BHARAT SANCHAR NIGAM LIMITED
(A Govt. Of India Enterprise) Sanjay Place Telephone Exchange Agra
Submitted To:
Submitted by:
Mr. A. S. Yadav D.E. (Outdoor) Sanjay Place, Agra
Prashant Kumar Bhagaur B.Tech (ECE) IIIrd Year, VIth Semester Bhagwan Parshuram Inst. Of Tech.
ACKNOWLEDGEMENT We reached the horizons of dictionary, but still we are short of words to express our gratitude towards esteemed Mr. Chhotey Lal,
Divisional Engineer (Indoor), Bharat
Sanchar Nigam Limited, Sanjay Place, Agra and Mrs. Krishna Verma, Sub Divisional this
Engineer,
for giving us
great opportunity to work under their guidance and
providing all necessary facilities required in the training period. We are also indebted to our training guide Mr. Rakesh Kumar and Mr. Girish Kumar, Junior Telecom Officer, for giving us the necessary knowledge and for their moral support, cooperation, individual guidance, infinite patience and the zeal and enthusiasm they created in us. Last but not the least the almighty is to be thanked For giving us this golden opportunity to work at Bharat Sanchar Nigam Limited, Sanjay Place, Agra.
INTRODUCTION Today, we are all familiar with the telephone as a means of communicating with one another. But, it was only about 125 years ago that the telephone was invented. The names comes from two words, -tele, meaning ‘at a distance ‘ and ‘phonic’ referring to ‘sound’. A telephone is a device that enables us to hear sound at a distance, just a€s television enables us to see at a distance.
Reis’s Telephone (1860) : the diaphragm man Who really invented telephone? There are quite a few candidates, and the leading one is a German science teacher called Philipp Reis. Reis began work on the telephone in 1860, inspired by an 1854 paper by a French investigator named Bourseul, who suggested: ‘Speak against one diagram and let each vibration ‘make or break’ the electric contact. The electric pulsations
there by produced will set the other diaphragm working, and [it then reproduces] the transmitted sound.’
Bell, Alexander Graham (1847-1922) Alexander Graham Bell was born on 3 March 1847. He was born in Scotland, but moved to America as an adult. He is known as the inventor of telephone. Bell has a strong imagination and an inquiring mind and created his first invention when he was 15 years old. He made a model of a skull and voice box and with a pairs of bellows blew air through this instrument, to produce human like sounds. Bell’s passion for invention and experimentation grew with age. One of his ideas was that of sending sound along electric telegraph wires. Graham called this the ‘harmonic telegraph’. By October 1875 Bell has designed the telephone and by March 1876 it was built. Bell’s telephone finally carried real articulate speech Alexander Graham Bell died on 2 August, 1922, at the age of 75. By the time of his death there were around 14 million telephones in the USA alone.
The first telephones arrive in Britain (1877) : ‘greatest by far of all the marvels’ Sir William Thomson (later lord Kelvin) exhibited Bell’s primitive telephone before the British Association of science assembly at Glasgow in September 1876, describing it as “the greatest by far of all the marvels of the electric telephone”. The first pair of practical telephones seen in Great Britain arrived in July 1877, brought here by William Preece, Chirf Electrician of the post Office. A few months later, Bell’s ‘ perfected ’ type of telephone was exhibited in another meeting of the British Association in Plymouth.
Bell form his company (1877) : but sales are slow… Within a year of making the first telephone call, bell and his financial backers – Thomas Sanders and Gardiner G Hubbard – had formed the Bell Telephone Company in the United States. Unsurprisingly, early demand for the telephone was not great and prior to forming their company Bell and his Partners had struggled in their attempts to promote the new invention. At one point they
even offered to sell the patents to the Western Union Telephone Company – Elisha Gray’s employers – for $100,000. The offer was spurned – a decision the Western Union was soon to regret bitterly.
First British long distance calls (1878) : a royal introduction Alexander Graham Bell demonstrated his telephone to Queen Victoria on January 14, 1878, at Osborne House on the [s]e of Wight. During the demonstration Bell made calls to London, Cowes and Southampton. These were the first publicly witnessed long - distance calls in the UK.
Bell’s British company is founded (1878) : from south to north Less than sixteen months after making his first British demonstration, the UK rights to the Bell patents had been taken up by a new British Company, The Telephone Company (Bell’s Patents) Ltd. One of the first telephone lines to be erected was from London’s Hay’s Wharf, south of the Thames, to Hay’s Wharf Office on the north bank.
As more telephones were provided, it was obviously necessary to allow interconnection to all other lines to the system and TELEPHONE EXCHANGES came into being.
Note all the operators were woman; boys were employed initially but proved too liable to distraction. Separate receivers were used and the transmitters were mounted on the face of the switchboard. The temporary connection were made by flexible multiple wires which became known as ‘cords’ and were used over and over again.
In 1912 the General Post Office (GPO) took over most of the private telephone companies that had sprung up throughout the country. A network of lines between exchanges in the larger cities grew. These were called ‘TRUNKS’ or junction.
Many of the Trunk Lines used Telegraph lines, some in the underground cables many on the overhead routes similar to the one shown below. These followed the main roads of the time with other going along the network of canals that linked the main centres of commerce and industry.
3. THE ADVENT OF AUTOMATIC TELEPHONE SWITCHING SYSTEMS
For a long time after the introduction of the telephone, subscribers, as they were called, depended on all their connections on switchboards with human operators,
usually girls. But as the system grew, the operators became very particularly at peak periods and delays in answering calls were an increasing source of annoyance. Wrong connections were frequent and in the rush to connect calls, conversations were occasionally cut off by mistake. One of the earliest subscribers in America was Alimon B. Strowger, a funeral parlor proprietor of Kansas City who, it is said, discovered that his local telephone operator was married to his rival undertaker to whom she diverted Strowger’s calls. In 1889 he invented a ‘Step-by-Step’ automatic system and all early automatic exchanges were based on “Strowgers” principle. Using a dialing disc, which become known as a DIAL which was in use throughout the world until the advent of the keypad in the 1980’s.
By turning the dial, whose fingers holes are marked with the digits, the caller ‘routes’ his connection through several switching stages, selecting the exchange, the group of lines, and finally the number he wants, at the same time switching on the current operating the bell and the ringing signals (tone) or the ‘engaged’ tone. Today, this system has been extended and developed that one can ring up, without human help, telephones n most other countries and continents.
It does not matter if those in other countries use a different system of automatic routing such as , for instance, the ‘panel’ system that used to be used in New York and other American towns, in which the connection made by motor driven unites; the ‘Crossbar’ which works with relays and matrix layout; the ‘Rotary’ system
which has electro- magnetic clutches or the ever developing ‘ Electronic Systems’ based on electronic computer techniques which establish the connection within a fraction of second and are much more reliable than the earlier electromechanical system.
Electronic Exchange To overcome the limitation of the manual switching, automatic exchanges having electromechanical components were developed. Strowger’s exchange, the first automatic exchange having direct control feature appeared in 1892 in Ea.porte(Indiana). Though it improved the performance of manual exchange it still had a no. of disadvantages like a large no. of mechanical parts, limited availability, inflexibility , bulky in size etc. as a direct result of further research and development, crossbar exchange having an indirect control system, appeared in 1926 in Sundsvall Sweden. The crossbar exchange improved upon many shortcoming of the older system. However, much more improvement was
expected and the revolutionary change in the field of electronic provided it a largest no. of moving parts in register marker, translator etc. was replaced en-block by a single computer. This made exchange smaller in size, volume and weight, faster and reliable, highly flexible, noise free, easily managed with no preventive maintenance, etc. The first electronic exchange employing space division switching (analogue switching) was commissioned.
Definition of Switching In a telecommunications network, switching means routing traffic by setting up temporary connection between two or more network points. This is done by device located at different locations on the networks, called switches (or exchange). The basic structure of a telecommunications network therefore comprises transmission media, interconnected by exchanges. “Packet” and “Circuit” switching are two techniques used by telecommunication network.
Definition of packet switching Process where-by message are broken into finite-size packets that always are accepted by the network. The message packets are forwarded to the other party over a multitude of different circuit paths. At the other end of circuit, the packets are reassembled into the message, which is then passed onto the receiving terminal. Each packet than transmitted individually and can even follows
different routes to its destination. Once all the packets forming a message arrive at the destination, they are recompiled into the original message.
Definition of circuit switching: A method of communicating in which a dedicated communications path established between two devices through one or more intermediate switching nodes. Unlike packet switching, digital data are sent as a continuous stream of bits. Bandwidth is guaranteed, and delay is essentially limited to propagation time. The telephone system used circuits switching.
Time Division Multiplexing: The time division multiplexing is define as “the multiplexing in which whole channel is given to a signal for particular interval of time”. In time division switching system the speed and supervisory signals are first converted into digital signal using PCM techniques. In PCM we transmit samples of information in discrete points of place of continuous information. These discrete points are called time slots or a channel. In between two consecutive samples of one speech signal we can send samples of other speech signals by multiplexing. Therefore some physical path can be used for many speeches in their respective time slots. This physical path connecting many channels is called a highway. Thus a TOM switches simply a transistor switch, which is switched on or off when required passing or
preventing the transmission of appropriate transistor base performs this.
Telecommunication: A telecommunication network established and realizes temporary connection in accordance with the instructions and information received from subscriber line and interexchange trunks, in fond of various signals. Therefore, it is necessary to interchange information between an exchange and it external environment i.e. between subscriber lines and exchange, and between different exchanges. Though these signals may differ widely in their implementation they are collectively known as telephone signals. A signaling system uses a language which enables two switching equipment in converse for the purpose of setting up calls. Like any other language it possesses a vocabulary of varying size and varying precision i.e. a list of signals which may also vary in size and a syntax in the form of a complex set of rules governing the assembly of these signals.
Basic diagram of telecommunication network
Functions of Exchange: • Subscriber lines and trunks connected to and from (both) some exchange. • Pair of trunks towards different exchanges. • Exchange of information with the external environment. Charging and billing. • Controlling the operation of switching network.
Working of telephone:
To convey sound information from one place to another telephone uses an electric current. Telephone Company’s send the steady current. Which is been shared by both of the user. When we send some voice signal the current through phone fluctuates which is related to air pressure fluctuation that are sound of your voice of microphone. So the fluctuation in current is also observed by receiver end. Now speaker response to that fluctuating current and reproduce the sound of your voice. The current which power the telephone is generated by 48volt battery in the control room. This voltage is sent through firm resistor and inductor. When telephone is on-hook state, the TIP is at about 0 volts, while “RING” is at -48 volts, and in off hook state TIP is at -20v and ring is at -28V dc resistance of telephone equipment is 200-300ohms. And current flowing is 20-50ma. The 48volts was selected because • It was enough to get through kilometers of the thin wires. • It is safe. • It is easy to generate. The line feeding the voltage was selected to be negative to make the electromechanical reactions on the wet telephone wiring to be less harmful. When wires at -ve potential compared to the ground the metal ions go from the ground to the wire, otherwise corrosion may occur.
Telecommunication carriers to reduce poor connection and noisy lines often do place small dc bias on along transmission pair, this is often referred as “SEALING” current. So it sealed the connection and so improved the transmission.
What is a full duplex operation: It is term to use to describe a communication channel, which is capable of both receiving and sending information simultaneously. Telephone set have only 2 wires which carry both speaker and microphone signals. But the signal path between telephones requires amplification using a 4 wire circuit. But because of the cost and maintenance factor we can’t use the 4 wires. So we use a device called “hybrid” which convert the 4 wire circuits to 2 wire local cabling. This is well balanced circuits which are capable of separating incoming audio to outgoing audio. Normally bandwidth of 3 kHz is used in telephone lines.
How much it is safe and what precaution we should take: Dangers regarding the telephone: •
The 48 volt dc voltage in telephone doesn’t cause immediate danger to the user, but the ac ringing signals (70-120 v ac) can give a nasty shock.
• Telephone wires also exposed to any lightening. • Ground potential difference. • Interference from power lines. So telephone is designed to be fully enclosed in insulating plastic case, which provides isolation. It works nicely if there are no metal contacts in telephone which are somehow connected to telephone lines. The telephone companies can’t know what kind of designed equipment we consumer are using. But it does specifically tell them, that at any time without warning and at their convenience, they might just put a variety of voltage current on a given loop. If the device is not designed to meet the regulations it can cause some dangers or problems in those situations.
Precautions Don’t put in series or in parallel, into a telephone loop:
• Batteries of any kind • Polarized capacitors • Diodes of any kind • ¼ watt resistors • Lamps
All of above are safety hazard. We should avoid the polarized capacitors and batteries, as both can make a favorable condition for explosion. While using resistor we should keep in mind that they must have a enough power handling capacity.
Some parameters of telephone lines: Telephone lines are balanced transmission line which can have up to 120 ma of dc current from voltage up to 56dc and 120V ac. Some parameters of telephone lines like resistors, capacitors and inductance don’t depend on the voltage and current on the line. Actually balancing the telephone line circuit is our major interest. As cross talk is related to how well your circuit is balance. Actually loop current does not affect the balance of circuit. There are many factors which can affect the balance:-
•
It depends on circuit connected to line ends.
It also depends on the quality of wire for example wet cable can cause balance problem if inner is in contact with water. •
Telephones: It consists of speaker, microphone & network interface circuit.
Speaker: It makes us able to hear the voice of other distant user. Actually it reflects the changes in current.
Microphone: It sends our voice signal to telephone lines and alter it transfers to other user. Carbon mics consists of small button of carbon powder connected to diaphragm. When sound flexed the diaphragm, the carbon grains change their resistance that’s why a variable current is produced which corresponds to voice signal.
Network Interface:
Telephone has a circuit which connects the microphone and speaker to the phone line that circuit we termed as network interface (telephone hybrid). Function of network interface: • It sends only the current changes to speaker which are caused by other user • It does not allow the current changes which are caused by our voice not send to speaker, they are send to microphone.
TONE RINGER Single tone ringer: It has a fixed frequency self resonance oscillator, which are turned ON and OFF by an alternative half of the ringing voltage. Such a ringer produces a very high frequency of the 2-3 KHz.
Disadvantages: • With the age the hearing sensitivity of people get reduced due to this high frequency. • The ability of the people to locate the source at high frequency is poor.
Multi tone Ringer: With the development of IC’s this multi tone ringer is being designed. It is more complex than a single tone ringer. Since, the input voltage to circuit varies over a wide range depending on how far the telephone is from exchange. It also depends on the no. of telephones connected to the same line.
Advantages: • Volume control and loudness control can be provided. •
Possible to obtain pleasant and alternating sound.
• Components used for electronic ringer are much smaller. •
They are more reliable.
Dialing system of exchange: When we lift the receiver hook switch contacts close ,dial impulse spring are connected in series with the line circuit , when the dial is operated , contact on these springs open and close the circuit, a no. of times corresponding to the digit dialed. Now a new system of dialing was introduced. In this system digits are transmitted as to tones simultaneously. Here one more term introduced that is duel tone multi frequency (DTMF). To facilitate transmission over the normal 2 wire pair, these tones frequencies lie within audible range. There are 8 frequencies defined in DTMF system, 4 in the low frequency group, and 4 in high frequency group. A valid digit is defined as one tone of the low frequency with one tone of high frequency group.
Types of Tones: Dial tones: When we lift the handset we heard a continuous low pitch that signal indicate that the telephone is connected to free selector and it is ready to make a call or in other words we can dial the no. if we dial before the selector is ready to receive a call, then no call will be made. This tone stopped when we dialed a digit.
Ring Tones: After the completion of dialing we heard an interrupted high pitch, which indicate the call has proceeded satisfactorily and ringing condition has been set up. It stops when the called subscriber answer our call.
Busy tones: This interrupted high pitch tone indicates that the connection cant be made now, but may be attempted later on. When heard before dialing it indicates the called subscriber is engaged.
Number un-obtained tones: An interrupted high pitch tone indicates that the call should be abandoned. When we heard during dialing it indicates that selector has been connected to nonworking level. When after complete no. is dialed then it indicates that line is not working or a temporarily unavailable.
Frequencies of these tones: Dial tone:
33cycles/sec
Ring tone:
4000cycles/sec .4 sec “ON”, 2 sec “OFF”
Busy tones:
400 cycles/sec. .75 sec “ON”, .75 sec
“OFF”
Number un-obtained tone:
400 cycles/sec. but for
this 2.5 sec “ON” 5 sec “OFF”
E-10B System: E-10B system is the culmination of a massive R&D effort in the field of digital system in France. The first E-10B was commissioned in June 81 at Brest in France. Version applied to India is the 384 PCM versions, which can handle a maximum traffic of 400 erlangs.
Connection with remote subscriber: Now with the help of this system we can extend the telecommunication facilities to remote area by setting up the Remote Line unit (RLU). In French these are called “urad” (Distant Electronic Spatial Concentrators). Each RLU has to parent to main exchange by 2-4 PCM systems. Each RLU connects to 1023 remote subscribers. URAD can be called as CSED has no stand alone capability. It requires main exchange for termination, where termination of their connection is there. Various ways in which we can use E-10B exchange:
•
Local Exchanges:
These exchanges terminate local subscriber lines and are connected to other exchanges in the local network.
•
Local Transient or Tendem Exchanges:
E-10B system can be used to carry pure transit traffic. In this case subscribers lines terminating equipments will not provided.
Tax: When used as the system provides for the termination of long distance circuits. The maximum capacity of an E-10B tax is limit to 11000 lines in the 384 pcm versions.
Characteristics of E-10B system: •
Capacity:
No. of switching PCM links:- 384
Processing capacity:- 1,90,000 BHCA
Traffic handling capacity:- 4000 erlanges
Subscriber exchange:-45000 lines and 500 circuits.
•
System:
Time division switching.
Pulse code modulation (PCM) to CCITT standards 2mbits PCM link.
30 telephone channels per PCM link.
8 bits per telephone channel.
Stored program control.
Dedicated processors for switching functions.
Non-dedicated processors for operation functions.
•Subscribers line:
Dial or push button VF telephone (CCITT standards).
Maximum loop resistances inclusive of telephone set 2400 ohms.
Ringing currents: 80V, 25 and 50 Hz.
Features of E-10B system:
PCM principles:
The system has been developed for 30 channels PCM.
Segregation of switching using dedicated microprocessor or mini computers: Switching function like reception of digits, their storage, analysis, routing of the calls etc. is performed by the control units, which have a decentralized architecture employing dedicated processors. Minicomputer located as
centralized operation and maintenance centre (OMC) performed functions like subscriber line and circuit group management, alarm and fault management, testing diagnostics etc. The OMC and switching centre are connected by PCM links.
Stored program control
The controlling function related to call processing, in E10B realized mostly in software, but they are carried out by stored instructions only.
Centralized management for a group of E10B exchange The O&M functions for a group of E-10B (max of 6 Exchange) are carried out by a single OMC which is connected to various exchanges by PCM links.
Environment condition:
•
Exchange:
Ambient temperature of air drawn into racks: 18-20 degree Celsius.
Relative humidity: 30% to 70%.
•
OMC
Air conditioned environment
Temperature: 15-18 degree Celsius.
•
Mechanical data:
Rack dimensions:
o
Height: 2m
o
Width: .75m
o
Depth: .5m
•
Power supply:
Exchange: -48v
OMC: 220v, 50hz
Power supply current: 23-60ma
Loop resistance: 1500 to 2400ohms
Note: All units required dc voltage only OMC require ac voltage.
Components of E-10B Exchange:
DSF: It is a control unit. It is standby charge recording equipment. It is not duplicated. It includes an interchange unit around processor els-48. Its functions are: •
Data save
• Regeneration: auxiliary memories of marker, translator and charging unit can be regenerated or reloaded. It is provided with a magnetic tape drive unit.
OC: It is a monitoring unit. It is the interface between exchange and OMC. Receivers and processors message from OMC addressed to exchange units and forward messages from these units to OMC. It is connected to message transmission equipment ETM at the OMC end.
BDA: It is the auxiliary equipment rack. It controls the distribution of recorded announce given by the recorded equipment provided in the rack. It displays alarm conditions and control of local alarm indications. Normally it has 8 input/output sockets through which we feed the message which we want to hear at respective point of time. By inserting the mike in that we can also feed the voice in that.
ETA: It is tone and frequency generator, frequency sender and receiver unit.
CSE: It is the subscriber connection unit. It has the space division electric line concentrator. It has several racks, according to the capacity. Each rack is divided into sub racks, normally 8 in numbers. Their numbering is from 07. Each sub rack has 8 cards numbering from 0-7. Each card has 16 telephone subscriber connections their numbering is from 0-15. So, in total one sub rack have 8 * 16 = 128 connections. Numbering is 0-127. So, one rack has 128*8=1024 connections. Normally first connection is not taken in use. So that’s why we use maximum of 1023 connections from one rack. Which we can termed as 1k capacity of a rack. Each subscriber is located with the help of EN (Equipment Number). The functions of CSE (URA) include management of subscribers. It is interface between subscribers and other units of E-10B exchange. It also does fault handling and identification of faulty units. Also it has 4 functional areas.
Subscriber connection unit: It delivers power to telephone set and detect loop condition of the line and send bell current and to transfer to test.
Connection network: 1024 incoming lines are connected to 120 outgoing channels.
Transmission unit: Here takes place the A/D and D/A conversion and transmission and reception of speech sample on LR links.
Current Logic: All the above three are managed by control logic through speech path system. There are two types of CSE racks depending on the place where they are placed: URAL/CSEL: local (in switching unit) URAD/CSED: remote logical unit.
URM: It is the interface between tank circuits and control unit. It handles 32 PCMS. 4 PCM are corresponds to 120 line at a time, which corresponds the maximum capacity of the rack. It acts as an interface between two exchanges. It control the transmission of speech channels between PCM links and network links and interchange with remote URA and other exchanges. It had an ITRA card.
BT (time base): It is control unit. It is not duplicated. It supplies basic timing signals to all other exchanges where secondary timing signals are derived for their operation.
MQ: mark the call, i.e. tells the type of call. MR and TX: both are charging unit.
CX: It is the main unit of exchange. It is the switching unit in the exchange. It is connected to connection units for speech paths and signaling. It is connected to control units (UGCX) for command or control or both, to OC for positioning. It interconnects calling, called time slots and enables transfer PF signals between them. It transmits tones. It directs DTMF. It transmits signaling to connection unit. It works on TST techniques.
OMC (operation and maintenance centre): E-10B exchange handles switching functions are handled by OMC. The OMC is designed around computer MITRA225.OMC has no direct role in call processing and so an E1 OB exchange will perform switching functions even without OMC, but OMC absolutely essential for management of E-10B exchange. ETM is an OMC peripheral providing the interface. ‘Subscriber management‘ Trunk group management ‘Routing and analysis management ‘Call charging management ‘Trunk testing ‘Traffic and load measurement ‘Alarm message processing ‘Fault tracing & clearing ‘Unit positioning.
ETM RACK:
It routes message sent by the OMC computer to destination exchanges and receives massages sent by the exchange and forward them OMC computer.
EWSD SYSTEM: Siemens communication inc. offers its customers a broad portfolio of communication products and services and is a leader in convergent technologies, products and services for wireless, fixed and enterprise networks. ESWD SYSTEM is manufactured by SIEMENS, GERAMANY Siemens AG (NYSE:SI) is one of the largest global electronics and engineering.
ISDN: ISDN stands for Integrated Service Digital Network. The ISDN standard defines various services (bearer and teleservices) and additional supplementary services which can be controlled by the subscriber by means of the functional and/or keypad protocol. The main characteristics of ISDN are: • Integration of the different telecommunication services (voices, text, data and image) on one subscriber access line. • Integration of existing separate network supporting these network services. •
Digital telephone network.
• Transparent digital transmission at 64 kits/ s within the network. • Send to end signaling for supporting more sophisticated supplementary services.
EWSD ISDN SERVICES: The primary aim of ISDN is to provide new digital services for voice and non-voice communication with high value supplementary services .Improved quality and greater ease of communication.
Following types of ISDN services are available: •
Bearer services:
Bearer services are used for unrestricted voice and text transformation. The technical specification of these services covers the transformation function of open system inter connection reference modal that is required for the transport of information. A bearer services only insures the transport of information between the respective user network interfaces.
•
Tele-services:
The Tele-services are services for the user and the user to host the communication including specification of communication functions of the terminals. The
communication functions comprise all the transmission functions and the communication protocol of the open system interconnections reference model for controlling the different communication processes.
•
Supplementary services:
The supplementary services augment the basic services. These services may be individually assigned according to the needs of the ISDN subscriber.
Traffic routing: Introduction The purpose of the traffic routing is to establish a connection between a calling and a calling party or a requested service on the basis of directory n. called. The EWSD system has a high performance traffic routing function that make optimum use of available network resources and are thus capable of handling even large volume of traffic with ease. The traffic routing function in ESWD are: •
Digital translation
•
End of selection evaluation
•
Destination translation
•
Routing
•
Trunk group selection
•
Trunk selection
•
Application of announcements and the intercept.
Network structure: A telecommunication network basically consists of the network nodes and the trunk groups connecting them.
Network nodes: The nodes in the network consist of the following types of the network nodes.
•
Local network nodes:
The local network node is the network node where the lines of subscribers in a particular local area terminate. There may be more than one local network node service local area. It sets up connection for calls within the local area and the forward request for the calls to the destination outside this area to the transit network node.
•
Transit network node:
Transit network nodes set up connections for long distance calls between different local areas.
•
Gateway network node:
Gateway nodes are the points of access between network in different countries or between network run by different operators in the same country.
Traffic handling: A number dependent traffic routine: It permits the identification of authorized and unauthorized subscriber during connection set up. The subscriber’s directory numbers and the carrier access code are used as identification criteria.
Multiple local networks in one network node: It allow subscribers form up to 3840 different local network to the connected to one network node via remote digital line units (DLU). This means that subscribers belonging to a region with analog network nodes can be provided with analog ISDN services on
economical basis. Such subscribers are then switched physically to the remove ISDN capable node.
Automatic alternative routing: It allows up to 16 alternatives routes to be accessed per destination. These routes are then searched in a fixed ascending hunting sequence to whether there is at least one free trunk in the selected trunk groups. The waiting call is allocated to the first route in the hunting sequence in which an idle trunk is found. If no idle are found on a selected route, this result in overflow to the next alternative route in the destination area. If the hunt for an idle route remains unsuccessful right up to the final route, call set up is aborted.
Automatic rerouting : This feature offers the traffic routing an additional means of successfully setting up a connection. If a connection attempt in a subsequent network node is unsuccessful automatic routing attempts to set up the connection via a route in different routing group. The routes of an automatic rerouting group are searched in ascending order, beginning with the direct route. Route with the same rerouting no. as the route on which the precious search for an idle trunk failed are skipped. It is used in: Long distance traffic, International traffic in transit, Network nodes.
Customer dependent routing:
This feature allows call to be routed via selected carriers or selected trunk group offering better transmission quality, for instance. This type of routing can be provided for the customers of one particular network operators, for the other operators in the network. The customers or network operators are given customers numbers, and these customers numbers are assigned to the corresponding subscribers and incoming trunk groups. Connection with special routing can then be identified be a means of the customer number. This feature customer dependent routing can be used in gateway network nodes and also in local network.
Distribution frames: MDF (main distribution frame): This frame provides: •
A fixed means of terminating the external cables.
• A means for mounting the protective devices for incoming circuits. • A means for cross connection the external circuits to the appropriate internal circuits. • A convenient point of interception for locating of faults. It acts as interface between the exchange side and line side. The MDF is properly earthed for the protection of the equipment. The external pairs are area wise
terminated on the line sides of the frames, while connection from the equipment s done on the exchange side in a numerical order. In exchange by interconnection at this frame with the help of jumper’s wire, allow us to give any exchange no. to any subscriber of any area easily. Now a rack type MDF has been standardized. It occupies less space and is convenient to install. It uses GD tube as fuse. So when external current reaches at certain maximum value which can damage our exchange, the fuse blows up, and disconnected the external circuit with exchange. In this manner it saves the exchange.
IDF (Intermediate Distribution Frame): An IDF provides: • A means of cross connecting an exchange line to chosen calling equipment. • A suitable point for connecting the exchange lines to answering multiple or final selector multiple in case of auto exchange. • A means for cross connecting circuits to or from other exchange to the appropriate signaling equipment. • A means for terminating and interconnecting the various ranks of switching equipment in an automatic exchange.
Combine Distribution Frame: At small exchange, separate main and intermediate distribution frames are not provided. Instead a combined and distribution frame that combines the facilities of those of MDF and IDF provided. The lower portion of this frame is equipped with fuse and protector mountings, while the IDF connection stripes are mounted above the adoption of combined frame effects considerable saving in the table costs between the protectors and the multiple side of IDF.
Power supply: Telephone exchange switches need the continuous supply of power to operate. One of the primary requirements of any telephone exchange is that, service shall be available to the subscriber at the time. The Exchange power supply system is designed to provide continues and uninterrupted telephone service even when the public electricity fails. So, they have made a provision for alternative source of supply at the time of power failure. •
Batteries of secondary cells.
• A combination of batteries of secondary cells and prime mover generator sets. The secondary cells in generator used are of lead acid type. These cells have electrode of lead immersed in electrolyte if Sulphuric acid in soluble container. Capacity
of battery is specified in terms of ampere-hours. If the capacity is 1000Ah, it can be discharged at rate of 100A for 10 h. The capacity of battery is decided by: • How long it is required to supply the power to maintain service in the event of main failure. The busy hour load of the exchange getting supply for the batteries. •
Batteries are connected by connecting the secondary cells in series. The batteries are floated (connected in parallel with rectifiers) across the exchange load. The floating voltage per cell can’t be higher than 2.34 V/cell because beyond this voltage battery will discharge. Mostly 24 cells battery with float voltage 2.5 -2.20 V/cell
DC INVERTOR: Actually every machine in exchange works on DC voltage except OMC. So, power supply systems have inverters which convert the main AC power to DC which is required. It consists of 5 modules. Each module can give 200 A of current. So total of 1000 A current we can get from that machine. From there power goes to distribution unit which distribute s the power to exchange according to their requirements.
Battery Conventional Battery:
These are old type of dilute H2S04 Battery. These have a 24 cells and each cell with the range of 1.9-2.1 volts. The voltage should not drop below 1.7 V and specific gravity should not fall below 1160. These are bulky in size and constant invigilation is required.
Non-Conventional Battery: These are valve regulated acid batteries. These VRLA batteries are maintenance free type and works on oxygen recombination principle. VRLA cells give very long life in floating service, in addition to deep discharge.
Voltage: VRLA cells are 2 volt units which are assembled, in modular racks to get 2v, 4v, 6v, 8v and 12v modules. For maximum service life, the recommended float voltage is 2.23 per cell. VRLA cells are normally to terminate discharge at higher end cell voltage due to reasons of equipment compatibility, providing higher rated capacities can do it.
Distortion telecommunication circuits: A signal is said to be distorted if after passing through a network, it is not an exact replica of the original signal in respect of its amplitude wave shape. Signal distortion is of three types:
•
Attenuation Distortion:
This term is employed to the case of the transmission system where there is a variation of gain or loss with frequency.
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Phase Distortion:
Phase distortion takes place when the time of propagation through a transmission system varies with frequency. Owning to this the output waveform may spear to be quite different from the input waveform.
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Non-linear Distortion:
Non linear distortion is the name given to the certain type of distortion that occurs when the transmission properties of system are dependent on the instantaneous magnitude of the applied system.
Cross talk: A telephone line seldom is consists of only one linear pair, they invariably may have pair carrying more than one speech signal. Now some current is induced from one pair to another. If the induced current attains sufficient magnitude than the speech flowing in the one disturbing circuit will be heard in another distributing circuit. This is called cross talk.
Trunk Exchange: The local exchange manual or automatic caters for interconnections of telephone in a limited area served by it. The limited area may be a suburb, a town or may be a metropolitan city. But this is only one need of a telephone subscriber. Sometime he wants to communicate with those in another town or city situated hundreds of kilometer away from him. The media which connects these areas is called trunk network. The trunks network which interconnects the local network of various limited telephones, networks covering the length and breadth of the country consists of trunk line connecting various towns of the country and the associated switching exchange. The lines of the circuit which interconnects various towns and cities if the country is known as trunk circuit and switching exchange which serves to connect one trunk line to another trunk line and to local exchange are called the trunk exchange.
