller. Since the J links were externally connected, switchroom
staff discovered that such failures could be found by making busy both
grids, grounding the controller's test leads, and then testing all 128
J links, 256 wires, for a ground.
Supervision and trunk signalling were the responsibility of trunk
circuits. The most common kinds (reverse battery one-way trunks) were
in plug-in trunk packs, two trunks per pack, 128 packs per Trunk Frame
(originally) on 16 shelves. Each trunk pack was originally about 3x5x8
inches (8x12x20 cm) with edge connector in the back. The later 1AESS
were made with shorter wire spring relays, making them less than half
as wide, with more complex leaf spring connector. Trunk Frames were in
pairs, the even numbered one having the Signal Distributor to control
the relays in both. Most trunks had three wire spring relays and two
scan points. They could supply regular battery or reverse battery to a
line, and on-hook or off-hook supervision to the distant end, or be
put into a bypass state allowing all functions (usually sending and
receiving address signals) to be performed by common control circuits
such as digit transmitters and receivers. Slightly more complex
trunks, for example those going to TSPS offices for operator control,
were packaged as only one per plugin unit.
Junctor Circuits were installed in similar frames, but were simpler,
with only two relays. They were used only in Line to Line junctors.
Large offices, in addition to these
Junctor Circuits, had Intraoffice
Trunks, which were of similar design but fit into the same Universal
Trunk Frames as interoffice trunks. They carried overflow traffic when
Junctor Groups of an office with many LN could not cope.
Digit transmitters, receivers, other complex service circuits, and
some complex trunks including those using E&M signaling, were
permanently mounted in relay racks similar to those of 5XB rather than
Scan and distribute
The computer received input from peripherals via magnetic scanners,
composed of ferrod sensors, similar in principle to magnetic core
memory except that the output was controlled by control windings
analogous to the windings of a relay. Specifically, the ferrod was a
transformer with four windings. Two small windings ran through holes
in the center of a rod of ferrite. A pulse on the Interrogate winding
was induced into the Readout winding, if the ferrite was not
magnetically saturated. The larger control windings, if current was
flowing through them, saturated the magnetic material, hence
decoupling the Interrogate winding from the Readout winding which
would return a Zero signal. The Interrogate windings of 16 ferrods of
a row were wired in series to a driver, and the Readout windings of 64
ferrods of a column were wired to a sense amp. Check circuits ensured
that an Interrogate current was indeed flowing.
Scanners were Line Scanners (LSC), Universal Trunk Scanners (USC),
Junctor Scanners (JSC) and Master Scanners (MS). The first three only
scanned for supervision, while Master Scanners did all other scan
jobs. For example, a DTMF Receiver, mounted in a Miscellaneous Trunk
frame, had eight demand scan points, one for each frequency, and two
supervisory scan points, one to signal the presence of a valid DTMF
combination so the software knew when to look at the frequency scan
points, and the other to supervise the loop. The supervisory scan
point also detected Dial Pulses, with software counting the pulses as
they arrived. Each digit when it became valid was stored in a software
hopper to be given to the Originating Register.
Ferrods were mounted in pairs, usually with different control
windings, so one could supervise a switchward side of a trunk and the
other the distant office. Components inside the trunk pack, including
diodes, determined for example, whether it performed reverse battery
signaling as an incoming trunk, or detected reverse battery from a
distant trunk; i.e. was an outgoing trunk.
Line ferrods were also provided in pairs, of which the even numbered
one had contacts brought out to the front of the package in lugs
suitable for wire wrap so the windings could be strapped for loop
start or ground start signaling. The original 1ESS packaging had all
the ferrods of an LSF together, and separate from the line switches,
while the later 1AESS had each ferrod at the front of the steel box
containing its line switch. Odd numbered line equipment could not be
made ground start, their ferrods being inaccessible.
The computer controlled the magnetic latching relays by Signal
Distributors (SD) packaged in the Universal Trunk frames, Junctor
frames, or in Miscellaneous Trunk frames, according to which they were
numbered as USD, JSD or MSD. SD were originally contact trees of
30-contact wire spring relays, each driven by a flipflop. Each
magnetic latching relay had one transfer contact dedicated to sending
a pulse back to the SD, on each operate and release. The pulser in the
SD detected this pulse to determine that the action had occurred, or
else alerted the maintenance software to print a FSCAN report. In
later 1AESS versions SD were solid state with several SD points per
circuit pack generally on the same shelf or adjacent shelf to the
A few peripherals that needed quicker response time, such as Dial
Pulse Transmitters, were controlled via Central Pulse Distributors,
which otherwise were mainly used for enabling (alerting) a peripheral
circuit controller to accept orders from the Peripheral Unit Address
Harvard architecture central processor or CC (Central
Control) for the 1ESS operated at approximately 200 kHz. It comprised
five bays, each two meters high and totaling about four meters in
length per CC. Packaging was in cards approximately 4x10 inches (10x25
centimeters) with an edge connector in the back. Backplane wiring was
cotton covered wire-wrap wires, not ribbons or other cables.
was implemented using discrete diode–transistor logic. One hard
plastic card commonly held the components necessary to implement, for
example, two gates or a flipflop.
A great deal of logic was given over to diagnostic circuitry. CPU
diagnostics could be run that would attempt to identify failing
card(s). In single card failures, first attempt to repair success
rates of 90% or better were common. Multiple card failures were not
uncommon and the success rate for first time repair dropped rapidly.
CPU design was quite complex - using three way interleaving of
instruction execution (later called instruction pipeline) to improve
throughput. Each instruction would go through an indexing phase, an
actual instruction execution phase and an output phase. While an
instruction was going through the indexing phase, the previous
instruction was in its execution phase and the instruction before it
was in its output phase.
In many instructions of the instruction set, data could be optionally
masked and/or rotated. Single instructions existed for such esoteric
functions as "find first set bit (the rightmost bit that is set) in a
data word, optionally reset the bit and tell me the position of the
bit". Having this function as an atomic instruction (rather than
implementing as a subroutine) dramatically sped scanning for service
requests or idle circuits. The central processor was implemented as a
hierarchical state machine.
Memory card for 128 words of 44 bits
Memory had a 44-bit word length for program stores, of which six bits
were for Hamming error correction and one was used for an additional
parity check. This left 37 bits for the instruction, of which usually
22 bits were used for the address. This was an unusually wide
instruction word for the time.
Program stores also contained permanent data, and could not be written
online. Instead, the aluminum memory cards, also called twistor
planes, had to be removed in groups of 128 so their permanent
magnets could be written offline by a motorized writer, an improvement
over the non motorized single card writer used in Project Nike. All
memory frames, all busses, and all software and data were fully dual
modular redundant. The dual CCs operated in lockstep and the detection
of a mismatch triggered an automatic sequencer to change the
combination of CC, busses and memory modules until a configuration was
reached that could pass a sanity check. Busses were twisted pairs, one
pair for each address, data or control bit, connected at the CC and at
each store frame by coupling transformers, and ending in terminating
resistors at the last frame.
Call Stores were the system's read/write memory, containing the data
for calls in progress and other temporary data. They had a 24-bit
word, of which one bit was for parity check. They operated similar to
magnetic core memory, except that the ferrite was in sheets with a
hole for each bit, and the coincident current address and readout
wires passed through that hole. The first Call Stores held 8
Kilowords, in a frame approximately a meter wide and two meters tall.
The separate program memory and data memory were operated in
antiphase, with the addressing phase of Program Store coinciding with
the data fetch phase of Call Store and vice versa. This resulted in
further overlapping, thus higher program execution speed than might be
expected from the slow clock rate.
Programs were mostly written in machine code. Bugs that previously
went unnoticed became prominent when 1ESS was brought to big cities
with heavy telephone traffic, and delayed the full adoption of the
system for a few years. Temporary fixes included the Service Link
Network (SLN), which did approximately the job of the Incoming
Register Link and Ringing Selection Switch of the 5XB switch, thus
CPU load and decreasing response times for incoming calls,
and a Signal Processor (SP) or peripheral computer of only one bay, to
handle simple but time consuming tasks such as the timing and counting
of Dial Pulses. 1AESS eliminated the need for SLN and SP.
The half inch tape drive was write only, being used only for Automatic
Message Accounting. Program updates were executed by shipping a load
of Program Store cards with the new code written on them.
The Basic Generic program included constant "audits" to correct errors
in the call registers and other data. When a critical hardware failure
in the processor or peripheral units occurred, such as both
controllers of a line switch frame failing and unable to receive
orders, the machine would stop connecting calls and go into a "phase
of memory regeneration", "phase of reinitialization", or "Phase" for
short. The Phases were known as Phase 1,2,4 or 5. Lesser phases only
cleared the call registers of calls that were in an unstable state
that is not yet connected, and took less time.
During a Phase, the system, normally roaring with the sound of relays
operating and releasing, would go quiet as no relays were getting
orders. The Teletype Model 35 would ring its bell and print a series
of P's while the phase lasted. For Central office staff this could be
a scary time as seconds and then perhaps minutes passed while they
knew subscribers who picked up their phones would get dead silence
until the phase was over and the processor regained "sanity" and
resumed connecting calls. Greater phases took longer, clearing all
call registers, thus disconnecting all calls and treating any off-hook
line as a request for dial tone. If the automated phases failed to
restore system sanity, there were manual procedures to identify and
isolate bad hardware or buses.
Head on view of 1AESS Master Control Center
The 1AESS version CC (Central Control) had a faster clock,
approximately one MHz, and required only one bay of space instead of
four. The majority of circuit boards were manufactured from metal for
better heat dissipation, and carried TTL SSI chips, usually attached
by hybrid packaging. Each finger at the back of the board was not a
mere trace on the circuit board, as usual in plug-in boards, but a
leaf spring, for greater reliability.
1AESS used memory with 26-bit words, of which two were for parity
checking. The initial version had 32 Kilowords of core mats. Later
versions used semiconductor memory. Program stores were arranged to
feed two words (52 bits) at a time to the
CPU via the Program Store
Bus, while Call Stores only gave one word at a time via the Call Store
Bus. 1A Program Stores were writable and not fully duplicated but were
backed up by the
File Stores. They were provided in multiplicity of
N+2, i.e. as many as were needed for the size of the office, plus two
hot standby units to be loaded from disk as needed.
In both the original version and 1A, clocks for Program Store and Call
Store were operated out of phase, so one would be delivering data
while the other was still accepting an address. Instruction decoding
and execution were pipelined, to allow overlapping processing of
consecutive instructions in a program.
File Stores had four hard drives each. These hard drives
were large, fast, expensive and crude, weighing about a hundred pounds
(40 kg) with 128 tracks and one head per track as in a drum
memory. They contained backups for software and for fixed data
(translations) but were not used in call processing. These file
stores, a high maintenance item with pneumatic valves and other
mechanical parts, were replaced in the 1980s with the 1A Attached
Processor System (1AAPS) using the
3B20D Computer to provide access to
File Store". The 1AAPS "1A
File Store" is just a disk
partitions in the
Common Network Interface (CNI) Ring became available it was
added to the 1AAPS to provide
Common Channel Signaling (CCS).
The 1AESS tape drives had approximately four times the density of the
original ones in 1ESS, and were used for some of the same purposes as
in other mainframe computers, including program updates and loading
Most of the thousands of 1ESS and 1AESS offices in the USA were
replaced in the 1990s by DMS-100,
5ESS Switch and other digital
switches, and since 2010 also by packet switches. As of late 2014,
just over 20 1AESS installations remained in the North American
network, which were located mostly in AT&T's legacy
AT&T's legacy Southwestern Bell states, especially in the Atlanta
GA metro area, the Saint Louis MO metro area, and in the Dallas/Fort
Worth TX metro area. In 2015, AT&T did not renew a support
Alcatel-Lucent (now Nokia) for the 1AESS systems still
in operation and notified
Alcatel-Lucent of its intent to remove them
all from service by 2017. As a result,
Alcatel-Lucent dismantled the
last 1AESS lab at the Naperville
Bell Labs location in 2015, and
announced the discontinuation of support for the 1AESS. In 2017,
AT&T completed the removal of remaining 1AESS systems by moving
customers to other newer technology switches, typically with Genband
switches with TDM trunking only.
The last known 1AESS switch was in Odessa, TX (Odessa Lincoln Federal
wirecenter ODSSTXLI). It was disconnected from service around June 3,
2017 and cut over to a
Genband G5/G6 packet switch.
Nonblocking minimal spanning switch
^ Ketchledge, R.: “The No. 1 Electronic Switching System” IEEE
Transactions on Communications, Volume 13, Issue 1, Mar 1965, pp 38-41
^ 1A Processor,
Bell System Technical Journal, 56(2), 119 (February
^ "NO. 1 ELECTRONIC SWITCHING SYSTEM"
^ D. Danielsen, K. S. Dunlap, and H. R. Hofmann. "No. 1 ESS Switching
Network Frames and Circuits. 1964.
^ a b J. G. Ferguson, W. E. Grutzner, D. C. Koehler, R. S. Skinner, M.
T. Skubiak, and D. H. Wetherell. "No. 1 ESS Apparatus and Equipment".
Bell System Technical Journal. 1964.
^ Al L Varney. "Questions About The No. 1 ESS Switch". 1991.
NANP Telephone Switches
Early Automatic & Crossbar Switches
Western Electric 1XB
Western Electric 5XB
Electronic Switching Systems
Western Electric 1ESS/1AESS
Western Electric 4ESS
Western Electric 5ESS
Northern Electric SP1
Northern Telecom DMS-100