Digital Switching System

Size: px

Start display at page:

Download "Digital Switching System"

Magnus Robinson
6 years ago
Views:

Digital Switching System ANSWER ANY 5 QUESTIONS Full Marks : 50 1. a) Why two wire to four wire conversion is required in telecommunication system?

1 Digital Switching System ANSWER ANY 5 QUESTIONS Full Marks : a) Why two wire to four wire conversion is required in telecommunication system? [2] *For long distance transmission signal need to be amplified. *In two wire communication amplification in both directions are not possible b) Draw a neat diagram and explain how hybrid transformer helps to reduce echo in four wire transmission line. [8] *Signal split and merge required for two wire to four wire conversion *How incoming signal is reflected on outgoing line *How additional circuitry helps to cancel reflected signal by opposite phase using transformer dot convention 2. a) If coupling between two wire and four wire line having loss of 3dB, balance return loss is L db and four wire line gain A db, then derive the expression for Talker echo and Listener echo. [8]

2 2wire to 2wire path attenuation is P = 3 A +3 = 6-A db Talker echo attenuation = 3 A + (L+6) A+3 =12 2A +L =2(6-A)+L=2P+L Listener echo attenuation = 3 A +(L+6)-A+(L+6)-A+3 = 18-3A+2L Relative Listener echo attenuation = 18-3A+2L (3-A+3)=12-2A+2L=2(6-A)+2L=2P+2L b)what should be ideal value of balance return loss in case of a perfect situation and why does it not happen? [2] *Ideal value for Balanced return loss is infinite. *due to variable impedance of customer side 2wire line and improper balancing circuit. 3. a) A four wire circuit has an overall loss ( two-wire to two-wire ) of 1 db and the balance return loss at each end is 6 db. Find i) The singing point ii) The stability margin iii)the attenuation of talker and listener echo. [6] Refer to 2 a) P= 1 db L=6 db i) singing point is when P=L =6 db ii) stability margin M = L +P = 7 db iii ) Talker echo attenuation =2P+L=8 db Listener echo attenuation =2P+2L = 14 db b) If four wire transmission delay is 50 milliseconds in one direction, what will be delay between echo in case of i) Talker echo ii) Listener echo. [2]

3 2 x 50mS = 100 ms c) What is cross talk and why does it take place? [2] Signal mixing of two conversions and due to that other s voice are heard. Due to weak insulation between wires, magnetic coupling causes cross talk. 4. a) Calculate total bit rate of E1 and T1 channel. [6] E1 ->(32channelx8bit)x8kHz = 2.048Mbps T1 ->( (24channelx8bit)+1bit sync) x8khz=1.544mbps b) Calculate interframe time-period for E1 and T1. [2] 1/8kHz = 125 usec for both c) Why companding is used? [2] To sample voice signal in nonlinear step sizes across the range, so that low level signals can be sampled with greater resolution. 5. a) What is primary multiplexing? [2] First level Multiplexing which is multiplexing of 64kbps channels. b) Why PDH is called plesiochronous? Why this system is getting outdated? [ 4+2] *Primary multiplexing is done in equipment using local crystal oscillator *clock frequencies of different equipments do not match exactly * in PDH system provision is kept to cope up with difference of frequencies in the form of bit stuffing *as clocks for higher order multiplexing are derived from primary multiplexed channel, the problem remains for higher order multiplexing also *The system is synchronous but not full system is not synchronous to a single frequency. There are some variations. So this is called Plesiochronous or semi synchronous system *Due to Plesiochronous nature the system is complicated *Accessing lower order channel from higher ordered multiplexed channel is not possible. *For drop / insert of a channel need full demux and mux activity which is costly.

4 c) In European PDH multiplexing of 4 channels of 8Mbps stream produces 34Mbps. Give two reasons for this extra bandwidth requirement. [2] *in specification each channel can have some variation in operating frequency in both positive and negative direction. To cope up with higher side extra bandwidth is required *For Plesiochronous nature extra bandwidth is required for bit stuffing *Multiplexing will have some overhead 6. a) Describe why tree topology is suitable for Telecommunication Network. [3] *star is the best topology for any telecom exchange. *When multiple exchanges are connected through a single node that become as star of star or a tree topology. So it is the most natural *for connecting exchanges parsed at various distances tree topology need minimum cable distance *the system becomes modular *equipments at different level of tree topology can focus on different feature / quality of telecommunication system b) Describe i) Local loop ii) Trunk Line iii) Tandem Line [3] i) circuit that goes from exchange to end user device ii) Communication line going from one exchange to higher level exchange ii)communication line going between same level or peer exchanges c) What do you understand by i) Core Network ii) PBX iii)central Office iv) PSTN [4] i) Core Network - network that carry traffic but do not face the end user. ii)pbx personal branch office which is a smaller version of exchange residing in office or building iii) Central office the exchange where end users devices are terminated iv)pstn Public Switch Telephone Network Network of exchanges which helps telephone users to get connected with each others. 7. a) What are power level in dbm i) 1mW ii) 50W iii) mw [3] i) 10log1 = 0dBm ii) 10log50x1000=10(log(0.5x10000))=10(-0.3+5)=47 dbm iii)10log(1/1000)=-10x3=-30 dbm

5 b)what fraction of power is received when signal is attenuated by i) 6dB ii) 20 db [2] i)1/4=0.25 ii)100 times c) A B C D > 4dB Loss > 10 db Loss > 5dB Gain > 10dBm power is fed at point A. i)how much power will be received at point D? [1] = 1 dbm ii) If C is reference point, then what is dbr at A and D? [2] At A 10+4=14 dbr At D 5 dbr iii)what are dbmo value at point A and D? [2] dbm0=dbm dbr At A dbm0=10-14=-4 At D dbm0=1-5=-4 8. a) Explain in brief how two wire circuit can carry both way voice traffic? [2] *in two wire circuit any signal from microphone will be heard at both end *as communication style is half duplex (either talk or listen ) one side remain inactive *receiver is designed to reduce effect of own voice b) Describe basic functions done by an operator to handle a call at manual switching board. [4] *Attending *Information receiving *Information processing *Routing *Busy test *interconnection *Alerting / ringing *Supervision *Call teardown *Billing c) What are role of a Regulatory body and Standard body in Telecommunication system? Give examples. [4] Regulatory body control operation of telecom operators in a country. Ex. FCC, TRAI Standard body Specification of technology required for interoperation. Ex ITU,ETSI

Part VI: Requirements for ISDN Terminal Equipment

Part VI: Requirements for ISDN Terminal Equipment Issue 9 November 2004 Spectrum Management and Telecommunications Policy Compliance Specification for Terminal Equipment, Terminal Systems, Network Protection Devices, Connection Arrangements and Hearing