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Telephone exchangeFrom Wikipedia, the free encyclopedia

This article needs additional citations for verification. Please helpimprove this articlebyaddingcitations to reliable sources. Unsourced material may bechallengedandremoved.(March 2010)

Atelephone operatormanually connecting calls with cord pairs at a telephone switchboard.

A telephone exchange ortelephone switch is atelecommunicationssystem used in large enterprises or in

thepublic switched telephone network. An exchange consists of electronic components and in older system

also human operators that interconnect (switch) telephone subscriber lines or virtual circuits of digital systems

to establishtelephone callsbetween subscribers. A telephone exchange is located in a central office (CO)

which is the physical building used to house theinside plantequipment including telephone switches.

An exchange area is the geographic region served by a particular switch, but is typically known as a rate

centerorwire centerin the US telecommunications industry. This establishes local calling areas, in which it is

not necessary to pay along-distancerate, but they typically cover more than one rate center even

inmicropolitanand smallmetropolitan areas. The exchange code orcentral office code, orprefix, is the set of

the initial digits of the subscriber number. Historically, the prefix had one, two, or three digits, the latter being

the firmly established dial plan since the mid-1900s.

Rate centers formobile phonestypically cover a much larger area thanlandlinerate centers

inurbanandsuburbanareas, and often include dozens of codes. In theUnited States, local exchange areas

together make up a legal entity calledlocal access and transport areas (LATA) under theModification of Final

Judgment(MFJ).

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Contents

[hide]

1 Historic perspective

o

1.1 Number plan details

2 Technologies

o 2.1 Manual service exchanges

o 2.2 Early automatic exchanges

2.2.1 Electromechanical signaling

2.2.2 Sounds

2.2.3 Maintenance tasks

2.2.4 Electronic switches

o

2.3 Digital switches

o 2.4 The switch's place in the system

3 Switch design

4 Switch control algorithms

o 4.1 Fully connected mesh network

o 4.2 Clos's nonblocking switch algorithm

5 Fault tolerance

6 Internet exchanges

7 See also

8 Notes

9 External links

[edit]Historic perspective

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Tivadar Pusks, inventor of the telephone exchange.

Exchange in Miskolc, Hungary

In the era of the electrical telegraph, post offices, railway stations, the more important governmental centers

(ministries), stock exchanges, very few nationally distributed newspapers, the largest internationally important

corporations and wealthy individuals were the principle users of such telegraphs.[1]Despite the fact that

telephone devices existed before the invention of the telephone exchange, their success and economical

operation would have been impossible on the sameschemaand structure of the contemporary telegraph, as

prior to the invention of the telephone exchange switchboard, early telephones were hardwired to and

communicated with only a single other telephone (such as from an individual's home to the person's business).

A telephone exchange is a telephone system located at service centers (central offices) responsible for a small

geographic area that provided the switching or interconnection of two or more individual subscriber lines for

calls made between them, rather than requiring direct lines between subscriber stations. This made it possible

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for subscribers to call each other at homes, businesses, or public spaces. These made telephony an available

and comfortable communication tool for everyday use, and it gave the impetus for the creation of a whole new

industrial sector.

One of the first people to build a telephone exchange wasHungarianTivadar Pusksin 1877 while he was

working forThomas Edison.[2][3][4][5]

The first experimental telephone exchange was based on the ideas of

Pusks, and it was built by theBell Telephone CompanyinBostonin 1877.[6]

George W. Coy designed and

built the first commercial telephone exchange which opened inNew Haven, Connecticutin January, 1878. The

switchboard was built from "carriage bolts, handles from teapot lids and bustle wire" and could handle two

simultaneous conversations.[7]

Charles Gliddenis also credited with establishing an exchange in Lowell, MA.

with 50 subscribers in 1878.

In Europe the earliest telephone exchanges were based inLondonandManchester, both of which opened

under Bell patents in 1879.[8]

The first in Germany was opened in Berlin 1881.[9]

Belgium had its

firstInternational Bellexchange (inAntwerp) a year later.

Later exchanges consisted of one to several hundredplug boardsstaffed bytelephone operators. Each

operator sat in front of a vertical panel containing banks of -inchtip-ring-sleeve(3-conductor) jacks, each of

which was the local termination ofasubscriber'stelephone line. In front of the jack panel lay a horizontal panel

containing two rows of patch cords, each pair connected to acord circuit. When acalling partylifted the

receiver, a signal lamp near the jack would light.[10]

The operator would plug one of the cords (the "answering cord") into the subscriber's jack and switch her

headset into the circuit to ask, "Number, please?" Depending upon the answer, the operator might plug the

other cord of the pair (the "ringing cord") into the called party's local jack and start the ringing cycle, or plug into

atrunkcircuit to start what might be a long distance call handled by subsequent operators in another bank of

boards or in another building miles away. In 1918, the average time to complete the connection for a long-

distance call was 15 minutes.[10]

In theringdownmethod, the originating operator called another intermediate operator who would call the called

subscriber, or passed it on to another intermediate operator.[11]

This chain of intermediate operators could

complete the call only if intermediate trunk lines were available between all the centers at the same time. In

1943 when military calls had priority, a cross-country US call might take as long as 2 hours to request and

schedule in cities that used manual switchboards for toll calls.

On March 10, 1891,Almon Brown Strowger, an undertaker inKansas City, Missouri, patented thestepping

switch, a device which led to the automation of telephone circuit switching. While there were many extensions

and adaptations of this initial patent, the one best known consists of 10 levels or banks, each having 10

contacts arranged in a semicircle. When used with a rotarytelephone dial, each pair of digits caused the shaft

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of the central contact "hand" of the stepping switch to first step (ratchet) up one level for each pulse in the first

digit and then to swing horizontally in a contact row with one small rotation for each pulse in the next digit.

Laterstepping switcheswere arranged in banks, the first stage of which was a linefinder. If one of up to a

hundred subscriber lines had the receiver lifted "off hook", a linefinder connected the subscriber's line to a free

first selector, which returned the subscriber adial toneto show that it was ready to receive dialed digits. The

subscriber's dial pulsed at about 10 pulses per second, although the speed depended on the standard of the

particular telephone administration.

Exchanges based on the Strowger switch were eventually challenged byother exchange typesand later

bycrossbartechnology. These exchange designs promised faster switching and would accept pulses faster

than the Strowger's typical 10 ppstypically about 20 pps. At a later date many also acceptedDTMF"touch

tones" or other tone signaling systems.

A transitional technology (from pulse to DTMF) had DTMF link finders which converted DTMF to pulse, to feedto older Strowger, panel, or crossbar switches. This technology was used as late as mid 2002.

[edit]Number plan details

Further information:Telephone number

[edit]Technologies

This article will use the following terms:

manual service for a condition where a human operator routes calls inside an exchange and a dial is not

used

dial service for an exchange where calls are routed by a switch interpreting dialed digits

telephone exchange for the building housing the switching equipment

telephone switch for the switching equipment

concentratorfor a device that concentrates traffic, be it remote or co-located with the switch

off-hookfor a tip condition or to describe a circuit that is in use (i.e., when a phone call is in progress)

on-hookfor an idle circuit (i.e., no phone call is in progress)

wire centerfor the area served by a particular switch or central office

Many of the terms in this article have conflicting UK and US usages.

central office originally referred to switching equipment and its operators. Now it is used generally for the

building housing switching and relatedinside plantequipment.

telephone exchange means an exchange building in the UK, and is also the UK name for a telephone

switch, and also has a legal meaning in U.S. telecoms.

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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.

[edit]Manual service exchanges

1924 PBX switchboard

With manual service, the customer lifts the receiveroff-hookand asks theoperatorto connect the call to a

requested number. Provided that the number is in the same central office, the operator connects the call by

plugging into the jack on theswitchboardcorresponding to the called customer's line. If the call is to another

central office, the operator plugs into the trunk for the other office and asks the operator answering (known as

the "inward" operator) to connect the call.

Most urban exchanges provided common-battery service, meaning that the central office provided power for

the telephone circuits. In common-battery systems, the pair of wires from a subscriber's telephone to the

exchange carry 48V (nominal) DC potential from the telephone company end across the conductors. The

telephone presents an open circuit when it ison-hookor idle.[12]

When a subscriber's phone is off-hook, it connects an electrical resistor across the line which causes current to

flow through the telephone and wires to the central office. In a manually operated switchboard, this current

flowed through a relay coil actuating a buzzer and lamp on the operator's switchboard. The buzzer and lamp

would tell an operator the subscriber's phone was off-hook, requesting service.[12]

In the largest U.S. cities, it took many years to convert every office to automatic equipment, such as panel

switches. During this transition period, it was possible to dial a manual number and be connected without

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requesting an operator's assistance. This was because the policy of theBell Systemwas that customers

should not need to know whether they were calling a manual or automated office.

If a subscriber dialed a manual number, an inward operator would answer the call, see the called number on a

display device, and manually connect the call. For instance, if a customer calling from TAylor 4725 dialed a

manual number, ADams 1233, the call would go through, from the subscriber's perspective, exactly as a call to

LEnnox 5813, in an automated exchange.

In contrast to the common-battery system, smaller towns with manual operator service often hadmagneto, or

crank, phones. Using a magneto phone, the subscriber turned a crank to generate ringing current, to gain the

operator's attention. The switchboard would respond by dropping a metal tab above the subscriber's line jack

and sounding a buzzer.Dry cellbatteries (normally two large No. 6 cells) in the subscriber's telephone provided

the DC power for conversation. Magneto systems were in use in one American small town, Bryant

Pond,Woodstock, Maineas late as 1983. In general, this type of system had a poorer call quality compared to

common-battery systems.

Many small town magneto systems featuredparty lines, anywhere from two to ten or more subscribers sharing

a single line. When calling a party, the operator would use a distinctiveringing signalsequence, such as two

long rings followed by one short. Everyone on the line could hear the rings, and of course could pick up and

listen in if they wanted. On rural lines which were not connected to a central office (thus not connected to the

outside world), subscribers would crank the correct sequence of rings to reach their party.

[edit]Early automatic exchanges

A rural telephone exchange building in Australia.

cal adjustments. Unlike modern switches, a circuit connecting a dialed call through an electromechanical switch

actually had DC continuity. The talking path was a physical, metallic one.

In all systems, subscribers were not supposed to notice changes in quality of service because of failures or

maintenance work. A variety of tools referred to as make-busys were plugged into electromechanical switch

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elements during repairs or failures. A make-busy would identify the part being worked on as in-use, causing the

switching logic to route around it. A similar tool was called a TD tool. Subscribers who got behind in payments

would have their service temporarily denied (TDed). This was effected by plugging a tool into the subscriber's

office equipment (Crossbar) or line group (step). The subscriber could receive calls but could not dial out.

Strowger-based, step-by-step offices in the Bell System were under continual maintenance. They required

constant cleaning. Indicator lights on equipment bays in step offices alerted staff to conditions such as blown

fuses (usually white lamps) or a permanent signal (stuck off-hook condition, usually green indicators.) Step

offices were more susceptible to single-point failures than newer technologies.

Crossbar offices used more shared, common control circuits. For example, a digit receiver (part of an element

called an Originating Register) would be connected to a call just long enough to collect the subscriber's dialed

digits. Crossbar architecture was more flexible than step offices. Later crossbar systems had punch-card-based

trouble reporting systems. By the 1970s,automatic number identificationhad been retrofitted to nearly all step-

by-step and crossbar switches in the Bell System.

[edit]Electronic switches

The firstElectronic Switching Systemswere not entirely digital. TheWestern Electric1ESS switchhadreed

relaymetallic paths which werestored-program-controlled. Equipment testing, changes to phone numbers,

circuitlockoutsand similar tasks were accomplished by typing on a terminal. Northern TelecomSP1, Ericsson

AKE, PhilipsPRX/A, ITT Metaconta, British TelecomTXEseries and several other designs were similar.

These systems could use the old electromechanical signaling methods inherited from crossbar and step-by-

step switches. They also introduced a new form of data communications: two 1ESS exchanges could

communicate with one another using a data link calledCommon Channel Interoffice Signaling, (CCIS). This

data link was based on CCITT 6, a predecessor toSS7.

[edit]Digital switches

A typical satellite PBX with front cover removed.

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Digital switches work by connecting two or more digital circuits together, according to a dialedtelephone

number. Calls are set up between switches using theSignalling System 7protocol, or one of its variants. In

U.S. and militarytelecommunication, a digital switch is a switch that performs time division switching of

digitized signals.[13]

This was first done in a few small and little used systems. The first product using a digital switch system was

made byAmtelco. Prominent examples includeITT System 12, NortelDMS-100, Lucent5ESS switch,

SiemensEWSDand EricssonAXE telephone exchange. With few exceptions, such asPAMswitches,[14]

most

switches built since the 1980s are digital. This article describes digital switches, including algorithms and

equipment.

A digital exchange (NortelDMS-100) used by an operator to offer local and long distance services inFrance. Each switch typically serves 10,000-

100,000+ subscribers depending on the geographic area

Digital switches encode the speech going on, in 8000 time slices per second. At each time slice, a

digitalPCMrepresentation of the tone is made. The digits are then sent to the receiving end of the line, where

the reverse process occurs, to produce the sound for the receiving phone. In other words, when someone uses

a telephone, the speaker's voice is "encoded" then reconstructed for the person on the other end. The

speaker's voice is delayed in the process by a small fraction of one second it is not "live", it is

reconstructed delayed only minutely. (See below for more info.)

Individuallocal looptelephone lines are connected to aremote concentrator. In many cases, the concentrator

is co-located in the same building as the switch. The interface between remote concentrators and telephone

switches has been standardised byETSIas theV5protocol. Concentrators are used because most telephones

are idle most of the day, hence the traffic from hundreds or thousands of them may be concentrated into only

tens or hundreds of shared connections.

Some telephone switches do not have concentrators directly connected to them, but rather are used to connect

calls between other telephone switches. These complex machines (or a series of them) in a central exchange

building are referred to as "carrier-level" switches ortandem switches.

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Some telephone exchange buildings in small towns now house only remote orsatellite switches, and are

homed upon a "parent" switch, usually several kilometres away. The remote switch is dependent on the parent

switch for routing and number plan information. Unlike adigital 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 atelephone service providerorcarrierand located in

their premises, but sometimes individual businesses or private commercial buildings will house their own

switch, called a PBX, orPrivate branch exchange.

Map of the Wire Center locations in the US

Map of the Central Office locations in the US

[edit]The switch's place in the system

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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 theoutside plant. In the middle 20th century, each subscriber

telephone number required an individual pair of wires from the switch to the subscriber's phone.

A typical central office may have tens-of-thousands of pairs of wires that appear on terminal blocks called

themain distribution frameorMDF. A component of the MDF is protection: fuses or other devices that protect

the switch from lightning, shorts with electric power lines, or other foreign voltages. In a typical telephone

company, a large database tracks information about each subscriber pair and the status of each jumper. Before

computerization of Bell System records in the 1980s, this information was handwritten in pencil in accounting

ledger books.

To reduce the expense of outside plant, some companies use "pair gain" devices to provide telephone service

to subscribers. These devices are used to provide service where existing copper facilities have been exhausted

or by siting in a neighborhood, can reduce the length of copper pairs, enabling digital services such

asISDNorDSL.

Pair gain ordigital loop carriers(DLCs) are located outside the central office, usually in a large neighborhood

distant from the CO. DLCs are often referred to asSubscriber Loop Carriers(SLCs), after aLucentproprietary

product.

DLCs can be configured as universal (UDLCs) or integrated (IDLCs). Universal DLCs have two terminals, a

central office terminal (COT) and a remote terminal (RT), that function similarly. Both terminals interface with

analog signals, convert to digital signals, and transport to the other side where the reverse is performed.

Sometimes, the transport is handled by separate equipment. In an Integrated DLC, the COT is eliminated.

Instead, the RT is connected digitally to equipment in the telephone switch. This reduces the total amount of

equipment required.

Switches are used in both local central offices and inlong distancecenters. There are two major types in

thePublic switched telephone network(PSTN):

1. Class 4 telephone switchesdesigned for toll or switch-to-switch connections.

2. Class 5 telephone switchesor subscriber switches, which manage connections from subscriber

telephones. Since the 1990s, hybrid Class 4/5 switching systems that serve both functions have

become common.

Another element of the telephone network is time and timing. Switching, transmission and billing equipment

may be slaved to very high accuracy10 MHz standardswhich synchronize time events to very close intervals.

Time-standards equipment may include Rubidium- or Caesium-based standards and aGlobal Positioning

Systemreceiver.

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[edit]Switch design

Long distance switches may use a slower, more efficient switch-allocation algorithm thanlocal central offices,

because they have near 100% utilization of their input and output channels. Central offices have more than

90% of their channel capacity unused.

Traditional telephone switches connected physical circuits (e.g., wire pairs) while modern telephone switches

use a combination ofspace-and time-division switching. In other words, each voice channel is represented by

atime 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.

Thestructure of a switchis 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 anonblocking minimal

spanning switch, or acrossover switch.

[edit]Switch control algorithms

[edit]Fully connected mesh network

One way is to have enoughswitching fabricto assure that the pairwise allocation will always succeed by

building afully connected mesh network. This is the method usually used in central office switches, which have

low utilization of their resources.

[edit]Clos's nonblocking switch algorithm

Main article:Nonblocking minimal spanning switch

The scarce resources in a telephone switch are the connections between layers of subswitches. The control

logic has to allocate these connections, and most switches do so in a way that isfault tolerant. See nonblocking

minimal spanning switch for a discussion of theCharles Closalgorithm, used in many telephone switches, and

a very important algorithm to the telephone industry.

[edit]Fault tolerance

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Composite switches are inherently fault-tolerant. If a subswitch fails, the controlling computer can sense it

during a periodic test. The computer marks all the connections to the subswitch as "in use". This prevents new

calls, and does not interrupt old calls that remain working. As calls in progress end, the subswitch becomes

unused, and new calls avoid the subswitch because it's already "in use." Some time later, a technician can

replace the circuit board. When the next test succeeds, the connections to the repaired subsystem are marked

"not in use," and the switch returns to full operation.

To prevent frustration with unsensed failures, all the connections between layers in the switch are allocated

usingfirst-in-first-out lists(queues). As a result, if a connection is faulty or noisy and the customer hangs up

and redials, they will get a different set of connections and subswitches. Alast-in-first-out(stack) allocation of

connections might cause a continuing string of very frustrating failures.

[edit]Internet exchanges

The telephone exchange concept has been adapted for use inInternet exchanges.Voice over IP(VoIP) traffic

may pass through both kinds of exchanges, depending on what kind of service the caller and the called

subscriber are using.

[edit]See also

History of telecommunication

List of telephone switches

Pair gain system

Full Availability, Limited Availability and Gradings

Softswitch

Stored Program Control exchange

Strowger switch

Telephone number

DSLAM

DSL

ISDN

PDHPlesiochronous Digital Hierarchy

PBXPrivate Branch Exchange or business-level switch

Telephone exchange names

First telephone exchange in UK - Faraday building

In UStelecommunicationjargon, a central office (C.O.) is acommon carrierswitching centerClass 5 telephone

switchesin which trunks andlocal loopsare terminated and switched.[15]

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Note: In the DOD, "common carrier" is called "commercial carrier."Synonyms exchange, local central

office,local exchange, local office, switching center (except in DODDefense Switched

Network(formerlyAUTOVON) usage), switching exchange, telephone exchange. Deprecated

synonymswitch.[15]

[edit]Notes

1. ^Private Telegraphs,The Sydney Morning Herald, credited toThe Times, April 19, 1878, p. 6.

2. ^http://www.hungarian-history.hu/mszh/epuskas.htm

3. ^"SZTNH". Mszh.hu. Retrieved 2012-07-01.

4. ^"Pusks, Tivadar". Omikk.bme.hu. Retrieved 2012-07-01.

5. ^"Welcome hunreal.com - BlueHost.com". Hunreal.com. Retrieved 2012-07-01.

6. ^Frank Lewis Dyer: Edison His Life And Inventions. (page: 71)

7. ^SeeNational Park Service "first switchboard" page.

8. ^http://www.mosi.org.uk/media/33871608/early%20manchester%20telephone%20exchanges.pdf

9. ^"Siemens History Site - Information & Communications". Siemens.com. Retrieved 2012-07-01.

10. ^abCalvert, J. B. (2003-09-07)."Basic Telephones". Retrieved 2007-09-13.

11. ^Calvert, J. B. (2003-09-07)."Basic Telephones, The Switchboard (ringdown is near bottom)". Retrieved 2006-09-13.

12. ^abConnected to a switch, an off-hook condition operates a relay to connect a dial tone and a device to collect dialed digits.

13. ^ This article incorporatespublic domain materialfrom theGeneral Services Administrationdocument "Federal Standard

1037C"(in support ofMIL-STD-188).

14. ^(Ronayne 1986, p. 12).

15. ^abSource: fromFederal Standard 1037C.

Ronayne, John P. (1986). Introduction to Digital Communications Switching(1st edition ed.). Indianapolis:

Howard W. Sams & Co., Inc.ISBN0-672-22498-4.

[edit]External links

Wikimedia Commons has

media related to:Telephone

exchanges

Hundreds of Telephone Central Office Pictures

Telephone Central Office History and Pictures

Telephone Central Office Building Pictures (historical preservation)

History of Central Offices

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Clive Feather's guide to the BT network

Basic Telephones Technology

Roger W. Haworth's guide to London (UK) Director Exchange Names

National Park Service's page about the first telephone exchange

patent 252,576 for the first telephone switchboard in 1881

A Telecom Exchange Tour in NZ

Picture collection Telephon and Exchange

Alexander Graham Bell patented the first Telephone instrumentcapable of practical use in 1876. This method was used in the firstcommercial instrument developed by Bell in 1876. In 1878, thefirst telephone exchange was established at New Haven.

In 1880, two Telephone Companies viz. The Oriental TelephoneCompany Ltd. and The Anglo-Indian Telephone Company Ltd.approached the Govt. of India for permission

to establish Telephone Exchanges in India. The permission was however refused on the grounds that the

establishment of Telegraphs was a Government monopoly and that the Government itself would undertakethe work in the event of sufficient demand. By 1881, Govt. of India changed their earlier decision and licencewas granted to the original Oriental Telephone Company Limited of England for opening TelephoneExchanges at Calcutta, Bombay, Madras, Karachi and Ahmedabad.

28th January, 1882, is a Red Letter Day in the history of Telephone in India. On this day Major E. Baring,Member of the Governor General's Council declared open the Telephone Exchange in Calcutta, Madras andBombay. The exchange at Calcutta named "Central Exchange" was opened at third floor of the building at 7,Council House Street. On 30-06-1882, the Central Telephone Exchange had 93 number of subscribers.

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A distant view of old Writers Building taken before the Dalhousie Institute was built within DalhousieSquare. The foundation of Dalhousie Institute was laid on 4th March 1865

On 1899, The Central Tele- phone Exchange wsa shifted to 1,Council House Street. The management of the Oriental Telephone Company was subsequently takenover by Bengal Telephone Company Limited. The telephone system in the city remained undermanagement of Private Company till 1941 when all the shares of the Private company were purchased by aPublic Enterprise. The capital expen- diture involved in this deal was Rs 117 lakhs only. From 1st April1943, the control of the Telephone system in Calcutta,Madras and Bombay was taken over directly by the

Indian Posts and Telegraphs Deptt. In 1985, Indian P & T was bifurcated and the control of Telephone hasbeen transferred to Deptt. Of Telecom. On 01-10-2002, the telephone system of Calcutta came underBharat Sanchar Nigam Limited (BSNL) alongwith all other circles except the city of Delhi and Mumbay whichare under Mahanagar Telephone Nigam Limited (MTNL).

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Dalhousie Institute Building - demolished on 1950 to make space for construction of TelephoneBhawan

Calcutta Telephone District CTD) is the largest metro district of BSNL. Calcutta Telephones is having aservice area of 1900 sq. k.m. covering the city of Kolkata and adjoining areas from five districts of WestBengal viz. Howrah, Hooghly, Nadia, North 24 Parganas and South 24 Parganas.

At the time of Independence there w