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Main article: "Signalling (telecommunications)

Circuits interconnecting 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 signalling communicated dialed digits. An early form called "Panel Call Indicator Pulsing used "quaternary pulses to set up calls between a "panel switch and a manual switchboard. 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 or office 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). Tip and ring were the voice-carrying pair, and named after the tip and ring on the three conductor cords on the manual operator's console.

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 acknowledgment 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 radio relays.

On "T-carrier digital transmission systems, bits within the T-1 data stream were used to transmit supervision. 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.


A 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 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 housed in prefabricated metal buildings. These facilities almost always had concrete floors. The hard surfaces reflected sounds.

During heavy use periods, it could be difficult to converse in a central 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 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 extent 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.

Bell System installations typically had alarm bells, gongs, or chimes to announce alarms calling attention to a failed switch element. A trouble reporting card system was connected to switch common control elements. These trouble reporting systems punctured cardboard "cards with a code that logged the nature of a failure. "Reed relay technology in "stored program control exchange finally quieted the environment.

Maintenance tasks[edit]

Electromechanical switching systems required sources of electricity in form of direct current (DC), as well as alternating ring current (AC), which were generated on-site with mechanical generators. In addition, telephone switches required adjustment of many mechanical parts. Unlike modern switches, a circuit connecting a dialed call through an electromechanical switch had DC continuity within the local exchange area via metallic conductors.

The design and maintenance procedures of all systems involved methods to avoid that subscribers experienced undue changes in the quality of the service or that they noticed failures. A variety of tools referred to as make-busys were plugged into electromechanical switch elements upon failure and during repairs. A make-busy identified the part being worked on as in-use, causing the switching logic to route around it. A similar tool was called a TD tool. Delinquent subscribers had their service temporarily denied (TDed). This was effected by plugging a tool into the subscriber's office equipment on Crossbar systems or line group in step-by-step switches. The subscriber could receive calls but could not dial out.

Strowger-based, step-by-step offices in the Bell System required continuous maintenance, such as 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[edit]

"Electronic switching systems gradually evolved in stages from electromechanical hybrids with "stored program control to the fully digital systems. Early systems used "reed relay-switched metallic paths under digital control. Equipment testing, phone numbers reassignments, circuit lockouts and similar tasks were accomplished by data entry on a terminal.

Examples of these systems included the "Western Electric "1ESS switch, Northern Telecom "SP1, Ericsson AKE, Philips "PRX/A, ITT Metaconta, "British GPO/BT "TXE series and several other designs were similar. Ericsson also developed a fully computerized version of their ARF crossbar exchange called ARE. These used a crossbar switching matrix with a fully computerized control system and provided a wide range of advanced services. Local versions were called ARE11 while tandem versions were known as ARE13. They were used in Scandinavia, Australia, Ireland and many other countries in the late 1970s and into the 1980s when they were replaced with digital technology.

These systems could use the old electromechanical signaling methods inherited from crossbar and step-by-step switches. They also 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. In European systems R2 signalling was normally used.

Digital switches[edit]

A typical satellite PBX with front cover removed.

Digital switches work by connecting two or more digital circuits, according to a dialed "telephone number or other instruction. Calls are set up between switches. In modern networks, this is usually controlled using the "Signalling System 7 (SS7) protocol, or one of its variants. Many networks around the world are now transitioning to "voice over IP technologies which use Internet-based protocols such as the "Session Initiation Protocol (SIP). These may have superseded TDM and SS7 based technologies in some networks.["citation needed]

The concepts of digital switching were developed by various labs in the United States and in Europe from the 1930s onwards. The first prototype digital switch was developed by "Bell Labs as part of the ESSEX project while the first true digital exchange to be combined with digital transmission systems was designed by LCT (Laboratoire Central de Telecommunications) in Paris.["citation needed] The first digital switch to be placed into a public network was the Empress Exchange in "London, England which was designed by the "General Post Office research labs.["citation needed] This was a tandem switch that connected three "Strowger exchanges in the London area. The first commercial roll-out of a fully digital local switching system was "Alcatel's E10 system which began serving customers in "Brittany in Northwestern France in 1972.["citation needed]

Prominent examples of digital switches include:

Alcatel developed the E10 system in France during the late 1960s and 1970s. This widely used family of digital switches was one of the earliest TDM switches to be widely used in public networks. Subscribers were first connected to E10A switches in France in 1972. This system is used in France, Ireland, China, and many other countries. It has been through many revisions and current versions are even integrated into "All IP networks.
Alcatel also acquired "ITT System 12 which when it bought ITT's European operations. The S12 system and E10 systems were merged into a single platform in the 1990s. The S12 system is used in Germany, Italy, Australia, Belgium, China, India, and many other countries around the world.
Finally, when Alcatel and Lucent merged, the company acquired Lucent's "5ESS and "4ESS systems used throughout the United States of America and in many other countries.
A digital exchange ("Nortel "DMS-100) used by an operator to offer local and long distance services in "France. Each switch typically serves 10,000-100,000+ subscribers depending on the geographic area

Digital switches encode the speech going on, in 8,000 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 someone uses a telephone, the speaker's voice is "encoded" then reconstructed for the person on the other end. The speaker's 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 remote concentrators and telephone switches has been standardised by "ETSI as the "V5 protocol. 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 connected to them, but rather are used to connect calls between other telephone switches. These complex machines (or a series of them) in a central exchange building are referred to as "carrier-level" switches or "tandem switches.

Some telephone exchange buildings in small towns now house only remote or satellite switches, and are homed upon a "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.

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

The switch's place in the system[edit]

Telephone switches are a small component of a large network. A major part, in terms of expense, maintenance, and logistics of the telephone system is "outside plant, which is the wiring outside the central office. While many subscribers were served with party-lines in the middle of the 20th century, it was the goal that each subscriber telephone station was connected to an individual pair of wires from the switching system.

A typical central office may have tens of thousands of pairs of wires that appear on terminal blocks called the "main distribution frame (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 neighborhood, can reduce the length of copper pairs, enabling digital services such as "Integrated Services Digital Network (ISDN) or "Digital Subscriber Line (DSL).

Pair gain or "digital loop carriers (DLCs) are located outside the central office, usually in a large neighborhood distant from the CO. DLCs are often referred to as "Subscriber Loop Carriers (SLCs), after a "Lucent proprietary product.

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.

Switches are used in both local central offices and in "long distance centers. There are two major types in the "Public switched telephone network (PSTN), the "Class 4 telephone switches designed for toll or switch-to-switch connections, and the "Class 5 telephone switches or subscriber switches, which manage connections from subscriber telephones. Since the 1990s, hybrid Class 4/5 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 Caesium-based standards and a "Global Positioning System receiver.

Switch design[edit]

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.

Traditional telephone switches connected physical circuits (e.g., wire pairs) while 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 8,000 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[edit]

Fully connected mesh network[edit]

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[edit]

Nonblocking minimal spanning switch

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 the Charles Clos algorithm, used in many telephone switches, and a very important algorithm to the telephone industry.

Fault tolerance[edit]

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 in progress end, the subswitch becomes unused, and new calls avoid the subswitch because it's already "in use." 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 (queues). As a result, if a connection is faulty or noisy and the customer hangs up and redials, they will get a different set of connections and subswitches. A "last-in-first-out (stack) allocation of connections might cause a continuing string of very frustrating failures.

Fire and disaster recovery[edit]

Second Avenue exchange, NYC

In July 1951, during "massive flooding in "Kansas and "Missouri, a manual switchboard in "Manhattan, Kansas was abandoned as water levels rose in the central office; operators regained access to the town's four trunk lines from a local "filling station on higher ground to send emergency messages and "radiotelephone was used to bypass damaged facilities.[19]

On "February 27, 1975 a fire at "New York Telephone's building at 204 "Second Avenue (at East 13th Street) in "Manhattan destroyed the "main distribution frame and damaged much of the underground cabling, "disconnecting 170,000 subscribers. This office connects many circuits to "Brooklyn which were disrupted. Equipment was redirected from other Bell System operating companies in multiple US states to establish temporary service and rebuild the destroyed exchange.[20]

In 1978, a central office fire in "Mebane, North Carolina knocked out every one of the small community's 3900 phones.[21]

In May 1988, a central office fire in the "Chicago suburb of "Hinsdale, Illinois knocked out 35,000 local subscribers, broke the link between the "FAA and "air traffic control at "Chicago O'Hare International Airport (then the world's busiest) and disrupted the Midwest's ability to communicate with the rest of the country. The office had a fire alarm but no "automatic fire suppression equipment. The facility was unattended and monitored remotely by an "Illinois Bell technician in "Springfield; it took an hour to notify firefighters of the blaze as the distant technician's attempts to call the fire department in Hinsdale did not get through. The fire had already knocked out the phone lines.[22]

In 1991, all twenty-eight exchanges serving "Kuwait were out of service in the wake of a 1990 "invasion by "Iraq; equipment had been looted and central offices destroyed. Service was initially restored via "satellite.[23]

On September 11, 2001 a "terrorist attack destroyed a central office in the "World Trade Center in "New York City and heavily damaged an adjacent exchange. The "Verizon Building at 140 West Street was restored by 3500 workers at a cost of "$1.2 billion,[24] after 200,000 voice lines and three million data circuits had been knocked out of operation.[25]

The central exchange, due to the system's design, is almost always a "single point of failure for local calls. As the capacity of individual switches and the "optical fibre which interconnects them increases, potential disruption caused by destruction of one local office will only be magnified. Multiple fibre connections can be used to provide redundancy to voice and data connections between switching centres, but careful network design is required to avoid situations where a main fibre and its backup both go through the same damaged central office as a potential "common mode failure.[22]

See also[edit]


  1. ^ "General Definitions". Verizon service. Verizon Enterprise Solutions. 
  2. ^ Private Telegraphs, "The Sydney Morning Herald, credited to "The Times, April 19, 1878, p. 6.
  3. ^ "Bo Leuf (2002). Peer to Peer: Collaboration and Sharing Over the Internet. Addison-Wesley. p. 15. "ISBN "9780201767322. 
  4. ^ Alvin K. Benson (2010). Inventors and inventions Great lives from history Volume 4 of Great Lives from History: Inventors & Inventions. Salem Press. p. 1298. "ISBN "9781587655227. 
  5. ^ TIVADAR PUSKÁS (1844 - 1893)
  6. ^ "SZTNH". Retrieved 2012-07-01. 
  7. ^ "Puskás, Tivadar". Retrieved 2012-07-01. 
  8. ^ "Welcome -". Archived from the original on 2012-03-16. Retrieved 2012-07-01. 
  9. ^ Frank Lewis Dyer: Edison His Life And Inventions. (page: 71)
  10. ^ 120 Year Telephone anniversary
  11. ^ See National Park Service "first switchboard" page.
  12. ^ "Early Manchester telephone exchanges" (PDF). Retrieved 2013-07-30. 
  13. ^ Francis S. Wagner: Hungarian Contributions to World Civilization - Page 68
  14. ^ a b Calvert, J. B. (2003-09-07). "Basic Telephones". Retrieved 2007-09-13. 
  15. ^
  16. ^ Calvert, J. B. (2003-09-07). "Basic Telephones, The Switchboard (ringdown is near bottom)". Retrieved 2006-09-13. 
  17. ^ Source: from "Federal Standard 1037C.
  18. ^ a b Connected to a switch, an off-hook condition operates a relay to connect the line to a dial tone generator and a device to collect dialed digits.
  19. ^ AT&T Tech Channel (2011-06-17). "AT&T Archives : Flood Waters". "AT&T. Retrieved 2013-07-30. 
  20. ^ "Miracle on Second Avenue: Reconnecting 170,000 Phone Customers in NYC After a Major Fire" (archive video, 0:22:40 including modern introduction). "AT&T. 
  21. ^ AT&T Tech Channel (2012-07-13). "AT&T Archives : The Town That Lost Its Voice". "AT&T. Retrieved 2013-07-30. 
  22. ^ a b Andrew Pollack (1988-05-26). "Phone System Feared Vulnerable To Wider Disruptions of Service" (PDF). New York Times. Retrieved 2013-07-30. 
  23. ^ "Operation Desert Switch" (archive video, 0:17:04). "AT&T. 1991. 
  24. ^ "Wall Street a year on: Annus horribilis". The Economist. 2002-09-05. Retrieved 2013-07-30. 
  25. ^ "Bond trading resumes, stocks remain on hold". The Mount Airy News. Sep 14, 2001. 

External links[edit]

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