Lab 2

1. A modem constellation diagram has data points at the coordinates: (1,1), (1,-1), (-1,1) and (-1,-1) (eg QPSK encoding). How many bps can a modem with this encoding achieve at 1200 baud?
2. A modem constellation diagram has data points at the coordinates (0,1) and (0,2). Does the modem use phase modulation or amplitude modulation?
3. Now suppose all the points lie in a circle centered on the origin. What kind of modulation is being used?
4. An ADSL system using DMT allocates 3/4 of the available data channels to the download stream. It uses QAM-64 modulation on each channel. What is the maximum capacity of the downstream link?
5. Why has PCM sampling time been set at 125 usec?
6. What is the percent overhead (percentage of bps used for control) on a T1 carrier for:
   1. A voice T1
   2. A data T1
7. The user data rate for OC-3 is stated on page 146 to be 148.608 Mbps. Show how this number is derived from the SONET OC-3 parameters.
8. What is the difference between packet switching and message switching?
9. D-AMPS has appreciably worse speech quality than GSM. Is this due to the requirement that D-AMPS be backwards compatible with AMPS, whereas GSM had no such constraint? Discuss your answer.
10. A CDMA receiver is within the range of four stations with the following chip sequences:
    • A: (-1 -1 -1 +1 +1 -1 +1 +1)
    • B: (-1 -1 +1 -1 +1 +1 +1 -1)
    • C: (-1 +1 -1 +1 +1 +1 -1 -1)
    • D: (-1 +1 -1 -1 -1 -1 +1 -1)
    The reciever receives the following chips: (-1 +1 -3 +1 -1 -3 +1 +1). Which stations transmitted and which bits did the transmitting stations send? (Hint: look at page 163, Fig 2-45 for six examples for these four stations)