Review Questions

R1. What are the advantages and the drawbacks of the NRZ code?

R2. What type of information is transmitted using ASK?

Digital data

R3. Why is ASK modulation not used in broadband channels?

The main advantage of ASK is its narrow spectrum which is not very important for broadband channels. Other encoding methods have advantages that are more important for such channels, e.g. a noise immunity.

R4. What parameters of the sinusoid change when using the QAM method?

1.        Amplitude

2.        Amplitude and phase correct

3.        Amplitude and frequency

4.        Frequency and phase

R5. How many bits are transmitted by one character of a code that has seven states?

Bits = log2 7 = 2.8

R6. Explain the reasons the bandwidth of 64 Kbps has been chosen for an elementary channel of digital telephone networks.

R7. Which method is used for improving self-synchronization properties of the B8ZS code?

The artificial distortion of zero strings by invalid characters.

R8. What are the differences between logical encoding and physical encoding?

R9. What principles serve as a basis for error detection and correction methods?

A.      Self-synchronization

B.       Redundancy - correct

C.       Maximization of the signal-to-noise ratio

R10. List the compression methods most suitable for text information. Why are they inefficient for the compression of binary data?

The variable-length codes, e.g. Haffman codes, are effective for text encoding because different symbols have different frequency of encountering in text information.

R11. What is Hamming distance?

The Hamming distance is the minimum number of bits by which any pair of allowed codes differs

R12. What value does the Hamming distance have for parity control methods?

2

R13. Is it possible to use frequency division multiplexing in the Ethernet network?

In principle yes; such method existed but currently it is not in use. All modern Ethernet versions use TDM.

R14. Which TDM mode is used in packet-switching networks?

Asynchronous

R15. Is it possible to combine different multiplexing methods? If yes, provide appropriate examples.

Yes, for example, DWDM and TDM can be used together. The former is used for a wavelength channel creation, wheras the latter is used for packet multiplexing inside of a wavelength channel.

R16. What are common features of the FDM and WDM methods?

R17. Based on which technique is the duplex mode implemented for the channel if both transmitters use the same frequency range simultaneously?

Time division multiplexing

Problems

Q1. Find the first two harmonics of the spectrum of the NRZ signal when transmitting the sequence 110011001100 if the transmitter frequency is 100 MHz.

25 MHz and 75 MHz

Q2. Which 16 codes would you choose for transmitting user information when using
3B/4B code?

Q3. Suggest redundant code with a Hamming distance of 3 bits.

Q4. Is it possible to reliably transmit the data through a channel with a bandwidth limits from 2.1 GHz to 2.101 GHz if the following transmission parameters are used: carrier frequency of 2.1005 GHz, ASK modulation with two amplitude values, and clock frequency of 5 MHz?

The bandwidth of a channel is 1 MHz (2100 2101). The signal spectrum when two first harmonics are taken into account is 10 MHz which is significantly wider than the banwidth of a channel so that data will be distorted.

Q5. Suggest variable-length codes for each of the A, B, C, D, F, and O characters if it is necessary to transmit the following message: BDDACAAFOOOAOOOO. Is it possible to achieve compression as compared to the use of:

                Traditional ASCII codes?

                Fixed-length codes taking into account only the presence of the previously listed characters?

Taking into account the frequency of symbols encountering one can choose the following codes:

O - 1, A - 01, D - 001, B - 001, C - 0001, F 00000

The use of these codes gives 35 bits for encoding of the message given. A use of ASCII codes results in 128 bits for the same message.

The use of fixed-length codes taking into account only 6 symbols needs three-bit codes so that the message will have a length of 48 bits.

Q6. How many times would the spectrum width of the NRZ signal increase if the transmitter clock frequency is doubled?

In 2 times