Exercise 4 (15.-19.2) (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) What does multiplexing mean?
b) 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?
c) For what is Pulse Code Modulation (PCM) used?
d) What is the difference between a modem and
a codec? Or is there any difference? (Both
make transformations
between analog and digital signals.)
e) What is SONET?
f) What are the differences between Narrowband ISDN and
Broadband ISDN? Do they
have something in common?
g) What are the most important interfaces
prescribed in ISDN? What has been the aim of
these nterfaces?
h) Why does ISDN need both NT1 and NT2?
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 machines 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. a) The following data fragment occurs in the middle of a data
stream for which the character stuffing algorithm is used:
DLE, STX, A, DLE, B , DLE, ETX. What is the output after stuffing?
b) How does the receiver distinguish
the end of the frame from the LDE ETX character in the middle
of the data stream?
c) What happens to the frame synchronization, if some
DLE-, STX- or ETX-character gets crooked?
.