Exercise 4 (15.-18.2) (Tanenbaum pp. 118-181)
1.Answer shortly the following questions ("review type" problem: answers
are found in lecture
notes or in Tanenbaum's book).
a) A cable TV system has 100 commercial channels, all
of them alternating programs with
advertising. Could
table TV be regarded as an example of TDM or FDM multiplexing?
b) For what is Pulse Code Modulation (PCM) used?
c) What is the difference between a modem and
a codec? Or is there any difference? (Both
make transformations
between analog and digital signals.)
d) What is SONET?
e) What are the differences between Narrowband ISDN and
Broadband ISDN? Do they
have something in common?
f) What is ADSL? What does it offer?
2. a) PCM-coding uses 8 bits. How does this affect "the quality of voice"?
b) Why has the PCM sampling time been set
at 125 us ?
c) Why transmission using PCM-technique is not
so error prone?
3. Suppose that x bits of user data are to be transmitted over a k -hop path in a packet-switched network as a series of packets, each containing p data bits and h header bits (x >> p+h). The bit rate of the lines is b bps and the propagation delay is neglible. What value of p minimizes the total delay ?
4. How long does it take to transmit an 8" x 10" image by facsimile over an ISDN B channel? The facsimile digitizes the image into 300 pixels per inch ans assigns 4 bits per pixel. Current FAX mach ines go faster than this over ordinary telephone lines. How do you think they do it?
5. a) In a typical celular telephone system with hexagonal cells, it
is forbidden to reuse a frequency band in an adjacent cell. If total of
840 frequencies are available, how many can be used in a given cell?
b) Sometimes when a cellular user crosses the boundary
from one cell to another the current call is abrubltly terminated, even
though all transmitters and receivers are functioning perfectly. Why?
6. Find out what kind of data transfer services are available in Finland.