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Public Switched Telephone Network (PSTN) Chapter 11/12 Text book Hioki Subscriber Loop: Fig below (fig 11-17 P275 Text book) shows a simplified schematic of a conventional Telephone handset and how it is connected to the Public Switched Telephone Network. The connection between your telephone and the world wide interconnecting network is a two-wire circuit that the phone company calls Subscriber Loop. The signals that your telephone company must send and receive are listed on page 275 of your Text book Hioki and are repeated below Ringing Signal 20 Hz, 90 v R.M.S ON Hook/OFF Hook Signal A current of about 20 ma OFF hook, 0 ma ON Hook Dial Tone 350Hz + 440 Hz Busy Tone 480Hz + 620 Hz Ring Back 440 Hz + 480 Hz Congestion 480 Hz + 620 Hz Receiver Off hook 1400 + 2060 + 2450 + 2600 RINGER: First device we encounter as we proceed from the Central Office (CO) or Local Office (LO) or Local Exchange (LE) into the telephone is the RINGER, which responds only to an A.C ringing signal and produces an audible ring. SWITCH HOOK: After that, in parallel with the Ringer is the Switch Hook, which has two contacts that are in the open position when the telephone is ON HOOK. The DC current in the loop is 0 ma when the telephone is ON HOOK because of the open circuit. The – 48 V battery at the LO is disconnected from the loop when the telephone is ON HOOK. When the handset is lifted OFF the HOOK, the Switch Hook contact closes, the battery is connected to the loop causing a 20 ma of DC current to flow through the loop into the telephone circuit. This is how the Telephone company knows that you have lifted the telephone Off the hook and are about to Dial a telephone number Hyder Khoja Page 1 6/29/2017 As soon as a subscriber lifts the handset off the hook and current begins to flow a special device in the LO is attached to the subscriber loop which produces a dial tone and also stores the dialed digits. As mentioned earlier, the RINGER responds to a.c signal only and does not interfere with the DC loop current Dialing: Next event is dialing. There are two kinds of dialing. PULSE and TONE Pulse dialing is disappearing so I will not cover it. However I would still like you to be familiar with the relevant voltages and currents and waveform. Please study the handout. In pulse dialing, the switch hook opens and closes as many times as the dialed digit, thereby interrupting the circuit as many times as the number being dialed. So if the number 5 is dialed it will generate 5 pulses. For “0” 10 pulses are generated. Pulse dialing is slow and time consuming. So they came up with Tone Dialing. Touch Tone Dialing Modern method of dialing Fig shows the keypad This is called DTMF (Dual Tone Multi Frequency) dialing It uses a unique set of two tones to represent each dialed digit. These tones are actually sine waves that are sent to the LO as each digit is pressed Consider the keypad diagram shown on p265 of your textbook. When a key is pressed, the tones corresponding to its ROW and COLUMN are transmitted. Fastest rate is 0.1s, typically about 0.25s/key pressed. And 0.25s between keys Frequencies are so chosen that (i) None of the tone is a harmonic or a close harmonic of any other frequency, e.g 2nd harmonic of 1209 is 2418 which doesn’t exist on the keypad. (ii) The amplitude modulation of one tone by another does not Hyder Khoja Page 2 6/29/2017 generate the sum and difference frequencies- 1447 + 697=2144 which is not found on the keypad. Note also that in DTMF, the sum of the amplitude of the two frequencies is transmitted not the sum of the frequencies. Ringer: Ringer responds to the ac signal sent from the LO Fig shows a schematic of the Bell 500 telephone (fig 11.5 p262 Hioki) You can see that the ringer is really a coil ( inductor ) in series with a capacitor. The capacitor blocks dc and passes only ac ringing signal. Thus the ringer does not interfere with the dc loop current and dc operation of the telephone circuit When the conversation begins, the signal voltage during talking is only a few volts at about 3ma. The voice frequencies transmitted are between 300 HZ to 3300 HZ, since most of the voice energy lies in this range. This is called Telephone Bandwidth ( BW ). Thus the Telephone BW is 3 KHz. Telephone lines are designed to carry frequencies in this frequency range. Telephone Switching System: Local Exchange and Hierarchy of Exchanges. Now that we understand how a telephone is hooked to the nearest Local Exchange or Local office (LE or LO), how a subscriber loop works as well as associated voice and control signals, we turn our attention to how a call is routed across the telephone network The first 3 digits of a telephone number represent the city code such as 416 for Toronto, 905 for Thornhil, Markham and Brampton etc. The next 3 digits specify Local Office address such as 675 for Rexdale, 453 in Brampton, 276 Mississauga etc. The last 4 digits indicate the customer address Thus 416 – 675 - 6111 Hyder Khoja Page 3 6/29/2017 City LO subscriber LO usually serves an area within a 10KM radius Thus a city can have a maximum of 000 to 999 (1000) Local Offices and each office can serve a maximum of 0000 to 9999 (10,000) customers. That’s why Toronto was split into 416 and 905 regions, because the number of exchanges grew more than 1000 due to population explosion. Fig below shows how a call is connected to another customer. 6610 1234 INTER OFFICE TRUNKS 1 BRAMPTON 1234 MISSISAUGA LO A LO B 4422 2 455 276 5678 5000 1 1 2 5678 REXDALE LO C 675 1100 9110 2457 Fig above shows 3 neighboring towns served by 3 Local offices. Hyder Khoja Page 4 6/29/2017 The lines connecting Local Offices are called Inter Office trunks or simply TRUNKS There may be more than one trunk installed between a pair of offices (depending upon the level of traffic) Consider the case when 455-5000 calls 455-4422 A Switch in the Local Office A makes a connection between 5000 and 4422. In good old days a human operator did it manually. Now a day it is done electronically under the control of a microprocessor or a computer. If 4422 is talking to some one, a busy signal is returned, which is a combination of audio tones 480 Hz and 620 Hz pulsing at a rate of one per second. If 4422 is not busy, a ringing signal is sent to 4422 and when the called party picks up the phone, a connection is made. LO also sends a ring back to the calling party. These control or call progress signals are manufactured electronically inside the Local office When the called party picks up the phone, the LO (i) Detects the off-hook by sensing the current (ii) Ringing and ring back ceases (iii) Conversation begins When the call is finished, both parties put the phone back on-hook. DC current stops. Connection is broken This was an example of a call within the confines of LO Interoffice Calls: Now suppose LO A 455-5000 calls LO C 675-1100 The prefix 675 will indicate to LO A that the call is for another LO and therefore routs the call to LO C via inter Office trunk 1 This is called Routing. The last 4 digits of the called party are transmitted down the trunk to the LO C. this process is called Inter Office Signaling. The Inter office Signaling techniques are a sophisticated area of growth and development in communications technology. It concerns with how to transmit control information between offices and allow inter office communication in the most efficient way. Hyder Khoja Page 5 6/29/2017 There are several schemes used by Network operators and Carriers such as Telus, Bell mobility, microcell, FIDO, Rogers Cantel etc. They fall in two major categories The Common Channel Interface Signaling (CCIS) and SS#7. The Signaling System #7 is the most commonly used mechanism in Telephone system as well as Wireless system. We shall learn more about these systems later in the program At this point, you should understand that the primary purpose of the signaling protocols is to transfer control information between parts of the Network for the control of connection establishment, management of the call in progress and disconnection When the LO C receives the customer number 1100, then the same process is repeated as described earlier for the call between 5000 and 4422 in the 455 Office. Now suppose, before this call is broken, LO A 453-5678 calls LO C 675-2457. Since another trunk is available, fortunately the call goes through. Suppose now, while both trunks are occupied453 4422calls 675 9110. This call cannot go through, even if the called party is not currently using the phone. This is called BLOCKING or CONGESTION caused by the insufficient resources either inside the LO or inadequate number of trunks For this kind of blocking, the caller will hear a busy tone, but it will pulse at twice the rate of a normal busy signal. This system of switching calls is called Circuit Switching, because the resources of the Network are dedicated to caller for the duration of the call and then switched to somebody else. Blocking is unavoidable in Circuit Switching because, the telephone company cannot provide a dedicated line for each customer. The cost becomes extremely prohibitive, while most of the circuits are not busy most of the time. The number of lines needed to provide a dedicated service to n customer = n( n-1)/2. Try calculating for 1000 customers???? North American Telephone Hierarchy: In order to provide efficient, cost effective service across the Nation, the Telephone Companies use a telephone Switching hierarchy as shown in Fig 12.1 12.2 P290/292 in you text book Hioki Hyder Khoja Page 6 6/29/2017 As shown, the customers are connected to the LO via subscriber’s loop. The Los are interconnected directly by Trunk lines or indirectly through a facility called TANDEM CENTER or TOLL CENTER 4 Classes of these Tandem centers exist in North America Toll Centers (Class 4) Primary Centers (Class 3) Secondary Centers (Class 2) Regional Centers (Class 1) When a direct trunk doesn’t exist between the called and the calling parties, the call is routed through one of these alternate paths. The sophisticated Switch software decides which the most efficient path based on the traffic condition at that time. Terms Used in Telecommunication Systems: Simplex Communication: Transmission in one direction only for example radio or TV broadcast. T R Half Duplex: Transmission in both directions but only in one direction at a time. This is called Time Division Switching. e.g human conversation, Intercome system, walkie-talkie etc Hyder Khoja T R R T Page 7 6/29/2017 Full Duplex: Transmission in both directions simultaneously. It uses either two separate lines or two different modulation frequencies for forward and reverse channels e.g Most modems use two sets of modulation frequencies, also most modern protocols are full duplex such as HDLC,LAP B etc R T T R 4 WIRE to 2 WIRE Hybrid: Show the picture of hybrid and discuss its function and implementation in the Telephone hand set and earlier subscriber loops. Also discuss the Modern technique of using CODEC for 2 Wire to 4 Wire conversions. Switched and Leased lines: Switched Line: Regular non-dedicated line. Time-shared with other customers Leased Lines: Also Called Private lines. Refers to renting a private dedicated line - Customer pays a fixed price per month. - Connection always exist, no need to dial the number - Low cost for high volume - Usually used by corporations to connect their branch offices - No blocking . Echo Suppressors: Voice operated Relays used to suppress echoes. Hyder Khoja Page 8 6/29/2017 Special Telecom Services: Call Forwarding: Transfers the call automatically to where you can be found. Call waiting: A short beep alerts you to an incoming call. 3-way call: Allows you to have a 3-way conversation. Return Call: Lets you know who was the last caller Caller ID or Call Display: Allows you to see the name and Telephone number of the person calling you. Caller ID Block : Blocks your number and name from appearing on Call ID screen. You can do some research and find some more special services that the Telephone companies are providing for an extra fee. Hyder Khoja Page 9 6/29/2017 DECIBELL (dB): - Indicates power level. - Ratio of output power to input power - DB = 10 log Po/Pi - If Po >Pi, it is called Gain - If Po<Pi, it is called Loss or Attenuation (a negative number) - In terms of voltage ratio, dB=20 log Vo/Vi (provided Ro=Ri). Can you prove it?? - Also dBm is defined as dBm=10 log Po/1mw. Thus dBm refers to output power with 1mw of reference input power.dBm therefore indicates absolute power in mw. - Consider the table shown DB Po/Pi dB Po/Pi 0 1 0 1 3 2 -3 0.5 6 4 -6 0.25 9 8 -9 0.125 10 10 -10 0.1 13 20 -13 0.05 20 100 -20 0.01 On the other hand consider the table below for dbm Dbm Po dbm Po 0 1mw 0 1mw 3 2mw -3 0.5mw 6 4mw -6 0.25mw Hyder Khoja Page 10 6/29/2017 9 8mw -9 0.125mw 10 10mw -10 0.1mw 13 20mw -13 0.05mw 20 100mw -20 0.01 The main advantage of dB (or dBm) is that while calculating power gain or loss of interconnected networks, the individual gains or losses can be simply added or subtracted. Example: I/P=0dbm O/P=? -30dbm 35dbm -22dbm 20dbm -2dbm O/P = 0 + (-30) + (+35) + (-22) + (+20) + (-2) = +1dbm Example: For the telephone transmission system shown below, calculate the gain needed for the output amplifier to produce an output voltage of 6V. Also calculate the output power level in dbm. Cable section S.G Cable loss 26 db loss Out put Amp = 6V Vo 29 db loss 10 dbm X A Line Amp 13db gain B Boaster Amp 20 db gain C 600 Ro GND Solution: Hyder Khoja Page 11 6/29/2017 (a) First calculate the output power level using the Equation Po = Vo2/Ro = 62 / 600 = 36 /600 = 60mw. Now compute the output power level in dbm = 10 log Po/1mw =10log60mw/1mw = 17.78 dbm (b) To calculate the gain of the output amplifier C, first calculate the total loss or gain up to point X. 10 +13 –26 +20 – 29 = -12 dbm There for the output amplifier gain required = 17.78 dbm – (-12 dbm) = 29.78 dbm. Assignment: For the Telephone transmission system shown, calculate the output voltage and the output power in dbm. GAIN 16 dbm Cable loss = 6dbm S.G V 1.1 V O/P Vo = ? 600 GND 600 GND GND ANS: Output power level = 13dbm Output voltage Vo = 3.48V Hyder Khoja Page 12 6/29/2017