Ch. 6 The Data Communications Interface
1. What is a parity error? framing error?
2. Briefly describe the basic principle of error-detection, and draw a diagram illustrating
the concept.
3. Briefly describe simple parity checking and compare with the basic CRC error
detection procedure.
4. What is meant by modulo-2 arithmetic?
5. Identify the following variables in the CRC algorithm: T, D, F, P, Q, R ,n, k.
6. The CRC algorithm can be summarized in three steps; the first deals with
multiplication, the second is a division process, and the third deals with the
addition of the CRC bits to the transmitted polynomial. Write out these three
steps using the variables defined in your text.
7. What is meant by the error pattern E? How is E related to the transmitted frame (T)
and the received frame (Tr)?
8. Rewrite the three steps for the CRC process as polynomial equations. What kind of
polynomials are these? What type of arithmetic is being used?
9. What makes a good “generating polynomial” P(x)?
10. List four examples of standard generating polynomials.
11. Explain how the CRC circuit in Figure 6.5 (page 195) operates.
12. Draw a general CRC architecture and explain its relationship to a generating
polynomial P(x) (see Figure 6.6, page 196).
13. What is meant by a forward error correction codeword?
14. Briefly describe traditional computer/terminal configurations (point-to-point and
multipoint.).
15.What is meant by full duplex and half duplex transmission?
16. What is meant by asynchronous vs. synchronous transmission? Start and Stop
transmission?
Ch. 7 Data Link Control
1. Describe each of the following requirements and objectives for data link control:
a. frame synchronization
b. flow control
c. error control
d. addressing
e. control and data on the same link
f. link management
2. What is flow control?
3. What is stop-and-wait flow control?
4. Why are large messages often broken into blocks before transmission?
5. What is the sliding window protocol? What advantage does it have?
6. What are the four basic “ingredients” or “ARQ mechanisms” that are common to
most error control schemes?
7. What does ARQ stand for?
8. Briefly describe the following versions of ARQ:: Stop-and-Wait ARQ, Go-back-N
ARQ, Selective reject ARQ.
9. What is meant by a primary, secondary, and combination station as related to
HDLC?
10.
11.
12.
13.
What is meant by an unbalanced and balanced configuration as related to HDLC?
What are the three data transfer modes of operation for HDLC?
Draw an HDLC frame? Describe the fields? How many bits are in each?
What is the flag field pattern? What is bit stuffing? What is bit stuffing used for?
What is meant by data transparency?
14. Briefly contrast information frames, supervisory frames and unnumbered frames in
HDLC operations.
15. Describe the three phases of HDLC operations.
Ch. 8 Multiplexing
1. Briefly describe FDM; draw a diagram of a transmitter, spectrum of the composite
signal, and the receiver.
2. What are some problems with FDM?
3. Briefly describe the AT&T standard for FDM.
4. Describe TDM in general.
5. What is Synchronous TDM.
6. Is link control needed in STDM? Why or why not?
7. Describe added-digit framing.
8. What is pulse stuffing? What is it used for?
9. Describe the DS-1 transmission format. Draw a diagram.
10. Describe bit-robbing.
11. How is digital data service provided using the DS-1 frame?
12. What is the North American STDM carrier system called? (Answer: T-Carrier
System)
13. What is SONET/SDH?
14. What is a Cable MODEM?
15. In a typical cable modem, in North America, what range of bandwidth is set aside
for user-to network data (upstream)? What range is set aside for network to
user data (downstream).
16. In ADSL, what does the term asymmetric mean?
17. Briefly describe how ADSL uses frequency division multiplexing.
Ch.16, Section 3 ARQ Performance Issues
1.
Consider a half-duplex point-to-point line using stop-and-wait flow control;
assume that the message has been broken into n frames; write an expression for
the total time to send the data (i.e., the message).
2.
What simplifying assumptions can be made related to processing time and the
time to transmit an ACK? What is the result?
3.
Line utilization is defined as the fraction of time that the line is actually
transmitting data. Show how equation 16,18 is derived for Stop and Wait ARQ.
4.
How is the parameter “a” defined? How do distance d, propagation speed V, the
length of a frame L and the data rate R all relate to “a”.
5.
Write an expression for the line utilization for a sliding window protocol, assuming
a window size of W (see equation 16.21).
6.
Describe in words the relative efficiency of the various ARQ schemes.
7.
Compare the following equations, in particular what happens when P is small?:
a. Utilization for Stop-and-Wait flow control vs. Stop-and-Wait ARQ.
b. Utilization for the sliding window flow control vs. Go-back-N ARQ
c. Utilization for the sliding window flow control vs. Selective-reject ARQ