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. www.phunc.com/~hybrid ----------------------------------------------- : . ___ ___ _____.___.____________________ ____________ hybrid@b4b0.org / | \\__ | |\______ \______ \/_ \______ \ hybrid@ninex.com / ~ \/ | | | | _/| _/ | || | \ hybrid@dtmf.org \ Y /\____ | | | \| | \ | || hy_ \ ---------------- \___|_ / / ______| |______ /|____|_ / |___/_______ / \/ \/ : \/ \/ . \/