brief guide to the evolution of
switching systems; a primer for
9x by hybrid (http://hybrid.dtmf.org)


Switching Systems
   
Before the phone network went automated, phone switching was achieved by
operators that manaully made the connections between subscriber lines on
huge panels of inter-connecting circuit boards. As telephony technology
progressed, so did the type of swithing techniques; the manaul switchboards
where replaced by electromechanical switches which took the place of the
manual switchboard operators.

These primitive electromacanical switching mechanisms used a series of
fingers that would rotate and then make contact with the circuit, therfore
connecting the subscriber line; these types of switches where called stepper
switches, in essance they where verical laders of rotary switches with
rottating contacts that would either step up or down. This switching
technology was invented in 1891, and has sinse past its sell-by-date by far.

Next in the line of switch evoloution was a new bread of electromacanical
switch, this time called the crossbar switch. Again, the crossbar switch was
an analog device which only supported mechanical switching functions. The
crossbar switches used multiple verticle and horiztontal paths with some
electromechanical relay switches for the interconnecting of the vertical
paths to the hozzitontal paths. The crossbar switching interface was refered
to as the TXC switch (Telephone eXchange Crossbar). There where various
hybrid's/variants of the TXE switching system such as the number 5 crossbar
switch (5XB) which where deployed throughout end-offices during the 193O's.
   
Now some more fammiler ground to cover; the next breed of switch that came
to dominate the network where the electronic switches. Like the previous
switches they where also electromacanical, the difference being that these
switches where controled by computers, and therefore adopted the form of
computer controlled electromechanical or electronic switching devices. These
switches where designed to handle/distrobute analog signals, and used a new
method of call handeling; Unlike the previous switches where each digit
dialed would be proccessed one at a time, these new switches stored the
dialed number in a register and then executed the dialed connection. We
refere to this this type of switch as a common control switch, it soon
beacame the first steeping stone towards ESS.

Now, we all know what ESS is right? (you damn well should d0). This breed of
switching technology was derived from the previous switch, with the exception
that they implement[ed] stored program control so trunk up calls. The first
ever switch to implement this new stored program control was the Number 1 --
Electronic Switching System, more comanly refered to as (1ESS). The 1ESS was
a computer controled crossbar switch, which implemented computers to instruct
the elecromechanical functions of the switch; Such a system is refered to as
TXD (Telephone eXchange Digital). At the time this method of switching was
considered to be very advanced and ahead of its time; The concept was infact
rather simple, but effective. The ESS switches had to use an identical or
'generic' program in each class of switch. The differences between offices
was determined by parameters used by the 'generic' program. Parameters are
the number and location of active lines and trunks, tone or rotary dialing,
etc.

During the 197Os when this type of switching architecture was at large, call
handeling traffic increased, so the next breed of switch implemented with an
upgraded CPU type, and morphed from 1ESS to 1AESS, but was still effectivly
a computer controlled crossbar switch.

As the demand for phone services grew, the switching systems advanced into
a newer breed of switching, it was this time that the famous 4/5ESS switches
where born and have sinse been used as the workhorses for the phone network.
The first computer controller digital switch was the 4ESS system, which was
specifically designed for toll switching and routing. It implemented the
previous 1AESS CPU and was coupled together with a TMS (Time Multiplexed
Switch) capable of handeling 5O,OOO[+] similtanous loop connections. The
switch was designed to handle digital signals, but at the time the local
offices had to patch the older local loop equipment to it by ringing
subscriber lines with a 9O volt AC current; Sinse the semiconnductors had a
hard time dealing with this, the new breed of ESS was born -- 5ESS.

In the previous ESS systems, the analog signals where switched at local
offices, but the new 5ESS system converted the analog signals into a digital
form, and stored program switching was born. The AT&T 5ESS switches are based
on a TST (Time Space Time) digital switching concept that are capable of
handeling over 1OO,OOO subscriber lines. The current switch are identified as
Telephone eXchange Electronic (TXE) because they employ electronic switching,
as opposed to electromechanical means such as Crossbar or step-by-step
switches.
   
Northern Telecom is another manufacturer of digital telephone switches
designated as DMS-1OO, DMS-2OO, and DMS-25O. Each is tailored to specific
switching functions on the phone network. Cellular switch vendors market PBXs
or CO switches reconfigured with software to support mobile subscribers.
Three of the major U.S. cellular switch equipment suppliers -- AT&T, Ericsson,
and Northern Telecom (of Motorola Nortel) -- are also leading suppliers of CO
switches.
   
Today the phone network is becoming increasingly advanced with new telephony
inovations developing all the time. At present the phone network is run via
advanced digital CO switches which support many fucncions such as CLASS
services (a basic example). We also see the mass implementation of Signaling
System 7 (SS7), Integrated Services Digital Network (ISDN), Custom Local Area
Signaling Services (CLASS -- the phone company delivering the number and/or
name of the calling party to the subscriber), Centrex, cellular
communications, and Advanced Intelligent Networks (AIN) are supported by CO
switch suppliers' products today.


http://hybrid.dtmf.org       shouts to substance, 9x, darkcyde, phunc, b4b0.
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