FREDERICK INSTITUTE OF TECHNOLOGY
SUBJECT: ACOE312 Data Communications and Computer Networks
DATE: 12/01/2005
TIME: 15:15-16:25
T E S T
INSTRUCTIONS TO CANDIDATES
Answer all Questions in the answer sheet.
A formula sheet is provided at the end of this paper.
1) With reference to OSI and TCP/IP models
(i) List only the four lower layers of both models, briefly explaining the functionality of each layer [16 marks]
(ii) State two critiques of each model [6 marks]
2) What is the maximum modulation rate of
(i) a Bipolar-Alternate Mask Inversion encoded signal with a bit rate of 5kbps
(ii) a Manchester encoded signals with a bit rate of 20Mbps [8 marks]
3) Describe the procedural specification for exchanging data between a DTE and a DCE device connected directly via an RS232 serial interface. You may use the following diagram to ease your explanation. [8 marks]
4) A user sends data via a modem over a PSTN line of 3kHz bandwidth. Assuming that the channel has a signal-to-noise power ratio of 9dB
(i) what is the theoretical maximum bit rate that can be achieved? [6 marks]
(ii) how would the data rata change if the channel was noise-free and quaternary signalling is used? [6 marks]
a)
b)
c)
d)
e)
f)
5) For the bit stream 100011010, sketch the waveforms for the following digital encoding schemes
(i) Non-return to zero inverted (NRZ-I)
(ii) Pseudoternary
(iii) Differential Manchester
Assume that the signal level for the preceding bit for NRZ-I was low and the most recent preceding 0 bit (pseudoternary) has a positive voltage. [
12 marks]
6) Suppose a file of 90kBytes of data is to be sent over a transmission line at 40kbps. Calculate the overhead in bytes and time in using asynchronous communication. Assume that each character is 8 bits long, which includes within these 8 bits apart from the data, one (1) start bit, one (1) stop bits and one (1) Parity bit. [10 marks]
7) Explain the operation of stop-and-wait and sliding window flow control techniques. [12 marks]
8) A channel has a data rate of 40kbps and a propagation delay of 40ms. For what range of frame sizes does sliding window flow control with a window size of 31 give an efficiency of at least 60%?
[10 marks]
9)
Explain why statistical TDM is considered to be a more efficient multiplexing technique than synchronous TDM. [6 marks]
Formula Sheet
SNRdB=10log10 (SNR)
GdB = 10 log10 G = 10 log10 (Pout/Pin) GdB = 20 log10 (Vout/Vin)
LdB = 10 log10 L = 10 log10 (Pin/Pout) LdB = 20 log10 (Vin/Vout)
C = B log2 (1+ SNR) C = 2B log2 M
T = 1/f c = λ f c = 3 x108 m/s
logaB=X => aX=B π = 3.14159265
logaB = (log10 B)/(log10 a)
G = (4πΑe)/λ2 = (4π f 2 Αe)/c2
A = π r2
r = d/2 Ae= 0.56A
Pt/Pr = (4πd)2/λ2 = (4π f d)2/c2
LdB = 10 log10 (Pt/Pr) = -20log10(λ) + 20log10(d) + 21.98 dB
B = R (d/V)
U = 1/(1+2a) a=tprop/tframe
U=W/(2a+1) for W<2a+1
1m = 10-3 1k = 103
1μ = 10-6 1M = 106
1n = 10-9 1G = 109
1p = 10-12 1T = 1012
-- End of paper --
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