Phone exchange is a telecommunication system used in public switched telephone networks or in large enterprises. The exchange consists of electronic components and in older systems as well as human operators that interconnect (switch switches) telephone subscriber lines or virtual circuits of digital systems to make phone calls between customers.
In a historical perspective, the term telecommunications has been used with different semantics over time. The term phone exchange is often used synonymously with headquarters (CO), the Bell System term. Often, a headquarters is defined as a building used to store in-house factory equipment from some potential phone exchanges, each serving a specific geographic area. Such areas are also referred to as exchanges. The location of the head office can also be identified in North America as a cable center, pointing to a facility from which the telephone receives a dial tone. For business and billing purposes, the telephone operator also defines the tariff centers, which in large cities may be a headquarters cluster, to determine a particular geographical location to determine distance measurement.
In the United States and Canada, the Bell System was established in the 1940s a uniform system for identifying headquarters with a three-digit headquarters code, used as a prefix for customer phone numbers. All headquarters in a larger area, usually collected by the state, are coded in the general numbering area. With the development of international and transatlantic phone lines, driven primarily by direct customer calls, similar efforts from systematic organization of telephone networks occurred in many countries in the mid-20th century.
For corporate or corporate use, private telephone exchange is often referred to as a private branch exchange (PBX), when it has a connection to a public switched telephone network. PBXs are installed in company facilities, usually housed with large office space or within the organization's campus to serve local personal phone systems and private leased line circuits. Smaller installations may deploy a PBX or a primary telephone system in the reception office.
Video Telephone exchange
History
In the era of electrical telegraph, post office, railway station, more important central government (ministry), stock exchange, very few newspapers distributed nationally, the most important international companies and the rich are the main users of the telegraph. Despite the fact that telephone devices existed before the invention of telephone exchanges, their success and economic operations would not have been possible on the same contemporary telegraph schemes and structures, such as before the invention of switchboard telephone exchanges, the initial phone had been implanted for and communicating with just one other phone (such as from one's home to that person's business).
The telephone exchange is a telephone system located at service centers (headquarters) responsible for a small geographic area that provides switching or interconnecting two or more individual customer lines for calls made between them, rather than requiring a direct connection between the customer stations. This allows customers to call each other at home, business, or public space. This makes the phone into a convenient communication tool available for everyday use, and it gives a boost to the creation of a new industry sector.
As with the invention of the phone itself, the honor of "first telephone exchange" has several claimants. One of the first to propose telephone exchange was Tivaman Hungary Puskás in 1877 when he worked for Thomas Edison. The first experimental telephone exchange was based on the ideas of Puskás, and it was built by the Bell Telephone Company in Boston in 1877. The world's first state-run telephone exchange opened on November 12, 1877 in Friedrichsberg close to Berlin under the direction of Heinrich von Stephan. George W. Coy designed and built the first US commercial telephone exchange opened in New Haven, Connecticut in January 1878. The switchboard was built from "train carriages, handles from the lid of the kettle and wire" and can handle two simultaneous conversations. Charles Glidden is also credited with building exchanges at Lowell, MA. with 50 customers in 1878.
In Europe, other early phone exchanges were based in London and Manchester, both opened under Bell patents in 1879. Belgium had its first Bell International bourse (in Antwerp) a year later.
In 1887 PuskÃÆ'¡s introduced a multiplex switchboard..
Then the exchange consists of one to several hundred plug boards managed by the switchboard operator. Each operator sits in front of a vertical panel that contains banks of inch-ring-sleeve (3-inch) jack jacks, each of which is the local stop of the subscriber's phone line. In front of the jack panel puts a horizontal panel containing two rows of patch cords, each pair connected to the wiring circuit.
When a calling party lifts the receiver, the local loop current turns on the signal light near the jack. The operator responds by plugging the back cable ( answering cable ) into the customer's jack and switching the headset to the circuit to ask, "Number, please?" For local calls, the operator inserts the front pair ( ringing cord ) into the so-called local jack and initiates the ringing cycle. For long distance calls, he attaches cables to the baggage circuitry to connect to other operators in other bank boards or at the remote headquarters. In 1918, the average time to complete the connection for long distance calls was 15 minutes.
The initial manual switchboard required operators to operate the keys and ringing keys, but in the late 1910s and 1920s, switchboard technology advances led to features that allow calls to be answered automatically as soon as the operator enters an answering cable, and the ringing will automatically start as soon as the operator inserts a cable that rings into the calling party's jack. The operator will be disconnected from the circuit, allowing it to handle another call, while the caller hears the audible ringback signal, so the operator does not need to periodically report that he keeps on the line.
In the ringdown method, the origin operator calls another intermediary operator who will call the referred customer, or forward it to another intermediary operator. This intermediate operator chain can complete calls only if an intermediate trunk line is available between all centers at the same time. In 1943 when a military call was given priority, a cross-country US call may take 2 hours to request and schedule in cities using a manual keypad for toll calls.
On March 10, 1891, Almon Brown Strowger, an administrator in Kansas City, Missouri, patented a stepped switch, a device that led to the automation of telephone circuit switching. Although there are many extensions and adaptations of these early patents, the best known consists of 10 levels or banks, each of which has 10 contacts arranged in a semicircle. When used with rotary dial phones, each pairs of numbers cause the central contact axis of the jumping switch to the first step (ratchet) rises one level for each pulse in the first digit and then swings horizontally in the line contact with one small spin for each pulse in the next digit.
Then the stepped switch is set in the bank, the first stage which is the linefinder . If one of a hundred subscriber lines has lifted the receiver "off hook", a linefinder connects the subscriber line to the first free picker, which returns the customer dial tone to indicate it is ready to receive the digits dialed. The subscriber calls pulsed at about 10 pulses per second, although the speed depends on certain telephone administration standards.
Exchange by Strowger switch is finally challenged by other types of exchange and then by crossbar technology. This exchange design promises faster switching and will receive pulses faster than the typical 10 pps Strowger - usually around 20 pps. In later days many also receive the "touch tone" DTMF or other tone signal system.
The transition technology (from pulse to DTMF) has a converter to convert DTMF into pulse, to feed it to an older Strowger switch, panel, or bar. This technology was used until mid 2002.
Maps Telephone exchange
Technology
Many of the terms used in telecommunications technology differ in meaning and usage among the various English-speaking regions. For the purposes of this article, the following definitions are made:
- Manual service is a condition in which human phone operators make calls into exchange without using a call.
- Call service is when the call exchange route by the switch interprets the digit dialed.
- The phone switch is the switching equipment of the exchange.
- The concentrator is a device that centralizes traffic, whether it is remotely or placed along with a switch.
- The off-hook condition represents the current series, for example, when a phone call is in progress.
- The on-hook condition represents the standby circuit, ie no phone call in progress.
- Cable center is the area served by a particular switch or headquarters.
Headquarters originally called switching equipment and its operators, is also commonly used for shaded buildings and in-plant equipment. In the US telecommunications jargon, a headquarters (C.O.) is a central switch operator of a Class 5 telephone switch where the local bars and loops are terminated and replaced. In the UK, a telephone exchange means an exchange building, and also a name for telephone switches. Manual service exchange
With service manual , the customer lifts the recipient in an off-hook state and asks the operator to connect the call to the requested number. Provided that the number is in the same head office, and located on the switchboard operator, the operator connects the call by connecting the ringing cable to the jack on the switchboard corresponding to the line of the called subscriber. If the so-called party channel is on a different keypad in the same office, or at a different head office, the operator plugged into the trunk for the board or the office of destination and requested the operator answering (known as the "B" operator) to connect the call.
Most urban exchanges provide general battery services, which means that the headquarters provides power to the subscriber telephone circuits for transmitter operation, as well as for automatic signaling with dialed calls. In a common battery system, a pair of wires from the customer's telephone to the stock carry a 48V (nominal) DC potential from the telephone company ending in the conductor. The phone presents an open circuit while being hooked or idle.
When the customer's phone is out-hooked, it presents an electrical resistance in the line that causes the current to flow through the phone and cable to the central office. In a manually operated switchboard, this current is flowed through the relay coil, and moves the buzzer or lamp on the switchboard operator, indicating the operator to perform the service.
In the largest cities, it takes years to turn every office into automatic equipment, such as a panel switch. During this transition period, after the number has been standardized into 2L-4N or 2L-5N formats (two-letter and four- or five-digit exchange names), it is possible to dial the number located in the manual exchange and connect without requesting operator assistance. Bell System Policy states that customers in big cities do not have to worry about the type of office, whether they call the manual or the office automatically.
When the customer dials the station number manually, the operator in the destination office answers the call after seeing the number on the indicator, and connects the call by inserting the cable to the outgoing circuit and calling the destination station. For example, if a calling customer calling from TAYLOR 4725 dialed a number served by a manual exchange, for example, ADAM 1383-W, the call was completed, from a customer perspective, exactly as a call to LEnnox 5813, in an automated exchange. The party letters W, R, J, and M are used only in manual exchanges with the jack-per-line party line.
Unlike the listing format of MAin 1234 for a two-letter auto office, a manual office, having a list like Hillside 834 or East 23, can be identified by a format in which the second letter is not capitalized.
The rural areas, as well as the smallest cities, have manual and signaling services achieved with a magneto phone, which has a crank for signal generators. To remind the operator, or another customer on the same channel, the customer cranked the crank to generate a ringing current. The switchboard responds by interrupting the circuit, which drops the metal tab above the subscriber line jack and the bell sounds. Dry cell battery, usually two N Â ° large. 6 cells in the customer's phone, providing direct current to the transmitter. Such a magneto system was used in the United States in late 1983, as in a small town, Bryant Pond, Woodstock, Maine.
Many small town magneto systems feature party lines, anywhere from two to ten or more subscribers sharing a single line. When calling a party, the operator uses a ringing code, a different ring signal sequence, such as two long rings followed by one short ring. Everyone on the phone can hear the signal, and can pick up and monitor other people's conversations.
Initial auto exchange
Automatic exchange , or call service , appeared in the early 20th century. Their goal is to eliminate the need for human switchboard operators who complete the connection required for phone calls. Automation replaces human operators with electromechanical systems and the phone comes with a dial where the caller sends the destination phone number to the automatic switching system.
The telephone exchange automatically senses the off-hook condition of the phone when the user removes the handset from a switchhook or cradle. The exchange provides a dial tone at that time to show the user that the exchange is ready to receive the digits being contacted. The pulse or DTMF tone generated by the phone is processed and the connection is made to the destination phone in the same exchange or to another remote exchange.
Exchange maintains a connection until one party hangs up. Monitoring this connection status is called surveillance. Additional features, such as billing tools, can also be put into exchange.
The Bell System dial service implements a feature called automatic number identification (ANI) that facilitates services such as automated billing, toll-free numbers 800, and 9-1-1 services. In the service manual, the operator knows where the call comes from the lamp in the switchboard jack field. Before ANI, long distance calls are placed into the queue of operators and the operator asks the number of the calling party and records them on the paper toll ticket.
The initial exchange is an electromechanical system using motors, drive shafts, rotating switches and relays. Some types of automatic exchanges are Strowger switches or step-by-step switches, All Relays, X-Y, switch panels, Rotary systems and crossbar switches.
Electromechanical Signals
Circuit interconnection switch is called trunks . Prior to Signaling System 7, the Bell System electromechanical switch in the United States communicated with each other over the rod using a variety of DC voltages and signal tones. It is rare to see it used today.
Some signals communicated dialed digits. The initial form called the Pulsing Panel Call Indicator uses a quartener pulse to set the call between the switch panel and the manual switchboard. Probably the most common form of communicating the scattered digit between the electromechanical switch is to send a call pulse, equivalent to pulsing pulsar pulsing, but sending over trunk circuits between switches.
In Bell System trunks, it is common to use 20 pulses per second between bar switches and crossbar bars. This is twice the rate of telephone calls Electrical System/West Bell. Using faster pulsation speeds makes baggage utilization more efficient because the switch spends half the time listening digit. DTMF is not used for signaling trunk.
Multi-frequency (MF) is the last of the pre-digital methods. It uses a set of different tones sent in pairs like DTMF. The invocation is preceded by a special keypulse (KP) signal and followed by start (ST). Variation of MF scheme The Bell System becomes the CCITT standard. Similar schemes are used in America and in several European countries including Spain. The string digits between switches are often abbreviated to improve further utilization.
For example, one button may only send the last four or five digits of a phone number. In one case, the seven digit number is preceded by a digit 1 or 2 to distinguish between two area codes or office codes (two digit savings per call). This increases the revenue per stem and reduces the number of required digit receivers in a switch. Each task in electromechanical switches is done in large metal pieces of hardware. Each second intersect cut off the interrupted call time means less rack of equipment to handle call traffic.
Examples of signals that communicate oversight or call progress include E and M signaling, SF signaling, and robbed-bit signaling. In the physical (not the carrier) E and the M rods, the rod is the four wires. Fifty bars will require a pair of hundred cables between switches, for example. The conductor in a common circuit configuration is given the name of the tip, ring, ear (E) and mouth (M). Tip and ring are sound carrier pairs, and are named from the ends and rings on three conductors in the manual operator console.
In a two-way rod with E and M signaling , a handshake occurs to prevent both switches from colliding with a calling call on the same rod at the same time. By changing this state of lead from ground to -48 volts, the switch goes through the handshake protocol. Using a DC voltage change, the local switch will send a signal to get ready for the call and the remote switch will reply with the acknowledgment to continue with the call pulsing. This is done with discrete and relay electronic logic.
This voltage change in the trunk circuit will cause pops or clicks that can be heard by the customer because electrical handshaking stepped through the protocol. Another handshake, to start time for billing purposes, causes a second set of blobs when the called party answers.
The second common form of signaling for monitoring is called single frequency or SF signaling . The most common form of this uses a 2,600Hz stable tone to identify luggage as idle. Circuit channels that hear 2,600 Hz sounds for a given duration will be wasted. (Needs of duration reduce counterfeiting.) Some systems use tone frequencies over 3,000 Hz, especially at the frequency of SSB division multiplex microwave radio relay.
In the T-carrier digital transmission system, bits in the T-1 data stream are used to transmit surveillance. With careful design, the corresponding bits do not change the sound quality significantly. Robbed bits are translated to changes in contact status (open and closed) by electronics in the channel's bank hardware. This allows direct current E and M signaling, or call pulses, to be sent between electromechanical switches through digital carriers that do not have DC continuity.
​​Sound
Characteristics of electromechanical switching equipment is that maintenance staff can hear mechanical rattling of Strowgers, switch panels or bar relays. Most Bell System headquarters are housed in reinforced concrete buildings with concrete ceilings and floors.
In rural areas some of the smaller switching facilities, such as community call offices (CDOs), are housed in prefabricated metal buildings. This facility almost always has a concrete floor. The hard surface reflects the sound.
During periods of heavy use, it may be difficult to speak in the central office switch room because the call clang is being processed in a large switch. For example, on Mother's Day in the US, or on Friday afternoons around 5 pm, a metal clatter can make the necessary sound. For wire spring relay markers, these sounds resemble hail that falls on a metallic roof.
On early morning Sundays, call processing may slow down if someone may be able to hear the individual calls being dialed and set up. There were also voices from the roar of electricity wailing and the buzzing of the buzzing generator. Some systems have a continuous and rhythmic "clack-clack-clack" of wire-spring relays that make reorder (120 ipm) and busy signals (60 ipm).
Installing the Bell System usually has an alarm, gong, or bell to announce an alarm that draws attention to a failed switch element. Card system reporting problems connected to change common control elements. The reporting system of this problem punctures card cards with code that records the nature of the failure. Reed relay technology in the exchange of stored program controls ultimately calms the environment.
Maintenance task
Electromechanical switching systems require a power source in the form of direct current (DC), as well as alternating current (AC), which is generated in locations with mechanical generators. In addition, telephone switches require adjustment of many mechanical parts. Unlike modern switches, the circuit connecting outgoing calls via an electromechanical switch has DC continuity in the local exchange area through a metal conductor.
The design and maintenance procedures of all systems involved methods to avoid customers undergoing undue changes in service quality or that they see failure. The various tools referred to as make-busy s are plugged into an electromechanical switch element after failure and during repair. Make-busy identifies the part that is being worked on as being used, causing the switching logic to route around it. The same tool is called the TD tool. Customers who are late in subscribing to their services are denied temporary (TDed). This is done by inserting the tool into the customer's office equipment on a Crossbar system or channel group in a step-by-step switch. Customer may receive a call but can not make outbound calls.
Strowger based step-by-step office in Bell System requires ongoing maintenance, such as cleaning. The indicator lights in the equipment room at the step office alert staff to conditions such as blown fuses (usually white lights) or permanent signals (jam conditions, usually green indicators). Office steps are more susceptible to one-point failures than newer technologies.
Crossover offices are used more together, general control circuits. For example, a digit receiver (part of an element called Original ) will connect to the call long enough to collect customer call digits. The crossbar architecture is more flexible than the step office. The crossbar system then has a punch card-based problem reporting system. In the 1970s, automatic number identification was installed to almost all step-by-step and bar switches in the Bell System.
Electronic switch
Electronic switching systems are gradually evolving gradually from electromechanical hybrids with program controls stored to a fully digital system. The initial system uses a reed relay-switched metal line under digital control. Equipment testing, telephone number assignment, circuit locking and similar tasks are completed with data entry at the terminal.
Examples of this system include Western Electric 1ESS switch, Northern Telecom SP1, Ericsson AX, Philips PRX/A, ITT Metaconta, British GPO/BT TXE series and some other similar designs. Ericsson also developed a computerized version of their ARF crossbar called ARE. It used a switching matrix of bars with a fully computerized control system and provides a variety of advanced services. The local version is called ARE11 while the tandem version is 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.
The system can use the old electromechanical signaling methods inherited from the crossbar switch and step by step. They also introduced a new form of data communication: two 1ESS exchanges can communicate with each other using a data link called Common Channel Interoffice Signaling (CCIS). This data link is based on CCITT 6, the predecessor for SS7. In the European system R2 signaling is usually used.
Digital switch
The digital switch works by connecting two or more digital circuits, according to the phone number being called or other instructions. Calls are set between switches. In modern networks, this is usually controlled using the Signaling System 7 (SS7) protocol, or one of its variants. Many networks around the world are now turning to voice over IP technology that uses Internet-based protocols such as Session Initiation Protocol (SIP). This may have replaced TDM and SS7 based technologies across multiple networks.
The concepts of digital switching were developed by various laboratories 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 a digital transmission system was designed by LCT (Laboratoire Central de Telecommunications) in Paris. The first digital switch placed into the public network was the Empress Exchange in London, England which was designed by the Postal General's research laboratory. This is a tandem switch that connects three Strowger exchanges in the London area. The first commercial roll-out of a fully local digital switching system was Alcatel's E10 system that began serving customers in Brittany at Northwestern France in 1972.
Prominent examples of digital switches include:
- The AX Ericsson phone exchange is the most widely used digital diversion platform in the world and can be found throughout Europe and in most countries around the world. It's also very popular in mobile apps. This highly modular system was developed in Sweden in the 1970s as a replacement for Ericsson's popular range of ARF, ARM, ARK, and ARE switches used by many European networks from the 1950s onwards.
- Alcatel-Lucent inherits three of the world's most iconic digital redirection systems: Alcatel E10, 1000-S12, and Western Electric 5ESS. Alcatel developed the E10 system in France during the late 1960s and 1970s. This widely used family of digital switches is one of the earliest TDM switches used widely in the public network. Customers were first connected to the E10A switch in France in 1972. This system is used in France, Ireland, China, and many other countries. It has been through many revisions and the current version is even integrated into All IP networks.
- Alcatel also acquired ITT System 12 which was then buying ITT operations in Europe. The S12 and E10 systems were merged into one 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 joined, the company acquired the 5ESS and 4ESS Lucent systems used throughout the United States and in many other countries.
- Nokia Siemens Networks EWSD originally developed by Siemens, Bosch, and DeTeWe for the German market is used worldwide.
- Nortel now Genband DMS100 is very popular with operators worldwide.
- GTD-5 EAX is developed by GTE Automatic Electric
- NEC NEAX is used in Japan, New Zealand, and many other countries.
- Marconi System X originally developed by GPT and Plessey is a type of digital exchange used by BT Group in the UK pay phone network.
- Telecommunication history
- List of phone switches
- Partner pairing system
- Full availability, limited availability and gradient
- Softswitch
- Plesiochronous digital hierarchy
- Name of telephone exchange
- Faraday Building - The first telephone exchange in the UK
- Telephone Center History and Drawings
- Building World Head Office Building
- A Clive Feather guide to the BT network
- Guide Roger W. Haworth for the Director of Exchange Name London (United Kingdom)
- patent 252,576 for first telephone switchboard in 1881
- The Telecommunications Exchange Tour in NZ
The digital switch encodes the ongoing speech, in 8,000 times the slice per second. At each slice, a digital PCM representation of the tone is made. The numbers are then sent to the receiving end of the line, where the opposite process occurs, to generate sound for the receiving phone. In other words, when someone uses the phone, the speaker's voice is "encoded" and then reconstructed to the person on the other end. The speaker's voice is delayed in the process by a fraction of a second - it's not "alive", it's reconstructed - just delayed meticulous. (See below for more info.)
Local individual loop phone lines connect to a remote concentrator. In many cases, the concentrator is located in the same building as the switch. The interface between the remote concentrator and the telephone switch has been standardized by ETSI as protocol V5. Concentrators are used because most phones are idle for most of the day, so traffic from hundreds or thousands of them can be concentrated into just tens or hundreds of joint connections.
Some telephone switches do not have concentrators directly connected to them, but are more used to connect calls between other telephone switches. These complex machines (or a series of them) in the central building of the exchange are referred to as "carrier-level" switches or tandem switches.
Some telephone exchange buildings in small towns now only use the remote or satellite switch, and are paired on the "parent" switch, usually several kilometers away. The remote switch depends on the master switch for routing and the plan number information. Unlike digital loop introductions, remote switches can route calls between the local phone itself, without using a trunk to the master switch.
Telephone switches are typically owned and operated by the telephone service provider or operator and located on-site, but sometimes individual businesses or private commercial buildings will place their own switches, called PBXs, or private branch exchanges.
System switch place
The telephone switch is a small component of a large network. The main part, in terms of cost, maintenance, and logistics of telephone systems outside the factory, which is the cable outside the headquarters. While many customers are served with party-lines in the mid-20th century, it is the goal that every customer's phone station is connected to a pair of individual cables from the switching system.
Typical headquarters may have tens of thousands of pairs of cables that appear on the terminal block called the main distribution frame (MDF). The MDF component is protection: a fuse or other device that protects the switch from lightning, shorts with power cords, or other foreign stresses. In a payphone company, a large database keeps track of information about each customer's pairs and the status of each jumper. Before the computerization of the Bell system recorded in the 1980s, this information was handwritten with pencils in accounting ledgers.
To reduce the cost of outsourced factories, some companies use "profit-sharing" devices to provide telephone services to customers. This device is used to provide services where existing copper facilities have been exhausted or with environmental placements, can reduce copper pair length, enable digital services such as Integrated Services Digital Network (ISDN) or Digital Subscriber Line (DSL).
Reinforcement pair or digital loop introduction (DLC) is located outside the head office, usually in a wide environment away from CO. DLC is often referred to as Customer Loop Carriers (SLC), after Lucent's exclusive product.
DLCs ​​can be configured as universal (UDLCs) or integrated (IDLCs). Universal DLC has two terminals, a central office terminal (COT) and a remote terminal (RT), which functions the same. Both terminal interfaces with analog signals, converted into digital signals, and transported to the other side where the reverse is done.
Sometimes, transportation is handled by separate equipment. In Integrated DLC , COT is omitted. Instead, the RT is digitally connected to the equipment on the telephone switch. This reduces the total amount of equipment needed.
Switches are used both at the local headquarters as well as in the remote centers. There are two main types in the public switched telephone network (PSTN), a Class 4 telephone switch designed for toll connections or switch-to-switch, and a Class 5 switch or a customer switch, which manages connections from subscriber phones. Since the 1990s, a Class 4/5 hybrid switching system that serves both functions has become common.
Another element of the telephone network is time and time. Moving, transmitting, and billing equipment can be shifted to a very high 10 MHz accuracy standard that syncs time events to very close intervals. Time standard equipment may include Rubidium or Cesium-based standards and a Global Positioning System receiver.
Switch design
Remote switches can use slower allocation flow algorithms that are slower and more efficient than the local headquarters, as they have almost 100% of their input and output channel utilization. Headquarters has more than 90% of their unused channel capacity.
Traditional telephone switches connect physical circuits (eg, cable pairs) while modern telephone switches use a combination of space and time switching. In other words, each voice channel is represented by a time slot (say 1 or 2) on the physical cable pair (A or B). To connect two voice channels (say A1 and B2) together, the telephone switch exchanges information between A1 and B2. It switches both time slots and physical connections. To do this, exchange data between time and connection slots 8,000 times per second, under the control of digital logic that rotates through the electronic list of current connections. Using both types of switches makes the modern switch much smaller than the space or time switch can be by itself.
The switch structure is an odd number of smaller and simpler subdivision layers. Each layer is interconnected by the wired network that runs from each subswitch, to a substritch layer next set. In some designs, the physical layer switching (space) alternates with the time transfer layer. The layer is symmetrical, because in the caller's phone system can also be called. Other designs only use time-switching, across switches.
The division sub-division reads the complete cycle of time slots into memory, and then writes them in different order, also under the control of cyclic computer memory. This causes some delay in the signal.
The space division substructure alters the electrical path, often using some variant of the minimum blocking switch that is not blocking, or the crossover switch.
Divert algorithm control
Built-in mesh network
One way is to have enough switching fabric to ensure that pairwise allocation will always work by building a fully connected mesh network. This is a method commonly used in central office switches, which have a low utilization of their resources.
The nonblocking switch algorithm Clos
The scarce resource in telephone switches is the connection between the sub-substructure layers. The control logic should allocate this connection, and most switches do it in an error-tolerant way. See nonblocking minimum spanning switch for discussion of Charles Clos algorithm, used in many telephone switches, and algorithms that are essential for the phone industry.
Fault tolerance
Composite switches are essentially fault tolerant. If a subswitch fails, the controlling computer can sense it during the periodic test. The computer marks all connections to the subswitch as "in use". This prevents new calls, and does not interfere with old calls that continue to work. When the call is in progress, the subswitch becomes unused, and the new call avoids the subswitch as it is "used." Some time later, a technician can change the circuit board. When the next test succeeds, the connection to the repaired subsystem is marked "not in use", and the switch returns to full operation.
To prevent frustration with uncensored failures, all connections between layers in the switch are allocated using the first incoming list (queue). As a result, if the connection is broken or noisy and the customer closes and reconnects, they will get a different set of connections and subswitches. The last allocation (pile) of connections can lead to a series of very frustrating failures.
Fire and disaster recovery
The central exchange, due to system design, is almost always a single point of failure for local calls. As the capacities of each switch and the associated fiber optic connections increase, the potential disturbance caused by the destruction of a local office will only be enlarged. Some fiber connections can be used to provide redundancy to voice and data connections between switching centers, but careful network design is needed to avoid situations where the main fiber and its backup passes through a faulty headquarters equal to the possibility of common mode failures.
See also
References
External links
Source of the article : Wikipedia
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