telephone switch
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Edward Mishael S. Dela Cruz
BS-ECE 4-1 2008-20041ECE 164 - Wire Communications
TELEPHONE SWITCH/ TELEPHONE EXCHANGE
In the field of telecommunications, a telephone exchange or telephoneswitch is a system of electronic components that connects telephone calls.
A central office is the physical building used to house inside plant equipmentincluding telephone switches, which make telephone calls "work" in thesense of making connections and relaying the speech information.The term exchange area can be used to refer to an area served by a
particular switch, but is typically known as a wire center in the US
telecommunications industry. The exchange code or Central Office Coderefers to the first three digits of the local number (NXX). It is sometimesconfused with the area code (NPA). In the United States, local exchange
areas together make up a legal entity called local access and transport
areas (LATA) under the Modification of Final Judgment (MFJ).
HISTORY OF TELEPHONE SWITCHINGCalling someone on the telephone is possible because of the use of
what communications engineers call “switching” technology. The numbers ona telephone keypad relay information to a device much like a computer,
which uses the numbers to direct your call to the appropriate destination.The first telephone switching technology was the manual switchboard. A
switchboard was usually a board or box where all the telephone lines in alocal area terminated. At the terminal points, plugs were installed so that
wires could be connected between any two lines. When a customer placed acall, a light, bell, or other signal alerted the operator, who answered the call.
The operator asked the caller whom they wanted to call, and used wires toconnect the caller and the callee. The first commercial switchboard opened in
1878 to serve the 21 telephone customers in New Haven, Connecticut.
Women telephone operators using a manual switchboard in Montrose,
Colorado, around 1915. Courtesy: Denver Public Library.
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Telephone switchboards grew larger and more elaborate as more and
more people got telephones in the late 19th century. They also began to bedesigned with more and more features, such as bells or lights that told the
operator when a line was busy. There were so many telephone subscribersby about 1910 that inventors and telephone companies around the world
began to look for ways to make telephone switching automatic. The mostfamous of the independent inventors was Almon Strowger. On 12 March
1889 in Kansas City, Missouri, he applied for a patent on a system of automatic telephone switching; the patent was granted on 10 May 1891. Itwas the first such system to meet with commercial success, being firstimplemented in La Porte, Indiana in 1892. But the automatic switching he
invented was not perfected for nearly a decade. After the turn of the
century, Strowger switches and dial telephones became common inindependent (non-Bell) telephone exchanges in the United States. But it wasonly in 1919 that the American Telephone and Telegraph (AT&T) began
giving customers in its Bell System exchanges telephones with dials on
them, and installing automatic switching equipment in its central offices. Ittook decades to complete the conversion, one exchange at a time. While all
telephones in Manhattan had dials by 1930, The last manual Bell telephones,on Catalina Island, California, were retired only in 1978.
A dial telephone sent electric pulses down the line to the central
switch, where they were detected and used to establish a connectionautomatically. Most areas of the United States had automatic switching and
dial telephones by about 1950, and operators were only necessary for long-distance calls. These, too, were mostly automated by about 1965. Most
automatic switching equipment used electromechanical apparatus to routecalls and make connections until the 1960s, when transistors replaced the
relays. And dial telephones themselves were replaced by touch tone phoneswith keypads. The first touch tone phones, which used musical tones instead
of pulses, entered service in 1963. The transistor was invented at AT&Twhen engineers were looking for a replacement for the electromechanicalrelay. Engineers working on the first computers were also interested in
telephone switching equipment, because the switching processes of computing were similar to automatic telephone switching. The term
“switchboard” is not used today, but has been replaced by “switch.” Today’s
telephone switches come in many different varieties to serve local, long-distance, international, satellite, and cellular telephone needs in addition todigital data.
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Manual service exchanges
1924 PBX switchboard
With manual service, the customer lifts the receiver off-hook and asksthe operator to connect the call to a requested number. Provided that thenumber is in the same central office, the operator connects the call by
plugging into the jack on the switchboard corresponding to the calledcustomer's line. If the call is to another central office, the operator plugs into
the trunk for the other office and asks the operator answering (known as the"inward" operator) to connect the call.
Most urban exchanges were common-battery, meaning that thecentral office provided power for the telephone circuits, as is the case today.
In common-battery systems, the pair of wires from a subscriber's telephoneto the switch (or manual exchange) carry -48VDC (nominal) from the
telephone company end, across the conductors. The telephone presents anopen circuit when it ison-hook or idle. When the subscriber goes off-hook,
the telephone puts a DC resistance short across the line. In manual service,
this current flowing through the off-hook telephone flows through a relay coilactuating a buzzer and lamp on the operator's switchboard. The buzzer and
lamp would tell an operator the subscriber was off-hook (requestingservice).
In the largest U.S. cities, it took many years to convert every office to
automatic equipment, such as panel switches. During this transition period,it was possible to dial a manual number and be connected without
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requesting an operator's assistance. This was because the policy of the Bell
System was that customers should not need to know whether they werecalling a manual or automated office. If a subscriber dialed a manual
number, an inward operator would answer the call, see the called number ona display device, and manually connect the call. For instance, if a customer
calling from TAylor 4725 dialed a manual number, ADams 1233, the callwould go through, from the subscriber's perspective, exactly as a call to
LEnnox 5813, in an automated exchange.
In contrast to the common-battery system, smaller towns with manualservice often had magneto, or crank, phones. Using a magneto set, the
subscriber turned a crank to generate ringing current, to gain the operator's
attention. The switchboard would respond by dropping a metal tab above thesubscriber's line jack and sounding a buzzer. Dry cell batteries (normally twolarge "No 6" cells) in the subscriber's telephone provided the DC power for
conversation. Magneto systems were in use in one American small town,
Bryant Pond, Woodstock, Maine as late as 1983. In general, this type of system had a poorer call quality compared to common-battery systems.
Many small town magneto systems featured party lines, anywhere from twoto ten or more subscribers sharing a single line. When calling a party, the
operator would use a distinctive ringing signal sequence, such as two longrings followed by one short. Everyone on the line could hear the rings, and
of course could pick up and listen in if they wanted. On rural lines whichwere not connected to a central office (thus not connected to the outside
world), subscribers would crank the correct sequence of rings to reach theirparty.
Early automatic exchanges
A rural telephone exchange building in Australia
Automatic exchanges, or dial service, came into existence in the early1900s. Their purpose was to eliminate the need for human telephone
operators. Before the exchanges became automated, operators had to
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complete the connections required for a telephone call. Almost everywhere,
operators have been replaced by computerized exchanges. A telephoneswitch is the brains of an automatic exchange. It is a device for routing calls
from one telephone to another, generally as part of the public switchedtelephone network.
The local exchange automatically senses an off hook
(tip) telephone condition, provides dial tone to that phone, receives thepulses or DTMF tones generated by the phone, and then completes aconnection to the called phone within the same exchange or to anotherdistant exchange.
The exchange then maintains the connection until a party hangs up,and the connection is disconnected. This tracking of a connection's status iscalled supervision. Additional features, such as billing equipment, may also
be incorporated into the exchange.
In Bell System dial service, a feature called automatic number
identification (ANI) was implemented. ANI allowed services like automatedbilling, toll-free 800-numbers, and 9-1-1 service. In manual service, the
operator knows where a call is originating by the light on the switchboard's jack field. In early dial service, ANI did not exist. Long distance calls would
go to an operator queue and the operator would ask the calling party'snumber, then write it on a paper toll ticket. See also Automatic Message
Accounting.
Early exchanges used motors, shaft drives, rotating switchesand relays. In a sense, switches were relay-logic computers. Some types of
automatic exchanges were Strowger (also known as Step-By-Step), AllRelay, X-Y, Panel and crossbar. These are referred to collectively
aselectromechanical switches.
Electromechanical signalingCircuits connecting two switches are called trunks.
Before Signaling System 7, Bell System electromechanical switches in
the United States communicated with one another over trunks using a
variety of DC voltages and signaling tones. It would be rare to see anyof these in use today.
Some signaling communicated dialed digits. An early formcalled Panel Call Indicator Pulsing used quaternary pulses to set up
calls between a panel switch and a manual switchboard. Probably themost common form of communicating dialed digits between
electromechanical switches was sending dial pulses, equivalent to
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a rotary dial's pulsing, but sent over trunk circuits between switches.
In Bell System trunks, it was common to use 20 pulse-per-secondbetween crossbar switches and crossbar tandems. This was twice the
rate of Western Electric/Bell System telephone dials. Using the fasterpulsing rate made trunk utilization more efficient because the switch
spent half as long listening to digits. DTMF was not used for trunksignaling. Multi-frequency (MF) was the last of the pre-digital methods.
It used a different set of tones sent in pairs like DTMF. Dialing waspreceded by a special key pulse (KP) signal and followed bya start (ST). Variations of the Bell System MF tone scheme becamea CCITT standard. Similar schemes were used in the Americas and in
some European countries including Spain. Digit strings between
switches were often abbreviated to further improve utilization. Forexample, one switch might send only the last four or five digits of a telephone number. In one case, seven digit numbers were preceded
by a digit 1 or 2 to differentiate between two area codes or office
codes, (a two-digit-per-call savings). This improved revenue per trunkand reduced the number of digit receivers needed in a switch. Every
task in electromechanical switches was done in big metallic pieces of hardware. Every fractional second cut off of call set up time meant
fewer racks of equipment to handle call traffic.
Examples of signals communicating supervision or call progressinclude E and M signaling, SF signaling, and robbed-bit signaling. In
physical (not carrier) E and M trunk circuits, trunks were four wire.Fifty trunks would require a hundred pair cable between switches, for
example. Conductors in one common circuit configuration were namedtip, ring, ear (E) and mouth (M). In two-way trunks with E and M
signaling, a handshake took place to prevent both switches fromcolliding by dialing calls on the same trunk at the same time. By
changing the state of these leads from ground to -48 volts, theswitches stepped through a handshake protocol. Using DC voltagechanges, the local switch would send a signal to get ready for a call
and the remote switch would reply with an acknowledgment to goahead with dial pulsing. This was done with relay logic and discrete
electronics. These voltage changes on the trunk circuit would cause
pops or clicks that were audible to the subscriber as the electricalhandshaking stepped through its protocol. Another handshake, to starttiming for billing purposes, caused a second set of clunks when the
called party answered. A second common form of signaling forsupervision was called single-frequency or SF signaling. The most
common form of this used a steady 2,600 Hz tone to identify a trunkas idle. Trunk circuitry hearing a 2,600 Hz tone for certain duration
would go idle. (The duration requirement reduced falsing). Some
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systems used tone frequencies over 3,000 Hz, particularly on
SSB frequency division multiplex microwave radio relays. On T-carrier digital transmission systems, bits within the T-1 data stream
were used to transmit supervision. By careful design, the appropriatedbits did not change voice quality appreciably. Robbed bits were
translated to changes in contact states (opens and closures) byelectronics in the channel bank hardware. This allowed direct current E
and M signaling, or dial pulses, to be sent between electromechanicalswitches over a digital carrier which did not have DC continuity.
SoundsA characteristic of electromechanical switching equipment is that
the maintenance staff could hear the mechanical clattering of Strowgers, panel switches or crossbar relays. Most Bell System centraloffices were housed in reinforced concrete buildings with concrete
ceilings and floors. In rural areas, some smaller switching facilities,
such as Community, were sometimes housed in prefabricated metalbuildings. These facilities almost always had concrete floors. The hard
surfaces reflected sounds.
During heavy use periods, it could be difficult to converse in acentral office switch room due to the clatter of calls being processed in
a large switch. For example, on Mother's Day in the US, or on a Fridayevening around 5pm, the metallic rattling could make raised voices
necessary. For wire spring relay markers these noises resembled hailfalling on a metallic roof.
On a pre-dawn Sunday morning, call processing might slow to
the extent that one might be able to hear individual calls being dialedand set up. There were also noises from whining power inverters and
whirring ringing generators. Some systems had a continual, rhythmic"clack-clack-clack" from wire spring relays that made reorder (120ipm) and busy (60 ipm) signals. In Bell System installations, there
were typically alarm bells, gongs, or chimes. These would annunciatealarms calling attention to a failed switch element. Another
noisemaker: a trouble reporting card system was connected to switch
common control elements. These trouble reporting systems wouldpuncture cardboard cardswith a code that logged the nature of afailure. Remreed technology in Stored Program Control
exchanges finally quieted the environment.
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Maintenance tasks
The maintenance of electromechanical systems was partly DC
electricity and partly mechanical adjustments. Unlike modern switches,a circuit connecting a dialed call through an electromechanical switch
actually had DC continuity. The talking path was a physical, metallicone.
In all systems, subscribers were not supposed to notice changesin quality of service because of failures or maintenance work. A varietyof tools referred to as make-busys were plugged into
electromechanical switch elements during repairs or failures. A make-
busy would identify the part being worked on as in-use, causing theswitching logic to route around it. A similar tool was called a TDtool. Subscribers who got behind in payments would have their service
temporarily denied (TDed). This was effected by plugging a tool into
the subscriber's office equipment (Crossbar) or line group (step). Thesubscriber could receive calls but could not dial out.
Strowger-based, step-by-step offices in the Bell System were
under continual maintenance. They required constant cleaning.Indicator lights on equipment bays in step offices alerted staff to
conditions such as blown fuses (usually white lamps) or a permanentsignal (stuck off-hook condition, usually green indicators.) Step offices
were more susceptible to single-point failures than newer technologies.
Crossbar offices used more shared, common control circuits. Forexample, a digit receiver (part of an element called an Originating
Register) would be connected to a call just long enough to collect thesubscriber's dialed digits. Crossbar architecture was more flexible than
step offices. Later crossbar systems had punch-card-based troublereporting systems. By the 1970s, automatic number identification hadbeen retrofitted to nearly all step-by-step and crossbar switches in the
Bell System.
Electronic switches
The first Electronic Switching Systems were not entirely digital.The Western Electric 1ESS switch had reed relay metallic paths which
werestored-program-controlled. Equipment testing, changes to phonenumbers, circuit lockouts and similar tasks were accomplished by
typing on a terminal. Northern Telecom SP1, Ericsson AKE,Philips PRX /A, ITT Metaconta, British Telecom TXE series and several
other designs were similar. These systems could use the old
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electromechanical signaling methods inherited from crossbar and step-
by-step switches. They also introduced a new form of datacommunications: two 1ESS exchanges could communicate with one
another using a data link called Common Channel Interoffice Signaling,(CCIS). This data link was based on CCITT 6, a predecessor to SS7.
Digital switches
Digital switches work by connecting two or more digital circuitstogether, according to a dialed telephone number. Calls are set up betweenswitches using the Signalling System 7 protocol, or one of its variants. In
U.S. and military telecommunication, a digital switch is a switch that
performs time division switching of digitized signals.[11] This was first done ina few small and little used systems. The first product using a digital switchsystem was made by Amtelco. Prominent examples include Nortel DMS-100,
Lucent 5ESS switch, Siemens EWSD and Ericsson AXE telephone exchange.
With few exceptions, most switches built since the 1980s are digital. Thisarticle describes digital switches, including algorithms and equipment.
Digital switches encode the speech going on, in 8000 time slices per
second. At each time slice, a digitalPCM representation of the tone is made.The digits are then sent to the receiving end of the line, where the reverse
process occurs, to produce the sound for the receiving phone. In otherwords, when you use a telephone, you are generally having your voice
"encoded" and then reconstructed for the person on the other end. Yourvoice is delayed in the process by a small fraction of one second — it is not
"live", it is reconstructed — delayed only minutely. (See below for moreinfo.)
Individual local loop telephone lines are connected to a remote
concentrator. In many cases, the concentrator is co-located in the samebuilding as the switch. The interface between remote concentrators andtelephone switches has been standardised by ETSI as the V5protocol.
Concentrators are used because most telephones are idle most of the day,hence the traffic from hundreds or thousands of them may be concentrated
into only tens or hundreds of shared connections.
Some telephone switches do not have concentrators directly connectedto them, but rather are used to connect calls between other telephone
switches. These complex machines (or a series of them) in a centralexchange building are referred to as "carrier-level" switches or tandems.
Some telephone exchange buildings in small towns now houseonly remote or satellite switches, and are homed upon a "parent" switch,
usually several kilometres away. The remote switch is dependent on the
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parent switch for routing and number plan information. Unlike adigital loop
carrier, a remote switch can route calls between local phones itself, withoutusing trunks to the parent switch.
Telephone switches are usually owned and operated by a telephone serviceprovider or carrier and located in their premises, but sometimes individual
businesses or private commercial buildings will house their own switch,called a PBX, or Private branch exchange.
Internet exchangesThe telephone exchange concept has been adapted for use in Internetexchanges. Voice over IP (VoIP) traffic may pass through both kinds of
exchanges, depending on what kind of service the caller and the called
subscriber are using.
Sources:
http://en.wikipedia.org/wiki/Telephone_exchange#Electromechanical_signaling
http://192.197.62.35/staff/mcsele/TelephoneSwitch.html
http://www.ieeeghn.org/wiki/index.php/Telephone_switching