Problem set 6 (29.-31.10.2002)

1. Find out what public wireless nerwork services (wireless LAN type) are available in Finland. Where are they already in use? What technology they deploy? Mention at least three different services.

2. Nyquist and Shannon
   1. What is the content of the theorems of Nyquist and Shannon? How do these theorems relate to each other?
   2. Television channels are 6 MHz wide. How many bits/sec can be sent if four-level digital signals are used? Assume a noiseless channel.
   3. A noiseless 4-lHz channel is sampled every 1 msec. What is the maximum data rate?
   4. A binary signal (getting only values 0 and 1) is sent over a 3 kHz channel whose noise-to signal ratio is 20dB, what is the maximum achievable data rate?

3. T1 and SONET
   1. If a T1 carrier system slips and loses track where it is, it tries to resynchronize using the 1st bit in each frame. How many frames will have to be inspected on the average to resynchronize with a probability of 0.001 of being wrong?
   2. SONET clocks are pretty exact and their timings are drifting apart only about 3 seconds in 100 years (a year = 365 days). How long does it take before the drift between two SONET clocks is equal to the width of 1 bit? What are the implications of this calculation in practice?

4. About modems
   1. How is a V.90 standard modem functioning? How is it possible to achieve 56 kbps rates, though the theoretical maximum for telephone network is only 35 kbps? And why is it only 56 kbps and not 64 kbps?
   2. A modem modulates digital bits first into analog form so that they can be transmitted in the analog local loop and at the receivers the modem demodulates the analog signal back to the digital bits. A codec samples the analog signal and encodes the samples into digital bits and at the other end the codec decodes the digital signals back to analog. How does the coding side of codec differ from the demodulation side of the modem or is the no difference at all? Both transform an analog signal to a digital one.

5. ATM
   1. Digitally encoded data is first packed into packets and then sent packet by packet forward. The time required to fill a packet is the packetization delay. Packetization delays greater than 20 ms can cause noticeable and unpleasant echo. A typical encoding and packetizing rate is 64 Kbps. How long is the packetizing delay for a maximum Ethernet frame of 1500 bytes? How long is the packetizing delay for a packet of 48 bytes.
   2. Why is ATM using small fixed lenght packets?

6. Your opinions about this course.
   1. What subjects were, in your opinion, the most interesting and important ones? What subjects were dull and unnecessary?
   2. How should the course be changed in the future? What should be more and what less? What missing subjects should be included? What items could be dropped from the course?
   3. Fill the teaching evalution form for this course (http://ilmo.cs.helsinki.fi/kurssit/servlet/Valinta).