Telecom switching (telephone exchange)

telecommunications switching

Telecommunications switching , also known as telephone switching or telephone switching . In telecommunications, a telephone switch is the electronic system that enables a connected telephone to communicate. The central processing room is the place where various equipment (including telephone switching machines) are placed, which connects telephone lines and transmits call information, also known as telephone building or telephone room .

An old manual telephone switchboard

Modern computer automatically controls fiber optic network switch

Operation Mechanism

In telecommunications , an exchange or central office is used to house switches that connect telephone calls . It establishes links and relays voice messages, enabling phone calls to function. Telecommunications switching refers to the services provided by switches.

Many telecommunications exchanges are long-distance offices that route long-distance calls. Each long distance office is assigned a two or three digit area code. In the past, the area code corresponded to the name of the exchange, for example, 869-1234 represents TOWnsend 9-1234, and in some places it may also represent TOWnsend 1234 (only uppercase letters and numbers are dialed when dialing).

history

Manual telephone operators are connecting telephones by unplugging and plugging wires on telephone switchboards.

In 1878, the first telecommunications exchange opened in New Haven , Connecticut . The switch board is made with "body screws", the microphone cover and telephone cord are attached to the handle. It can handle two sessions at the same time. (See U.S. National Park Service ( page archive backup , stored at)).

Telephone operators then work on combinations of one to several hundred patch panels . There may be one to three patch panels around each operator's seat, with rows of ¼-inch headphone jacks on the board and telephone lines on the front. Each line is the endpoint of a telephone line. When the caller lifts the phone, a light will light up near the corresponding socket, and the operator can then pick up the handset and ask "what's the phone number?". Then, if the phone number is within his range, a plug is inserted into a local jack, and the called party starts ringing. If it's not under his jurisdiction, he plugs into a handover circuit. At this time, the operator who may be in front of the other connection board, or the operator in another room thousands of miles away, will proceed to the next step.

On March 10, 1891, Tobeka, Kansas undertaker Almon Strogg invented the Strogg switch, also known as the step switch. This device brought circuit-switched telephone technology into the age of automation. Although there are many extensions and improvements on the basis of the original patent, the most well-known design is to arrange 10 layers of contactors, each with 10 different outputs, to form an arc, and then set a center contact connected to the input at the center of the circle. device. When used for dialing a telephone, every time two numbers are dialed, the first number makes the center contactor rise and fall to the corresponding floor, and the second number makes it rotate to one of the 10 contacts corresponding to the floor.

These step switches are arranged in a row, the first of which is used to detect "off-hook" subscriber lines, called a "line finder", and can detect up to 100 lines. A line finder connects the subscriber's line to a "dial tone" line to let the subscriber know that they are ready to dial. Then the user performs pulse dialing at a speed of about 10 pulses per second (different countries have different regulations).

Switches based on the Strogg design were later challenged by crossbar (aka crossbar) technology. The new telephone exchange guaranteed faster switching speeds and could accept dialing speeds of about 20 pulses per second, about twice as fast as Strogg's 10 pulses per second. Solid-state relay switches using dual-tone multi-frequency (DTMF) signaling (which could support touch-tone dialing) were the last winners before crossbars completely replaced steppers. In China, although several types of crossbar switches (some of which were developed by ourselves) were put into use, they were replaced by more advanced program-controlled switches before they became popular.

There is a transition from pulse dialing to DTMF using a DTMF "connection finder" that converts DTMF to pulses and sends them to older stepper or crossbar switches. This technique was still used until the late 1990s.

In December 1930, New York City became the first area in the United States to use the two-letter + five-number format, and it remained the only one until World War II . This format has since been adopted elsewhere in the United States (some areas such as Florida from the 1930s to the early 1950s had six-digit phone numbers, two letters representing the switch name followed by four numbers, such as "DUnkirk 0799"). Until the mid-1950s, the digits immediately following the name were never "0" or "1", and in fact "0" was not used at all except in the immediate vicinity of Los Angeles (the "BEnsonhurst 0" phone number prefix is ​​used in The Honeymooners. appeared as a fictional detail in a famous TV sitcom ).

In 1955, the Bell System attempted to standardize named area codes by organizing names for various number combinations into a "suggested name list". In 1961, the New York Telephone Company introduced a "chosen letter" switch, where the first two letters no longer begin any given name (e.g. "FL 6-9970"), followed in 1965 by all new telephone numbers nationwide containing numbers only ( Wichita Falls was the first in the US to do this, back in 1958). But until the 1970s, existing numbers continued to use the old method. A Chicago rug dealer often advertises on WGN radio with his phone number "NAtional 2-9000" (National 2-9000); not to mention that TYler 8-7100 (Tyler Village, a rural Texas town) is the name of a Detroit builder up.

Until the implementation of Subscriber Trunk Dialling (STD) in 1958, telephone numbers with letters were not used in most parts of the UK . Alpha dialing is only used in guided areas (Birmingham, Edinburgh, Liverpool, Glasgow, London and Manchester) and non-guided areas not adjacent to them, and these exchanges use a three-letter, four-digit format , until 1968 all changed to numbers.

Phone Number Fun Facts

Because the wiring between switches was fixed and difficult to rewire, some large metropolitan exchanges were used exclusively to handle calls with the same first two or three numbers in the North American standard 7-digit phone number. After the days of hand-switching at the patch panel, exchanges often used the first two letters of the exchange name as a numerical prefix for that exchange in ordinary telephone calls. For example CAstle (22, Castle), TRinity (87, Trinity ), Mutual (68, Union). Certain number combinations did not obey this naming convention, such as "57", "95", and "97", which was one of the factors that led to the eventual deprecation of the naming system. Another factor is the need for more numbers in case there aren't enough area codes .

A phone number with many 8's, 9's or 0's means it takes a long time to dial because the pulse of the step switch is time consuming. The phone company usually assigns these "big" numbers to payphones because they are usually rarely called.

To test the basic functionality of all switches in the link, all 5's (555–5555) are reserved as special "test" numbers (located right up, down, left, and right in the middle of the switch row). The "555" exchange number was never actually assigned a number, which is why today's TV and movie programs always use 555-xxxx numbers (in the past, these programs often used a specific combination of the last four digits to express something. For example, the West Coast uses "0079" and many other places use "9970". Examples include the TV series " Perry Mason " and the 1948 film "Sorry, Wrong Number"). In this way, it is impossible to use the fake number in the program to really dial someone, thus avoiding many misunderstandings. In 1982, the lyrics of the song 867-5309/Jenny by rock musician Tommy Tutone implied that 867-5309 was the phone number of a girl named Jenny. As a result, it became fashionable to call this phone number with the area codes of various places to say hello to Jenny, so many User receives many unwanted calls. In fact, many US phone companies either no longer use the number or keep it for testing purposes.

However, today only numbers starting with 555-01xx are available for fiction, and other 555 numbers can be assigned to "information providers" for use. A side effect is that the number of fictional numbers available is reduced to only 100, so the numbers used in movies and TV are also used. "958" and "959" exchange offices are also reserved in most territories, so very few companies or individuals use numbers starting with these digits (although this fact is not known to many people, so few people use these numbers in works of fiction) .

"PEnnsylvania 6-5000" (Pennsylvania), performed by the Glenn Miller Orchestra, just means the number of the Pennsylvania Hotel in New York. If you want to call this number, dial (212) 736-5000 instead, and you can find the hotel as well.

technology

This article uses the following terms:

• Manual connection service means that the operator manually connects the call to the exchange without dialing.
• Dial-up service Refers to the switching mode in which the dialed number is automatically connected by the switchboard.
• Telephone Exchange Refers to the room where equipment is placed and operated.
• Telephone exchange refers to the equipment used in the exchange process.
• Concentrator Refers to the concentrator itself, whether integrated in the switch or not.
• On-hook Refers to the state that the circuit is "in use".
• On-hook means that the circuit is in an idle state.

Manual Wiring Service

phone switch

In manual service , the user goes off-hook and asks the operator to connect to a number. If the called number is in the same central office, the operator establishes the call by plugging a line into the jack on the exchange that corresponds to the called line. If the call is to another central office, the operator inserts the other office's trunk line and asks that office operator to answer (called a "transfer" operator) and establish the call.

Most manual switching offices in major cities were common, and telephone lines were fed from central offices, as is the case today. When the user lifts the phone, the line status changes to on-hook, the light on the switchboard of the operator will light up, and the buzzer will sound. In a general power sharing system, a pair of cables from a user's telephone set to an exchange (or a manual exchange) is open when the telephone is on-hook or idle. There is a voltage of 48 volts DC across the wire from the telephone exchange to the endpoint. When the user goes off-hook, the phone applies a DC impedance between the two wires. During a manual exchange, current through the on-hook state flows through the buzzer and lights on the operator's console. The buzzer and light tell the operator that a customer is on the phone to request service. If it is connected to the exchange, when the phone is picked up, a relay will be closed to connect the subscriber line to the dial tone and the number receiving device.

In smaller towns, magnet phones , or crank phones, were generally used in the early days. The user turns the rocker to send an electric current to activate the call appearance, notifying the operator of an incoming call. A battery in the user's home provides the current required for the session. Magnet systems were still in use in some small towns in the United States until the late 1980s.

In a large city like New York City with hundreds of central offices, it would take many years to convert all of them from manual to dial-up service. In this process, in order to automate the transfer to the manual service, a special exchange is used which displays the number dialed by the user. For example, if a user at the MUrray Hill exchange picked up the phone and dialed a user at the CIty Island exchange, the user would not need to know that the destination phone number was manually wired. Dialing this number connects you to an incoming operator at the CIty Island switchboard, who can see the phone number from the switchboard and plug the line into the appropriate jack.

Program-controlled switching before digitization

Program-controlled switching , or dial-up service, has been around since the early 1900s. The aim is to reduce human telephone operators. Before the advent of program-controlled switching, operators had to make connections for every telephone call. Operators are now replaced by program-controlled switches almost everywhere. The telephone exchange is the nerve center of the program-controlled exchange. It routes a call from one telephone to another and is generally part of the Public Switched Telephone Network (PSTN).

The local exchange automatically senses the off-hook status of a phone , provides a dial tone for that phone, accepts pulse or DTMF tones, and then connects the incoming call to the called phone on the same exchange, or another exchange at a remote location.

The switch then remains connected until one party hangs up, at which point the connection is broken. Tracking the state of a connection is called monitoring. It is also possible to allow the switch to integrate more functional components, such as billing equipment.

The Bell System's dialing service implemented a feature called Automatic Number Identification (ANI). ANI can support services such as automatic billing, toll-free 800 numbers, and 911 (emergency callers). In the manual service, the operator can determine who is calling by the light on the switchboard near which earphone jack. There was no ANI in the early dial-up service. Long distance calls are handled by a team of operators who ask for the calling party's phone number, which is then written on a paper ticket.

Early switches included motors, cams, rotary switches, and relays . In a way, a switch is a computer for relay logic. There are some types of automatic switches, such as Strow format (also known as step switch), full relay type, XY type, switchboard type and crossbar switch, collectively referred to as electromechanical switches.

electromechanical signaling

The circuit connecting two switches is called an interoffice trunk . Prior to SS7, Bell System 's electromechanical switches used a variety of voltages and tones on mainlines in the United States. These are rarely seen today.

Certain signaling is communicated by dialed digits . An early form was called a switchboard call indication pulse , which used some pulses in a non-standard format to set up a call between two switchboard switches. Perhaps the more common form of dialed digital communication between electromechanical switches is to send a dial pulse, which is equivalent to a dial pulse sent in reverse, but on the trunk circuit between the switches. On Bell System trunk lines, crossbar switches and crossbars typically transmit at 20 pulses per second. This is twice as fast as dialing a Nortel/Bell system telephone. Since switches spend half the time listening for digits, using a faster pulse rate allows the trunk infrastructure to operate more efficiently. DTMF is not used for trunk signaling. Before digitization, multi-frequency (MF) was the last form of signaling. It uses a different set of tones than DTMF, but also in pairs. Dial a special key pulse (KP) signal before dialing, and add a start (ST) after dialing. One chime of the Bell System MF tone system became the CCITT standard. The same system is also used in the United States and some European countries, including Spain. Strings of numbers transmitted between switches are often abbreviated to improve functionality. For example, a switch might only send the last four or five digits of a phone number. In one case, the seven-digit number is preceded by one or two digits to distinguish between two area codes (saving two digits per call). This can increase trunk efficiency and reduce the number of digital transceivers required on the switch. Every electromechanical switch is made of many huge metal parts. If every second can be divided in half, you can use fewer racks to handle more traffic.

Other signaling used for communication monitoring or call process includes E and M signaling , single frequency signaling, robbed bit signaling (RBS) and so on. In terms of physical connection rather than the number of transmitted signal paths, there are four lines in the main line of E and M signaling. Fifty trunks between switches would then require one hundred pairs. These four lines are named "top, bell, eye, ear" respectively. When using E and M signaling , a handshake process is required to prevent two switches from initiating dial-up calls on the same trunk line at the same time. The handshake protocol begins when the line voltage transitions from ground to -48 volts. Through the DC voltage change, the local switch sends a signal that it is ready to accept calls, and the remote switch replies with an acknowledgment that it is about to send a dial pulse. This is accomplished with relay logic and discrete electronic snow. These voltage changes on the trunk circuit will cause current sounds such as pop and ga to be heard on the subscriber line, indicating that the handshake protocol is in progress step by step. Another handshake process is used to initiate timing for billing purposes. It causes some additional noise when the called party answers.

Another common monitoring signaling is called single frequency signaling or SF signaling . The most common form is to use a fixed 2600Hz tone to indicate that the trunk is idle. When the switch hears the 2600 Hz sound for a period of time, it will enter the idle state (the requirement for the duration can reduce the chance of error). Some systems use sound frequencies above 3000 Hz, especially the SSB frequency multiplexing microwave band. On a T-1 digital carrier, the format of the numbers is called Alternate Mark Inversion (AMI), and the signaling is sometimes carried by taking bits from the T-1 digital stream. If carefully designed, the stripped bits will not affect the quality of the device. This directly supports E and M signaling between two electromechanical switches via a digital carrier, or pulse dialing, when DC power cannot be continuously supplied.

Various voices in the electromechanical telephone network

A feature of electromechanical switching equipment is that maintenance personnel can hear stepping mechanisms and crisscrossing relays. Most central offices of the Bell System have a reinforced concrete structure, with ceilings and floors made of concrete. In rural areas, certain small switching facilities such as Communications Dialing Offices (CDOs) are sometimes housed in iron houses. These facilities always use concrete floors. This solid surface reflects sound.

During busy times, the large switches process calls with such clicking noises that it is difficult to hear each other's small talk. For example, around five o'clock in the evening on a Monday or Friday in the United States, the rattling sound of metal rattles will force you to raise your voice. The noise sounded like hailstones hitting a metal roof. In the early hours of Sunday morning, call processing could slow down, allowing individual calls to be dialed and lines connected to be heard. Additional noise sources are whining power converters and humming ring generators. WireSpring relays used in some systems emit a continuous rhythmic "click-click-click" sound, which is signaling both rearrangement (120 times per minute) and busy (60 times per minute). In the Bell System telephone network, there are generally several alarm sounds such as warning bells, gongs, or gongs. They can signal an alarm when certain switch components fail. Another source of noise is the punch card fault reporting system connected to the switch general control unit. The fault-reporting system punches holes in the card against an internal debug code to record what actually happened when the fault occurred.

Management and Maintenance

The maintenance of the electromechanical system is partly DC electrical and partly mechanically adjusted. Unlike modern switches, the DC current flowing through the switch is constant after the call is made. Voice is transmitted over a physical wire made of metal.

In all systems, users are not expected to notice the impact on quality of service due to maintenance failures. Many "busy" facilities are built into electromechanical switches as components that, when the system fails, mark all affected lines of the switch as active so that the switching logic can be routed elsewhere. The TD tool plays a similar role. When the user fails to pay the phone bill in time, the service will be temporarily suspended. This affects the user's local equipment (crossbar) or line group (stepper). User can accept calls but cannot dial out.

Maintenance work continued on the Bell System's Strogg step-by-step exchange. They need constant cleaning. The indicator light on the equipment panel in the stepping station is used to remind the operator of some situations. For example, if the fuse is broken, the white light will usually be on, or if something is stuck (for example, the machine is always on-hook), the green light will usually be on. Stepping bureaus are more prone to single points of failure than newer technologies.

Crossbars use a more general and generic control circuit. For example, the digital transceiver ( part of the source register component) is only connected to the call for as long as it needs to accept the number dialed by the user. Crossbar structure is more flexible than stepping. Later crossbar systems had perforated fault recording systems. By the 1970s, the Bell System had added automatic number recognition to nearly all stepper and crossbar switches.

electronic switch

The first electronic switches were not digital. Western Electric's 1ESS is an electronic switch that uses metal wires. It uses a microprocessor and Stored Program Control Switching (SPC).

Modifying the phone number, testing or making the line busy are all completed by inputting corresponding commands on the computer terminal. The 1ESS can support normal electromechanical signaling used by crossbar and stepping telephone switches. These switches introduce a new form of data communication: two 1ESS switches can communicate with each other through a method called Common Channel Interoffice Signaling (CCIS). This data connection method is the predecessor of the No. 7 signaling system .

There is no essential difference between the early electronic program-controlled switching system and the electromechanical switch. They both use space-divided voice switching, and all the signals exchanged are analog signals. Therefore, this type of program-controlled switch is also called an analog program-controlled switch.

digital switch

According to the dialed telephone number, the digital exchange connects two or more digital virtual circuits to complete the exchange. The SS7 protocol (or its variants) is used to establish a call between two switches. According to Federal Standard 1037C (American National Standard) and MIL-STD-188 (U.S. Military Standard), in U.S. and military communications , a digital switch refers to a device that performs time-division multiplex switching of digital signals. All switches built after the 1980s are digital, so this distinction in terminology makes no difference. This section describes digital switches, including algorithms and devices.

Digital switches encode speech in extremely short time slices (many per second). In each time slice, a digital representation of the speech signal is generated in real time. The resulting digits are sent to the receiver, which in turn produces sound in the receiver's phone. In other words, when you use the phone, your voice is encoded and reproduced on the other side. During the whole process, your voice is only very small, usually only a fraction of a second delay. In other words, the phone call isn't really real-time communication, since the sound needs to be reproduced, but only delayed for a very short time.

Each local loop telephone line is connected to a remote concentrator. The concentrator is in many cases located in the same building as the switch. The interface between the concentrator and the telephone exchange is standardized by the European Telecommunications Standards Institute as the V5 protocol.

Some telephone exchanges are not directly connected to the concentrator, but are only used to interface with other exchanges. Usually there will be one or more composite machines in a central office called "carrier class" switches or tandems.

Telephone exchanges are usually owned and operated by telecommunications service providers, or "intermediaries," and may also be located in their own buildings. But sometimes independent enterprises or private business units also place their own switches in their own places, called private branch exchanges (PBX).

The role of switches in telecommunication systems

Telephone exchanges are only a small part of a vast telecommunications network. The toughest job and most expensive part of the phone system is the wiring outside the central office. In early telecommunications systems, each subscriber number required a wire to be drawn from the switchboard to the subscriber's phone. An ordinary exchange may have tens of thousands of pairs connected to the terminal box (called the main distribution frame, MDF). MDF has protective components such as discharge tubes to prevent short circuit of the power line or excessive external voltage. A typical phone company has a large database that keeps track of every pair of subscriber lines. Before the computerization of the Bell System's system of records in the 1980s, the information was written in pencil on ledgers.

In order to reduce the cost of external circuits, some telecommunications companies use "wire pair capacity" equipment to provide users with telephone services. These devices are used to reduce the length of copper pairs and enable digital services such as ISDN or DSL when existing copper lines are exhausted or when there are subscribers nearby. Pair augmentation or Digital Loop Carrier (DLC) is placed outside the central office, usually next to large subscribers at some distance from the central office.

DLC is often referred to as Subscriber Loop Carrier (SLC), after Lucent gave its line-pair capacity proprietary equipment a proprietary name, the early SLC system (SLC-1) used an analog carrier between the central office and remote communication between. Subsequent systems (SLC-96, SLC-5) and other vendors' DLC products such as line cards convert the analog signal to digital (using PCM encoding). This digital signal can be sent to the central office via copper wire, fiber optic cable, or other transmission medium. Other components include a ring generator and a backup battery to generate the ring current.

DLC lines can be configured as universal (UDLC) or integrated (IDLC). Universal DLC has two terminals, a central office terminal (COT) and a remote terminal (RT), whose functions are identical. Both terminals face the analog signals directly and convert them into digital signals and send them to the other party. Sometimes, transfers are handled by separate devices. In the integrated DLC, no COT is used. Instead, RTs are connected digitally to telephone exchanges. This reduces the amount of equipment required. Certain standards such as Telcordia's TR/GR-008 and TR/GR-302 cover DLC.

Switches are also used between two central offices and in long-distance central offices.

exchange design

Long-distance switches may use slower but more efficient switch allocation algorithms than central offices because their input and output channel utilization is close to 100%. The central office has more than 90% of the channel capacity idle.

While traditional telephone exchanges are connected to physical circuits (i.e. wire pairs), modern telephone exchanges use multiplex devices and time-division switching. In other words, each voice channel is a time slot (indicated by 1 or 2) on a physical wire pair (indicated by A or B). In order to connect two voice channels (eg A1 and B2), the telephone exchange will exchange the information on A1 and B2, if the exchange has both physical connections and time slots. To do this, it needs to exchange data between slots and connections 8,000 times per second. This is done under the control of a digital logic circuit that cycles through the currently connected electronic list. The use of these two types of switching technologies makes modern switches much smaller than switches that do not use multi-channel and time-division technology.

The switch fabric uses an odd number of layers of smaller, simpler sub-switches. Each sub-switch of each layer is connected to each other, and then connected to the sub-switch of the next layer. In most designs, the physical switching layer and the time switching layer are interleaved. The load on these layers is balanced because the calling party can also act as the called party in the telephone system.

The time-division switch reads a time slot cycle into the memory completely, and then writes it in a different order, also under the control of a cycle computer memory. This causes some signal delay.

A multiway switch switches circuits, often using some variant of nonblocking minimum span switching, or using a cross-cut switch.

Exchange Fabric in the People's Republic of China

Entry: Telephone area codes for the People's Republic of China

Mainland China adopts a four-level tandem radiation long-distance telephone network.

• Bureau C1 is a regional central bureau or an inter-provincial central bureau, established in 8 major cities: Shanghai , Guangzhou , Beijing , Shenyang , Nanjing , Xi'an , Chengdu , and Wuhan .
• C2 bureaus are provincial central bureaus, which are set up in provincial capital cities that are not C1 bureaus.
• C3 bureau is the regional central bureau.
• C4 Bureau is the county-level switching center.

Structurally, the C1 office is all interconnected and radiates to all the C2 offices, the C2 office radiates to the C3 office, and the C3 office radiates to the C4 office. In addition, there are direct circuits from Beijing to the central offices of each province, and the central offices of each province (C1 jurisdiction) are basically interconnected. In addition, there are other direct lines to improve efficiency. [1]

Cities such as Beijing and important provincial capitals use three-digit area codes, Beijing is "010", and important provincial capitals start with "02". Others use four bits. Ordinary fixed-line telephone numbers do not use 1 or 0 at the first place, and 9 is used for special service stations in this area, such as audio stations, etc. The first digit of a long distance area code is assigned as follows (with exceptions):

• North China: 3
• Northeast: 4
• East China: 5, 6
• Central South: 7
• Southwest: 8
• Northwest: 9

Switch Control Algorithm

fully connected to the network

One way to ensure that phone connections are always successful is to build fully connected networks, where all switches are interconnected. This method is usually used for switches inside the central office, and its resource utilization is low.

Clos non-blocking switching algorithm

Layer idle connections between sub-switches are scarce resources in switching systems. Control logic must distribute these links, and most switches do this in a fault-tolerant manner . See Nonblocking Minimum Span Switching for a discussion of the Charles Crews algorithm, which is used in many telephone exchanges and is perhaps one of the most important algorithms in modern industry.

fault tolerance

Composite switches are inherently very fault tolerant. If a sub-switch crashes, the controlling computer will find out during periodic testing. The computer will then mark all links to that subswitch as "occupied". This prevents new calls from entering the switch and does not interrupt active calls. This way, after these calls are over, the subswitch is not used anymore. After a while, a technician will come and replace the circuit board. If the next test is successful, the connection to the repaired sub-switch will be marked as "idle" and the switch will resume full functionality.

In order to prevent some undetectable errors, the links between all layers in the switch are allocated through a first-in-first-out queue. This way, if a user goes off the phone and redials, they are served by a different link between the subswitches. The last-in-first-out link allocation may cause users to fail to dial up frequently.