This report is on Industrial Attachment under BTCL, Rajshahi.

1. Heaven’s light is our guide
RAJSHAHI UNIVERSITY OF ENGINEERING & TECHNOLOGY
( RUET ), RAJSHAHI
DEPARTMENT OF ELECTRICAL & ELECTRONIC
ENGINEERING
Ahosan Habib
Roll: 1501113
Section: B
Course No.: EEE 4100
Industrial Training

2. DECLARATION
This is to certify that the attachment work entitled “REPORT on INDUSTRIAL ATTACHMENT at BTCL,
Rajshahi has been carried out by Ahosan Habib, Roll: 1501113 under my supervision in the Department of
Electrical and Electronic Engineering at Rajshahi University of Engineering & Technology, Rajshahi.
Attachment Supervisor
………………………….
(Tasneem Sarkar Joyeeta)
Department of Electrical and Electronic Engineering
Rajshahi University of Engineering & Technology
Rajshahi 6204, Bangladesh

3. Acknowledgement
I accomplished the attachment successfully. Many people have given me their blessings and the
heart pledged support. At first, I would like to thank the Almighty Allah for guiding me to the right
path. I wish to express my gratitude to my parents for overwhelming love and care they bestow
upon me without whose proper guidance it would have been impossible for me to be in this
platform.
I am deeply grateful and humble to my respected supervisor, Tasneem Sarkar Joyeeta, Lecturer,
Department of Electrical and Electronic Engineering, Rajshahi University of Engineering &
Technology for her guidance, help and encouragement and kind consideration in many ways
throughout this industrial attachment.
My special and profound respect to all my teachers for their encouragement and support through
my entire life.
September, 2022 Author
RUET, Rajshahi Ahosan Habib
Roll:1501113

4. Chapters
Chapter One: Introduction of BTCL …………………………………….. 1-2
Chapter Two: Objectives ………………………………………………….. 3-3
Chapter Three: Important Services of BTCL …………………………… 4-14
Chapter Four: Constructional Features of BTCL ………………………. 15-22
Chapter 5: Cables used in BTCL …………………………………………. 23-31
Chapter Six: Equipments used by BTCL ………………………………… 32-42
Chapter Seven: Operational Features of BTCL …………………………. 43-50
Chapter Eight: CARDS USED BY BTCL ………………………………... 51-51
Chapter Nine: Salient Tariff of BTCL ……………………………………. 52-52
Chapter Ten: Strength & Opportunities of BTCL ………………………. 53-53
Chapter Eleven: Conclusion ……………………………………………….. 53-53
Pages

5. List of Figures:
i) Fig. 3.1. Schematic diagram flow chart of ADSL …………………. 13
ii) Fig.4.1. DC & AC Distribution System ……………………………. 16
iii) Fig 4.2. Server System ……………………………………………... 17
iv) Fig.4.3. PCM Coder ………………………………………………... 18
v) Fig.4.4. Slots with Subscriber Cards ……………………………….. 19
vi) Fig4.5. Subscriber Card …………………………………………….. 19
vii) Fig.4.6. Slots with Subscriber Cards ……………………………….. 20
viii) Fig.4.7. Ports for Subscriber Line ………………………………….. 21
ix) Fig.4.8. Subscriber Lines …………………………………………... 21
x) Fig. 5.1. CAT5 & CAT6 Cable .……………………………………. 24
xi) Fig.5.2. FTTP Cable ……………………………………………….. 25
xii) Fig. 5.3. FTTC Cable ………………………………………………. 26
xiii) Fig. 5.4. FTTB Cable ………………………………………………. 27
xiv) Fig. 5.5. Submarine Cable ………………………………………….. 28
xv) Fig.5.6. Underground Cable ………………………………………... 30
xvi) Fig.5.7. Splitter Cable ………………………………………………. 31
xvii) Fig.6.1. Passcot Cable connection ………………………………….. 32
xviii) Fig.6.2. SFP ………………………………………………………… 33
xix) Fig.6.3. ADSL Modem ……………………………………………… 35
xx) Fig.6.4. Optical Power Meter ……………………………………….. 37
xxi) Fig.6.5. LC-LC Passcot Cable ………………………………………. 38
xxii) Fig. 6.6. Ethernet Media Converter …………………………………. 40
xxiii) Fig. 6.7. ONU ……………………………………………………...... 42
xxiv) Fig.7.1. Analog & Sampled Signal ………………………………..... 44
xxv) Fig.7.2. Quantized Signal …………………………………………… 45
xxvi) Fig.7.3. QPSK Signal ……………………………………………...... 46
xxvii) Fig.7.4. Circuit Switching …………………………………………... 48
xxviii) Fig.7.5. Message Switching ……………………………………... 48
xxix) Fig.7.6. Packet Switching …………………………………………… 49
Pages

6. 1
Chapter One: Introduction of BTCL
In the field of communication, telecommunication gives us the superior facilities by
which long distance communications have become possible. "Tele" means "Long
Distance" and "Communication" means "Exchange of Information". The imparting
or exchanging of information by speaking, writing, or using some other medium is
called telecommunication. In Bangladesh, the telecommunication system is
controlled by BTCL (Bangladesh Telecommunication Company Limited)
Bangladesh Telecommunication Company Limited (BTCL) is the largest
telecommunication company in Bangladesh. The company was founded as
the Bangladesh Telegraph & Telephone Board (BTTB) following Bangladesh's
independence in 1971. Formerly it was under control of Bangladesh Government
but on July 1, 2008 the BTTB became a public limited company and was renamed
as "BTCL". The Bangladesh government initially owned all BTCL shares, but stated
it would sell the shares to the public at 2009. The value of BTCL is estimated to be
at 150 billion tk. Now BTCL has a total of 12,636 officials and staff. We went for
our industrial attachment at BTCL, Rajshahi.
Without BTCL, Bangladesh can’t think of improving its telecommunication system.
Following Bangladesh's independence, the Bangladesh Telegraph and Telephone
Department was set up under the Ministry of Posts and Telecommunications in 1971.
This was converted into a corporate body named 'Telegraph and Telephone Board'
by promulgation of Telegraph and Telephone Board Ordinance, 1975. Pursuant to a
1979 ordinance, the Telegraph and Telephone Board was converted into a
government board named the Bangladesh Telegraph and Telephone Board (BTTB).
On 1 July 2008, BTTB transformed to a government-owned Public Limited
Company under a new name of Bangladesh Telecommunications Company Limited
BTCL. BTCL has launched a 24-hour call center for customers. BTCL runs a red
telephone exchange for the VIPs which is secured and always live. BTCL provides
land-line telephone services in Bangladesh's urban areas, including domestic long
distance calling and international services as well as internet services.

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BTCL Rajshahi
We did our industrial training in BTCL, Rajshahi. Here they taught us different
functions of BTCL. Mr. MD. Abdul Mannan, Divisional Engineer of BTCL,
Rajshahi appointed their three officials to train us.

8. 3
Chapter Two: Objectives
There are various objectives for doing the attachment at BTCL.
❑ To learn about BTCL
❑ To know about "Telecommunication System"
❑ To get familiar with a "Switching Centre" or an "Exchange"
❑ To know how the internet connections are established
❑ To know the tariff system of BTCL
❑ To know about different Equipments and their operation
❑ To get familiar about the history of BTCL

9. 4
Chapter Three: Important Services of BTCL
❑ Land-line telephone services
❑ Internet services
❑ Gateway services: International Gateway (IGW), Interconnection exchange
(ICX), International Internet Gateway (IIG)
❑ Web services: Domain Name Registration, Web Hosting, DNS Parking
❑ Transmission services: Transmission Bandwidth, Tower Facility, Co-location
Facility, IPLC Local Loop
❑ PSTN (Public Switched Telephone Network)
❑ GPON (Gigabyte Passive Optical Network)
❑ LLI (Leased Line Internet)
❑ ADSL (Assymetric Digital Subscriber Line)
❑ ISP ( Internet Service Provider)
3.1 Land Line Telephone Services
A landline is a telephone connection that works on analog technology such as copper
wires to complete calls. Essentially, your voice travels along a physical line to
connect with the other caller.
As technology has evolved, new types of landline phone services have been
introduced, such as voice over internet protocol (VoIP) that works through an
internet connection, and wireless landline service that connects to cell towers in your
area.
While landline connections were often more reliable, they lacked the flexibility that
wireless phones offered and its speed is lower to establish a call than wireless
connection. That’s why BTCL is seen providing Land Line connections hardly now-
a-days.

10. 5
3.2 PSTN
The full form of PSTN is Public Switched Telephone Network. The public switched
telephone network, or PSTN, is the world's collection of interconnected voice-
oriented public telephone networks. PSTN is the traditional circuit-
switched telephone network. PSTN comprises all the switched telephone networks
around the world that are operated by local, national or international carriers. These
networks provide the infrastructure and services for public telecommunication.
PSTN is the mother network service. This is the system that has been in general use
since the late 1800s. It's the aggregation of circuit-switching telephone networks that
has evolved from the days of Alexander Graham Bell.
A public switched telephone network is a combination of telephone networks used
worldwide, including telephone lines, fiber optic cables, switching centers, cellular
networks, satellites and cable systems. A PSTN lets users make landline telephone
calls to one another. A PSTN is made up of switches at centralized points on a
network that function as nodes to enable communication between two points on the
network. A call is placed after being routed through multiple switches. Voice signals
can then travel over the connected phone lines. It was the main mode of home
internet access but now-a-days newly introduced internet methods make PSTN
obsolete. The PSTN phone line is used with traditional dial-up network modems to
connect a computer to the internet. Dial-up internet connections support up to 56
Kbps. In the early days of the internet, this was the main method for home internet
access. The traditional PSTN has a hierarchical architecture and a star structure. The
individual subscriber lines are connected to a local exchange, which communicates
with trunk exchanges as well as main and central exchanges. The lines within a local
exchange typically have the same area code. A user who wants to call a number
outside the local exchange has to add an area code. To make an international call, a
user has to dial the country code. The code for Dhaka is 02.
Actually, PSTN is nothing but telephones. PSTN phones are widely used and
generally still accepted as a standard form of communication. However, they have
seen a steady decline over the last decade. In fact, there are currently just 972 million

11. 6
fixed-line telephone subscriptions in use worldwide, the lowest tally this century so
far.
Working steps of PSTN telephone:
Step 1 – 1st
telephone set converts sound waves into electrical signals. These signals
are then transmitted to a terminal via a cable.
Step 2 – The terminal collects the electrical signals and transmits these to the central
office.
Step 3 – The central office routes the calls in the form of electrical signals through
fiber optic cable. The fiber optic conduit then carries these signals in the form of
light pulses to their final destination.
Step 4 – Your call is routed to a tandem office (a regional hub responsible for
transmitting calls to distant central offices) or a central office (for local calls).
Step 5 – When your call reaches the right office, the signal is converted back to an
electrical signal and is then routed to a terminal.
Step 6 – The terminal routes the call to the appropriate telephone number. Upon
receiving the call, the telephone set converts the electrical signals back to sound
waves.
PSTNs are all about switching, which forms the backbone of traditional phone
networks. When a call is made, switches create a wire circuit between two
telephones, with this particular connection lasting as long as the duration of the call.

12. 7
3.3 GPON
Gigabyte Passive Optical Networks (GPON’s) are networks which rely on optical
cables to deliver information. GPON’s are currently the leading form of Passive
Optical Networks. GPONS offer up to a 1:64 ratio or 1:128 ratio on a single fiber.
As opposed to a standard copper wire in most networks, GPON’s are 95% more
energy efficient. In addition to efficiency, gigabyte passive optical networks provide
a low cost solutions to adding users through splitters which make GPON’s desirable
in populated areas. It uses OLT (Optical Line Transfer).
A TDMA, time division multiple access, format is used in GPON in order to
designate bandwidths to each ONU. Depending on the service provider and how
they decide to allocate the bandwidth, each ONU can receive a downstream rate of
2.488 Gbits/s.
The upstream rate however will receive less than its maximum due to the sharing of
other ONUs. The ONUs will communicate with the OLT and it will determine the
distance and time delay for every subscriber.

13. 8
3.4 LLI (Leased Line Internet)
It alludes to a reliable communication network that perfectly connects two or more
sites. In other words, a leased line exists between a service provider and consumer
as an agreement of service. Through this dedicated line of a network, data flows
continuously from one end to another. And the service providers bestow a fixed rate
on this connection of data flow monthly.
The dedicated leased line connection is endowed with the cable of the optical fiber
so that it holds the capacity of delivering the best quality connectivity speed and
bandwidth. Leased line connectivity is commonly used for:
• Establishing a connection to the internet
• Placing and carrying phone calls
• Making it feasible for employees and other staff to work from home on their
personal computers.
• Establishing the connection and linking computers and servers in various and
distinctive workplaces.
BTCL delivers a bidirectional line for telecommunications through LLI and it helps
establish connectivity between two or more locations. And the renting value differs
from provider to provider based on monthly rent. Various businesses require strong
and consistent internet connectivity, speed and bandwidth to enhance business
processes and other dedicated applications. Other businesses solely depend on
telecommuting or remote employees for accessing office applications and delivering
the right data to other sources. The leased lines come with various options as they
significantly fall into certain dedicated categories.

14. 9
The several options of leased lines are:
EFM or Ethernet in the First Mile
EFM is also known as Ethernet broadband. This connection sets up a network and
delivers data to businesses through the traditional copper network, thereby providing
a full-fibre ethernet connection in place of exchange. EFM uses aggregated copper
pairs for providing seamless connectivity. There are many coppers, so if one fails,
the other takes over and comes into action.
These kinds of leased lines are used by those businesses that do not possess a cabinet
of local fibre. The speed of this connectivity subsides unnaturally and is much lesser
than that of the regular optical fibres.
Full Fibre
The full fibre internet connection is better known as Ethernet Access Direct and
Fibre Ethernet. The performance, speed and bandwidth of this connectivity are
excessive and are endowed with the name of “big daddy” in the world of internet
connectivity. It is established as a direct and superfast ethernet connection with
optical fibre cables.
It connects the optical fibre cables from the specific business premises with 100%
dedication to the ISP, thereby delivering speeds up to 1Gbps. This leased line
connection is the most expensive as it is immensely reliable for providing the fastest
speed.
EoFTTC or Ethernet over FTTC
The EoFTTC is almost very similar to that of the Ethernet in the First Mile, but it
comes in at a much lower price when compared. The cables are made up of a mixture
of copper and dedicated fibre. Businesses extensively use this leased line
connectivity to access the local fibre-based cabinets.
There are chances that the connectivity speed might drop and disrupt because of the
distance of the business set up from the local cabinet. You will come across many
service providers with this symmetric connection. Sometimes it is also found that
the EoFTTC can increase the download speed far beyond the upload speed.

15. 10
3.5 IIG (International Internet Gateway)
IIGs will serve as a gateway for routing International incoming and outgoing Internet
based data traffic. The gateway will be connected with the existing Submarine cable
as main link and with the Satellite Earth Station/VSAT as back up until another
Submarine cable is available. All ISPs shall be connected to global internet through
this IIGs. IIG Licensee will arrange both submarine cable bandwidth, and backup
satellite bandwidth. It can be used for transferring both voice signals and data signal.
3.6 ICX (Interconnection Exchange)
ICXs are connected with IGWs and Access Network Service (ANS) operators.
According to IDLTS Policy, all International Voice Calls including VoIP will be
routed (Terminated to and Originated from Bangladesh) through Interconnection
Exchanges (ICXs) and International Gateways (IGWs). Domestic inter operator
voice call services will be routed through ICXs. ICX improves quality of voice
services; ensures balanced traffic among all operators and easy access for telecom
operators to national telecom network. It also helps keeping records of all calls,
thereby helping authority to identify and monitor any calls, identifying, monitoring
& locating movement of miscreants. ICX can help to reduce mobile phone stealing
by implementing International Mobile Equipment Identification (IMEI). By
enabling number portability ICX can facilitate the service of changing mobile
network without changing mobile number.
3.7 ISP (Internet Service Provider)
ISP is an acronym that stands for Internet Service Provider. An Internet Service
Provider is a company that provides Internet access to organizations and home users.
BTCL is also an internet service provider. The internet provided by BTCL is very
fast and even RUET is using internet from BTCL. An ISP provides Internet access,
usually for a fee.
ISPs also offer services like web hosting, domain name registration, and email
services. BTCL Internet service providers sell access to their networks to ISPs. Tier
2 ISPs then sell Internet access to organizations and home users. However,
sometimes BTCL ISPs may sell Internet access directly to organizations and
individuals.

16. 11
3.8 Co-Location Facilities
A co-location facility is a data center facility in which a business can rent space
for servers and other computing hardware. BTCL provides co-location facilities and
provide towers to flow microwave and other waves for a monthly basis rent. A co-
location provides the building, cooling, power, bandwidth and physical security,
while the customer provides servers and storage. Space in the facility is often leased
by the rack, cabinet, cage or room. Many co-location providers have extended their
offerings to include managed services that support their customers' business
initiatives. For some organizations, colocation might be an ideal solution, but there
can be downsides to this approach. Distance can translate into increased travel costs
when equipment must be manually handled and co-location customers can find
themselves locked into long-term contracts, which might prevent them from
renegotiating rates when prices fall.
3.9 Domain Registration
BTCL PROVIDES .bd domain register. No other ISP provider can give .bd domain.
A domain name registrar provides domain name registrations to the general public.
A domain name registrar is a business that handles the reservation of domain names
as well as the assignment of IP addresses for those domain names. Domain names
are alphanumeric aliases used to access websites. BTCL sells .bd domain register.

17. 12
3.10 Asymmetric digital subscriber line (ADSL)
The full form of ADSL is Asymmetric Digital Subscriber Line. ADSL means to
provide internet service through cupper wire cable. ADSL is a communication
technology that offers faster connection speeds over traditional telephone lines than
dial-up internet provides. It powers many internet connections worldwide and
enabled the broadband internet speeds that drove Web 2.0 and beyond. operates
over the same copper telephone lines that have powered voice calls between
landlines for decades. ADSL transmits digital data over those same lines at high
speed, letting you do everything from sending emails to watching YouTube videos.
For asymmetric digital subscriber line connections to work correctly, several
technologies must operate in tandem. A user needs an ADSL modem, which is
connected by a traditional twisted pair of copper telephone wires and connects to a
local telephone exchange. At that end, the copper wire and others also from the
local area are connected to a digital subscriber line access multiplexer (DSLAM).
The user's ADSL modem transmits and receives data to and from the DSLAM. It
uses frequencies that differentiate the digital data from potential voice calls taking
place on the same phone line. Those two signals are split at the exchange and
handed off to different networking technologies further up the chain, connecting
the user to the telephone or web server at the other end.
ADSL has a hard speed ceiling of around 50 Mbps due to the available bandwidth
of the copper wires that form the basis of the connection. However, there's a lot
that can affect the speed of an ADSL connection, and much of it is not in the hands
of the consumer.
In ADSL, the exchanges process calls, The main exchange in Bangladesh is situated
in Dhaka Sher-E-Bangla Nagar. Every exchange is connected through underground
cupper wire cable. If any call is processed it will be connected to Dhaka exchange
firstly and then from there the call will be processed and come to Rajshahi exchange.
In ADSL, QPSK modulation technique is applied. ADSL was mostly famous before
optical fiber was introduced.

18. 13
Fig. 3.1. Schematic diagram flow chart of ADSL
DSLAM: Digital Subscriber Line Access Multiplexer or DSLAM is a network
device that is used by the Internet Service Provider or ISPs often via locating them
in telephone exchanges or distribution points, in order to connect multiple DSL
(Digital Subscriber Line) to the internet or high-speed digital communicating
channel using the multiplexer (a multiplexer also known as MUX is a network
device that combines many signals into one by allowing multiple signals to travel
via the same communication link). The main aim of the DSLAM technology is to
provide the customer with a high-speed internet connection and this is achieved by
directing multiple incoming requests to the DSLAM to a single multiplexer from
where the device connects to the Internet through its own high-speed capability, the
completion of the task/browsing session is completed by the multiplexer receiving
the response from websites browsed by the user. The DSLAM equipment collects
the data from its many modem ports and aggregates their voice and data traffic into
one complex composite "signal" via multiplexing. Depending on its device
architecture and setup, a DSLAM aggregates the DSL lines over its Asynchronous
Transfer Mode (ATM), frame relay, and/or Internet Protocol network, i.e., an IP-
DSLAM using Packet Transfer Mode - Transmission Convergence (PTM-TC)
protocol(s) stack.
In BTCL, negative voltage is applied to cards usually -48V so that metal corrosion
becomes lower and it becomes risk free for the people.

19. 14
3.11 Dial-up Internet
Dial-up internet service is a service that allows connectivity to the internet through
a standard telephone line. By connecting the telephone line to the modem in your
computer and inserting the other end into the phone jack, and configuring the
computer to dial a specific number provided by your internet service provider (ISP)
you are able to access the internet on your computer.

20. 15
Chapter Four: Constructional Features of BTCL
There are several departments in BTCL. These are:
❑ Maintenance Department
❑ Networking Department
❑ Official Department
❑ Department for the Customers
Here, maintenance & networking departments will be discussed jointly. There are
several sections which are used for networking and maintenance.
1. Power Supply Section
2. Server Section
3. Coding Section
4. Switching Section
5. Transmission Section
6. Maintenance & Troubleshooting Section
4.1 Power Supply Section
This section provides required power to run all the networking and maintenance
equipment’s. There are two power distribution system. One is ac distribution and
another is dc distribution. 220 volts ac voltage is fed by the ac and dc distributor.
Then required dc and ac voltage is taken from the distributor. All the equipment’s
are initiated and run at a negative voltage around -48 volts. Negative voltage is used

21. 16
to protect the equipment’s from being damaged due to instantaneous voltage
increase.
Fig.4.1. DC & AC Distribution System

22. 17
4.2 Server Section
This section is used to provide internet connection. It is used to allocate required
bandwidth. Optical fiber is mainly used as cable because of its ability to transfer data
at a high speed. There are incoming & outgoing ports. Submarine cable is directly
connected with the incoming port & outgoing port is used to give the bandwidth to
the users. Internet is provided with general cable or optical fiber cable. This will be
discussed later.
Fig 4.2. Server System

23. 18
4.3 Coding Section
This section is used for voice processing and signaling purposes. Applicable method
is Pulse Code Modulation (PCM). It is an Analog to Digital Converter (ADC). Low
frequency voice (analog) signal is digitalized here. In this section there is also used
Pulse Code Demodulator which converts digital signal into voice (analog) signal.
Fig.4.3. PCM Coder

24. 19
4.4 Switching Section
This is the most important section. Switching is done here in digital switching
method. All the switching process is preprogrammed by microprocessor. It should
be mentioned that there are several exchanges. Each are used for different purposes.
Incoming & outgoing trunks, subscriber lines are connected in this section. These
lines are two wire lines. There are many slots for the subscribers. Suppose, there are
50 slots each can handle 4 cards then 200 simultaneous calls can happen. Primary of
the Switching section is connected with Transmission section and secondary is
connected with Coding section.
Fig.4.4. Slots with Subscriber Cards Fig4.5. Subscriber Card
These slots & cards were used earlier. There is no application of these equipment’s
nowadays.

25. 20
The following slots & cards are used nowadays.
Fig.4.6. Slots with Subscriber Cards

26. 21
4.5 Transmission Section
This section is directly connected to subscriber lines & trunk lines. These lines are
further connected with switching section. The main purpose of this section is the
connectivity with the outside world.
Fig.4.7. Ports for Subscriber Line
Fig.4.8. Subscriber Lines

27. 22
4.6 Maintenance & Troubleshooting Section
This section is used for looking after the whole system, fault tracing, troubleshooting
& repairing. Maintenance officials are monitoring the system whether there is
problem or not. Sometimes users are facing problems with their connection. They
notify it to the office section. Then troubleshooting process is started. There are
different priority criteria’s for the different customers. When there is a problem in a
subscriber line, the card which is allocated for this subscriber in the server is taken
out. Then it is repaired and has brought into the action again. There are several
computers used in a control room for facing various types of subscriber problems.

28. 23
Chapter 5: Cables used in BTCL
5.1 Copper Cable
BTCL provides PSTN services by Cu cable. Generally it uses CAT5 and CAT6
copper cables. If the customer needs more bandwidth then CAT5 cable is used and
if customer need fast ethernet then BTCL provides CAT6 cable. But it should be
remembered that if the customer’s distance from BTCL is higher, then CAT5
connection is provided and CAT6 is provided I short distance.
5.1.1 CAT5
CAT5 is currently the industry standard for network and telephone wiring. CAT5
contains four pairs of 24-gauge copper wiring and ends in a standard RJ-45 jack.
This type of copper wire cabling is known as twisted pair cabling and is only
recommended for a maximum distance of 100 meters or 328 feet. While it does
contain four lines of copper wire, two of these copper wires lie dormant. CAT5
cables come in two different varieties: solid and stranded. Solid supports longer
distances of ethernet usage and is often used in professional buildings such as an
office park. Stranded CAT5 cables are more flexible and better for shorter distances.
5.1.2 CAT6
CAT6 cabling is slowly taking over as the new industry standard in network
cabling. Category 6 cabling is another variation of twisted pair cabling. Similar to
CAT5, it uses four pairs of copper wire ending in an RJ-45 jack. However, unlike
CAT5, CAT6 uses all four pairs of copper wire to produce faster internet speeds.

29. 24
CAT6 decreases crosstalk throughout the cable by using a tighter twist in the copper
wire. Besides that, CAT6 is considerably thicker than CAT5.
CAT6 supports 10 Gigabit ethernet, but only over a distance of 164 feet. CAT6A
cables are CAT6’s successor and do support 10 Gigabit ethernet over the full
distance of 328 feet. After 164 feet, CAT6’s speed is limited to the same as CAT5e
at 1 Gigabit.
Fig. 5.1. CAT5 & CAT6 Cable

30. 25
5.2 Optical Fiber
Optical fiber is a flexible, transparent fiber made by drawing glass (silica) or plastic
to a diameter slightly thicker than that of a human hair. BTCL provides LLI and
GPON through optical fiber. Fiber optics is the technology used to transmit
information as pulses of light through strands of fiber made of glass or plastic over
long distances.
Optical fibers are about the diameter of a strand of human hair and when bundled
into a fiber-optic cable, they’re capable of transmitting more data over longer
distances and faster than other mediums. It is this technology that provides homes
and businesses with fiber-optic internet, phone and TV services. A fiber-optic cable
contains anywhere from a few to hundreds of optical fibers within a plastic casing.
Also known as optic cables or optical fiber cables, they transfer data signals in the
form of light and travel hundreds of miles significantly faster than those used in
traditional electrical cables. And because fiber-optic cables are non-metallic, they
are not affected by electromagnetic interference and its speed is much higher than
CAT5 and CAT6.
5.2.1 FTTP
The fastest type of fiber network is called Fiber to the Home (FTTH) or Fiber to the
Premises (FTTP) because it’s a 100% fiber-optic connection with optical fiber cables
installed to terminals directly connected to houses, apartment buildings and
businesses.
Fig.5.2. FTTP Cable

31. 26
5.2.2 FTTC
Fiber to the Curb (FTTC) is a partial fiber connection because the optical cables run
to the curb near homes and businesses and copper cables carry the signals from the
curb the rest of the way.
Fig. 5.3. FTTC Cable

32. 27
5.2.3 FTTB
Fiber to the Building (FTTB) is when fiber cable goes to a point on a shared property
and the other cabling provides the connection to offices or other spaces.
Fig. 5.4. FTTB Cable

33. 28
5.3 Submarine Cable
BTCL provides IIG connection through submarine cables. A submarine
communications cable is a fiber optic cable laid on the sea bed between land-based
stations to carry telecommunication signals across stretches of ocean and sea. For
IIG, Internet has to pass a long way over sea that’s why submarine cable is used. The
cables are laid using ships that are modified specifically for this purpose,
transporting and slowly laying the ‘wet plant’ infrastructure on the seabed. These
special ships can carry thousands of kilometers of optical cable out to sea. A special
subsea plow is also used to trough and bury submarine cables along the seabed closer
to shorelines where naval activities, such as anchoring and fishing, are most
prevalent and could damage submarine cables. Today there are more than 400 subsea
cables in operation. Some connecting nearby islands can be shorter than 50 miles
long. Others, traversing the pacific, can reach more than 10,000 miles in length.
Some connect singles points across a body of water, others have multiple landing
points connecting multiple countries. While subsea cables might not face danger
from cows or construction, risks to subsea cables do exist. Subsea cables have a
greater tolerance for failure than terrestrial fiber, but issues that do arise can be
costly, difficult, and slow to fix to the time and effort required to first reach and then
lift cables to the surface.
Fig. 5.5. Submarine Cable

34. 29
5.4 Underground Cable
BTCL mostly uses underground cables for long distance services. It is also
interconnected with other branches through underground cables. underground
cables, of course, come with their own set of advantage and limitations. Aside from
better general appearance and lesser interference with other amenities, the
advantages include smaller voltage drops and lesser probability of fault occurrence.
On the other hand, they have higher production and installation costs, and hence are
used wherever overhead lines aren’t viable due to practical limitations or risks
involved. Hence it is employed in specific places such as urban areas with high
population densities and across water bodies (as submarine cables). underground
cable will consist of a conductor/s covered by a number of insulating and protective
layers necessary for its satisfactory operation.
Underground cables are usually classified according to their Voltage ratings.
They’re grouped as follows:
1. Low tension cables which have a maximum voltage handling capacity of
1000V
2. High tension cables which have a maximum voltage handling capacity of
11kV
3. Super tension cables which have a maximum voltage handling capacity of
33kV
4. Extra high tension cables which have a maximum voltage handling capacity
of 66kV
5. Extra super voltage cables which are used for applications with voltage
requirement above 132kV.

35. 30
Classification Of 3 Phase Underground Cables
1. Belted cables: As the name suggests, it has an additional layer of oil-impregnated
paper which is wound around the insulated conductors. Such an arrangement is
useful for low and medium voltage levels up to 11 kV.
2. Screened cables: Used only in particular applications with specialised
construction, these Underground cables can be further divided as H-type and S.L-
type cables.
3. Pressure cables: These are used when the voltage requirement exceeds 66kV and
solid cables can't be used. Either pressurized gas or pressurized oil is used in these
cables.
Fig.5.6. Underground Cable

36. 31
5.5 Splitter
BTCL uses splitter to maintain power so that power can’t be down. A cable splitter
is a small device that allows two separate coaxial cables to be run from a single outlet
or similar type of cable source. These devices are often used in homes or businesses
where only a single wall outlet for a cable signal is available, but multiple
televisions, modems, or similar devices are needed for connection. A cable splitter
can potentially degrade the signal quality coming from a cable outlet, and multiple
splitters on a single line should be avoided. Though there are many different types
of wires and cables used in television and computer setup, the term “cable” by itself
typically indicates coaxial cable. Coaxial cable is used in “cable television” and
“cable Internet” to transfer data from a wall socket to a television, receiver,
or modem. It is of moderate thickness and circular, with a single cylindrical prong
at the male end. A cable splitter is a device that allows multiple cables to be
connected to a single wall socket, and is often used for connecting more than one
television to an outlet.
Fig.5.7. Splitter Cable

37. 32
Chapter Six: Equipments used by BTCL
6.1 Passcot
The yellow wires in a telephone cabinet are called passcot. They are mainly optical
link which has come from switch. There are optical fibers inside these optical links
but for easy calling, they are named as passcots. The blue part that are situated in
front of the passcots are called connectors. Connectors are connected to SFP.
Fig.6.1. Passcot Cable connection
6.2 SFP
SFP stands for small form-factor pluggable. It’s also commonly called mini-GBIC
(gigabit interface converter). SFP is a popular transceiver for three main reasons.
First is the small form factor. Its size allows it to be used in tight networking spaces
to provide fast communication between switches and important networking
components. The second reason for its proliferation is the variety of SFP connection
options. SFP works with copper or fiber optics. The networks that can’t utilize SFP
are scarce. Finally, SFP is hot swappable. That makes it ideal for expanding or
adjusting existing networks without having to redesign the entire cable
infrastructure. SFP is very important for BTCL. The smallest area coverage by SFP
is 10 kilometers for BTCL in Bangladesh. SFP is designed to work with the bulk of
modern networks. On the copper side, primarily 1000BASE-T and 1000BASE-TX
modules are found for gigabit Ethernet networking. SFP modules are excellent for
bridging communications between switches in compact environments, provided
everything is within 100 meters.
On the fiber optics side of the equation, options are massive. SFP modules are made
to support single mode and multi-mode fiber. It works with simplex and duplex.
Wavelength options range from 850 nm to 1550nm. Networking ranges are
Yellow Wires
named Passcots
Yellow Wires
named Passcots
Connectors SFP

38. 33
anywhere from around 10 km to over 100 km for Rajshahi BTCL. In all, there’s an
SFP module for every job.
Fig.6.2. SFP
Rajshahi
Exchange
Switch
Optical
Link
IMS
Dhaka
Godagari
Exchange

39. 34
6.3 ADSL MODEM:
A digital subscriber line (DSL) modem is a device used to connect
a computer or router to a telephone line which provides the digital subscriber
line (DSL) service for connection to the Internet, which is often called DSL
broadband. The modem connects to a single computer or router, through an Ethernet
port, USB port, or is installed in a computer PCI slot.
The more common DSL router is a standalone device that combines the function of
a DSL modem and a router, and can connect multiple computers through
multiple Ethernet ports or an integral wireless access point. Also called a residential
gateway, a DSL router usually manages the connection and sharing of the DSL
service in a home or small office network.
DSL modems are various types but BTCL, Rajshahi uses ADSL Modems.
Asymmetric digital subscriber line (ADSL) modem is a type of digital subscriber
line (DSL) modem technology that enables faster data transmission
over copper telephone lines than a conventional voiceband modem can provide. In
ADSL, bandwidth and bit rate are said to be asymmetric, meaning greater toward
the customer premises (downstream) than the reverse (upstream). Providers usually
market ADSL as an Internet access service primarily for downloading content from
the Internet, but not for serving content accessed by others.
Currently, most ADSL communication is full-duplex. Full-duplex ADSL
communication is usually achieved on a wire pair by either frequency-division
duplex (FDD), echo-cancelling duplex (ECD), or time-division duplex (TDD). In
BTCL, FDD is used. FDD uses two separate frequency bands, referred to as the
upstream and downstream bands. The upstream band is used for communication
from the end user to the telephone central office. The downstream band is used for
communicating from the central office to the end user.
In ADSL modem, the modem and the DSLAM communicate by a protocol
called discrete multitone modulation (DMT), which is a form of frequency division
multiplexing.[2]
The modem only uses frequencies above 8 kHz, to avoid interfering
with normal phone service. The bandwidth of the line between 8 kHz and about
1 MHz is divided into 247 separate channels, each 4 kHz wide.[2]
A
separate carrier signal carries information in each channel. Thus the system acts like
247 separate modems operating simultaneously. The bits of the incoming digital
data are split up and sent in parallel over the channels. Each data stream is sent using
an error-correcting code to allow minor bit errors due to noise to be corrected at the
receiving end. Most of the channels are unidirectional, carrying download data from
the DSLAM to the modem, but some on the low frequency end are bidirectional, to

40. 35
carry the smaller quantity of upload traffic. The modem constantly monitors the
transmission quality on each channel, and if it is too impaired it will shift the signal
to other channels. The modem is constantly shifting data between channels searching
for the best transmission rates.[2]
Thus interference or poor quality lines will
generally not interrupt transmission, but only cause the data rate of the modem to
degrade.
Fig.6.3. ADSL Modem

41. 36
6.4 Optical Power Meter
An optical power meter (OPM) is a device used to measure the power in
an optical signal. The term usually refers to a device for testing average power
in fiber optic systems. Other general purpose light power measuring devices are
usually called radiometers, photometers, laser power meters light meters or lux
meters.
A typical optical power meter consists of a calibrated sensor,
measuring amplifier and display. The sensor primarily consists of
a photodiode selected for the appropriate range of wavelengths and power levels. On
the display unit, the measured optical power and set wavelength is displayed. Power
meters are calibrated using a traceable calibration standard.
Optical power meters are available as stand-alone bench or handheld instruments or
combined with other test functions such as an Optical Light Source (OLS), Visual
Fault Locator (VFL), or as sub-system in a larger or modular instrument. Commonly,
a power meter on its own is used to measure absolute optical power, or used with a
matched light source to measure loss.
A typical OPM is linear from about 0 dBm (1 milli Watt) to about -50 dBm (10 nano
Watt), although the display range may be larger. Above 0 dBm is considered "high
power", and specially adapted units may measure up to nearly + 30 dBm ( 1 Watt).
Below -50 dBm is "low power", and specially adapted units may measure as low as
-110 dBm. Irrespective of power meter specifications, testing below about -50 dBm
tends to be sensitive to stray ambient light leaking into fibers or connectors. So when
testing at "low power", some sort of test range / linearity verification (easily done
with attenuators) is advisable. At low power levels, optical signal measurements tend
to become noisy, so meters may become very slow due to use of a significant amount
of signal averaging.
To calculate dBm from power meter output : The linear-to-dBm calculation method
is: dB = 10 log ( P1 / P2 ) where P1 = measured power level ( e.g. in mWatts ), P2
= reference power level, which is 1 mW
Optical Power Meter calibration and accuracy is a contentious issue. The accuracy
of most primary reference standards (e.g. Weight, Time, Length, Volt, etc.) is
known to a high accuracy, typically of the order of 1 part in a billion. However the
optical power standards maintained by various National Standards Laboratories, are
only defined to about one part in a thousand. By the time this accuracy has been
further degraded through successive links, instrument calibration accuracy is usually
only a few %. The most accurate field optical power meters claim 1% calibration

42. 37
accuracy. This is orders of magnitude less accurate than a comparable electrical
meter.
Optical power meters usually display time-averaged power. So for pulse
measurements, the signal duty cycle must be known to calculate the peak power
value. However, the instantaneous peak power must be less than the maximum meter
reading, or the detector may saturate, resulting in wrong average readings. Also, at
low pulse repetition rates, some meters with data or tone detection may produce
improper or no readings. A class of "high power" meters has some type of optical
attenuating element in front of the detector, typically allowing about a 20 dB
increase in maximum power reading. Above this level, an entirely different class of
"laser power meter" instrument is used, usually based on thermal detection.
Fig.6.4. Optical Power Meter

43. 38
6.5 LC-LC Passcot Cable
LC-LC patch cord has become an important ingredient of high-density cabling
network infrastructure. To make LC-LC fiber patch run a long way, the
manufacturers have already invented LC-LC patch cables with different features to
meet various requirements in data centers and increase the network performance. LC
stands for Lucent Connector, sometimes also known as “little connector.” It is one
of the smallest fiber optic connectors in the market. It is half the size of a Square
Connector (SC) which is the most commonly used fiber optic connector in the
market. LC uses latch locking. LC is usually used in high density patch panel. High
density patch panel provides the flexibility for future expansion. LC is the preferred
connector for single mode fiber optic cables. Single mode is able to transmit a higher
bandwidth over a longer distance. LC-LC fiber patch cable with two LC fiber
connectors terminated at both ends is the most commonly used fiber optic cable type
in the industry. Compared with other common fiber optic cables, LC fiber cables
offer high density and reliable performance in most applications. Standard LC fiber
patch cables can be divided into single mode (OS1/OS2) and multimode
(OM1/OM2/OM3/OM4/OM5), duplex and simplex fiber cable types.
Fig.6.5. LC-LC Passcot Cable

44. 39
6.6 Ethernet Media Converter
An Ethernet media converter is a device designed to interconnect different
networking media such as fiber and coaxial cables to facilitate communication
between them. It often comes in the form of a small box where the two different
networking cables can be plugged in. An Ethernet media converter allows Ethernet
communication to work properly despite the difference in the network cabling media
being used. Typically, it is used to connect fiber media from an optical fiber network
to a more conventional copper-based Ethernet network.
There are essentially three types of Ethernet media converters:
• Standalone unit - This is a small portable box that can simply be placed
anywhere the connection is made. This is used in places where only one or
two conversions are needed. Because it is light weight, it can be mounted
easily on a wall using strong double-sided tape or other forms of mounting, or
simply left on the surface as it is inconspicuous enough.
• Card-based chassis - This is often used in large main distribution frame (MDF)
or intermediate distribution frame (IDF) locations where multiple optical fiber
links are converted into unshielded twisted pair (UTP) links so that they can
directly plug into conventional switches or routers. This can be as large as a
single-to-double (2U) rack server.
• DIN-Rail mount industrial - This is used in large manufacturing plants with
harsh conditions and limited space and power options.

45. 40
Fig. 6.6. Ethernet Media Converter
BTCL uses Ethernet Media Converter AF1100SF series. AF1100SF series is an
Ethernet media converter that converts media transmission between UTP Copper
cablings and fiber SFP.

46. 41
6.7 ONU
ONU (Optical Network Unit) is also called optical modem, which is a device similar
to the baseband modem. But the difference is ONU accesses optical fiber. It converts
the digital signal into analog signal through modulation at the transmitting end and
transmits the analog signal into digital signal.
Electronic signals are divided into analog and digital signals. Telephone
transmission adopts analog signals and PCs transmit digital signals. Therefore, ONU
is used to modulate the signals if the computer connects to the Internet through the
telephone line.
ONU converts optical signals transmitted via fibers to electrical signals. These
electrical signals are then sent to individual subscribers. In general, there is a distance
or other access network between ONU and end user's premises. Furthermore, ONU
can send, aggregate, and groom different types of data coming from the customer
and send it upstream to the OLT. Grooming is the process that optimizes and
reorganizes the data stream so it would be delivered more efficiently. OLT supports
bandwidth allocation that allows making smooth delivery of data float to the OLT,
which usually arrives in bursts from the customer. ONU could be connected by
various methods and cable types, like twisted-pair copper wire, coaxial cable, optical
fiber, or through Wi-Fi.

47. 42
Fig. 6.7. ONU

48. 43
Chapter Seven: Operational Features of BTCL
BTCL is an organization which gives mainly telephone & internet services.
Telecommunication means communication at a distance. The telephone system
consists of two distinct principle, one is Switching & Coding and another is
Transmission. In BTCL, transmission is done by the Co-axial cable or Optical fiber
cable. It is a simple transmission system which includes source, destination and
medium. These will be discussed here:
7.1 Source, Destination & Medium
Source: It is the element where information is produced. Human voice or
surrounding sounds are the sources which has to be transmitted.
Medium: It is the path by which digitalized signal or analog signal is transmitted.
Co-axial & Optical fiber cable are used here.
Destination: It is the element which receives transmitted signal.
7.2 Pulse Code Modulation (PCM)
Pulse Code Modulation (PCM) refers to the process of converting an analog signal
into a digital signal. PCM works in two steps: Sampling & Quantization.
Step-1 (Sampling): It is the process of generating pulses of zero width and of
amplitude equal to the instantaneous amplitude of the analog signal.

49. 44
Fig.7.1. Analog & Sampled Signal
Time
V
o
l
t
a
g
e
7
6
5
4
3
2
1
0
L
e
v
e
l
s Time

50. 45
Step-2 (Quantization): It is the process of dividing the maximum value of the
analog signal into a fixed number of levels in order to convert the PAM into a binary
code.
Fig.7.2. Quantized Signal
There are 32 ports in the PCM machine. 30 ports are used for the subscriber, one
port is used for signaling and the remaining one port is for the voice. So, if there is
more than 32 calls then call drop will be occurred. To minimize this problem, module
distribution system is used. One wire is connected to a module which is connected
to a number of subscribers.
7.3. QPSK Modulation
Quadrature phase shift keying (QPSK) is a phase shift modulation technique, and
it’s particularly an interesting one because it actually transmits two bits per symbol.
In other words, a QPSK symbol doesn’t represent 0 or 1 rather it represents 00, 01,
10, or 11. This two-bits-per-symbol performance is possible because the carrier
variations are not limited to two states. In QPSK, the carrier varies in terms of phase,
not frequency, and there are four possible phase shifts. First we recall that
modulation is only the beginning of the communication process; the receiver needs
to be able to extract the original information from the modulated signal. Next, it
Time
V
o
l
t
a
g
e

51. 46
makes sense to seek maximum separation between the four phase options, so that
the receiver has less difficulty distinguishing one state from another. We have 360°
of phase to work with and four phase states, and thus the separation should be 360°/4
= 90°. So our four QPSK phase shifts are 45°, 135°, 225°, and 315°.
Fig.7.3. QPSK Signal
QPSK is advantageous in terms of bandwidth efficiency. QPSK system can use a
baseband signal of the same frequency, yet it transmits two bits during each period.
Thus, its bandwidth efficiency is higher by a factor of two.
7.4 Subscriber Signaling
Signaling means informing the subscriber (Calling and Called subscriber) about the
condition of the communication. Various signaling tones are used for signaling.
These are Dial tone, Ringing tone, busy tone, Number unobtainable tone, Call-in-
progress tone. Characteristics of these tones are different.

52. 47
7.5 Subscriber Loop Design
It is the design of the connection of the subscriber to the exchange. Telephone
channel involves two unidirectional path or wire, one used for transmission and other
used for reception. Asymmetrical Digital Subscriber Lines (ADSLs) uses this same
2 wire local.
7.6 Switching Method
Switching is a method of establishing connection between two subscribers. Users
can't have direct lines. They are connected to the exchange or switching center.
Switching center makes the contact between two users. BTCL uses microprocessor
based electronic switching. Multiple subscribers are connected to the switching
center. Only a number of users can be connected simultaneously. Switching centers
draw an attention to this so that minimum numbers of call block shall be occurred.
There are three typical switching techniques available for digital traffic.
a) Circuit Switching: In Circuit switching, two nodes communicate with each other
over a dedicated communication path. In this, a circuit is established to transfer the
data. These circuits may be permanent or temporary. Applications that use circuit
switching may have to go through three phases. The different phases are:
❑ Establishing a circuit
❑ Transferring the data
❑ Disconnecting the circuit

53. 48
Fig.7.4. Circuit Switching
Circuit switching was designed for voice applications. Telephone is the best suitable
example of circuit switching. Before a user can make a call, a virtual path between
the called subscriber and the calling subscriber is established over the network.
b) Message Switching: In message switching, the whole message is treated as a data
unit. The data is transferred in its entire circuitry. A switch working on message
switching, first receives the whole message and buffers it until there are resources
available to transfer it to the next hop. If the next hop is not having enough resource
to accommodate large size message, the message is stored and the switch waits.
Fig.7.5. Message Switching

54. 49
In this technique, the data is stored and forwarded. The technique is also called
the Store-and-Forward technique. This technique was considered a substitute to
circuit switching. But the transmission delay that followed the end to end delay of
message transmission added to the propagation delay and slowed down the entire
process.
c) Packet Switching: The packet switching technique is derived from message
switching where the message is broken down into smaller chunks called Packets.
The header of each packet contains the switching information which is then
transmitted independently. The header contains details such as source, destination
and intermediate node address information. The intermediate networking devices
can store small size packets and don’t take many resources either on the carrier path
or in the internal memory of switches. Individual routing of packets is done where a
total set of packets need not be sent in the same route. As the data is split, bandwidth
is reduced. This switching is used for performing data rate conversion.
Fig.7.6. Packet Switching

55. 50
The line efficiency of packet switching can be enhanced by multiplexing the packets
from multiple applications over the carrier. The internet which uses this packet
switching enables the user to differentiate data streams based on priorities.
Depending upon the priority list, these packets are forwarded after storing to provide
quality of service.

56. 51
Chapter Eight: CARDS USED BY BTCL
In BTCL, each number has individual port and each port has individual card. Each
card contains 64 subscriber lines. These cards are very important for transferring
signals.
8.1 PTWVM CARD
PTWVM card is used for transferring voice signals only. It consists of 64 subscriber
lines.
8.2 ACWV CARD
ACWV card is used for transferring both voice and data signals. It is used to provide
ADSL connection and Internet services.
8.3 PRWGS CARD
PRWGS Card is essential. It is also called power card. It takes electricity and provide
to supply system of BTCL.
8.4 SCXN CARD
Each port contains two SCXN Cards. One card is for read and another one is for
write operation. If any fault occurs at the one SCXN card then another card is used
as alternative.

57. 52
Chapter Nine: Salient Tariff of BTCL
9.1 Voice
i. Telephone new connection (Dhaka Met/Ctg Met/Other):
2000/1000/600 Tk
ii. BTCL-BTCL (country-wide) 08AM-08PM : 0.30
Tk/min
iii. BTCL-BTCL (country-wide) 08PM-08AM : 0.10
Tk/min
iv. BTCL-cellphone/other operator (country-wide) : 0.80
Tk/min
9.2 Internet
i. Dial-up Internet (Click2Net) (56 Kbps max) : 0.10
Tk/min
ii. Dial-up Internet unlimited package (56 Kbps max) : 250
Tk/Month
iii. ADSL Internet (0.512/1/1.5 Mbps) : 300/500/700Tk/Month
iv. Leased Line Connection (2/4/6/10 Mbps) :750/1100/1500/2000
Tk/Month

58. 53
Chapter Ten: Strength & Opportunities of BTCL
❑ Nationwide established Network and Infrastructure
❑ Trained and Expert Manpower
❑ Known Brand to mass people
❑ Government Support
❑ Formation of Department of Telecommunication (DoT) is under process
❑ Fast growing Telecom and ICT sector
❑ Innovative and New projects with next generation Networks
Chapter Eleven: Conclusion
BTCL has become one of the most important companies around the country. Its
popularity is growing day by day. It gives us an easy method of exchanging
information via telephone system. Its widespread internet service has drawn great
attention of the customers. Rules and regulation of this company are well stacked.
BTCL provides reliable services to the people of Bangladesh.

59. 54
Fig. Group pic during session