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| Full Availability, Limited Availability And Gradings |
Full Availability, Limited Availability and GradingsWhen customers of the PSTN make telephone calls, they commonly make use of a telecommunications network called a switched-circuit network. In a switched-circuit network, devices known as switches are used to connect the caller to the callee. Each switch has a number of inlets and outlets and by connecting a specific inlet to the correct outlet each switch helps to complete an end-to-end circuit between users [1].
In a modern circuit-switched network, switches can connect any inlet to any outlet; this is known as Full Availability.
Due to previous technological challenges, older switches can only connect some inlets to some outlets; this is known as Limited Availability [2, 3]. In Limited Availability Switches, the circuits inside the switch are arranged into Grading groups [1]. An example of a Limited Availability Switch can be seen in Figure 1.
Figure 1 depicts five users whose calls are being routed through this switch. The switch contains eight inlets and 15 outlets. Not all inlets can connect to any of the outlets. The Grading Scheme divides the 15 outlets into eight individuals, four pairs, two quads and one common. To improve efficiency, incoming calls are assigned to the first available outlet, such as outlets 1, 6 and 7. If another call is received on the same inlet, then that call is assigned to the first available outlet further away. An example of this occurs in outlets 5 and 11, where the first caller was assigned to outlet 5, but the second caller was assigned to the next available outlet viz. 11. It should be noted that a particular inlet can only receive a maximum of four calls as there are only four available outlets to each inlet. This assumes that these outlets are not already in use by another inlet. Thus, this Switch has an availability of four (columns)[1].
References
[1] Kennedy I., Lost Call Theory, Lecture Notes, ELEN5007: Teletraffic Engineering, School of Electrical and Information Engineering, University of the Witwatersrand, 2005
[2] Akimaru H., Kawashima K., Teletraffic: Theory and Applications, Springer-Verlag London, 2nd Ed., 1999, p 6
[3] Farr R.E., Telecommunications Traffic, Tariffs and Costs: An Introduction For Managers, Peter Peregrinus, 1988, p 90.
category: telephony
Telephone
The telephone or phone (Greek: tele = far away and phone = voice) is a telecommunications device which is used to transmit and receive sound (most commonly voice and speech) across distance. Most telephones operate through transmission of electric signals over a complex telephone network which allows almost any phone user to communicate with almost any other.
telephone network
Introduction
telephone network]]
There are four principal means by which an end user using a telephone handset may connect to a telephone network: a traditional fixed phone "landline", which uses dedicated physical wire connections connected to a single location; wireless and radio telephones, which use either analog or digital radio signals; satellite telephones, which utilize telecommunications satellites; and voice over internet protocol (VoIP) telephones, which use broadband internet connections.
Between end users, transmissions across a network may be carried by fiber optic cable, point to point microwave or satellite relay.
Until relatively recently, a "telephone" generally referred only to landlines. Cordless and mobile phones are now common in many places around the world, with mobile phones expected to gradually displace the conventional landline telephone. Unlike a mobile phone, a cordless telephone is considered to be landline because it is only useable within a short distance of a small personal or domestic base station connected to a fixed phone line.
The identity of the inventor of the electric telephone remains in dispute. Antonio Meucci, Philip Reis, and Alexander Graham Bell, amongst others, have all been credited with the invention.
History
invention]
The very early history of the telephone is a confusing morass of claim and counterclaim, which was not clarified by the huge mass of lawsuits which hoped to resolve the patent claims of individuals. There was a lot of money involved, particularly in the Bell Telephone companies, and the aggressive defense of the Bell patents resulted in much confusion. Additionally, the earliest investigators preferred publication in the popular press and demonstration to investors instead of scientific publication and demonstration to fellow scientists.
It is important to note that there is probably no single "inventor of the telephone". The modern telephone is the result of work done by many hands, all worthy of recognition of their addition to the field.
Early development
The following is a brief summary of the history of the invention of the telephone:
- 1849 Antonio Meucci, an Italian living in Havana, demonstrates a device later called a telephone. (The demonstration involves direct electrical connections to people. See Physiophony)
- 1854 Charles Bourseul publishes a description of a make-break telephone transmitter and receiver but does not construct a working instrument.
- 1854 Meucci demonstrates an electric telephone in New York. [http://chem.ch.huji.ac.il/~eugeniik/history/meucci.html]
- 1860 Johann Philipp Reis demonstrates a make-break transmitter after the design of Bourseul.
- 1860 Meucci supposedly demonstrates his telephone on Staten Island.
- 1861 Reis manages to transfer voice electrically over a distance of 340 feet, see Reis' telephone.
- 1871 Meucci files a patent caveat (a statement of intention to patent).
- 1872 Elisha Gray founds Western Electric Manufacturing Company.
- July 1873 Thomas Alva Edison notes variable resistance in carbon grains due to pressure, but shelves the discovery.
- 1874 Gray demonstrates his liquid transmitter telephone at the Highland Park Presbyterian Church.
- 2 June 1875 Alexander Graham Bell first transmits voice.
- 1 July 1875 Bell first uses a bi-directional capable telephone (Both the transmitter and the receiver were identical membrane instruments.)
- 14 February 1876 Bell files his first patent on the telephone.
- Two hours later Gray files his patent caveat.
- 30 January 1877 Bell patents the electro-dynamic transmitter, receiver telephone telephone
Later history
1877
The history of additional inventions and improvements of the electrical telephone includes the carbon microphone (later replaced by the electret microphone now used in almost all telephone transmitters), the manual switchboard, the rotary dial, the automatic telephone exchange, the computerized telephone switch, Touch Tone® dialing (DTMF), and the digitization of sound using different coding techniques including pulse code modulation or PCM (which is also used for .WAV files and compact discs).
Newer systems include IP telephony, ISDN, DSL, mobile cellular phone systems, cordless telephones, and the third generation cell phone systems that promise to include high-speed packet data transfer.
The industry has divided into telephone equipment manufacturers and telephone network operators (telcos). Operating companies often hold a national monopoly. In the United States, the Bell System was vertically integrated. It fully or partially owned the telephone companies that provided service to about 80% of the telephones in the country and also owned Western Electric, which manufactured or purchased virtually all the equipment and supplies used by the local telephone companies. The Bell System divested itself of the local telephone companies in 1984 in order to settle an antitrust suit brought against it by the United States Department of Justice.
In 1926 Bell Labs and the British Post Office engineered the first two-way conversation across the Atlantic.
The first commercial transatlantic telephone call was between New York City and London and occurred on January 7, 1927.
Digital Telephony
The Public Switched Telephone Network (PSTN) has gradually evolved towards digital telephony which has improved the capacity and quality of the network. End-to-end analog telephone networks were first modified in the 1970s by upgrading long-haul transmission networks with SONET technology and fiber optic transmission methods. Digital transmission made it possible to carry multiple digitized switched circuits on a single transmission medium (known as multiplexing). While today the end instrument remains analog, the analog signals reaching the aggregation point (Serving Area Interface (SAI) or the central office (CO) ) are typically converted to digital signals. Digital loop carriers (DLC) are often used, placing the digital network ever closer to the customer premises, relegating the analog local loop to legacy status.
Wireless phone systems
While the term "wireless" means radio and can refer to any telephone that uses radio waves it is primarily used for cell phones. In the United States wireless companies tend to use the term wireless to refer to a wide range of services while the cell phone itself is called a mobile phone, mobile, cell phone or simply cell with the trend now moving towards mobile.
The changes in terminology is partially due to providers using different terms in marketing to differentiate newer digital services from older analog systems and services of one company from another.
Cordless telephone
marketing
Cordless telephones, first invented by Teri Pall in 1965, consist of a base unit that connects to the land-line system and also communicates with remote handsets by low power radio. This permits use of the handset from any location within range of the base. Because of the power required to transmit to the handset, the base station is powered with an electronic power supply. Thus, cordless phones typically do not function during power outages. Initially, cordless phones used the 1.7 MHz frequency range to communicate between base and handset. Because of quality and range problems, these units were soon superseded by systems that used frequency modulation (FM) at higher frequency ranges (49 MHz, 900 MHz, 2.4 GHz, and 5.8 GHz). The 2.4 GHz cordless phones can interfere with certain wireless LAN protocols (802.11b/g) due to the usage of the same frequencies. On the 2.4 GHz band, several "channels" are utilized in an attempt to guard against degradation in the quality of the voice signal due to crowding. The range of modern cordless phones is normally on the order of a few hundred meters.
Cellular phone
Modern mobile phone systems are cell-structured. Radio is used to communicate between a handset and nearby cell sites.
When a handset gets too far from a cell site, a computer system commands the handset and a closer cell site to take up the communications on a different channel without interrupting the call.
Radio frequencies are a limited, shared resource. The higher frequencies used by cell phones have advantages over short distances. Connection distance is somewhat predictable and can be controlled by adjusting the power level. By only using enough power to connect to the "nearest" cell site phones using one cell site will cause almost no interference with phones using the same frequencies on another cell site. The higher frequencies also work well with various forms of multiplexing which allows more than one phone to connect to the same tower with the same set of frequencies.
Cordless/mobile phone
There are phones that work as a cordless phone when near their corresponding base station (and sometimes other base stations) and work as a wireless phone when in other locations but for a variety of reasons did not become popular.
Some kinds of cordless phones work like cellular phones but only within a small private network covering a building or group of buildings. These kinds of systems using VoIP are popular in hospitals and factories where the same wireless network can be used for both data and voice.
VoIP Telephony
VoIP phone]]
Also known as Internet telephony or Voice over IP (VoIP), digital telephony is a disruptive technology that is rapidly replacing traditional telephone networks. In Japan and Korea up to 10% of subscribers, as of January 2005, have switched from analog to digital telephone service. A recent Newsweek article suggested that Internet telephony may be "the next big thing." [http://msnbc.msn.com/id/6831938/site/newsweek/]
Digital telephones use a broadband Internet connection to transmit conversations as data packets. In addition to replacing the PSTN, digital telephony is also competing with mobile phone networks by offering free or lower cost connections via WiFi hotspots. As mentioned above VoIP is also used on private wireless networks which may or may not have a connection to the outside telephone network.
Telephone equipment research labs
Bell Labs is a noted telephone equipment research laboratory, amongst its other research fields.
Telephone operating companies
In some countries, many telephone operating companies (commonly abbreviated to telco) are in competition to provide telephone services. Some of them include those in the following list. However, the list only includes providers of copper wires from the exchange to the user, not those who only supply "Voice over IP" or only transport voice signals between exchanges. See also: List of telephone operating companies
Trivia
- The modern handset came into existence when a Swedish lineman tied a microphone and earphone to a stick so he could keep a hand free.
- The folding portable phone was an intentional copy of the fictional futuristic communicators (which in use actually more closely resembled walkie-talkies, Nextel-style) used in the television show Star Trek.
See also
Telephone equipment
- 431A
- 610
- Answering machine
- Cordless telephone
- Modem
- Payphone
- Pen register
- Photophone
- Telautograph
- Telecommunications Device for the Deaf (TDD or TTY)
- Telegraph
- Switchboard
- Telex
- Teletype
- Electronic Switching System
Telephone equipment manufacturers
Several manufacturers build telephones of all kinds. Some of these are:
- Alcatel
- Avaya
- Conair (makers of Southwestern Bell Freedom Phone)
- Ericsson
- Huawei
- Kyocera
- Lucent
- Marconi
- Mitel
- Motorola
- Nokia
- Nortel
- palmOne / Handspring
- Samsung
- Siemens AG
- Sony Ericsson
- Unical Enterprises (makers of Northwestern Bell Phones)
- US Electronics (makers of BellSouth Products)
Telephone technology
- Asymmetric Digital Subscriber Line (ADSL)
- AIOD leads
- ANAC
- ANI
- Assistive technology
- Automatic redial
- Call capture
- Call forwarding
- Call waiting
- Caller ID
- Computer telephony integration (CTI)
- Customer premises equipment (CPE)
- Dial tone
- Digital subscriber line (DSL)
- Direct dial
- Direct distance dialing
- Dual tone multi frequency (DTMF)
- Interactive Voice Response (IVR)
- Last Call Return (
- 69)
- Telephone feature code (
- code)
- Party line
- Plain old telephone service (POTS)
- Ringing signal
- Videotex
- Voice over Internet Protocol (VOIP)
Telephone system, organization, and structure
- Area code
- Office code
- Basic exchange telecommunications radio service
- Bell System
- Call center
- Competitive local exchange carrier (CLEC)
- Foreign exchange service
- Incumbent local exchange company (ILEC)
- Key system
- Local exchange company (LEC)
- Public Switched Telephone Network (PSTN)
- Regional Bell operating company (RBOC)
- Post office
- Private line
- Private branch exchange (PBX)
- Station set
Telephone hacking and exploitation
- Blue box
- Bomb threat
- Crank (or prank) call
- Demon dialing
- (Phone) phreaking
- Speed dialer
- Telephone fraud
- War dialing
US-specific terminology
- Competitive local exchange carrier (CLEC)
- Federal Standard 1037C
- Interexchange carrier (IXC)
- List of telephony terms
- Local access and transport area (LATA)
- Local exchange carrier (LEC)
- Modification of Final Judgment
- Federal Regulations - Part 68
- Regional Bell operating company (RBOC)
- US telephony
Telephone terminology
- Call originator
- Call waiting
- Called party
- Caller
- Calling party
- Circuit busy
- COCOT
- Deaf
- Emergency telephone number
- End instrument
- Fax
- Help desk
- Hook
- Hook Flash
- Hunt Group
- Infrastructure
- Interactive voice response (IVR)
- Line
- Local loop
- Long-distance operator
- Operator assistance
- Person-to-person
- Red telephone, Red telephone box,
- Ringer equivalency number (REN)
- Ringing signal
- Rural radio service
- Smartphone
- Station-to-station
- Telemarketing
- Telephone booth
- Telephone call
- Telephone card
- Telephone directory
- Telephone exchange
- Telephone tapping
- Telephone User Interface (TUI)
- Telephony Application Programming Interface (TAPI)
- Trap and trace
- TWX
- Vertical service code
- Voicemail
- Western Union
- Wide Area Telephone Service (WATS)
- WATS line
- Wireless network
- Wi-Fi
- Zenith number
Telephone Standards
Wired Standards
- RJ-11
- BS_6312
Wireless Standards
There are many standards for common carrier wireless telephony, often with incompatible standards used in the same nation:
- First generation - Analog
- marine and mobile radio telephony
- AMPS
- CDPD
- NMT
- Satellite systems- digital
- Inmarsat
- Iridium (satellite)
- Second generation (2G) - Digital
- CDMA IS-95A
- GSM, (different frequencies for different continents: see GSM article)
- iDEN
- TDMA IS-136
- 2.5G
- CDMA IS-95B
- GPRS
- EDGE
- PDC-P
- Third generation (3G)
- CDMA 2000
- UMTS, also called W-CDMA
- TD-SCDMA
Patents
- [http://patft.uspto.gov/netacgi/nph-Parser?Sect1=PTO1&Sect2=HITOFF&d=PALL&p=1&u=/netahtml/srchnum.htm&r=1&f=G&l=50&s1=174465.WKU.&OS=PN/174465&RS=PN/174465 US174465] -- Telegraph -- A. G. Bell
References
- Huurdeman, Anton A. (2003). The Worldwide History of Telecommunications, IEEE Press and J. Wiley & Sons, 2003. ISBN 0471205052
Category:Telephony
Category:Office equipment
ja:電話
ms:Telefon
simple:Telephone
th:โทรศัพท์
Circuit switchedA circuit switched network is one where a dedicated connection (circuit or channel) must be set up between two nodes before they may communicate. For the duration of the communication, that connection may only be used by the same two nodes, and when the communication has ceased, the connection must be explicitly cancelled.
Early telephone systems are a suitable example. The subscriber would ask the operator to connect them to another subscriber, whether on the same exchange or via an inter-exchange link and another operator. In any case, the end result was a physical electrical connection between the subscriber's telephones for the duration of the call. The copper wire used for the connection could not be used to carry other calls at the same time — even if the subscribers were in fact not talking and the line was silent.
In later years it became possible to multiplex multiple connections over the same physical conductor, but nonetheless each channel on the multiplexed link was dedicated to one call at a time. Circuit switching can be relatively inefficient because capacity is wasted on connections which are set up but (however momentarily) not in use.
Circuit switching contrasts with packet switching which splits data (for instance, digital representation of sound, or computer network data) into chunks which are separately routed over a shared network.
In telecommunication, the term circuit switching has the following meanings:
1. A method of routing traffic between an originator and a destination through switching centers, from local users or from other switching centers, whereby a continuous electrical circuit is established and maintained between the calling and called stations until it is released by one of those stations.
The method of establishing the connection and monitoring its progress and availability may utilize a separate control channel as in the case of ISDN or not as in the case of the Public Switched Telephone Network.
2. A process that, on demand, connects two or more data terminal equipments (DTEs) and permits the exclusive use of a data circuit between them until the connection is released. The acronym CSD (Circuit Switched Data) is also used, as in GSM's original data transfer method HSCSD.
See also
- Federal Standard 1037C
- MIL-STD-188
External links
- [http://www.wired.com/wired/archive/4.10/atm.html Netheads vs Bellheads by Steve Steinberg]
- [http://www.pbs.org/cringely/pulpit/pulpit20040624.html Out of Sight, Out of Mind, Big Old Stupid Telephone Companies Are Throwing Away Their Only Real Asset] Opinion piece on packet switching vs circuit switching
Category:Telecommunications
Category:Computer networks
Telephone exchangeIn the field of telecommunications, a telephone exchange (or telephone switch) is a piece of equipment that connects phone calls. It is what makes phone calls "work" in the sense of making connections and relaying the speech information.
The term exchange can also be used to refer to an area served by a particular switch. And more narrowly, it can refer to the first three digits of the local number.
In the past, the first two or three digits would map to a mnemonic exchange name, e.g. 869–1234 was formerly TOwnsend 9–1234, and before that (in some localities) might have been TOWnsend 1234 (only the capital letters and numbers being dialed).
In December of 1930, New York City became the first locality in the United States to adopt the two-letter, five-number format; it remained alone in this respect until well after World War II, when other municipalities across the country began to follow suit (in some areas, most notably much of California, telephone numbers in the 1930s through early 1950s consisted of only six digits, two letters which began the exchange name followed by four numbers, as in DUnkirk 0799). Prior to the mid-1950s, the number immediately following the name could never be a "0" or "1;" indeed, "0" was never pressed into service at all, except in the immediate Los Angeles area (the "BEnsonhurst 0" exchange mentioned in an episode of the popular TV sitcom The Honeymooners was fictitious).
In 1955, the Bell System attempted to standardize the process of naming exchanges by issuing a "recommended list" of names to be used for the various number combinations. In 1961, New York Telephone introduced "selected-letter" exchanges, in which the two letters did not mark the start of any particular name (example: FL 6-9970), and by 1965 all newly-connected phone numbers nationwide consisted of numerals only (Wichita Falls, Texas had been the first locality in the United States to implement the latter, having done so in 1958) Pre-existing numbers continued to be displayed the old way in many places well into the 1970s. A Chicago carpet retailer frequently advertised their number NAtional 2-9000 on WGN until the 1990s; not to mention, the number TYler 8-7100 for a Detroit construction company.
Most of the United Kingdom had no lettered telephone dials until the introduction of Subscriber Trunk Dialling (STD) in 1958. Only the director areas (Birmingham, Edinburgh, Glasgow, Liverpool, London and Manchester) and the non-director areas adjacent to them had lettered dials, and the exchanges used the three-letter, four-number format until conversion to all-figure numbering in 1968.
In the United States, the word exchange can also have the technical meaning of a local access and transport area under the Modification of Final Judgment (MFJ).
Historic perspective
Modification of Final Judgment
The first telephone exchange opened in New Haven, Connecticut in 1878. The switchboard was built from "carriage bolts, handles from teapot lids and bustle wire" and could handle two simultaneous conversations (see [http://www.cr.nps.gov/nhl/DOE_dedesignations/Telephone.htm National Park Service]).
Later exchanges consisted of one to several hundred plug boards manned by operators. Each operator sat in front of from one to three banks of ¼-inch phone jacks fronted by several rows of phone cords, each of which was the local termination of a phone subscriber line. A calling party (known as the 'subscriber'), would lift the receiver, a light near the plug would light, and the operator would switch into the circuit to ask "number please?". Depending upon the answer, the operator might plug the plug into a local jack and start the ringing cycle, or plug into a hand-off circuit to start what might be a long distance call handled by subsequent operators in another bank of boards or in another building miles away.
On March 10, 1891, Almon Strowger, an undertaker in Topeka, Kansas, patented the Strowger switch, a device which led to the automation of the telephone circuit switching. While there were many extensions and adaptations of this initial patent, the one best known consists of 10 layers or banks of 10 contacts arranged in a semi-circle. When used with a dial telephone, each pair of numbers caused the shaft of the central contact "hand" to first step up a layer per digit and then swing in a contact row per digit.
These step switches were arranged in banks, beginning with a "line-finder" which detected that one of up to a hundred subscriber lines had the receiver lifted "off hook". The line finder hooked the subscriber to a "dial tone" bank to show that it was ready. The subscriber's dial pulsed at 10 pulses per second (depending on standards in particular countries).
Exchanges based on the Strowger switch were challenged by crossbar technology. These phone exchanges promised faster switching and would accept pulses faster than the Strowger's typical 10 pps — typically about 20 pps. The advent of DTMF tone-signalling solid-state switches cut off the crossbar's takeover before it could really get going.
A transitional technology (from pulse to DTMF) had DTMF "link finders" which converted DTMF to pulse and fed it to conventional strowger or crossbar switches. This technology was used as late as the mid to late 1990s.
Historic trivia
Because the switches were hard-wired together and fairly hard to re-wire (re-grade), telephone exchange buildings in many larger cities were dedicated to circuits that began with the first two or three numbers of the (in North America) standard 7 digit phone numbers. In a holdover from the days of plug-board exchanges, the exchanges were typically named with a name whose first two letters translated to the digits of the exchange's prefix on a common telephone dial. Examples: CAstle (22), TRinity (87), MUtual (68). Certain number combinations were not amenable to this naming process, such as "57," "95" and "97;" it was in part due to this factor that the name system was eventually abandoned, as more numbers were needed to prevent a given area code from running out of available numbers.
Because the pulses in a Strowger switch exchange took time, having a phone number with lots of 8s or 9s or 0s meant it took longer to dial. The phone companies typically assigned such "high" numbers to pay phones because they were rarely dialed to.
To test the basic functioning of all of the switches in a chain, a special "test" number was reserved that consisted of all 5s (555–5555) — half-way up and in on each bank. The "555" (or KLondike) exchange was never assigned any real numbers, which is why today's TV and movie shows use 555-xxxx numbers for their phone numbers (previously, such productions often used numbers that ended in certain four-number combinations that were typically set aside for similar uses — "0079" on the West Coast and "9970" in many other places; examples include the TV series Perry Mason and the 1948 film Sorry, Wrong Number). That way there was no possibility that a fake number from a show would actually reach someone, thus avoiding the scenario which arose in 1982 with Tommy Tutone's hit single 867-5309/Jenny, which led to many customers who actually had that number receiving a plethora of unwanted calls. In fact, many US phone companies either no longer assign this number, or have relegated to internal testing purposes.
However, today only numbers beginning with 555–01 are reserved for fiction and other 555-numbers can be allocated to "information providers". A side effect of the fictional-number pool being reduced to 100 numbers is that the same ones now often recur in different movies or TV shows. The "958" and "959" exchanges have also been reserved for similar purposes in most localities, and as a result very few individuals or businesses have telephone numbers beginning with those sets of digits either (although this fact is not as well known, so such numbers have not been used in a fictional context).
The number in the Glenn Miller Orchestra's hit 'PEnnsylvania 6-5000' was and is the number of the Hotel Pennsylvania in New York. If you call the number, now written as (212) 736-5000, you still get the hotel.
Technologies
In U.S. and military telecommunication, a digital switch is a switch that performs time-division multiplexing switching of digitized signals. Source: from Federal Standard 1037C and from MIL-STD-188. All switches built since the 1980s are digital, so for practical purposes this is a distinction without a difference. This article describes digital switches, including algorithms and equipment.
This article will use the terms:
- telephone exchange for the building
- telephone switch for the switching equipment
- concentrator for the concentrator, whether or not it is co-located with the switch
Manual telephone exchanges
With all-manual calling, the customer calls the operator and asks the operator for the number, and provided that the number is in the same central office, the operator connects the call by plugging into the jack on the switchboard corresponding to that customer's line. If the call is to another central office, the operator plugs into the trunk for the other office and ask the operator answering (known as the "inward" operator) to connect the call.
Most manual telephone exchanges in cities were common-battery, meaning that the central office provided power for the telephone circuits, as is the case today. A customer lifting their receiver would change their line status to "tip," thereby lighting a light on the operator's switchboard. In smaller towns, early telephones were often magneto, or crank, phones, where the subscriber turned a crank to generate current to activate the "tip" condition, notifying the operator of the call. Batteries at the subscriber's home provided the current to allow conversation. Magneto systems were in use in some small towns in the U.S. as late at the 1980s.
In large cities, such as New York City, with hundreds of central offices, it took many years to convert the whole city to dial service. To help automate service to manual offices during the transition to dial service, a special type of switchboard, which would display the number dialed by the customer, was used. For instance, if a customer in the MUrray Hill exchange picked up the phone and dialed a number in the CIty Island exchange, the customer would never need to know the destination number was in a manual exchange. Dialing that number would connect to the CIty Island exchange inward operator, who would see the number displayed on the switchboard, and plug into the line.
Automatic telephone exchanges
These came into existence in the early 1900s. They were designed to replace the need for human telephone operators. Before the exchanges became automated, operators had to complete the connections required for a telephone call. Almost everywhere, operators have been replaced by computerized exchanges.
The local exchange automatically senses an off hook (tip) telephone condition, provides dial tone to that phone, receives the pulses or DTMF tones generated by the phone, and then completes a connection to the called phone within the same exchange or to another distant exchange.
The exchange then maintains the connection until a party hangs up, and the connection is disconnected. Additional features, such as billing equipment, may also be incorporated into the exchange.
Early exchanges used motors, shaft drives, rotating switches and relays.
Some types of automatic exchanges were Strowger (also known as Step-By-Step), All Relay, X-Y, Panel and Crossbar.
Telephone switches
A telephone switch is the brains of an exchange. It is a device for routing calls from one telephone to another, generally as part of the public switched telephone network. They work by connecting two or more digital virtual circuits together, according to a dialed telephone number. Calls are setup between switches using the Signalling System 7 protocol, or one of its variants.
Digital switches encode the speech going on, in extremely minute time
slices — many per second. At each time slice, a digital representation
of the tone is made. The digits are then sent to the receiving end of
the line, where the reverse process occurs, to produce the sound for
the receiving phone. In other words, when you use a telephone, you
are generally having your voice "encoded" and then reconstructed for
the person on the other end. Your voice is very slightly delayed
in the process (probably by only a small fraction of one second) —
it is not "live", it is reconstructed — delayed only minutely. (See below for more info.)
Individual local loop telephone lines are connected to a remote concentrator. In many cases, the concentrator is co-located in the same building as the switch. The interface between concentrators and telephone switches has been standardised by ETSI as the V5 protocol.
Some telephone switches do not have concentrators directly connected to them, but rather are used to connect calls between other telephone switches. Usually a complex machine (or series of them) in a central exchange building, these are referred to as "carrier-level" switches or tandems.
Some telephone exchange buildings in small towns now house only remote switches, and are homed "parent" switch, usually several kilometres away. The remote switch is dependent on the parent switch for routing and number plan information. Unlike a digital loop carrier, a remote switch can route calls between local phones itself, without using trunks to the parent switch.
Telephone switches are usually owned and operated by a telephone service provider or "carrier" and located in their premises, but sometimes individual businesses or private commercial buildings will house their own switch, called a PBX, or Private Branch Exchange.
The switch's place in the system
Telephone switches are a small part of a large network. The majority of work and expense of the phone system is the wiring outside the central office, or the "Outside Plant".
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 ISDN or 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 Lucent's proprietary name for their pair gain products. Early SLC systems (SLC-1) used an analog carrier for transport between the remote site and the central office. Later
systems (SLC-96, SLC-5) and other vendors' DLC products contain line cards that convert the analog signal to a digital signal (usually PCM). This
digital signal can then be transported over copper, fiber, or other transport
medium to the central office. Other components include ringing generators to provide ringing current and battery backups.
DLCs can be configured as universal (UDLCs) or integrated (IDLCS). Universal DLCs have two terminals, a central office terminal (COT) and a remote terminal (RT), that function similarly. Both terminals interface with analog signals, convert to digital signals, and transport to the other side where the reverse is performed. Sometimes, the transport is handled by separate equipment. In an Integrated DLC, the COT is eliminated. Instead, the RT is connected digitally to equipment in the telephone switch. This reduces the total amount of equipment required. Several standards cover DLCs, including Telcordia's TR/GR-008 & TR/GR-303.
Switches are used in both local central offices and in long distance centers.
Switch design
Long distance switches may use a slower, more efficient switch-allocation algorithm than central offices, because they have near 100% utilization of their input and output channels. Central offices have more than 90% of their channel capacity unused.
While traditionally, telephone switches connected physical circuits (e.g., wire pairs), modern telephone switches use a combination of space- and time-division switching. In other words, each voice channel is represented by a time slot (say 1 or 2) on a physical wire pair (A or B). In order to connect two voice channels (say A1 and B2) together, the telephone switch interchanges the information between A1 and B2. It switches both the time slot and physical connection. To do this, it exchanges data between the time slots and connections 8000 times per second, under control of digital logic that cycles through electronic lists of the current connections. Using both types of switching makes a modern switch far smaller than either a space or time switch could be by itself.
The structure of a switch is an odd number of layers of smaller, simpler subswitches. Each layer is interconnected by a web of wires that goes from each subswitch, to a set of the next layer of subswitches. In most designs, a physical (space) switching layer alternates with a time switching layer. The layers are symmetric, because in a telephone system callers can also be callees.
A time-division subswitch reads a complete cycle of time slots into a memory, and then writes it out in a different order, also under control of a cyclic computer memory. This causes some delay in the signal.
A space-division subswitch switches electrical paths, often using some variant of a nonblocking minimal spanning switch, or a crossover switch.
Switch control algorithms
Fully-connected mesh network
One way is to have enough switching fabric to assure that the pairwise allocation will always succeed by building a fully-connected mesh network. This is the method usually used in central office switches, which have low utilization of their resources.
Clos's nonblocking switch algorithm
The scarce resources in a telephone switch are the connections between layers of subswitches. The control logic has to allocate these connections, and most switches do so in a way that is fault tolerant. See nonblocking minimal spanning switch for a discussion of Charles Clos's algorithm, used in many telephone switches, and arguably one of the most important algorithms in modern industry.
Fault tolerance
Composite switches are inherently fault-tolerant. If a subswitch fails, the controlling computer can sense it during a periodic test. The computer marks all the connections to the subswitch as "in use". This prevents new calls, and does not interrupt old calls that remain working. As calls are ended, the subswitch then becomes unused. Some time later, a technician can replace the circuit board. The next test succeeds, the connections to the repaired subswitch are marked "not in use", and the switch returns to full operation.
To prevent frustration with unsensed failures, all the connections between layers in the switch are allocated using first-in-first-out lists. That way, when a disgusted customer hangs up and redials, they will get a different set of connections and subswitches. A last-in-first-out allocation of connections might cause a continuing string of very frustrating failures.
See also:
- DSL
- ISDN
- PDH
- PBX Private Branch Exchange or business-level switch
The definition below is very technical, and a lot of it appears to be US-specific:
In telecommunication, a central office (C.O.) is a common carrier switching center in which trunks and local loops are terminated and switched.
Note: In the DOD, "common carrier" is called "commercial carrier." Synonyms exchange, local central office, local exchange, local office, switching center (except in DOD DSN [formerly AUTOVON] usage), switching exchange, telephone exchange. Deprecated synonym switch.
Source: from Federal Standard 1037C
Usage
Many of the terms in this article have conflicting UK and US usages.
- central office originally referred to the switching equipment itself. Now it is used generally for the building housing switching and related equipment.
- telephone exchange means an exchange building in the UK, and is also the UK name for a telephone switch, and also has a technical meaning in U.S. telecoms.
- telephone switch is the U.S. term, but is in increasing use in technical UK telecoms usage, to make the CO/switch/concentrator distinction clear
See also
- List of switches
- Pair gain system
- Full Availability, Limited Availability and Gradings
- Softswitch
External links
- [http://www.telcodata.us Paul Timmins' database of North American Exchange and Operator information]
- [http://www.davros.org/phones/btnetwork.html Clive Feather's guide to the BT network]
- [http://ourwebhome.com/TENP/TENproject.html Telephone Exchange Name Project]
- [http://www.rhaworth.myby.co.uk/phreak/tenp_01.htm Roger W. Haworth's guide to London (UK) Director Exchange Names]
- [http://www.cr.nps.gov/nhl/DOE_dedesignations/Telephone.htm National Park Service's page about the first telephone exchange]
Category:Telephony
Articles about telephony.
Category:Telecommunications
Category:Electronic engineering
ja:Category:電話
Wikipedia:Votes for deletion/Neo-new wave
This page is an archive of the proposed deletion of the article below . Further comments should be made on the appropriate discussion page (such as the article's talk page or on a Votes for Undeletion nomination). No further edits should be made to this page.
The result of the debate was Delete. Redwolf24 07:19, 31 July 2005 (UTC)
Delete: it admits to being a neologism in the second section: was used in a small blurb in People magazine...It has time soon enough to be picked up by the public. - Splash 05:13, 26 July 2005 (UTC)
- Delete Unless anybody has actually heard any member of any of the bands mentioned in the article have referred to their music as such. Then I'll change my vote. Hamster Sandwich 05:19, 26 July 2005 (UTC)
- Save Hot Hot Heat is considered neo-new wave. Google it. See this link http://www.spunoutcentral.com/franzferdinand.html.
- Keep. The term has some currency; googling for "neo-new wave" yields 4000 hits. Wile E. Heresiarch 05:38, 26 July 2005 (UTC) Vote revised below. Wile E. Heresiarch 01:59, 28 July 2005 (UTC)
- I meant to say: only [http://www.google.co.uk/search?q=%22Neo-new+wave%22&hl=en&hs=JEc&lr=&client=firefox-a&rls=org.mozilla:en-GB:official&start=450&sa=N about 450 of them are unique]. Which suggests that some outlet(s) uses it loads and it isn't much around elsewhere. - Splash 05:47, 26 July 2005 (UTC)
- Yes, good point. I'm changing my vote to delete. Wile E. Heresiarch 01:59, 28 July 2005 (UTC)
- Keep being of recent coinage doesn't seem like enough to delete it. It otherwise looks like a OK article. Splash, can you point me to some policies, guidelines, or discussions that talk about neologisms. I might change my mind.Delete Thank you, Splash. Those are quite convincing; the article doesn't say "Neo-new wave is a style of music", it says "Neo-new wave is the name of a style of music." We don't include articles on names for things, we include articles on concepts. JesseW 08:08, 26 July 2005 (UTC) 23:28, 26 July 2005 (UTC)
- Most importantly, there is Wikipedia:Neologism, but WP:NOT a dictionary and WINAD also help. There is also WP:NOR, which helps to cover made-up terms: you can't make them up and then put them in WP, because they'd be original dicdefs. Also, the whole 'neologism' thing is used extensively in VfD; take a look through any day's debate, and you'll see neologisms being deleted because they are neologisms. Oh, and don't be fooled by a few hundred Googles; any made-up word can collect that many — we don't want it unless it has so much attention is is encyclopedic at the moment: becoming so in future means the word should only get its article in future, and then only if it has more to say than a dicdef. - Splash 16:50, 26 July 2005 (UTC)
- Weak keep. Seems like a term on its way up, in popularity terms. Would be prepared to change my vote if Wikipedia is shown to have strict rules on neologisms, but I see no objection to them once they reach this level of popularity. Agentsoo 10:13, 26 July 2005 (UTC)
- See my response above: we don't keep neologism until they are established parts of language and only then if their article is more than a dicdef. - Splash 16:50, 26 July 2005 (UTC)
- Delete. I've heard my hipster friends call bands "neo-new wave", but god knows my hipster friends are only notable for their odd fashion sense, and not for being barometers of English usage. If the term ever becomes widespread, re-create the article. Fernando Rizo T/ C 17:09, 26 July 2005 (UTC)
- Merge with New wave music and redirect. — mendel ☎ 19:57, July 26, 2005 (UTC)
- Delete as neologism. -- Carnildo 23:06, 26 July 2005 (UTC)
- Delete imaginary music genres. Note that exactly zero of the bands listed as examples use this term in their own articles. See Wikipedia talk:WikiProject Music/Notability and Music Guidelines. — Wahoofive ( talk) 01:51, 27 July 2005 (UTC)
- Delete neologism. JamesBurns 09:00, 27 July 2005 (UTC)
- Delete nn neologism. Ashmodai 10:56, 27 July 2005 (UTC)
- Comment The page was just blanked by its original author. -- Icelight 22:19, July 28, 2005 (UTC)
darmowe statystyki wegetarianizm tapety wygaszacze gry keno zawory metalowe |
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