Telephone - Good To Know Stuff

 

Today let's reminisce what was telephone and how it worked back then.

Telephone

• Telephone is common usage in Telecom, as it comprises telegram, television, fax, etc. 
• Teledensity is term which shows how many people are using telephone lines.
• A term commonly used to describe the number of telephone lines per some unit of the population (often per 100 people); the density of telephone. It also shows company's wealth.
• The different between telephone and computer is
• The old computer may not support to the current environment, whereas old model telephone can support to the environment but with the same features what it had.
• CTI => Computer Telephony Integration
• Voice Call recording system uses the following
• communicating with telephone
• on hook and off hook process

Analog Phones

• Analog telephones are dependent on voltages. It works on flow of voltage. It doesn't sends any packets.
• Types of Phones
• Rotary telephone
• Dial plan telephone
• Connection Settings
• Exchange ---- twisted pair wire ----> wall jack / rosette ---- twisted pair wire ----> local phone

• The local phone has two parts :
• headset
• connected with coiled wire
• twisted pair wire (copper wire) 
• red wire
• black wire
• jack
• base unit(box) or dialing box :
• Consists a place where the handset is kept, which is called "cradle".
• Consists two "hook switches", but now we have only one.
• Power
• The exchange gives power to phone.
• 50V dc power is sent to phone, when it is on hook state.
• The "multimeter" can measure the voltage passed.
• Dial Tone
• When the handset is picked up, a tone is played at the first is called dial tone.
• To check whether connection is established.
• To check the working state of the phone.
• If the dial tone is heard that means it is ready to use.
• Ring back
• The ring received after dialling the number.
• Handset place on the hook is called as on hook.
• Handset picked up then it is off hook.
• 70V AC is given when incoming call has come.
• While off hook is there then maximum voltage is given 20V to 28V.
• Flash => Getting the attention of the PBX. After that PBX will accept the numbers to perform some actions(hold). In the PBX certain numbers will be configured to respond that number.
• If the hook switches are pressed less than a second then we may get the attention of PBX.
• Oscilloscope is used to see the voltages.
• Rotary Dialler : The phone's dialler will be in a circle.

Architecture of Base Unit

                             

• hybrid : It will transform one type of signal to another. It is also meant as transducer. It is the very costly part of the base unit. The quality of the voice depends on this. The quality of the voice is evaluated here.
• capacitor : It will filter the AC from DC. It sends AC for ringing (incoming call). 
• In the on hook, the telephone will receive 50V.  When the incoming call comes the PBX will send 70V to our phone. When we make a call it will get a ring back tone from the PBX and the calling telephone will get 70V for ringing.
• The problem in this system is that, if the phone is disconnected from the wall jack then we too will get the ring back, it will just check the wall jack's connection not the wall jack to phone connection.
• In rotary phone, if we press the hook continuously, then there are chances of call to be made.
• When we pick up the handset, then the hook switch mechanism closes the switch. Then the voltage will be passed to the hybrid.
• To dial we will rotate the dialler, if we rotate zero then from the zero position it will open and close the dial switches(before hook switch) 10 times. All this will send pulse to the exchange, then the exchange will realise the pulse and calculate the number. All these pulses will be sent within a second. The one of the use of wall jack is that we can connect many telephones, else we can connect only one.

How Telephone Call Works

• After picking up the handset : Hear the dial tone passed by the PBX, this means the exchange is ready to serve.
• Dial the number, the dial tone will be stopped.
• Numbers are sent to the PBX in pulses, there it is counted and identifies the numbers. 

  Eg: The number dialled is 432.

  The pulse will be as following :

  _   _   _   _        _   _   _        _   _

               _| |_| |_| |_| |______| |_| |_| |______| |_| |_

                       4          |       3       |      2

                                  |               |

                                  V               V

• Inter digit Time : This is the time between the each digit pressed.
• Receive the ring back. And the other side receives 70V AC for ringing.
• The dial tone received back and voice will be of 400Hz purely which is modulated 20Hz.
• Then the connection is established, the voice will passed in pulses.
• All the combinations will be sent within a seconds, milliseconds or micro seconds.

Digital Phone

• Digital phone will send packets to communicate instead of sending voltages. 
• All the communication has been sent in 0s and 1s. 
• In digital, the header information like source address, destination address, etc., is used to send send and receive the packets.
• It is more intelligent and fast. 
• It gives lot of features of the analog phones.
• It gives constant power supply. 
• Unlimited signalling will be given.
• It has good speed to respond the request.
• Digital phones are fully dependent on PBX. So, if we use digital phone then the digital line has been made between phone and PBX.
• Noise tolerance.

VoIP

• It needs the network connection.
• It sends the data compressed in the packets format.
• It is more intelligent and faster compared to analog and digital.

Features of Telephones

• Ringing alert
• The LED will glow, if there are many telephones then to identify which phone is ringing.
• Ring volume controller 
• To control the volume, to minimize and maximize the call's volume.
• Automatic gain controller 
• To tune the voice.
• Flash button 
• To get the attention of PBX.
• Pulse and tone mode facility
• Can change the modes, but after the call completion it will reset to the original one.
• Hold button 
• To put a call in hold.
• Pause button 
• To pause a call, using this we can skip the IVR's options.
• Mute button
• To put a call in mute, this will avoid the user to hear us.
• Redialing 
• Wall mountable phones 
• Phones kept on the walls.
• Memory 
• Save the contact details.
• Hands free dialing 
• Echo cancellation 
• Subtracting the incoming and outgoing voice to avoid noise.
• Ringer Equivalent number 
• To obtain power consumption of the telephone.
• Caller id.

Telephone Exchange

                      Telephone Exchange

                  +------------+

      111(Phone) <------| *        * |-------> 101(Phone)

      112(Phone) <------| *        * |-------> 102(Phone)

      113(Phone) <------| *        * |-------> 103(Phone)

      114(Phone) <------| *        * |-------> 104(Phone)

      115(Phone) <------| *        * |-------> 105(Phone)

      116(Phone) <------| *        * |-------> 106(Phone)

                  +------------+

• A telephone exchange system acts as an intersection between two phone lines. 
• This is because a direct phone line between two establishments doesn’t actually exist.
• In the telephone exchange previously man intervention was needed. 
• This problem was solved by Mr. Strowger.  
• He designed the automated branch exchange.
• The communication can be done in only two ways analog(pulse) and digital(tone mode).
• Pulse Mode:
• Pulse mode refers to a way of signal transmission that involves sending electrical pulses down the telephone line.
• Pulse duration is short, measured in milliseconds or microseconds.
• Pulse is complicated.
• Tone Mode:
• Tone dialing refers to a way of transmitting signals in telecommunications systems that uses audio tones of different frequencies.
• Every digit when we press will have a frequency, which will be unique, to identify the number.
• The frequency is the combination of two frequencies. Since it will be difficult for the user to mishandle it, as this is very difficult to make the artificial voice.
• The tone is simple.
• There are phones now-a-days where we can change to Tone Mode or Pulse mode.

Telecommunication

Telecommunications such as radio, telephone, and television all use a method called multiplexing--shortened to "muxing"--to transmit and receive information.

Multiplexing

• Multiplexing was designed to send numerous analog signals or digital streams through one common transmission line.
• Multiplexing is the sharing of a communications channel through local combining of signals at a common point.
• One output, many input.

A------------+

B------------+ Multiplexing 

C------------+------------------- 

D------------+

De-Multiplexing

• Demultiplex (DEMUX) is the reverse of the multiplex (MUX) process – combining multiple unrelated analog or digital signal streams into one signal over a single shared medium, such as a single conductor of copper wire or fiber optic cable.
• Many output, one input.
• It is used for reducing the cost.

                         +------------- 

         De-Multiplexing +------------- 

        -----------------+------------- 

                         +------------- 

                         +------------- 

• Ex:
• A organization has 100 employees, get only one line from the exchange. From that make 100 connections.

TDM (Time Division multiplexing)

• This is also one of the methods in multiplexing which will work based on the time.
• The method of combining more than one independent data streams into a single data signal and transmitting that single data signal through a multiplexer to a demultiplexer is known as time-division multiplexing.

Ex :

----

   000|010 

A ---------+         +----------- A

   011|001 +   +------------+         +-------------+   +

B ---------+---+ Multiplexer+---------+ Demultiplex +---+----------- B

   111|100 +   +------------+         +-------------+   +

C ---------+            +----------- C

• Multiplexer read the data from the start(A) to end(B) within a second. If any channels are empty, it doesn't consider and send the data in a sequence. 
• Demultiplexer have control to identify the empty channels and separate the data using the given duration(one second). So first it reads the data 000 and 011 etc.
• First it takes all the data from the channels which has to be sent in the first one second. That will be passed into de-multiplexer. Then de-multiplexer separates the information based on the time. Here the time is one second but in real life it will be less than microseconds.
• Multiplexer does a multiplexing and De-Multiplexer does the de-multiplexing work. In between these two, one controlling terminal will be there for controlling the data. While passing it sends duration channels. Controlling terminal decides how many channels send that data.

Signalling

Analog Signals Features

• Accuracy
• Caller id is passed between rings.
• Analog Signal

      _           _           _           _           _

     / \         / \         / \         / \         / \

    /   \       /   \       /   \       /   \       /   \

----+   +-     -+   +-     -+   +-     -+   +-     -+   +------

          \   /       \   /       \   /       \   /

  \_/         \_/         \_/         \_/

Digital Signals Features

• Noise tolerance
• Avoid data loss
• Caller id is passed in packets.
• Digital Signal

                                           

    +---+   +---+   +---+   +---+   +---+  

    |   |   |   |   |   |   |   |   |   |  

----+   +---+   +---+   +---+   +---+   +------

PCM - Pulse code module

• This is a technique for converting analog signals into digital form.
• PCM samples the signal 8000 times a second, each sample is represented by 8 bits for a total of 64 Kbps.
• There are two standards for coding the sample level :
• Mu-Law 
• A-Law 

Sampling

• Sampling is a way to convert the analog to digital.
• Sampling rate means samples taken per second or hertz, the rate at which the samples of an analog signal are converted into digital form.
• To get original voice the sampling rate must be high.
• It requires double of bandwidth rate. Since the limiting factor is the bandwidth at which speech can be transmitted. 
• The 8000 samples per minute is taken for 64kbps.
• Example, the bandwidth telephone line we take as 64kbps at a sampling rate of 8kHz and 8-bits per sample.
• 8000 samples per second * 8-bits per sample = 64000bps = 64kpbs. 

Companding

• This is a technique used to compress data. It compresses the 16 bit data into 8 bit.
• This is formed from the words COMpressing and exPANDING. 
• Companding reduces the noise and crosstalk levels at the receiver by sending the compressed data and then expanding it at the receiving end to increase the signal quality when it is digitizing an analog signal.

CoDec

• The conversion of signals from analog to digital is performed by a coder. 
• On the receiving side, the process will be reversed using a decoder so that the signals can be used by an analog telephone.

PBX

• It is abbreviated as Public Branch eXchange.
• It is a switch station for telephone systems. It consists mainly of several branches of telephone systems and it switches connections to and from them (linking phone lines).
• To buy telephone for every employee of a organisation it will charge on the following basis :
• Wiring
• Rental
• Maintenance
• Deposit
• Call charge
• The calls made within the organisation costs heavily. Using PBX we multiple the phones.  Hence PBX is less expensive than connecting an external telephone line to every telephone in the organization.
• It's easier to call someone within a PBX, dial only 3 or 4 digits.
• It is like a hub, all the calls are routed to and from it.
• Two types of PBX:
• Soft PBX --> asterisk, freeswitch
• Hard PBX --> avaya, nortel, alcatel

PRI

• This is a type of ISDN.
• This is like a multiplexor, from a signal wire connected to the PBX 30 channels will be separated.
• This is a wire.
• This is an international communications standard for sending voice, video and data.

                           ------------ 701

                                    |

                                    |

                           ------------ 702

                                    |

                                    |

                           ------------ 703

   |

PRI --------- PBX  -------- |

                           ------------ 704

                                    |

                                    |

                           ------------ 705

                                    |

                                    |

                           ------------ 706

Cadence

• It is the pause between the rings.
• ring -- pause -- ring
• In America it is 4 seconds pause and 2 seconds ring.
• This differs based upon the country and PBX.

MSN

• The abbreviation is multiple subscriber number.
• These numbers will be given by exchange provider.
• E.G : from 700 to 799
• There can be many connections provided but only 30 can be simultaneously connected. 

Pilot Number

• It is a unique number given with a PRI connection.
• Example :
• From ABC, we are making a call and changing our profile details.
• We have changed our number to some other number which is not provided from MSN or it can be an invalid number.
• Then we will get a number as our caller number which is called pilot number.

ISDN 

• It stands for integrated service digital network. 
• It is communication standard which can be used between telephone exchange to telephone exchange and extension to extension.
• For communicating to the different exchange, one standard has to be followed that is ISDN.
• Types:
• BRI - Basic rate interface
• PRI - Primary rate interface 
• In BRI there are 2B channels and ID channel. In PRI there are 30B channels and 2D channels. 
• B channel -> Transfer the data(information)
• The speed of the B channel in BRI is 64 KBPS 
• D channel -> Control the data passed through B channel.
• The speed of the D channel in BRI is 16 KBPS 
• PRI Standards :
• E1 : European country introduces this standard. Indians are also using this standard.
• T1 : North America uses this type.
• J1 : Japan uses this type.

Type  Channels    Speed 

-----------------------------------

E1   (30B + 2D)  (64 Kbps + 64 Kbps )

T1   (23B + 1D)  (64 Kbps + 64 Kbps )

J1   Same as E1  (64 Kbps + 64 Kbps )

• For all the B channel only one D channel has been used. It is called as NFAS which will be used in T1 type standard.
• G7F1 is a standard which is used by B channel to send the data. The mulaw supply sampling rate is 8000 sp/s. For mulaw the sampling has been get as 16 bit and stored as 8 bit.
• G931 is a signalling method for controlling all the B channel. This standard is not only used for this. It is used globally in all the channels. Before sending the data it sends the packets in the channels for controlling the data.
• DID : It stands for direct inward dialling. It is a facility when the Multiple Service Numbers has been used. 
• Ex: In our organization MSN numbers are 43902701 - 43902799.
• ACD : It stands for Automatic Call Distribution which is used to route the calls to the appropriate systems.

Alcatel PBX

• The standard number is 4400.
• It has lot of wires such as power, extension, earth and PRI.
• It also has floppy, processor, hard disk.
• For knowing the status of extension LED's will be there.
• It transfers data in 0s and 1s.
• It uses the Unix OS.
• We can enable or disable any extension. 
• We can give the commands such as ping, ssh, traceroute, etc.
• If we need to add any device to the PBX, there no need to restart the PBX, since if we restart the data can be lost.
• It has the slots to add PRI.
• Main distribution framework : It is used to distribute the signals.

Facilities in the PBX

• Call parking
• It gives the numbers to park the calls. The parking is done based on the PBX.
• Call transferring
• Two types of transfer:
• Attended transfer (manual process needed to transfer the call to other person).
• Blind transfer (automated).
• Enquiry facility
• This is used to find out the line is busy or not.
• Queuing call
• If one number received more than one call at the same time, then the calls are made to be stand in the queue.
• Call waiting
• Conferencing
• Conference calls can be made.
• Wireless phone
• Noise tolerance 
• The noise and volume can be adjusted.
• Hunt Group
• Group of the extension numbers and call can be made to the group  number. 
• The call may be go simultaneously or one by one.
• Pick Group
• If you are in the same group then you can attend others call by typing # followed by the person's extension number.
• Chain dialling
• It is used to redirect the calls if one is not ready to attend.
• Heart Line
• Dedicate line for the extension. In this feature user don't need to type the numbers. 
• When the user picks up the handset, call goes automatically.
• Call follow me switch
• It is used to redirect the calls if one is not ready to attend with in a time.
• Call barring
• It is used to restrict the incoming call and outgoing call.
• Dynamic call barring
• It is used to restrict the call by some time.
• Call detailed record
• It is used to get call related informations. It is mainly used for billing purposes.
• Call budget
• It is used to restrict the minutes to speak.
• Call barging 
• It is used to hear the other's communication without their knowledge.
• Production from barging
• If we set this option no one can overhead of your conversation.
• DND ( Do Not Disturb )
• Toll Free Numbers ( Company will be charged )
• DSS (Digital station selector)  console
• It is used to control the calls and also used to restrict the extension numbers.

Asterisk

• It is open source soft PBX.
• Asterisk give the interface to configure all the things in easy.
• AsteriCon
• It has the details of asterisk meeting which happens every year.
• Switch matrix
• It is used to identify the capacity of PBX. How much matrices have based on that only the connection has been made.
• Least call routing
• The way of finding shortest path.

Credits and References

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https://networkencyclopedia.com/plain-old-telephone-service-pots/