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?

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