國立暨南國際大學 97 學年度第一學期小考試卷

(考試時間: 14:10-14:30)

1. (20%) Spell out the following terminologies in data transmission and give brief explanations.
1. AM
2. RF
3. TDM
4. UV
   
   
   
   
   
2. (10%) When using even parity, what is the parity bit for the character 'P', whose binary representation is 1010000 in ASCII code?
   
   
   
   
3. (10%) Draw a figure to illustrate the multiplex mechanism, and use an example to explain how a demultiplexor determines the destination of a data packet.
   
   
   
   
   
   
   
4. (10%) Consider the following figure which shows the wave that results from phase shift modulation (PSM).  Suppose a sine wave completes a cycle in 2π radians, what are the values of the 2 shifts indicated by the arrows?
   
   
   
   
5. (10%) Consider the following source code (/usr/src/usr.bin/cksum/sum2.c) which shows the algorithm to compute the cksum of a file on UNIX.
   
           uint32_t cksum;
           int nr;      
           u_char *p;
           u_char buf[8192];
   
           /*
            * Draft 8 POSIX 1003.2:
            *
            *   s = sum of all bytes
            *   r = s % 2^16 + (s % 2^32) / 2^16
            *   cksum = (r % 2^16) + r / 2^16
            */
   
           cksum = 0;
           while ((nr = read(fd, buf, sizeof(buf))) > 0)
                   for (p = buf; nr--; ++p)
                           cksum += *p;
           if (nr < 0)
                   return (1);
   
           cksum = (cksum & 0xffff) + (cksum >> 16);
           cksum = (cksum & 0xffff) + (cksum >> 16);
   
   
   
   Note that this algorithm compute the summation of every character as an 8-bit integer (instead of every pair of characters as a 16-bit integer shown in Figure 7.6 in the textbook).  Apply this algorithm to a file containing the following string
   
   

   H

   e

   l

   l

   o

   w

   o

   r

   l

   d

   .

   \n

   48

   65

   6C

   6C

   6F

   20

   77

   6F

   72

   6C

   64

   2E

   0A

   
   and predict the final checksum.