1) Why telecom power supply has -48V dc power supply?

• Negative sign indicates The positive grounded
• 
  
• he negative voltage on the line was better than positive to prevent electro-chemical reactions from destroying the copper cable quickly, if cables get wet
  
• 
  
• Low enough not to cause serious danger if somebody touches the telephone wires
  
• 
  

2)In 1989, AT&T introduced its 900 services to the general public. What was its 900 number service option called?

MultiQuest

3) What is the need of CPG message in ISUP protocol?

Consider any call fowarding scenario.... As soon as switch

receives Setup, it checks that the Called_Party has

forwarded option activated and so tries to forward the

call. Meanwhile, Switch sends CPG message towards

originator saying that Call is in Progress.

A message, sent in either direction during the setup or

active phase of the call(ex.inbetween ACM and ANM),

indicating that an event, which is of significance, and

should be relayed to the originating or terminating access,

has occurred.

4) What is a superheterodyne receiver?

superhetrodyne receiver is one which has same carrier

frequency as the transmitter otherwise the original signal

recovery will not be possible.

5) What is protocol testing ? Types of  tool used in telecom testing ?

Protocol testing means to test the functionality of the node(piece of software) which should

compliance to some standard message flow. For this one should take a tester(Testing Unit)

which should send standard messages to the node(Item under test)

Different types of telecom testing Tools used

GlomoSim simulator: It has the ability to test 100 nodes on single cpu and for thousands of

nodes multiprocessor.

Abacus5000 for SIP testing

Etherpeek IP

Nethawk SS7

Wireshark SS7/IP

K1297-G20(Tektronics)

6) What is MTU ?

In computer networking, the term Maximum Transmission Unit (MTU) refers to the size (in bytes) of the largest packet  that a given layer of a communications protocol can pass onwards. MTU parameters usually appear in association with a communications interface (NIC, serial port, etc.). The MTU may be fixed by standards (as is the case with Ethernet) or decided at connect time (as is usually the case with point-to-point serial links). A higher MTU brings higher bandwidth efficiency. However large packets can block up a slow interface for some time, increasing the lag on other
packets.

7) A 2MB PCM(pulse code modulation) has how many channels?

30 voice channels, 1 signaling channel, & 1 synchronization channel.

8) What is the nominal voltage required in subscriber loop connected to local exchange?

   - 48 volts. This -48 volts is required by the copper wires to prevent it from corroded & eluded

9) VOICE is sampled at which Frequency?

8 KHz as the sampling frequency will be double the maximum frequency component for no aliasing effect, as per the nyquist theorm. as the maximum frequency allowed by the filter is just 4khz. so the sampling frequency is 8KHz

9) how many Max number of satellite hops allowed in voice communication?

2 hops

10) what is the significance of SGM (segmentation) message in ISUP?

When messages exceed the maximum size of the SS7 packet (272 octets) the message must be segmented.

This message type is used to send an additional segment to the destination signaling point.

11)Protocol used in Google Talk.

a)SIP

b)H.323

c)XSALT

d)XMPP