Question 36: The network’s Subnet Mask is 255.255.255.224, determine
its broadcast address if that a computer which has the address
192.168.1.1?
A. 192.168.1.255 B. 192.168.1.15 C. 192.168.1.31 D. 192.168.1.96
Đáp án: C
Question 37: Consider building a CSMA/CD network running at 1 Gbps
over a 1–km cable with no repeaters. The signal speed in the cable is
200000 km/sec. What is the minimum frame size?
A. 500 bits B. 5000 bits C. 1000 bits D. 10000 bits
Đáp án: D
Hướng dẫn:
Thời gian cần truyền đi cho 1 km là: 1/200000 = 5 x 10^-6 = 5 µsec
Thời gian truyền cả đi và về 1 km là: 5 x 2 = 10 µsec
Số bit truyền được là: 10^9 bps x 10 x 10^-6 sec = 10^4 bits = 10000
bits
Question 38: What is EIRP of a dipole antenna fed with a power of 100
mW?
0.1W≈ 100mW B. ≈A.
0.16W D. A, B and C are wrong≈C.
Đáp án: D
Hướng dẫn: (dựa vào slide 29 – Chương 3)
100 mW = 0,1 W
Gain = 0 dBd (do chuyển từ anten lưỡng cực sang đẳng hướng)
Do đó công suất nhận là 0,1W và công suất phát là ERP = 0,1W.
Ta có: 2,14 = 10log10 (EIRP / ERP)
EIRP / ERP = 1,639.⇒
EIRP = 16,39 W⇒
Question 39: We need to send 56 kbps over a noiseless channel with a
bandwidth of 4 kHz. How many signal levels do we need?
A. 1 B. 64 C. 2 D. 128
Đáp án: D

Hướng dẫn:
Áp dụng công thức Nyquist, ta có:
• 56000 = 2 x 4000 x log2L
log 2 L = 7, suy ra L = 2^7 = 128.
Question 40: Which modulation does a constellation diagram correspond
to?
A. QAM-4 B. PSK-4 C. PSK-2 D. QAM-2
Đáp án: B (câu này có cái hình)
- Mấy cái ghi logarit past từ word qua đây lỗi. Ví dụ log2L hiểu là
log cơ số 2 của L.
- Có phải dịch câu hỏi ra ko vậy, chắc khỏi nha.
Question 1: How many frequencies does a full-duplex QAM-64 modem use?
A. 1 B. 12 C. 6 D. 2
Đáp án: D
Question 2: An analog cellular system has 20 MHz of band-width and
uses two channels simplex of 25 kHz to provide a transfer service of
full-duplex. What is the number of full-duplex channels available by
cell for a reuse factor K = 4?
A. 400 B. 200
C. 100 D. A, B and C are wrong
Đáp án: C
Hướng dẫn:
Băng thông: 20 000 000 Hz
Băng thông của kênh là: 25 000 x 2 (simplex channel) = 50 000 Hz
(duplex channel)
Số kênh: 20 000 000/50 000 = 400 (channels)
K = 4: Số kênh có trên tế bào (cell) là: 400/4 = 100 (channels)
Question 3: A modem constellation diagram similar to figure. How many
bps can a modem with these parameters achieve at 1200 baud?
A. 4800bps B. 2400bps

C. 1200bps D. A, B and C are wrong
Đáp án: B
2 x 1200 = 4800 bps
Question 4: Ten signals, each requiring 4000Hz, are multiplexed on to
a single channel using FDM. How much minimum bandwidth is required for
the multiplexed channel? Assume that the guard bands are 400Hz wide.
A. 40800Hz B. 40400Hz
C. 44000Hz D. A, B and C are wrong
Đáp án: D
Hướng dẫn:
Có 10 tín hiệu, mỗi tín hiệu 4000 Hz. Dó đó, cần 9 dải tần (guard
bands)
Vậy, băng thông tối thiểu cần để ghép kênh là: 4000x10+400x9 = 43600
Hz.
Question 31: One class A address borrowed 21 bits to divide subnet,
the Subnet Mask is?
A. 255.255.255.248 B. 255.255.224.0 C. 255.255.192.0 D. 255.255.248.0
Mượn 21 bit =>2 octet giữa mượn tổng cộng 16 bit(255.255), octet
cuối mượn 5 bit:128+64+32+16+8=248
=>Câu A
Question 32: The radius of a cell in cellular network is 1600m. What
is the distance between centers of adjacent cells?
1600m≈ 2770m B. ≈A.
3200m D. A, B and C are wrong≈C.
Khoảng cách = r x căn bậc 2 của 3 = 2770 (đánh dấu căn nó lại
ko hiện lên :D)
=>Câu A
Question 33: Consider a noiseless channel with a bandwidth of 3000 Hz
transmitting a signal with 16 signal levels. The maximum bit rate can
be?

A. 48000 bps B. 3000 bps C. 12000 bps D. 6000 bps
Công thức Nyquist:BitRate = 2 x 3000 x log2(16)=24000
=>câu E(chắc dáp án sai ^^)
Question 34: A group of N stations share a Slotted ALOHA channel of 1
Mb/s. Each station emits in average a frame of 12000 bits/second. By
taking again the results obtained for Smax (maximum efficiency)
determine the maximum value of N that one can have?
700≈ 300 B. ≈A.
600 D. A, B and C are wrong≈C.
1 byte = 8 bit
1Mb = 1024 x 1024 byte
=>N = 1024 x 1024 x 8 / 12000 = 700
=>Câu B
Question 35: One takes the antenna gains into account; microwaves
transmitter has an output power of 0.1W at 2 GHz. The EIRP of the
transmitted signal is 10W. What is the wavelength of receiving
antennas parabolas?
0.64m≈ 1.2m B. ≈A.
0.32m D. A, B and C are wrong≈C.
lamda = c/f = 3x10^8 / 2x10^9 = 0,15(m)(đánh dấu lamda nó lại ko
hiện lên :D)
=>Câu D
Question 11: A typical Bluetooth data frame describe in figure. Which
field determines the active devices?
A. Header B. Access code C. Address D. Checksum
Ans: C
Question 12: If the diameter of receiving antennas parabolas is 30 cm,
what is the frequency concerned?
A. 1GHz B. 1MHz
C. 1kHz D. A, B and C are wrong

Ans: D
Question 13: The signal can be written .
What is the modulation type?
A. ASK-2 B. ASK-4 C. PSK-2 D. QAM-2
Ans: A
Question 14: The output power at transmitter is 1W, power received is
100mW. The gain is?
10 dB B. 100 C. 10 dB D. 1−A.
Ans: D
Question 15: A telephone line has a bandwidth of 4kHz. The
signal-to-noise ratio is 3162. For this channel the capacity is?
40000 bps≈ 46480 bps B. ≈A.
37944 bps D. A, B and C are wrong≈C.
Ans: A
C = B log2 (1 + SNR)
B is the bandwidth .SNR is the signal to noise ratio, capacity is the
capacity of the channel in bits per second
Câu 6-10:
Question 6: Suppose a transmitter produces 50W of power. The transmitter’s power is applied to a unity
gain antenna with a 900MHz carrier frequency. The close-in distance is d
0
= 100m. The value P
r
(d
0
) may be
predicted from equation, where G
t
, G
r

are antenna gain at transmitter, receiver, and L  1 is the system loss
factor. If P
r
is in unit of dBm, the received power is given by P
r
(d) = 10log
10
[P
r
(d
0
)/0.001] + 20log
10
(d
0
/d),
where dd
0
. What is the received power at a free space distance of 10km?
A.  24.5dBm B.  64.5dBm
C.  +64.5dBm D. A, B and C are wrong

Answer: unity gain antenna: Gain = 0;

= c / f = 3 * 10^8 / 900000000 = 1/3
L = (4d / ) ^ 2
=> Pr (d0) = Pt * ^4 / (4d) ^ 4 = 2.475 *10^-10 (mW)
=> Pr(d = 10km) = -106.0635447 dBm.



Question 7: A channel has a bit rate of 5 kbps and a propagation delay of 20 msec. For what range of frame
sizes does stop-and-wait give an efficiency of at least 50%?
A. frame size < 100 bits B. 100 bits < frame size < 200 bits
C. frame size > 200 bits D. frame size < 200 bits

Answer: Tf / Tt >= 50% (1)
Tf = Fs / Bw (2) ; Tt = 20*10^-3 + Tf (3)
From (1) and (3) => Tf >= 0.02 (4);
From (4) and (2) => Fs >= 102.4 (bits);
With:
Tf: Time to transmit the frame; Tt: Total time include propagation delay time;
Fs: frame size; Bw: bandwidth

Question 8: Need to share with each subnet have 510 hosts maximum in a class B address, should use the
Subnet Mask?
A. 255. 255. 255.0 B. 255. 255. 255.192 C. 255. 255.252.0 D. 255.255.254.0

Answer: hosts = 510 => 9bits host => 23 bits net, IP class B => borrow 7bits => 255.255.254.0

Question 9: Suppose that 1 MHz is dedicated to the control channels and that only a control channel is
necessary by cluster; determine a reasonable distribution of the voice traffic channels for each cell, for the 9
reuse factors given previously.
A. 8 cells with 53 voice channels and 4 cells with 54 voice channels
B. 8 cells with 71 voice channels and 1 cell with 72 voice channels
C. 4 cells with 91 voice channels and 3 cells with 92 voice channels
D. A, B and C are wrong
Question 10: The signal can be written.
What is the modulation type?
A. QAM-4 B. PSK-4 C. PSK-2 D. QAM-2


This yields the four phases π/4, 3π/4, 5π/4 and 7π/4 as needed.
Còn câu 9 nợ lát nữa post nhe, tranh thủ mấy câu này trước! ^^
Mình đang làm từ câu 26 - 30, mà bí 2 câu 26 và 30, có bạn nào giúp ko, tìm mãi ko thấy cái nào dính tới 2
câu này hết
3 câu làm xong :
Question 27: In the cellular network, Q = D/R called co-channel reuse ratio. Suppose i = 3 and j = 1. What is
the Q?
A.  6 B.  6.24
C.  4.58 D. A, B and C are wrong
Đáp án : C. Theo slide 13 chương 5
Question 28: For the network address 172.16.0.0/16. How many subnets this network can be split up?
A. 16 B. 2
14
C. 2
16
D. A, B and C are wrong
Đáp án : C
Question 29: The telephone line is now to have a loss of 20dB. The input signal power is measured as 0.5W,
and the output noise level is measured as 4.5W. Calculate the output SNR in dB?
A.  30.5dB B.  +30.5dB
C.  89dB D. A, B and C are wrong