   #copyright

Telephone exchange

2007 Schools Wikipedia Selection. Related subjects: Engineering

   Map of the Wire Center locations in the US
   Enlarge
   Map of the Wire Centre locations in the US
   Map of the Central Office locations in the US
   Enlarge
   Map of the Central Office locations in the US

   In the field of telecommunications, a telephone exchange or telephone
   switch is a system of electronic components that connects telephone
   calls. A central office is the physical building used to house inside
   plant equipment including telephone switches, which make phone calls
   "work" in the sense of making connections and relaying the speech
   information.

   The term exchange can also be used to refer to an area served by a
   particular switch (typically known as a wire centre in the US
   telecommunications industry). More narrowly, in some areas it can refer
   to the first three digits of the local number. In the three-digit sense
   of the word, other obsolete Bell System terms include office code and
   NXX. In the United States, the word exchange can also have the legal
   meaning of a local access and transport area under the Modification of
   Final Judgment (MFJ).

Historic perspective

   A Verizon Central Office in Lakeland, Florida at night.
   Enlarge
   A Verizon Central Office in Lakeland, Florida at night.
   A telephone operator manually connecting calls with patch cables at a
   telephone switchboard. Computers make most connections now.
   Enlarge
   A telephone operator manually connecting calls with patch cables at a
   telephone switchboard. Computers make most connections now.

   The first telephone exchange opened in New Haven, Connecticut in 1878.
   The switchboard was built from "carriage bolts, handles from teapot
   lids and bustle wire" and could handle two simultaneous conversations.

   Later exchanges consisted of one to several hundred plug boards staffed
   by operators. Each operator sat in front of from one to three banks of
   ¼-inch bantam jacks fronted by several rows of phone cords, each of
   which was the local termination of a phone subscriber line. A calling
   party (known as the subscriber), would lift the receiver, a light near
   the plug would light, and the operator would switch into the circuit to
   ask, "number please?" Depending upon the answer, the operator might
   plug the plug into a local jack and start the ringing cycle, or plug
   into a hand-off circuit to start what might be a long distance call
   handled by subsequent operators in another bank of boards or in another
   building miles away. In 1918 the average time to complete a
   long-distance call was 15 minutes. In the ringdown method, the
   originating operator called another intermediate operator who would
   call the called subscriber, or passed it on to another intermediate
   operator. This chain of intermediate operators could complete the call
   only if all of the intermediate trunk lines were available at the same
   time. In 1943 when military calls had priority, a cross-country US call
   might take as long as 2 hours to request and schedule in cities that
   used manual switchboards for toll calls.

   On March 10, 1891, Almon Strowger, an undertaker in Topeka, Kansas,
   patented the Strowger switch, a device which led to the automation of
   the telephone circuit switching. While there were many extensions and
   adaptations of this initial patent, the one best known consists of 10
   layers or banks of 10 contacts arranged in a semi-circle. When used
   with a dial telephone, each pair of numbers caused the shaft of the
   central contact "hand" to first step up a layer per digit and then
   swing in a contact row per digit.

   These step switches were arranged in banks, beginning with a
   line-finder which detected that one of up to a hundred subscriber lines
   had the receiver lifted "off hook". The line finder hooked the
   subscriber to a "dial tone" bank to show that it was ready. The
   subscriber's dial pulsed at 10 pulses per second (depending on
   standards in particular countries).

   Exchanges based on the Strowger switch were challenged by crossbar
   technology. These phone exchanges promised faster switching and would
   accept pulses faster than the Strowger's typical 10 pps—typically about
   20 pps. The advent of dual-tone multifrequency ( DTMF) tone-signalling
   solid-state switches (i.e., touch tone dialing) cut off the crossbar's
   takeover before it could really get going.

   A transitional technology (from pulse to DTMF) had DTMF link finders
   which converted DTMF to pulse and fed it to conventional Strowger or
   crossbar switches. This technology was used as late as the mid to late
   1990s.

Number plan trivia

   See Telephone number

Technologies

   This article will use the terms:
     * manual service for a condition where a human operator routes calls
       inside an exchange and a dial is not used.
     * dial service for an exchange where calls are routed by a switch
       interpreting dialed digits.
     * telephone exchange for the building.
     * telephone switch for the switching equipment.
     * concentrator for the concentrator, whether or not it is co-located
       with the switch.
     * off-hook for a tip condition or to describe a circuit which is in
       use.
     * on-hook for an idle circuit.
     * wire centre for the area served by a particular switch or central
       office.

   Many of the terms in this article have conflicting UK and US usages.
     * central office originally referred to the switching equipment
       itself. Now it is used generally for the building housing switching
       and related inside plant equipment.
     * telephone exchange means an exchange building in the UK, and is
       also the UK name for a telephone switch, and also has a legal
       meaning in U.S. telecoms.
     * telephone switch is the U.S. term, but is in increasing use in
       technical UK telecoms usage, to make the CO/switch/concentrator
       distinction clear

Manual service exchanges

   1924 PBX switchboard
   Enlarge
   1924 PBX switchboard

   With manual service, the customer goes off-hook and asks the operator
   for the number. Provided that the number is in the same central office,
   the operator connects the call by plugging into the jack on the
   switchboard corresponding to the called customer'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 manual telephone exchanges in cities were common-battery, meaning
   that the central office provided power for the telephone circuits, as
   is the case today. A customer lifting their receiver would change their
   line status to off-hook, thereby lighting a light and sounding a buzzer
   on the operator's switchboard. In common battery systems, the pair of
   wires from a subscriber's telephone to the switch (or manual exchange)
   are open circuit when the telephone is on-hook or idle. There is -48VDC
   coming from the telephone company end across the conductors. When the
   subscriber goes off-hook, the telephone puts a DC resistance across the
   line. In manual service, this current flowing through the off-hook
   telephone would flow through a relay coil actuating a buzzer and lamp
   on the operator's console. The buzzer and lamp would tell an operator
   the subscriber was off-hook, (requesting service).

   In large cities with hundreds of central offices such as New York City,
   it took many years to convert the whole city to dial service. To help
   automate service to manual offices during the transition to dial
   service, a special type of switchboard, which would display the number
   dialed by the customer, was used. For instance, if a customer in the
   MUrray Hill exchange picked up the phone and dialed a number in the
   CIty Island exchange, the caller would not need to know the called
   party was in a manual exchange. Dialing that number would connect to
   the CIty Island exchange inward operator, who would see the number
   displayed, and ring the destination.

   In contrast to the common battery system, smaller towns with manual
   service 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 the subscriber's line, or sounding a buzzer. Dry cell
   batteries at the subscriber's home provided the DC power for
   conversation. Magneto systems were in use in some small towns in the
   U.S. as late as the 1980s. In general, his type of system had a poorer
   call quality compared to common-battery systems.

   Many small town magneto systems featured party lines, anywhere from two
   to ten or more subscribers sharing a single line. When calling a party,
   the operator would use a distinctive ring sequence, such as two long
   rings, 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 which were not connected to a central office (and thus not
   connected to the outside world), subscribers would crank the correct
   sequence of rings to reach their party.

Pre-digital automatic exchanges

   Automatic exchanges, or dial service, came into existence in the early
   1900s. Their purpose was to eliminate the need for human telephone
   operators. Before the exchanges became automated, operators had to
   complete the connections required for a telephone call. Almost
   everywhere, operators have been replaced by computerized exchanges. A
   telephone switch 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 switched telephone network.

   The local exchange automatically senses an off hook (tip) telephone
   condition, provides dial tone to that phone, receives the pulses or
   DTMF tones generated by the phone, and then completes a connection to
   the called phone within the same exchange or to another distant
   exchange.

   The exchange then maintains the connection until a party hangs up, and
   the connection is disconnected. This tracking of a connection's status
   is called 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
   automated billing, 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's number, then write it on a paper toll
   ticket. See also Automatic Message Accounting.

   Early exchanges used motors, shaft drives, rotating switches and
   relays. In a sense, switches were relay-logic computers. Some types of
   automatic exchanges were Strowger (also known as Step-By-Step), All
   Relay, X-Y, Panel and crossbar. These are referred to collectively as
   electromechanical switches.

Electromechanical signaling

   Circuits connecting two switches are called trunks. Before Signalling
   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 any of these in
   use today.

   Some signaling communicated dialed digits. An early form called Panel
   Call Indicator Pulsing used some unknown-format pulses to set up calls
   between two Panel switches. Probably the most common form of
   communicating dialed digits between electromechanical switches was
   sending dial pulses, equivalent to a rotary dial's pulsing, but sent
   over trunk circuits between switches. In Bell System trunks, it was
   common to use 20 pulse-per-second between crossbar switches and
   crossbar tandems. This was twice the rate of Western Electric/Bell
   System telephone dials. Using the faster pulsing rate made trunk
   utilization more efficient because the switch spent half as long
   listening to digits. DTMF was not used for trunk signaling.
   Multi-frequency (MF) was the last of the pre-digital methods. It used a
   different set of tones sent in pairs like DTMF. Dialing was preceded by
   a special keypulse (KP) signal and followed by a start (ST). Variations
   of the Bell System MF tone scheme became a 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. For example, 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, (a two-digit-per-call savings). This improved revenue
   per trunk and 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 progress include
   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 named tip,
   ring, ear (E) and mouth (M). In two-way trunks with E and M signaling,
   a handshake took place to prevent both switches from colliding by
   dialing calls on the same trunk at the same time. By changing the state
   of these leads from ground to -48 volts, the switches stepped through a
   handshake protocol. Using DC voltage changes, the local switch would
   send a signal to get ready for a call and the remote switch would reply
   with an acknowledgement to go ahead 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 electrical handshaking stepped through its protocol.
   Another handshake, to start timing for billing purposes, caused a
   second set of clunks when the called party answered. A second common
   form of signaling for supervision was called single-frequency or SF
   signaling. The most common form of this used a steady 2,600 Hz tone to
   identify a trunk as idle. Trunk circuitry hearing a 2,600 Hz tone for a
   certain duration would go idle. (The duration requirement reduced
   falsing.) Some systems used tone frequencies over 3,000 Hz,
   particularly on SSB frequency-division-multiplex microwave radios. On
   T-1 digital carriers, a digital format called Alternate Mark Inversion,
   (AMI) was sometimes used to pass signaling by robbing bits from the T-1
   data stream. By careful design, the appropriated bits did not change
   voice quality appreciably. Robbed bits were translated to changes in
   contact states (opens and closures) by electronics in the channel bank
   hardware. This allowed direct current E and M signaling, or dial
   pulses, to be sent between electromechanical switches over a digital
   carrier which did not have DC continuity.

Sounds

   A characteristic of electromechanical switching equipment is that the
   maintenance staff could hear the mechanical clattering of Strowgers or
   crossbar relays. Most Bell System central offices were housed in
   reinforced concrete buildings with concrete ceilings and floors. In
   rural areas, some smaller switching facilities, such as Community Dial
   Offices (CDOs), were sometimes housed in prefabricated metal buildings.
   These facilities almost always had concrete floors. The hard surfaces
   reflected sounds.

   During heavy use periods, it could be hard to talk over the clatter of
   calls being processed in a large switch. For example, on Mothers Day in
   the US, or on a Friday evening around 5pm, the metallic rattling could
   make raised voices necessary. For Wire spring relay markers these
   noises resembled hail falling on a metallic roof.

   On a pre-dawn Sunday Morning, call processing might slow to the point
   that one might be able to hear individual calls being dialed and 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 (120 ipm)
   and busy (60 ipm) signals. In Bell System installations, there were
   typically alarm bells, gongs, or chimes. These would annunciate alarms
   calling attention to a failed switch element. Another noisemaker:
   trouble reporting card systems were connected to switch common control
   elements. These trouble reporting systems would puncture cardboard
   cards with a cryptic code that logged the nature of a failure. Remreed
   technology in Stored Program Control exchanges finally quieted the
   environment.

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, metallic one.

   In all systems, subscribers were not supposed to notice changes in
   quality of service because of failures or maintenance work. A variety
   of 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 the switching logic to
   route around it. A similar tool was called a TD tool. 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). The subscriber 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 permanent signal (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. For
   example, a digit receiver (part of an element called an Originating
   Register) would be connected to a call just long enough to collect the
   subscriber's dialed digits. Crossbar architecture was more flexible
   than step offices. Later crossbar systems had punch-card-based trouble
   reporting systems. By the 1970s, automatic number identification had
   been retrofitted to nearly all step-by-step and crossbar switches in
   the Bell System.

Electronic switches

   The first electronic switches were not digital. The Western Electric
   1ESS was an electronic switch with metallic paths. It was
   stored-program-controlled. Changes to phone numbers, testing, or making
   circuits busy were accomplished by typing on a terminal. A 1ESS could
   use the normal electromechanical signaling methods used by crossbar and
   step-by-step switches. Northern Telecom SP1, Ericsson AKE, Philips
   PRX/A, ITT Metaconta, and several other designs were similar. These
   systems introduced a new form of data communications: 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 virtual
   circuits together, according to a dialed telephone number. Calls are
   setup between switches 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 multiplexing switching
   of digitized signals. All switches built since the 1980s are digital,
   so for practical purposes this is a distinction without a difference.
   This article 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 digital PCM 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 other words, when you use a telephone, you are generally
   having your voice "encoded" and then reconstructed for the person on
   the other end. Your voice 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 more info.)

   Individual local loop telephone lines are connected to a remote
   concentrator. In many cases, the concentrator is co-located in the same
   building as the switch. The interface between concentrators and
   telephone switches has been standardised by ETSI as the V5 protocol.

   Some telephone switches do not have concentrators directly connected to
   them, but rather are used to connect calls between other telephone
   switches. Usually a complex machine (or series of them) in a central
   exchange building, these are referred to as "carrier-level" switches or
   tandems.

   Some telephone exchange buildings in small towns now house only remote
   or satellite switches, and are homed "parent" switch, usually several
   kilometres away. The remote switch is dependent on the parent switch
   for routing and number plan information. Unlike a digital loop carrier,
   a remote switch can route calls between local phones itself, without
   using trunks to the parent switch.

   Telephone switches are usually owned and operated by a telephone
   service provider 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.

The switch's place in the system

   Telephone switches are a small part of a large network. The majority of
   work and expense of the phone system is the wiring outside the central
   office, or the Outside plant. In early incarnations, each subscriber
   telephone number required an individual pair of wires from the switch
   to the subscriber's phone. A typical central office may have
   tens-of-thousands of pairs of wires that appear on terminal blocks
   called the main distributing frame or MDF. A component of the MDF is
   protection: fuses or other devices that protect the switch from
   lightning, shorts with electric power lines, or other foreign voltages.
   In a typical telephone company, a large database tracks information
   about each subscriber pair and the status of each jumper. Before
   computerization of Bell System records in the 1980s, this information
   was handwritten in pencil in accounting ledger books.

   To reduce the expense of outside plant, some companies use " pair gain"
   devices to provide telephone service to subscribers. These devices are
   used to provide service where existing copper facilities have been
   exhausted or by siting in a neighbourhood, can reduce the length of
   copper pairs, enabling digital services such as ISDN or DSL. Pair gain
   or digital loop carriers (DLCs) are located outside the central office,
   usually in a large neighbourhood distant from the CO.

   DLCs are often referred to as Subscriber Loop Carriers (SLCs), after
   Lucent's proprietary name for their pair gain products. Early SLC
   systems (SLC-1) used an analog carrier for transport between the remote
   site and the central office. Later systems (SLC-96, SLC-5) and other
   vendors' DLC products contain line cards that convert the analog signal
   to a digital signal (usually PCM). This digital signal can then be
   transported over copper, fibre, or other transport medium to the
   central office. Other components include ringing generators to provide
   ringing current and battery backups.

   DLCs can be configured as universal (UDLCs) or integrated (IDLCS).
   Universal DLCs have two terminals, a central office terminal (COT) and
   a remote terminal (RT), that function similarly. Both terminals
   interface with analog signals, convert to digital signals, and
   transport to the other side where the reverse is performed. Sometimes,
   the transport is handled by separate equipment. In an Integrated DLC,
   the COT is eliminated. Instead, the RT is connected digitally to
   equipment in the telephone switch. This reduces the total amount of
   equipment required. Several standards cover DLCs, including Telcordia's
   TR/GR-008 & TR/GR-303.

   Switches are used in both local central offices and in long distance
   centers. There are two major types: 1. switches designed for toll or
   switch-to-switch connections, and; 2. subscriber switches, which manage
   connections from subscriber telephones and other switching systems.
   Since the 1990s, hybrid switching systems that serve both functions
   have become common.

   Another element of the telephone network is time and timing. Switching,
   transmission and billing equipment may be slaved to very high accuracy
   10 MHz standards which synchronize time events to very close intervals.
   Time-standards equipment may include Rubidium- or Cesium-based
   standards and a Global Positioning System receiver.

Switch design

   Long distance switches may use a slower, more efficient
   switch-allocation algorithm than local central offices, because they
   have near 100% utilization of their input and output channels. Central
   offices have more than 90% of their channel capacity unused.

   While traditionally, telephone switches connected physical circuits
   (e.g., wire pairs), modern telephone switches use a combination of
   space- and time-division switching. In other words, each voice channel
   is represented by a time slot (say 1 or 2) on a physical wire pair (A
   or B). In order to connect two voice channels (say A1 and B2) together,
   the telephone switch interchanges the information between A1 and B2. It
   switches both the time slot and physical connection. To do this, it
   exchanges data between the time slots and connections 8000 times per
   second, under control of digital logic that cycles through electronic
   lists of the current connections. Using both types of switching makes a
   modern switch far smaller than either a space or time switch could be
   by itself.

   The structure of a switch is an odd number of layers of smaller,
   simpler subswitches. Each layer is interconnected by a web of wires
   that goes from each subswitch, to a set of the next layer of
   subswitches. In most designs, a physical (space) switching layer
   alternates with a time switching layer. The layers are symmetric,
   because in a telephone system callers can also be callees.

   A time-division subswitch reads a complete cycle of time slots into a
   memory, and then writes it out in a different order, also under control
   of a cyclic computer memory. This causes some delay in the signal.

   A space-division subswitch switches electrical paths, often using some
   variant of a nonblocking minimal spanning switch, or a crossover
   switch.

Switch control algorithms

Fully-connected mesh network

   One way is to have enough switching fabric to assure that the pairwise
   allocation will always succeed by building a fully-connected mesh
   network. This is the method usually used in central office switches,
   which have low utilization of their resources.

Clos's nonblocking switch algorithm

   The scarce resources in a telephone switch are the connections between
   layers of subswitches. The control logic has to allocate these
   connections, and most switches do so in a way that is fault tolerant.
   See nonblocking minimal spanning switch for a discussion of Charles
   Clos's algorithm, used in many telephone switches, and arguably one of
   the most important algorithms in modern industry.

Fault tolerance

   Composite switches are inherently fault-tolerant. If a subswitch fails,
   the controlling computer can sense it during a periodic test. The
   computer marks all the connections to the subswitch as "in use". This
   prevents new calls, and does not interrupt old calls that remain
   working. As calls are ended, the subswitch then becomes unused. Some
   time later, a technician can replace the circuit board. When the next
   test succeeds, the connections to the repaired subsystem are marked
   "not in use", and the switch returns to full operation.

   To prevent frustration with unsensed failures, all the connections
   between layers in the switch are allocated using first-in-first-out
   lists. That way, a disgusted customer who hangs up and redials will get
   a different set of connections and subswitches. A last-in-first-out
   allocation of connections might cause a continuing string of very
   frustrating failures.

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