TRUE/FALSE
1. Data can be sent either in serial format or in parallel format.
ANS: T
2. For practical reasons, virtually all data communications is done in serial format.
ANS: T
3. The first binary code was invented in 1914.
ANS: F
4. A character code is the same as a data code.
ANS: T
5. The old Baudot code is no longer used.
ANS: F
6. How fast data can be transferred is independent of the character code used.
ANS: F
7. Both synchronous and asynchronous transmission require bit timing.
ANS: T
8. There is no "framing" in asynchronous transmission.
ANS: F
9. A "mark" is the same as a binary 1.
ANS: T
10. Digital modulation is sometimes called "keying".
ANS: T
11. Due to the "bursty" nature of most channel noise, simple parity is more or less useless.
ANS: T
12. An advantage of using a UART is that "buffer overflow" cannot happen.
ANS: F
13. Synchronous transmission is much more efficient than asynchronous transmission.
ANS: T
14. In synchronous transmission, the data stream is used to "lock" the receiver's clock onto the transmitter's clock.
ANS: T
15. Bit-oriented protocols are being replaced by the newer character-oriented protocols.
ANS: F
16. In synchronous transmission, control characters in the data require special handling, but flag sequences in the data do not.
ANS: F
17. HDLC is very similar to SDLC.
ANS: T
18. HDLC is a bit-oriented protocol.
ANS: T
19. When receiving digital data, it is possible to detect errors, but not to correct them.
ANS: F
20. In digital data transmission, bad frames are usually retransmitted.
ANS: T
21. CRC codes are particularly good at detecting burst errors.
ANS: T
22. Hamming codes allow errors to be corrected without requiring retransmission.
ANS: T
23. All schemes to detect errors require adding extra bits to the data being transmitted.
ANS: T
24. Huffman codes are as good as CRC codes at detecting errors, but are faster.
ANS: F
25. Run-length encoding is a data-compression technique.
ANS: T
26. A "cipher" is, essentially, a secret code.
ANS: T
27. Both "private-key" and "public-key" encryption always require the addition of extra bits to the data.
ANS: T
28. An advantage of public-key encryption is that it is not "computation-intensive", meaning a computer doesn't have to do many numerical calculations to unencrypt (decrypt) the data.
ANS: F
29. A "digital signature" does not require the use of encryption.
ANS: F
MULTIPLE CHOICE
1. In practical terms, parallel data transmission is sent:
a. | over short distances only | c. | over any distance |
b. | usually over long distances | d. | usually over a coaxial cable |
ANS: A
2. The five-level teletype code was invented by:
a. | the Morkum Company | c. | Western Union |
b. | the Teletype Company | d. | Emile Baudot |
ANS: D
3. Data codes are also called:
a. | character codes | c. | they do not have any other name |
b. | character sets | d. | both a and b |
ANS: C
4. Digital data that is not being used to carry characters is called:
a. | FIGS data | c. | numerical data |
b. | binary data | d. | all of the above |
ANS: B
5. Character codes include:
a. | alphanumeric characters | c. | graphic control characters |
b. | data link control characters | d. | all of the above |
ANS: D
6. ASCII stands for:
a. | American Standard Character-set 2 |
b. | American Standard Code for Information Interchange |
c. | American Standard Code 2 |
d. | Alphanumeric Standard Code for Information Interchange |
ANS: B
7. BS, FF, and CR are examples of:
a. | nonstandard character codes | c. | control characters |
b. | escape characters | d. | none of the above |
ANS: C
8. LF stands for:
a. | Line Feed | c. | Line Forward |
b. | Link Feed | d. | Link Forward |
ANS: A
9. UART stands for:
a. | Universal Asynchronous Receiver-Transmitter |
b. | Unidirectional Asynchronous Receiver-Transmitter |
c. | Unaltered Received Text |
d. | Universal Automatic Receiver for Text |
ANS: A
10. In asynchronous transmission, the transmitter and receiver are:
a. | frame-by-frame synchronized using the data bits |
b. | frame-by-frame synchronized using a common clock |
c. | frame-by-frame synchronized using the start and stop bits |
d. | not synchronized at all, hence the name "asynchronous" |
ANS: C
11. In asynchronous transmission, the time between consecutive frames is:
a. | equal to zero | c. | equal to the start and stop bit-times |
b. | equal to one bit-time | d. | not a set length |
ANS: D
12. In synchronous transmission, the frames are:
a. | about the same length as ten asynchronous frames |
b. | much longer than asynchronous frames |
c. | 128 bytes long |
d. | 1024 bytes long |
ANS: B
13. Synchronous transmission is used because:
a. | no start and stop bits means higher efficiency |
b. | it is cheaper than asynchronous since no UARTS are required |
c. | it is easier to implement than asynchronous |
d. | all of the above |
ANS: A
14. In synchronous transmission, the receiver "syncs-up" with the transmitter by using:
a. | the clock bits | c. | the CRC bits |
b. | the data bits | d. | a separate clock line |
ANS: B
15. To maintain synchronization in synchronous transmission:
a. | long strings of 1s and 0s must not be allowed |
b. | transmission must stop periodically for resynchronization |
c. | the clock circuits must be precisely adjusted |
d. | the channel must be noise-free |
ANS: A
16. BISYNC:
a. | is an IBM product | c. | requires the use of DLE |
b. | is a character-oriented protocol | d. | all of the above |
ANS: D
17. HDLC:
a. | is an IBM product | c. | is identical to SDLC |
b. | is a bit-oriented protocol | d. | all of the above |
ANS: B
18. The use of flags in SDLC requires:
a. | "bit-stuffing" | c. | FEC |
b. | different flags at either end of a frame | d. | ARQ |
ANS: A
19. The initials ARQ are used to designate:
a. | automatic request for resynchronization | c. | automatic receiver queue |
b. | automatic request for retransmission | d. | automatic request for queue |
ANS: B
20. ARQ is used to:
a. | correct bit errors | c. | put data into a temporary buffer |
b. | correct synchronization problems | d. | none of the above |
ANS: A
21. FEC stands for:
a. | Fixed Error Control | c. | Forward Error Correction |
b. | Forward Error Control | d. | False Error Condition |
ANS: C
22. VRC is another name for:
a. | FEC | c. | LRC |
b. | ARQ | d. | parity |
ANS: D
23. CRC stands for:
a. | Control Receiver Code | c. | Cyclic Redundancy Check |
b. | Correct Received Character | d. | Cycle Repeat Character |
ANS: C
24. Huffman codes:
a. | allow errors to be detected but not corrected |
b. | allow errors to be detected and corrected |
c. | allow alphanumeric data to be corrected |
d. | allow alphanumeric data to be compressed |
ANS: D
25. Run-length encoding is used to:
a. | encrypt data | c. | correct data |
b. | compress data | d. | none of the above |
ANS: B
26. Public-key encryption:
a. | allows the use of digital signatures | c. | avoids the "password problem" |
b. | is used to convey symmetric keys | d. | all of the above |
ANS: D
27. SDLC stands for:
a. | Synchronous Data Link Control | c. | Synchronous Data Link Character |
b. | Synchronous Data Line Control | d. | Synchronous Data Line Character |
ANS: A
28. HDLC is:
a. | a bit-oriented protocol | c. | an ISO standard |
b. | based on SDLC | d. | all of the above |
ANS: D
COMPLETION
1. Parallel transmission can be used only for ____________________ distances.
ANS: short
2. The term "baud" was named after Emil ____________________.
ANS: Baudot
3. Data codes are also called ____________________ codes.
ANS: character
4. The ____________________ code is a 7-bit code commonly used in communication between personal computers.
ANS: ASCII
5. The two letters ____________________ designate the code character used to advance a printer to the next page.
ANS: FF
6. An asynchronous frame begins with the ____________________ bit.
ANS: start
7. An asynchronous frame ends with the ____________________ bit.
ANS: stop
8. At the end of an asynchronous frame, the line will be at the ____________________ level.
ANS:
mark
binary 1
9. An integrated circuit called a ____________________ is used in an asynchronous communication system to convert between parallel and serial data.
ANS: UART
10. When receiving digital data, ____________________ are used to hold data until they can be read.
ANS: buffers
11. Synchronous communication is more ____________________ than asynchronous since there are fewer "overhead" bits.
ANS: efficient
12. There must be sufficient 1-to-0 ____________________ to maintain synchronization in synchronous transmission.
ANS: transitions
13. Clock sync is derived from the stream of ____________________ bits in synchronous transmission.
ANS: data
14. In the ____________________ protocol, each frame begins with at least two SYN characters.
ANS: BISYNC
15. In HDLC, each frame starts with an 8-bit ____________________.
ANS: flag
16. The first eight bits of an SDLC frame are ____________________.
ANS: 01111110
17. BCC stands for ____________________ check character.
ANS: block
18. DLE stands for data link ____________________.
ANS: escape
19. HDLC uses bit-____________________ to prevent accidental flags.
ANS: stuffing
20. ____________________ errors cause many consecutive bits to be bad.
ANS: Burst
21. FEC stands for ____________________ error correction.
ANS: forward
22. An ____________________ scheme corrects errors by requiring the retransmission of bad blocks.
ANS: ARQ
23. Parity fails when an ____________________ number of bits are in error.
ANS: even
24. CRC codes are particularly good at detecting ____________________ errors.
ANS: burst
25. Huffman coding and run-length encoding are examples of data ____________________.
ANS: compression
26. A ____________________ is an encoding scheme that is not public in order to protect data.
ANS: cipher
27. A ____________________ is often used to generate an encryption key because it is easier to remember.
ANS: password
28. If the key is ____________________ enough, private-key encryption can be quite secure.
ANS: long
29. Messages cannot be ____________________ using a public key.
ANS: decrypted
30. Because it is ____________________-intensive, public-key encryption can be slow.
ANS: computation
SHORT ANSWER
1. How many different characters could be encoded using a six-bit code?
ANS:
64
2. What is the numerical difference between ASCII 'a' and ASCII 'A' if you treat them as hexadecimal (hex) numbers?
ANS:
20 hex (32 decimal)
3. The ASCII codes for the characters '0' through '9' are what hex numbers?
ANS:
30H to 39H
4. If an asynchronous frame is used to send ASCII characters in the form of bytes (8 bits), what is the shortest time it could take to send 1000 characters if each bit in a frame is 1 msec long?
ANS:
10 seconds
5. Suppose an asynchronous frame holds 8 bits of data, a parity bit, and two stop bits (it could happen). Calculate the efficiency of the communication system.
ANS:
66.7%
6. Suppose a synchronous frame has 16 bits of non-data in the front and a 16-bit BCC at the end. The frame carries 1024 bytes of actual data. Calculate the efficiency of the communication system.
ANS:
97.0%
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