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13 January 2013

Compiled By… Md. Abdullah Al Kawser (Sohel) B Sc in Telecommunication & Electronic Engineering

Email: m.abdullah.al.kawser@gmail.com

BTCL

21 days at BTCL… I got the opportunity to take a 21 days long internship 21 days at BTCL… opportunity at Bangladesh Telecommunications Company I got the opportunity to take a 21 days long internship opportunity at Limited (BTCL) as an undergraduate trainee. It was a great Bangladesh Telecommunications Company Limited (BTCL) as an experience to trainee. be Ittrained theto be digital switching undergraduate was a greatwith experience trained with the digital switching telecommunication I am very happyto to say telecommunication systems. systems.

I am very happy saythatthat I completed aa very successful training training period at BTCL. I would haveI have completed very successful period atlike BTCL. to thank from the bottom of my heart to those technical engineers who I would like to thank from the bottom of my heart to trained us. This book is written during the 21 days internship period. those technical engineers who trained us. This book is written during the 21 days internship period.

Acknowledgements…

I have to acknowledge those technical engineers who grantAcknowledgements… us to take a successful internship opportunity at BTCL. The internship was an engineers excellent match I have to acknowledge those technical who grant us towith take a our successful internship opportunity at BTCL. The special internshipthanks was an to… academic background knowledge.

Very excellent match with our academic background knowledge. Very special thanks to…

Md. Abdul Malek Md. Abdul Malek Divisional DivisionalEngineer Engineer Dinajpur Telecom. Division, Dinajpur Dinajpur Telecom.

Division, Dinajpur Bangladesh Telecommunications Company Limited (BTCL) Bangladesh Telecommunications Company Limited (BTCL) Md. Moktafi Mahmud Shah Engineer Md. Sub-Divisional Moktafi Mahmud Shah Dinajpur Telecom. Division, Dinajpur Sub-Divisional Engineer Company Limited (BTCL) Bangladesh Telecommunications

Dinajpur Telecom. Division, Dinajpur Bangladesh Telecommunications Company Limited (BTCL) Page 2

BTCL

About BTCL Bangladesh Telecommunications Company Limited (BTCL) is one of the largest Public Switched Telephone Network (PSTN) operators in Bangladesh. BTCL, formerly BTTB, began its journey on July 1, 2008. In addition to the land-line telephone service, BTCL also provides high speed broadband internet services.

Presently BTCL has about 1 million subscribers across the country, where the total number of telephone subscribers is around 45 millions in Bangladesh.

Now BTCL has more than 700 exchanges, 18 Trunk Automatic Exchanges (TAX), 3 Satellite Earth Stations, 7 International Trunk Exchanges (ITX) and countrywide microwave and optical fiber network to provide services to its valued subscribers. Such an infrastructure consists of modern digital exchanges, digital microwave systems, high capacity optical fiber transmission backbone and high-speed broadband internet service. Digital exchanges have been installed in all districts, majority of the upazillas so far. The backbone transmission networks of BTCL connecting various districts are now fully digital. All the subscribers of digital exchanges are able to get connection to internet. For high-speed internet and data communication, BTCL is maintaining a strong Digital Data Network (DDN) in the country. BTCL offers the lowest rate for local calls and it also lowered connection & other telephone related fees to very cheap.

BTCL forayed into the mobile telephony market of Bangladesh by launching a separate company called Teletalk. The company also uses its fiber-optic network and copper wires to distribute the internet.

Timeline of BTCL

1853 1885 1933 1962 1971 1975 1979 1998 2001

2008

Formation of Posts and Telegraph Department in British India Telegraph Act-1885 Wireless Act-1933 Renamed as Pakistan Telegraph and Telephone Department Renamed as Bangladesh Telegraph & Telephone Department under Ministry of Posts & Telecommunications Telegraph and Telephone Board Ordinance Bangladesh Telegraph and Telephone Board (BTTB) Ordinance with right to issue license for Telecom & Wireless services.

Telecommunication Policy-1998 Telecommunication Act-2001 to form Bangladesh Telecommunication Regulatory Commission (BTRC). Jobs of former Frequency and Wireless Board (under Ministry of Posts and Telecommunications) were vested upon BTRC. BTTB was converted into BTCL (Bangladesh Telecommunications Company Limited) from 1 July 2008. Bangladesh Government owns all shares of BTCL. Shares will be offloaded later to the public.

Reference: http://www.btcl.gov.bd Page 3

BTCL

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BTCL

Telephone Number Identities A telephone number is a number which uniquely identifies a number subscription within the Public Switched Telephone Network (PSTN). It is similar to the MSISDN (Mobile Subscriber Integrated Services Digital Network Number). A telephone number is limited to 15 or 16 digits, prefixes not included. It consists of…

Telephone Number = CC + NDC + SN CC = Country Code NDC = National Destination Code SN = Subscriber Number For Example: A subscriber in Dinajpur may have a telephone number like +88053161800 or a subscriber in Dhaka may have a telephone number like +88029010853.

Here Country Code (CC) National Destination Code (NDC) Subscriber Number (SN) +88 0531 61800 +88 02 9010853

Dual Tone Multi Frequency (DTMF) Dual Tone Multi Frequency (DTMF) is a multi-frequency dialing system used in telephony system to identify the keys or numbers we dialed by the push button of a telephone set.

When we press the buttons on a telephone keypad, a connection is made that generates two tones at the same time. A row tone and a column tone. These two tones identify the key we pressed on the keypad. This allows the telephone exchange switching equipment to know what numbers we have dialed by these tones, and will switch our call accordingly to the relevant number.

For example: When we press the digit 1 on the keypad, we generate the tones 1209 Hz and 697 Hz. Similarly, pressing the digit 2 will generate the tones 1336 Hz and 697 Hz.

1 4 7 *

2 5 8 0

3 6 9 #

1209 Hz

1336 Hz

1477 Hz

697 Hz 770 Hz 852 Hz 941 Hz

Fig: DTMF Keypad

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BTCL

Pulse Code Modulation The most common technique to change an analog signal to digital data (digitization) is called Pulse Code Modulation (PCM). Once a call has been set up between two subscribers, the caller starts speaking. At this point the voice signal has to be converted into a digital signal. A PCM has three processes. o Sampling o Quantizing o Encoding

Sampling The analog signal is sampled by the Nyquist sampling rate = 2 Х fmax (where, fmax is the maximum voice frequency = 4 KHz). Hence, the sampling rate is 8 KHz.

Quantizing The output of the sampling is converted into a series of amplitude pulses.

Encoding After each sample is quantized, it is encoded to digital streams with an 8 bit rate.

Thus, PCM produces a total of 8 KHz Х 8 = 64 Kbps stream of data with excellent voice quality. This process allowed long distance calls to be placed on the E1 lines for transmission. One voice call takes up one channel.

E1 Line In telecommunications, E1 line, a worldwide European E-carrier standard, is a physical wire that can be used to carry digital transmission. The E1 line data rate is 2.048 Mbps which is split into 32 time slots. Thus, an E1 line can carry 32 simultaneous voice conversions.

0

1

2

…

…

14

15

16

15 Voice Channels (1 to 15)

17

18

…

…

30

31

15 Voice Channels (17 to 31)

Fig: E1 32 Time Slots

Time slot 0 is used for synchronization channel Time slot 1 to 15 is used for voice channel Time slot 16 is used for signaling channel Time slot 17 to 31 is used for voice channel.

An E1 line has 32 channels out of which 30 channels are voice channels. Each channel has a date rate of 64 Kbps (PCM). Hence, the total bandwidth of E1 line is (64 Х 32 = 2.048 Mbps). For E1 lines, the line coding technique is always HDB3 (High Density Bipolar of order 3).

In the E1 PCM system, signaling information can be transmitted by two different methods: the common channel signaling (CCS) method and the channel associated signaling (CAS) method.

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Infrastructure of BTCL Telephone Exchange The Infrastructure of BTCL telephone exchange mainly deals with the connection from the digital switching room to the subscriber premises. The amount of equipment employed depends on several factors and obviously it involves very high costly equipments for providing underground ducts and manholes. Subscriber lines from the exchange are terminated at the Main Distribution Frame (MDF).

This is mainly to allow easy configuration and testing of lines. These are then taken on a primary cable down to the cable vault and then through manholes to a cabinet, where they are terminated again. Any faulty of the cable pair can be changed or repaired easily under the manhole. One man can get down to the manhole and can do the repairs or joints of the cable. These lines are further extended on a secondary cable, through manholes and hand-holes to a distribution point. These loops are then extended to subscriber premises via aerial cables and drop wires. The backbone network is illustrated in the figure below.

BTCL Exchange Side

Subscriber Side

Digital Switching Room Drop Wires

Pole

Main Distribution Frame (MDF)

Manhole

Distribution Point (DP)

Cabinet

Primary Cables

Secondary Cables

Under Ground (UG) Cable Primary Cable Side Secondary Cable Side

Arial Cable

Manhole

Under Ground (UG) Cable

Fig: BTCL Backbone Infrastructure Mainly there are two types of cables… o o

Arial Cable (Drop Wires) Under Ground (UG) Cables (Primary and Secondary Cables)

The underground cables lay at a depth to 2.5 feet below the ground level. Most of these cables are PVC jelly filled, in order to avoid water seepage into the cable and also to allow better heat dissipation. Normally copper is used as the conductor for those cables.

Digital Switching Room A telephone switch is the brain of a telephone exchange. It is a device for routing calls from one telephone to another, generally as part of the PSTN (Public Switched Telephone Network). BTCL Switching Room contains electronic switching equipments that interconnects and manages telecommunications wiring between the MDF and switching equipments. Cables are running through from switching room to the centralized MDF for proper functioning. Both Alcatel and ZTE switching equipments are installed in two different rooms on the same floor.

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BTCL

Main Distribution Point (MDF) A Main Distribution Frame (MDF) is a signal distribution frame or cable rack where subscriber pairs and switching pairs are interconnected and managed the telecommunications wiring between itself and any number of subscribers. The MDF provides a flexible interconnection mechanism which is very useful in reallocating cable pairs and subscriber numbers. It is a testing point where incoming cables (from switching room) are connected to the out-going cables (to cabinets).

Fig: Main Distribution Frame

Primary Cable Several cabinets that connect to the Main Distribution Point (MDF) of a local exchange through primary cables where all telephone lines get terminated.

Cabinet Cabinet is termination point between the cables coming from the MDF and DP side. Cabinet consist of two sides called primary and secondary. The primary side comes from the MDF and the secondary side comes from the Distribution Point.

Primary cable is inserted from the left side of the cabinet and secondary cable from the right side. Thus, faulty cable pair can be changed easily.

Fig: Cabinet

Secondary Cable Several Distribution Points are connected to a single cabinet through secondary cables. While modeling Out Side Plant (OSP) network, a ratio is maintained for primary and secondary cable pair distribution. No. of secondary cable pair = 1.3 Х No. of primary cable pair. For example: If the number of primary cable pair is 100, then number of secondary cable pair will be 130.

Distribution Point (DP) The Drop Wire cables are terminated in a box which is called a Distribution Point (DP) as the pairs are distributed to subscribers from here. When a new connection is provided, a pair of cable is drawn from Distribution Point (DP) to home. Distribution Points (DP) are frequently located at different areas of a district. Any of the 10 pairs or 20 pairs drop wires going to Distribution Point (DP) can be given to any of the customers from the designated areas. Laying Fig: Distribution Point and managing this Distribution Point (DP) from the exchange may be a bit easier than dealing with individual pairs.

Drop Wire (DW) At the subscriber end, the drop wires are taken to a Distribution Point (DP).

The drop wires are the individual pairs that must be installed between the Distribution Point (DP) and telephone subscriber premises. The drop wire connects to a telephone set. Drop wires are used for outdoor connections. It consists of two conductors laid in parallel and sheath color is black.

Fig: Drop Wire

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Structure of MDF (Main Distribution Frame) Main Distribution Frame (MDF) is one of the flexible points in the lines running from the exchange to the subscriber end. Simply, the interconnection between the telephone exchange and subscribers takes place at the MDF. A Main Distribution Frame (MDF) integrates the functions of both Combined Distribution Frame (CDF) and Cable Chamber as shown in the figure.

Main Distribution Frame (MDF)

Digital Switching Room

Combined Distribution Frame (CDF)

Cable Chamber Primary Cables

Primary Cables

Towards Cabinet Under Ground (UG) Cables

Fig: Components of Main Distribution Frame (MDF)

Combined Distribution Frame (CDF) The MDF is basically a long steel rack accessible from both vertical and horizontal terminating sides.

Combined Distribution Frame (CDF) contains two types of terminating sides. o o

Horizontal Side Vertical Side

Horizontal Side Horizontal Side contains the permanent telephone number lines entering the MDF from the switching room.

Vertical Side Vertical Side is used to terminate the permanent telephone number lines from the horizontal side to the desired cabinet contained in the MDF rack. This cabinet consists of fiber optic transmission equipment and customer access equipment.

There is a well-defined method for identifying all the cabinets within the MDF (Main Distribution Frame). They are coded as cabinet 1, cabinet 2… cabinet 21 or A, B, C… Z. Each cabinet designates a specific location area within the district/city. Both vertical and horizontal side cables are terminated systematically and join them with a wire called jumper wire. By this we can change or shift a number easily. Installing a jumper requires a cross connection between the horizontal and vertical sides to a specific cabinet terminating point corresponding to the desired location area of the subscriber.

Fig: Main Distribution Frame

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BTCL

With disciplined administration the Main Distribution Frame (MDF) can hold over a hundred thousand jumpers, changing dozens of them every day.

If a subscriber moves his house to a nearby area served by the same telephone exchange but a different cabinet and distribution point, he can be permitted to retain the same telephone number by a suitable jumper connection between the horizontal and vertical sides to a specific cabinet terminating point corresponding to the new location area of the subscriber at the MDF. The operator identifies a free line and then it is connected to an allocated loop. Similarly the wire pair Fig: Operator setting a Jumper Connection released by him can be given to another subscriber.

Apart from the normal telephone lines, the lines from other mobile operators are also terminate at the Main Distribution Frame (MDF). In addition to those lines, ADSL (Asymmetric Digital Subscriber Line) broadband internet lines are also fixed to the horizontal side at the Main Distribution Frame (MDF). Thus the same telephone number is given for both voice call and internet data service.

To identify the normal telephone line, it is used red and white color code wires. As same as for data line is used blue, white combination. For the ADSL line, it is used black and white wires.

Cable Chamber When the external cables coming from different areas reach the BTCL telephone exchange entering the building into an underground room called a Cable Chamber. The MDF lines are bought through cable chamber room where they are joined to underground cables.

Fig: MDF lines are bought through underground cables

Fig: MDF lines are turned upwards to the MDF rack

This chamber incorporates protecting devices and provides for a flexible arrangement for connecting underground lines to MDF lines. Rather immaculate cable chamber, the underground cables enter the cable chamber room from the left and then turn upwards to the MDF equipments lines.

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Different Types of Exchanges BTCL has four types of exchanges. These are… o o o o

Local Exchange Trunk Automatic Exchange (TAX) International Trunk Exchange (ITX) Tandem Exchange

Local Exchange A local exchange is a telephone exchange that connects local calls; all the local telephones connected to it are referred to as a local exchange. Here, a call takes place between two subscribers under the same exchange. To make local calls, subscribers do not have dial area codes and are usually billed at the lowest rate. Local exchange is sometimes referred to as Sub-TAX exchange.

Trunk Automatic Exchange (TAX) A TAX is a telephone exchange to interconnect all the local exchanges. Thus, when an NWD (Nation Wide Dialing) call is made, TAX decides where and how to route the call to the desired local exchange. To make NWD calls, subscribers must have to press telephone area code followed by the subscriber number.

International Trunk Exchange (ITX) An ITX is a telephone exchange to route international calls. When an ISD (International Subscribers Dialing) call is made, ITX decides where and how to route the call to the desired country. To make ISD calls, subscribers must have to dial country area code followed by the subscriber number.

Tandem Exchange A tandem exchange establishes trunk to trunk connections but has no local subscriber lines are connected. Like TAX, Tandem exchange is also used to inter-connect local exchanges as well as other operator exchanges. In addition, when a call takes place from a telephone exchange to a different mobile or PSTN operator, tandem exchange becomes the gateway.

Call Routing BTCL uses a variety of routes to send telephone calls from one exchange to another exchange. There are basically three types of calls. These are… o o o

Local Call (Requires subscriber telephone number) NWD (Nation Wide Dialing) Call (Requires NWD code & subscriber number) ISD (International Subscribers’ Dialing ) Call (Requires ISD code & subscriber number)

For local calls, a subscriber dials a local number to reach a particular telephone number. NWD call (One district to another) uses telephone area code (beginning with 0) and the subscriber telephone number. ISD call includes foreign country area code (beginning with 00) followed by the subscriber telephone number.

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BTCL

Local to Local Call When a subscriber dials a telephone number (for example: 65001), it is received by the local telephone exchange (Dinajpur BTCL Exchange) through Distribution Point, Cabinet and Main Distribution Frame (MDF) as shown in the figure.

Drop Wire (DW) Distribution Point Secondary Cable

(DP)

Cabinet

Primary Cable

Main Distribution Point (MDF)

Caller 61800 Tip Cable

For Example: Dinajpur BTCL Telephone Exchange

Local Telephone Exchange Tip Cable

Drop Wire (DW) Distribution Point

(DP)

Secondary Cable

Cabinet

Primary Cable

Main Distribution Point (MDF)

Called 65001

Fig: Block Diagram of a Local to Local Call The local exchange decides where the call to be forwarded to the relevant subscriber. When a number beginning with ‘6’ is pressed, the ALCN 214 06286 BALF 02 card considers it a local telephone call. The card also checks whether the relevant telephone is free or busy. If the telephone is busy, a busy tone is sent to the caller subscriber (for example: 61800). If the telephone is not busy the ring tone is sent to the called subscriber (for example: 65100). The exchange then maintains the connection until a party hangs up, and the connection is disconnected.

Local to NWD Call When a subscriber (for example: 61800) dials an NWD (Nation Wide Dialing) number, consists of a telephone area code (02) followed by the subscriber number (for example: 9010853), it is immediately sent to the nearest local exchange (Dinajpur BTCL Exchange). The local exchange decides where the call to be routed. When a number beginning with ‘0’ is pressed, the ALCN 214 06286 BALF 02 card considers it either a NWD (beginning with 0) or a mobile phone call. It waits for the second digit. If it is 2, as in our example, it considers it as an NWD calls.

Thus, the call is forwarded to the TAX (for example: Bogra TAX) where all local exchanges are interconnected. The TAX now decides how to route the call to the relevant subscriber via multi exchange or tandem exchange and finally to the called subscriber through the local exchange. The overall process is shown in the following figure.

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BTCL

To Local Exchange

Dinajpur BTCL Exchange

Fiber Optic Cable STM-4

Via Rangpur BTCL Exchange

Fiber Optic Cable STM-4

Caller 61800 (Ramnagor, Dinajpur)

Bogra TAX

Fiber Optic Cable STM-4

Via Sirajganj BTCL Exchange For Example: Sher-e-Bangla Nagor Tandem Exchange To Subscriber

Mirpur BTCL Exchange

To Local Exchange

Tandem Exchange

Fiber Optic Cable STM-4

Via Tangail BTCL Exchange

Fiber Optic Cable STM-4

Called 029010853 (Mirpur, Dhaka)

Fig: Block Diagram of a Local to NWD Call (DP & Cabinet are not shown)

Local to ISD Call When a subscriber wants to make a call to an ISD (International Subscribers Dialing) number, consists of a country area code followed by the subscriber number. It is immediately sent to the nearest local exchange (Dinajpur BTCL Exchange) as shown in the figure.

International Route (Either Satellite or Undersea Submarine)

Satellite

For Example: Mohakhali ITX To Local Exchange

Dinajpur BTCL Exchange

International Trunk Exchange (ITX)

Dinajpur, Bangladesh

SEA-ME-WE 04 Undersea Submarine Cable Cable Undersea Submarine

USA Operator (AT&T)

To Subscriber

Central Exchange

To Local Exchange

International Trunk Exchange (ITX)

To Foreign ITX

New York, USA

Fig: Block Diagram of a Local to ISD Call (DP, Cabinet and TAX are not shown)

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The local exchange decides where the call to be routed. When a number beginning with ‘0’ is pressed, the ALCN 214 06286 BALF 02 card considers it either an ISD (beginning with 00) or a NWD phone call. It waits for the second digit. If it is 0, it assumes it as an ISD calls. Thus, the signaling control is forwarded to the ITX (International Trunk Exchange) where the call is routed to the designated county (for example: New York, USA). There are two possible ways in which the call may be routed to the foreign operator: either satellite or submarine cable. The foreign ITX forwards the call to the subscriber through the central exchange.

Local to Mobile Phone Call When a subscriber makes a call to a mobile number (for example: 01715205000), this is immediately sent to the nearest local exchange (Dinajpur BTCL Exchange). The local exchange decides whether the call should be forwarded to another exchange, TAX or tandem exchange. Usually, the mobile operators are connected to the telephone network via the tandems. So, as shown in the figure, when a number beginning with 017 is pressed, the ALCN 214 06286 BALF card considers it either a mobile call (beginning with 0) or a NWD phone call. It waits for the second digit. If it is 017, it treats the number as a mobile phone number and transfers the signaling control to the tandem. The tandem considers it as a Grameen Phone number and chooses the suitable GMSC to the GSM network.

For Example: Mohakhali Tandem Exchange To Local Exchange

Dinajpur BTCL Exchange

Tandem Exchange

Caller 61800 GSM Network BTS - Base Transceiver Station BSC/TRC - Base Station System/Transcoder Controller MSC/VLR - Mobile services Switching Center/Visitor Location Register HLR - Home Location Register GMSC - Gateway Mobile services Switching Center GSM Network

BSC/TRC 01715205000

GMSC

MSC/VLR

HLR

BTS

Fig: Block Diagram of a Local to Mobile Phone Call The GMSC analyzes and queries the number with the HLR to determine which MSC/VLR the mobile is located in and routes the call to the designated MSC/VLR. The MSC/VLR knows which LA (Location Area) the mobile is located in. A paging message is sent to the BSC/TRC controlling the LA. The BSC/TRC distributes the paging message to all the BTS. When the mobile detects the paging message, a signaling channel SDCCH (Stand Alone Dedicated Control Channel) is assigned for call setup procedures. The mobile phone rings, if the subscriber answers, the connections are established.

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BTCL Switching System BTCL Switching systems and associated signaling systems are very important to the operation of telecommunication networks. The term switching implies establishing a connection between calling and called subscribers in order to achieve a successful call. Digital switching system enables establishment of a call or connection dynamically. Circuit switching is the technique used to build a communications network in which a dedicated channel or circuit is used for the duration of a call.

Time Division Multiplexing (TDM) is often used in circuit switching.

However, BTCL switching system consists of two units… o o

Remote Switching Unit (RSU) Main Switching Unit (MSU)

Basically, BTCL digital transport network consists of microwave radio systems & optical fiber networks. This network is connected to digital switching exchanges scattered in every corner of the country. Also, BTCL transmission network is supported by national & international telecommunications links and serves as the foundation for the telecommunications infrastructure in the country.

Major equipments of BTCL Switching System A Digital switching room is a system of electronic components that connects telephone calls. BTCL has two types of major digital switching equipments. These are… o o

Alcatel A1000 S12 (System 12) ZTE ZXJ10 (V.10.0)

Alcatel is one of the largest global telecommunication equipment company headquartered in France. BTCL uses Shanghai Bell’s Alcatel A1000 S12 (System 12) digital switching system. Alcatel Shanghai Bell is a joint venture between China and France. Shanghai Bell Alcatel S12 system is the most Fig: Alcatel, Shanghai Bell popular switching system that provides the most advanced full range of telecommunication solutions.

ZTE (Zhongxing Telecommunication Equipment Corporation) is another type of digital switching system headquartered in China. From circuit-based network to packet-based NGN, ZTE Corporation always provides a costeffective solution in the leading position in the telecom field.

Fig: ZTE, China

In Dinajpur BTCL, Alcatel system is designed to serve 8000 subscribers while ZTE system can serve 2000 subscribers. Both Alcatel and ZTE switching equipments are installed on the 2 nd floor. Both of them supports all the existing signaling systems, provides telephony, ISDN, cellular radio and both have auto recovery feathers and can handle busty traffic. BTCL allocates telephone numbers from the exchange and programs it in the exchange.

BTCL digital exchange includes the following hardware sub-systems…

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Digital Switching Room BTCL Switching Room contains electronic switching equipments that interconnects and manages telecommunications wiring between the MDF and switching equipments. Cables are running through from switching room to the centralized MDF for proper functioning. Both Alcatel and ZTE switching equipments are installed in two different rooms on the same floor.

Fig: Switching Room

Alcatel A1000 S12 Hardware

Switching Rack and Cards A switching rack is a rectangular box. There are several racks depending upon the type of the exchange which is coded as A-01, A-02, B-01, B-02, C-01, C-02 and D-01, D-02 etc. Each rack performs its own processing functions. Each rack contains up to 8 rows known as shelf. Each shelf is numbered as 1, 3, 5 …61, 63 known as slots in which subscriber cards are inserted. In addition, each switching rack has two Magneto Optical (MO) drives (Drive C & Drive D) like computer hard drives to keep track of subscriber’s billing information.

Fig: Switching Racks

These cards are sometimes referred to as Boards (for example: ALCN Boards) one or more shelves may contain spare. Each subscriber card can handle up to 16 subscribers.

Logic cards, switching and control cards, power cards are the most common cards contained almost in very rack. All these are maintained using system software that runs on a UNIX based operating system with CLI (Command Line Interface) console. Different cards contain in different racks have several information.

For example: ALCN 214 06286 BALF 02

Fig: ALCN Boards

Here Name of the Card Part Number PCS Position ALCN 214 06286 BALF 02 04 : 41 ALCN (Analogue Line Circuit N) 214 specifies the model of the card/board. Part Number (06286 BALF) and PCS (02) together indicates the specification of the card. Position (04:41) information is not included in the card, it tells an engineer where to plug-in the card (Shelf no. 04 and Slot no. 41).

Printed Board Assembly (PBA) In Alcatel A1000 S12 system, rack consists of several cards or boards known as Printed Circuit Boards (PBAs). PBA is a flat circuit board where chips and other electronic components are mounted. It interconnects components via copper pathways. A PBA is just like a computer motherboard.

Fig: Printed Board Assembly Page 20

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Digital Exchange Module A module is a separate unit of hardware or a segment of a larger device or arrangement that is designed to be separately installed, replaced or serviced. Each modules has 8 ALCN 214 06286 BALF 02 boards. As stated earlier, each ALCN board can handle 16 subscribers, meaning that each module can handle 128 subscribers.

In Alcatel A1000 S12, there are several modules that are designed to perform specific tasks. The most common modules are… o o o o o

Analog Subscriber Module (ASM) Digital Trunk Module (DTM) Service Circuit Module (SCM) Clock and Tone Module (CTM) Peripheral & Load module (P&L)

Analog Subscriber Module (ASM) ASM provides the line end circuit for subscribers. There are several boards (cards) under the ASM module. These are…

Name of the Board (Card)

ALCN 214 06286 BALF 02

RNGF 214 06274 AAMA 03

TAUC 214 06230 DHCA 02

RLMC 214 06326 AAAA 03

Analog Subscriber Module (ASM) Abbreviation & their Functions ALCN - Analogue Line Circuit N  Line protection (High Voltage/Over Current)  Connects call, sends ringing current, detects off hook and on hook state.  Analog to Digital (A/D) or Digital to Analog (D/A) speech conversion (PCM) with 8 KHz sampling rate.  Controls gain and cancels echo  Dial pulse and ring trip detection  Injecting of remote metering tone with noise suppression  Subscriber loop supervision RNGF - Ring PBA type F  Generates ringing current and send through any of the lines.

TAUC - Test Access Unit type C  Observes the line voltage.  Serves as an encoder for the transmission of audio signal samples.  Takes samples of the signal received to evaluate for Digital Signal Processing (DSP) RLMC - Rack Alarm PBA type C o Generates alarm

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Digital Trunk Module (DTM) DTM acts an interface between the signaling and the exchange control. It can detect frame alignment. Boards under the DTM module are…

Name of the Board (Card) DTUA 214 06260 ABCB 07 DTUE 214 06260 ABCB 07 DTRI12 214 06304 BACB 04 DTRF 214 06328 ACAA 35

Digital Trunk Module (DTM) Abbreviation DTUA - Digital Trunk Unit type A DTUE - Digital Trunk Unit type E DTRI - Digital Trunk PBA type I DTRF - Digital Trunk PBA type F

Service Circuit Module (SCM) SCM performs switching and signal processing functions.

Name of the Board (Card) SWCH 211 32150 BAAA 05 DSPA 214 06280 ABDA 07 MCUA 214 37608 BBCB 14 MCUB 211 37608 BBCB 14 MCUE 214 07902 BAAA 23 MCUG 214 07940 ABBA 07

Service Circuit Module (SCM) Abbreviation SWCH - Switching Matrix DSPA - Digital Signal Processor type-A MCUA - Module Control Unit type-A MCUB - Module Control Unit type-B MCUE - Module Control Unit type-E MCUG - Module Control Unit type-G

Clock and Tone Module (CTM) CTM generates digital signal and synchronizes with the reference clock signal

Name of the Board (Card) MCUB 211 37608 BBCB 14 RCCC 214 06920 AAAA 11 CCLC 214 06922 ABBA 03 DSGA 211 32224 ADDA 04

Clock and Tone Module (CTM) Abbreviation MCUB - Module Control Unit type-B RCCC - Reference Clock Control type C CCLC - Central Clock PBA type C DSGA - Digital Signal Generator type A

Peripheral & Load module (P&L) PLM serves as memory control and maintains the alarm of the PBA.

Name of the Board (Card) MCUB 211 37608 BBCB 14 DMCA 214 06234 EBAA 3C MMCA 214 06298 AACA 04 CLMA 211 37652 AABA 01

Peripheral & Load module (P&L) Abbreviation MCUB - Module Control Unit type-B DMCA - Direct Memory Control type A MMCA - Man Machine Communicator Type A CLMA - Central Alarm PBA type A Page 22

BTCL

Others Other important modules may have the following boards…

Other PBAs Name of the Board (Card) CLTD 211 32162 ACAA 03 DIAB 214 07936 ABAA 03 RCLA 211 37820 AABA 02 RLMC 214 06326 AAAA 03 DSGA 211 32224 AADA 04 TSAB 214 06306 AACA 02

Abbreviation CLTD (Clock & Tone Distribution) DIAB (Dynamic Announcement Module) RCLA (Rack Clock type A) RLMC (Rack Alarm PBA type C) DSGA (Digital Signal Generator type A) TSAB (Test Signal Analyzer type B)

Billing System The IBS 2000 (Immediate Billing System 2000) uses items to keep track of the services and products that are the source of revenue for BTCL exchanges. It also has special feathers to standardize subscriber’s billing to internal and external customers by consistently charging each customer the same price for the same service. IBS-2000 permits us to know the immediate bill by voice query service. The IBS-2000 is made of three modules…

Voice Module Voice module gives the voice query ticket to the subscriber to know the immediate meter reading of total call and last call.

Server Module Server module maintains communication between others modules and Alcatel A1000 S12 Billing system. The module also allows to browse and rollback data from the database.

Account Module This module is used for dump, convert, and import files of exchange billing.

Incorporating to manage the subscriber’s information such as monthly meter reading, query and print the bill both monthly bill and weekly bill, and copy above mentioned information on floppy disk as text file.

Fig: IBS 2000

How Billing is done?

A Magneto Optical (MO) disk is used to store subscriber billing information. One MO has two sides (A & B). Each side has 1.3 GB memory.

As stated earlier, Alcatel A1000 S12 switching rack has two MO drives (Drive C & Drive D) contained in the 8th shelf. Another MO drives contains in the account module of IBS 2000.

Fig: MO Disk Page 23

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The billing information is obtained by simply taking MO disk from switching rack and then inserting MO disk in the account module of IBS 2000.

BTCL makes two types of billing info…

Bulk Billing Bulk Billing is made once a month. It can be made on the last day or on the first day of a month. This monthly telephone bill is sent to Dhaka as a text file in a floppy disk or as a bill on paper.

Detail Billing Detail Billing has to make once a week. Detail Billing gives all detail information of the subscriber called numbers. Detail Billing must be made weekly. If the length of detail bill is large, exchange will not allow to call local, NWD or ISD.

ZTE ZXJ10 (V.10.0) Hardware

Switching Rack and Cards Like Alcatel A1000 S12 switching rack, ZTE has several racks which are coded as RACK 01, RACK 02, and RACK 03 etc. Each rack contains up to 7 rows known as shelf. Each shelf is numbered as 1, 2, 3 …26, 27 known as slots in which different types of cards are inserted.

Power cards, logic cards, switching and control cards are the most common cards contained in every rack. Each ZTE card can handle 24 subscribers. All these are maintained using system software that runs on a server based operating system Windows Server 2000 with GUI (Graphical User Interface) console.

Unlike Alcatel A1000 S12, ZTE card/board does not contain any model or specification information. The most important card/boards are ASLC, RASL, IASL, FASL, MIT, SPI, MP, SWEM, COMM, PEPD, MON, DTI, ASIG-3, POWER etc.

Fig: ZTE Switching Rack & Cards

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Distribution Frames All subscribers’ lines are wired to the Main Distribution Frame (MDF) where subscriber pairs are connected to the switching lines. In addition to MDF, BTCL also uses other distribution frames for transmission network management and maintenance. Two major types of distribution frames are…

Digital Distribution Frame (DDF) A Digital Distribution Frame is a cross-connection system to which digital interfaces from line systems and the exchange (or other network equipment) are connected with the help of an E1 line (2 Mbps). BTCL may easily change transmission connections between equipment lines. DDF rack is the interface to distribute or to terminate E1 lines (2 Mbps) coming from switching room or going to the MDF. Other function involves testing, isolating and cross patching E1 lines.

Fig: Digital Distribution Frame

Optical Distribution Frame (ODF) An optical distribution frame contains two fields of optical fiber connectors. The optical cables of the network are connected to one connector field and the other one is connected to optical line terminal equipment. Cross-connections are made between two connector fields with optical fibers. Two pairs are used to detect any faulty with the spare one. ODF rack provides splicing, distribution which increases maintenance efficiency.

Fig: Optical Distribution Frame

Synchronous Transport Module (STM) Frame STM-1 (155 Mbps) and STM-4 (622 Mbps) frames provide fiber optic transmission to provide high speed communication for transporting large amount of telephone calls and data traffic over the same fiber without synchronization problems. It supports information payload.

Fig: STM-4 Rack

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Power Distribution and Management Power distribution and management is important for telephone switching equipments to proper functioning. Power Distribution Board, which supplies 220 Volt AC is fed to the rectifier. A Rectifier is an electrical device that converts alternating current (AC), which periodically reverses direction, to direct current (DC), which flows in only one direction. Generally -48 V is taken from rectifier which is provided to the switching equipments. During the period of load shedding, BTCL uses a 137.5 kVA Engine Generator which supplies electrical power to the rectifier. The backup batteries also provide 48V to switching equipments when load shedding occurs. The overall system is shown below.

Power Distribution Board (PDB)

-

220V

Rectifier

-

- 48 V

Alcatel & ZTE Switching Equipments Engine Generator (137.5 kVA)

Backup Battery -

- 48 V

Fig: Block Diagram of BTCL Power Distribution

Why -48V used in switching? o o o

Positive voltages cause comparatively more corrosion in metal than negative voltages.

Negative voltages are safer for human body while doing Telecom activities.

Lightning may cause positive voltages in the equipment circuitry. In that case, negative voltages (lack of electrons) neutralize positive charges and prevent excessive heat. o Negative voltage is safer for long telephone line for transmitting power through it. o The reason for electing -48V in reference to ground is to avoid the nasty galvanic effects in telephone cabling. +48V would have nastier effect on wet cabling.

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BTCL Nationwide Transmission Link

Microwave Link For long distant communications, BTCL uses countrywide microwave radio links to provide services to its valued subscribers. The microwave link is used to connect remote and regional telephone exchanges without the need for optical fiber lines. BTCL microwave towers are placed every few miles to carry telephone signals connecting almost all the districts across the country. Earlier BTCL had a number of analog microwave links (for example: Bogra-Phulbari-Dinajpur-Thakurgan). Later on digital SDH microwave links were installed throughout the country. The modulation technique Quadrature Amplitude Modulation (QAM) is used for analog microwave transmission and Quadrature Phase Shift Keying (QPSK) is used for digital transmission. Now-a-days, most of the microwave radio links are replaced by fiber optic link.

Optical Fiber Link The demand of fiber optic links increases along with installation of new digital switching equipments. BTCL optical fiber backbone link covers the total telecommunications network throughout the country. All district telephone exchanges as well as upazillas are now connected with optical link (STM-1/STM-4) even for short distance communication. The north-west part Dinajpur to Rangpur (STM-4), Rangpur to Bogra (STM-4), and Bogra to Dhaka (STM-4) via the Jamuna Bridge is fully dependent on optical link. The intra-city exchanges of Dhaka are based on fiber optic link. In addition, Dhaka to Chittagong optical link uses STM-16. Dhaka to Sylhet and Dhaka to Khulna backbone link is also optical. The nationwide optical link joins to the country’s submarine cable network using SEA-ME-WE 4 with 10 Gbps bandwidth to route international voice calls and internet data traffic.

Fig: BTCL Fiber Optic Link

Why Optical Link?

The microwave link of BTCL is limited to distance within the LOS (Line Of Sight) about 20/30 kilometers. Beside this, the use of microwave link requires high gain antennas and powerful transmitters and signals get weakened after travelling a long distance. Sometimes these links cannot be accommodated to handle busty traffic. To overcome these problems, the underlying network of transmission and reception system of BTCL is now based on optical fiber link. This link can carry huge amount of voice, data, and image and video signals with less transmission loss with secure or reliable delivery.

BTCL now has an optical fiber backbone link that covers the total telecommunication system from country to a global submarine optical fiber communication system.

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However, to handle high-speed communication and busty traffic, many intra-city and district to district communication are now based on optical fiber link (STM-1, STM-4), even for short distance communication.

BTCL Worldwide Transmission Link

Satellite/Terrestrial Microwave Link The satellite/terrestrial microwave link is one of the important transmission links for global telecommunications. Bangladesh has currently 4 satellite base stations…

Betbunia satellite base station, 40 km from Chittagong-Rangamati highway, is connected by a 2 GHz 140 Mbps PDH microwave link to international gateway. This station is connected from Dhaka through a STM-16 optical fiber transmission. Mohakhali satellite base station, located in Dhaka, connects the all the ITX (International Trunk Exchange) in Dhaka. Talibabad satellite base station, 30 km north to Dhaka-Mymensingh highway, is connected to the international switch through a single hop 6 GHz microwave link. One satellite base station is located in Sylhet. This station is directly connected to the international gateway switch of British Telecom in the UK.

Undersea Submarine Cable Link To provide global telecommunications, Bangladesh Submarine Cable Company Limited (BSCCL), a fiber optic submarine cable telecommunication company handles the country’s submarine cable SEA-ME-WE 4 (South East Asia - Middle East - Western Europe 4), is the gateway between the high capacity optical fiber network of Bangladesh and the rest of the world communication. Bangladesh is Fig: Undersea Submarine Cable connected to the submarine cable with the landing station at the Kolatoli beach, Cox's Bazar.

The SEA-ME-WE 4 cable runs a total length of 18,800 kilometers long, and provides the primary Internet backbone between South East Asia, the Indian subcontinent, the Middle East and Europe.

The cable also carries telecommunications with 17 KPs (Key Points) and 4 segments through Singapore, Malaysia, Thailand, Bangladesh, India, Sri Lanka, Pakistan, United Arab Emirates, Saudi Arabia, Egypt, Tunisia, Italy, Algeria and France.

BSCCL also has an alternative backbone submarine cable link (25 Kilometers) that connects Darshana of Bangladesh with Krishno Nagar of India for any emergency case when the cable causes any damage, faulty or any serious problem.

Fig: Four segments of SEA-ME-WE-4 Page 29