Mu'alla Central Summer Training

Al-Mu'alla Central Office Summer Training Report

ACKNOWLEDMENTS

I would like to thank the following people for their help and cooperation in the summer training: the staff at Al-Mu'alla central office for their help, and patience; staff and teachers at the faculty of engineering for their encouragement.

I would specially like to thank the students at the department of electronics and communications, faculty of engineering, Aden for their help and contribution in the development of this report, and Mr. Sameh for his help and advice.

Ibrahim Abdul-Hakim

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INTRODUCTION

Summer training is a good and successful way for students to get a good background

and practical information about their specialist, and also makes them know about their

specialist jobs.

This was my first summer training and it was in the public telecommunications

corporation, Al-Mu'alla central office, Aden.

Al-Mu'alla central office is considered to be one of the oldest and the most important

central offices in Yemen, since it is the main central office in the economical capital

of Yemen-Aden. All telephone exchanges in Aden districts are connected to each

other through it, and also it is their bridge to reach the telephone exchanges outside of

the province.

In the first training day, we were told that we will not be helping the staff in their

serious works, but our mission will be only to see, ask, and listen. Their only excuse

for this was that because this is a very sensitive central office and any fail in any of its

units may cause a serious problem with the government and the national security.

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Figure1: A central office building.


From our first tour we knew that the central office has various departments and

sections such as the switching department, where multiple digital switching systems

have been installed and are in service, transmission department, power supply

department, security department, customer services department, billing department,

etc...

Each of these departments has its own staff of engineers and technicians with one of

them promoted as a head for the department.

Most of the training was theoretic and incomplete, so that we had to read more about

some basics of communications from the World Wide Web (internet). We have not

studied any communication subjects yet, so that it was a little bit confusing for us.

How Telephone Calls Work

A calling party wishing to speak to another party will pick up the telephone's handset,

operating a

lever which

closes the

switchhook

(A4), which

powers the

telephone by

connecting the

transmitter (microphone), receiver (speaker), and related audio components to the

line. The off-hook circuitry has a low resistance (less than 300 ohms) which causes a

direct current (DC), which comes down the line (C) from the telephone exchange. The

exchange detects this current, attaches a digit receiver circuit to the line, and sends a

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Figure 2: Schematic of a landline telephone installation.


dial tone to indicate readiness. On a modern push-button telephone, the caller then

presses the number keys to send the telephone number of the called party. The keys

control a tone generator circuit that makes DTMF tones that the exchange receives. A

rotary-dial telephone uses pulse dialing, sending electrical pulses, that the exchange

can count to get the telephone number. If the called party's line is available, the

exchange sends an intermittent ringing signal (about 60~90 volts alternating current

AC) to alert the called party to an incoming call. If the called party's line is in use, the

exchange returns a busy signal to the calling party. However, if the called party's line

is in use but has call waiting installed, the exchange sends an intermittent audible tone

to the called party to indicate an incoming call.

The phone's ringer (A7) is connected to the line through a capacitor (A6), which

blocks direct current but passes alternating current. So, the phone draws no current

when it is on hook (a DC voltage is continually connected to the line), but exchange

circuitry (D2) can send an AC voltage down the line to ring for an incoming call.

(When there is no exchange, telephones often have hand-cranked magnetos to make

the ringing voltage.) When a landline phone is inactive or "on hook", the circuitry at

the telephone exchange detects the absence of direct current and therefore "knows"

that the phone is on hook (therefore, only AC current will go through) with only the

alerting device electrically connected to the line. When a party initiates a call to this

line, the exchange sends the ringing signal. When the called party picks up the

handset, they actuate a double-circuit switchhook which simultaneously disconnects

the alerting device and connects the audio circuitry to the line. This, in turn, draws

direct current through the line, confirming that the called phone is now active. The

exchange circuitry turns off the ring signal, and both phones are now active and

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connected through the exchange. The parties may now converse as long as both

phones remain off hook. When a party "hangs up", placing the handset back on the

cradle or hook, direct current ceases in that line, signaling the exchange to disconnect

the call.

Calls to parties beyond the local exchange are carried over "trunk" lines which

establish connections between exchanges. In modern telephone networks, fiber-optic

cable and digital technology are often employed in such connections. Satellite

technology may be used for communication over very long distances.

Today the end instrument often remains analog but the analog signals are typically

converted to digital signals at the central office.

In the past the exchange was a manual service, so to call a party, the customer lifts the

receiver off-hook and asks the operator to connect the call to a requested number.

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

by plugging into the jack on the switchboard corresponding to the called customer's

line. If the call is to another central office, the operator plugs into the trunk for the

other office and asks the operator answering (known as the "inward" operator) to

connect the call.

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Chapter 1:

Switching Department

This is the operations room of the central office. It hosts the digital switches with their

accessories of terminals, alarming systems and maintaining equipments.

The staff of this department consists of communication engineers and communication

technicians with one of them promoted as a head for the department.

We visited this department in the first and the third weeks. In the switching

department there are various types and models of digital switching systems, such as

FETEX (Japanese), OCB (French), Huawei (Chinese), etc…

Some of these switching systems are in service since the early 1990s and still living.

In the first day of the summer training, one the staff members led us in a tour inside

the department and showed us the Japanese switching system which is called FETEX

and it is a product of Fujitsu Company. The switches room was wide and cold in order

to achieve the stability requirements of the switching systems and the other systems in

the same room.

The CSN is the place where customer cards

are arranged and their data is stored. There are

two types of CSN:

1. CSNL, the subscribers cards are in

the same central office (local).

2. CSND, the subscribers cards are

distant from the central office.

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Figure 3: CSN of the subscribers.


The switching systems are running 24 hours a day, and they response to thousands of

telephone calls every minute, and due to these high processing, the temperature of

their heating sinks rapidly raise. To condition and stabilize the temperature of the

switches room, an air conditioning system is used. Otherwise the switching systems

will burn and get damaged.

Switches

Switch structure

The structure of a switch is an odd number of layers of smaller, simpler subswitches.

Each layer is interconnected by a web of wires that goes from each subswitch, to a set

of the next layer of subswitches. In most designs, a physical (space) switching layer

alternates with a time switching layer. The layers are symmetric, because in a

telephone system callers can also be callees.

A time-division subswitch reads a complete cycle of time slots into a memory, and

then writes it out in a different order, also under control of a cyclic computer memory.

This causes some delay in the signal.

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Figure 4: A rack in the switching room.


Electronic switches

The first Electronic Switching Systems were not entirely digital. Equipment testing,

changes to phone numbers, circuit lockouts and similar tasks were accomplished by

typing on a terminal. These systems could use the old electromechanical signaling

methods inherited from crossbar and step-by-step switches (Strowger).

Digital switches

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

to a dialed telephone number. Calls are set up between switches using the Signalling

System 7 protocol, or one of its variants. a digital switch is a switch that performs

time division switching of digitized signals. The first product using a digital switch

system was made by Amtelco. With few exceptions, most switches built since the

1980s are digital.

Maintenance tasks

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

because of failures or maintenance work. A variety of tools referred to as make-busys

were plugged into electromechanical switch elements during repairs or failures. A

make-busy would identify the part being worked on as in-use, causing the switching

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

behind in payments would have their service temporarily denied (TDed). This was

effected by plugging a tool into the subscriber's office equipment (Crossbar) or line

group (step). The subscriber could receive calls but could not dial out.

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Chapter 2:

Transmission Department

The transmission department is the department where all incoming and outgoing

signals are maintained, sent, or received. Its staff consists of communication engineers

and communication technicians with their head.

In the last day of the training –the third week, we were permitted to go to see this

department. In the second week we were fed with two lectures about transmission and

about few technologies used to transmit signals.

Transmission Technologies

There are many technologies used in signals transmission such as the PDH

technology, SDH, DWDM, etc…

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Figure 5: Snapshots from the transmission department.


PDH Technology

The Plesiochronous Digital Hierarchy is a technology used in telecommunications

networks to transport large quantities of data over digital transport equipment such as

fibre optic and microwave radio systems. The PDH networks run in a state where

different parts of the network are nearly, but not quite perfectly, synchronised.

PDH allows transmission of data streams that are nominally running at the same rate,

but allowing some variation on the speed around a nominal rate.

PDH is typically being replaced by Synchronous Digital Hierarchy (SDH) or

Synchronous optical networking (SONET) equipment in most telecommunications

networks.

Multiplexing in signals transmission case

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Figure 7: Multiplexing block diagram.

Figure 6: Multiplexing block diagram.


In PDH, Multiplexing is done on transmission and Demultiplexing is done on

reception.

SDH Technology

The Synchronous Digital Hierarchy is a standard technology for synchronous data

transmission on optical media. It is the international equivalent of Synchronous

Optical Network. Both technologies provide faster and less expensive network

interconnection than traditional PDH (Plesiochronous Digital Hierarchy) equipment.

In digital telephone transmission, "synchronous" means the bits from one call are

carried within one transmission frame. "Plesiochronous" means "almost (but not)

synchronous," or a call that must be extracted from more than one transmission frame.

The SDH technology is more advanced than PDH because it supports remote

monitoring and maintenance and overcomes the withdraws of the SDH technology in

the number of channels. It is also more efficient and it supports line and ring

connections between different exchange centrals.

An OTDR is a maintenance device which is used to determine breaks in the fiber-

optic cables, it determine at which distance is the break from a maintaining place.

DWDM Technology

We did not learn about the DWDM transmission technology, but some of its features

are that it uses one optical fiber cable for transmission and reception, and it dose not

need to multiplex before transmitting.

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Pulse-code modulation (PCM)

Pulse-code modulation (PCM) is a method used to digitally represent sampled

analog signals. It is the standard form for digital audio in computers and various

Blu-ray, Compact Disc and DVD formats, as well as other uses such as digital

telephone systems. A PCM stream is a digital representation of an analog signal, in

which the magnitude of the analogue signal is sampled regularly at uniform intervals,

with each sample being quantized to the nearest value within a range of digital steps.

PCM streams have two basic properties that determine their fidelity to the original

analog signal: the sampling rate, which is the number of times per second that

samples are taken; and the bit depth, which determines the number of possible digital

values that each sample can take.

Time slot (0) is used for the FAW (Frame alignment word), or for alarm,

time slot (16) is used to determine the signaling system or protocol used.

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Figure 8: Sampling and quantization of a signal (red) for 4-bit PCM.

Figure 9: 2Mbit PCM frame with 32 time slots.


The number of channels in the PCM determines the maximum number of telephone

calls it can handle at a specific moment. In 2Mbit frame the maximum telephone calls

that can be handled at a specific moment are 30, because time slot 0 and time slot 16

are allocated for FAW and the signaling protocol respectively.

Signaling System 7 (SS7) is the standard protocol currently used between most

exchange centrals.

Each time slot contains 8 bits, then

Total bits of frame = 8 bits x 32 time slots = 256 bits

Frame duration = 256 bit / 2 Mb = 125 us

Frequency = 1 / T = 1 / 125 us = 8 KHz

One time slot duration = 125 / 32 = 3.9us

Bit rate = 1/488 = 2,048 bits

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Chapter 3:

Power Supply Department

The exchange central services must be online twenty four hours a day and seven days

a week without any interruption in its state. So that, there has to be a stable source of

power supplying the central office rooms, switches, devices and conditioning systems.

To achieve this stability, there are multiple solutions used in parallel to avoid any

interruption in the power supplying the central. These solutions are as follows:

1. Public electricity service.

2. Standby power generators.

3. UPS systems.

Public electricity service:

It is the electricity provided by the government electricity service company, or

by any electricity service provider.

Standby power generators:

They are power generators which are used as an alternate source of power in

case the public electricity service is urgently switched off. They are usually operated

using fuel such as petrol and diesel.

UPS systems:

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Uninterruptible Power Supply are smart UPS systems which function is to

keep the central most important devices online in case the power is urgently switched

off. A series of efficient batteries are installed with the UPS systems to increase its

power capacity. Inverters are used with the UPS systems in case an AC power source

is required.

The central switching devices require about 48-54VDC, so that they get their power

either from the rectified AC power (Public electricity service, or standby power

generators), or from the UPS systems in case of emergency. Both rectified AC source

and UPS systems source are parallelized to the central DC loads.

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Figure 10: Power system block diagram of the central.

Central DC loads

Central AC loads

Public electricity

service

Standby power

generators

UPS systems

RectificationInversion


SUMMARY

This summer training showed us that the technical experience is the key in the

operation of the exchange centrals in Yemen. All the problems are either solved using

the terminal command line (console) in case it is a software issue, or by replacing

some electronic cards in case it is a hardware issue. Every thing is ready and there is

no chance for engineers and technician to create, repair, or even make new solutions.

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FIGURES LIST

Figure Page

Figure1: A central office building. 1

Figure 2: Schematic of a landline telephone installation. 4

Figure 3: CSN of the subscribers. 7

Figure 4: A rack in the switching room. 8

Figure 5: Snapshots from the transmission department. 10

Figure 6: Multiplexing block diagram. 11

Figure 7: Multiplexing block diagram. 11

Figure 8: Sampling and quantization of a signal (red) for 4-bit PCM. 13

Figure 9: 2Mbit PCM frame with 32 time slots. 13

Figure 10: Power system block diagram of the central. 16

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REFERENCES

- Gnanasivam, P. 2006. Telecommunications, Switching, And

Networks, 2nd edition. New Age International Publishers.

- Wikipedia. The online encyclopedia. http://www.wikipedia.com.

- Google images service.

- Definitions from Whatis.com.

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Documents

Mu'alla Central Summer Training