COSC 6377 Spring 2002 Midterm

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COSC 6377 Spring 2002Midterm 1

• Student Name
• Student ID
• Student Signature

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• 1. (5 points)
• Ideal pulses are sent through a channel at the
  Nyquist rate. Eight levels are used for each
  pulse. What is the minimum bandwidth required so
  that the channel can transmit at a rate of
  6.0106 bits/second?
• Answer (Show your calculation )
• 6.0 106 2 W 3
• W 1.0 106 Hertz or 1 MHz
• 2. (5 points)
• Suppose that a frequency band of 1M Hz wide is
  divided into 10 channels of equal bandwidth.
• a. What bit rate is achievable in each channel?
  Assume all channel have the same SNR, which is
  equal to 7. (7 is a numerical value. Not DB)
  (The following identity can be used log22x x)
• b. How does the bit rates of (a) change if we
  suppose that FDM requires a guard band between
  adjacent channels? Assume the guard is 10 of the
  channel bandwidth.
• Answer (Show your calculation)
• (a) Bit rate (1.0 106) (log2(17))/10
  (1.0 106) (log28)/10 (1.0 106) 3 /10
• 3.0 105 bits per second
• (b) Bit rate (0.9) 3.0 105 2.7 105
  bits per second

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• 3 (a). (8 points)
• Given the Manchester encoding of a bit sequence
  in the following figure, complete the
  corresponding differential Manchester encoding.
  Both of them should represent the same bit
  sequence (Notethe trailing element of the
  preceding symbol of the bit sequence is high)
• 3.b (2 points)
• From pulse rate requirements perspective, which
  of the two encoding scheme is better Polar NRZ
  or Manchester Encoding assuming no multi-level is
  used?
• Answer
• Polar NRZ

Manchester encoding
time
Differential Manchester encoding
time
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• 4. Wave and frequency
• One of the wireless systems, the PCS (Personal
  Communications Service) systems exist near the
  2-GHz range.
• 4 (a) (3 points)
• What is the wavelength of a 2 GHz signal in cm?
• (a) wavelength C/f (3.0 108 meters/second)
  / (2.0 109 Hz)
• 0.15 meter 15 cm
• 4 (b) (2 points)
• If a signals frequency is 1990 MHz, what Band
  is it in based on the IEEE Radar Band
  Designation?
• Answer
• (b) L Band
• 4 (c) (5 points)
• A necessary condition of the CDMA spreading
  sequences is that these sequences are orthogonal
  sequences. Are the following four spreading
  sequences orthogonal ? If not, why(show your
  calculation)?
• (1, 1, 1, 1)
• (1, -1, -1, -1)
• (1, 1, -1, -1)
• (1, -1, -1, 1)
• Answer
• Because the inner product of sequence 2 and
  sequence 3 is not zero, sequence 2 and sequence
  3 are not orthogonal.

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• 5. (5 points)
• Odd-parity is used by the transmitter. When a
  receiver receives the following binary data
  units, the receive puts each data unit through a
  odd-parity checking function. Which ones will be
  accepted by the receiver? (Select all that apply)
• (a) 11111110
• (b) 11011110
• (c) 11101100
• (d) 11011000
• (e) 11001001
• Answer
• (a), (c)
• 6 (5 points) Internet checksum
• Assuming that a header has only four
  non-zero16-bits words listed below, calculate the
  internet checksum .
• word 1 10000000 00000001
• word 2 10000000 00000001
• word 3 10000000 00000001
• word 4 00000000 00000001

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• 7 (a) (5 points)
• Given x21 as the CRC generator, what is the
  value of CRC check bits (remainder) for the
  information bits 0101 ? (No points will be given
  unless the calculation is shown)
• Answer 00
• 010000
• 101 010100
• 000
• 101
• 101
• 000
• 000
• 000
• 000
• 00
• 7 (b) (5 points)
• Does the CRC generator x21 contain x1 as a
  factor? Can it detect all odd errors? (No points
  will be given unless the calculation is shown)
• Answer x21 contains x1 as a factor.
• 11

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• 8 (a). (3 points)
• SONET uses both positive stuffing method and
  negative stuffing method to support the
  synchronization between STS-1 frame and SPE
  frame.
• Which one of the two stuffing methods will stuff
  a byte with dummy data?
• Answer
• Positive stuffing.
• 8 (b) (3 points)
• A regenerator of the SONET can operate in what
  SONET layers ? (Select all correct answers)
• (a) line
• (b) photonic
• (c) section
• (d) path
• Answer
• (b) (c)
• 8 (c) (4 points)
• what is the maximum number of VT2s (VT stands
  for virtual tributary) that STS-1 can
  accommodate?
• (a) 4
• (b) 21
• (c) 22

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• 9 (a) (3 points)
• An Ethernet switch can function in which layer?
  (Select all correct answers)
• a. physical
• b. network
• c. data link
• d. transport
• Answer (a), (c)
• 9 (b) (3 points)
• LAN switches use three switching methods to
  route frames cut-through, store-and-forward, and
  fragment-free.
• Which statements about switching methods are
  true (Select all correct answers.)
• (a) Latency of cut through switching varies
  with frame size.
• (b) The latency of fragment-free switching is
  between latency of cut-through switching and
  latency of store-and-forward switching.
• (c) Both store-and-forward switching and
  cut-through switching performs CRC error
  checking for each frame.
• (d) Latency of store-and-forward switching
  varies with frame length.
• Answer (b) (d)
• 9 (c) (4 points)
• What is the result of segmenting a network with
  a bridge (Choose all that apply.)
• a. It increases the number of collision
  domains.
• b. It decreases the number of collision domains.

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• 10. Consider a (6,3) linear Hamming code in which
  the first three bits of the codeword b consists
  of three information bits b1, b2, and b3, and the
  three check bits b4, b5, and b6 are given by
• b4 b1
• b5 b2
• b6 b1 b2 b3
• 10.(a) (4 points)
• Calculate the check bits and weights in the
  following table Answer

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• 10 (b) (2 points)
• What is the minimum distance d of the code
  provided in (a)?
• Answer
• 2
• 10(c) (2 points)
• What is the maximum number of errors that can
  be detected by code provided in (a)?
• Answer
• 1
• 10(d) (2 points)
• What is the maximum number of errors that can
  be corrected by code provided in (a)?
• Answer
• 0

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• 11 (a) (1 point)
• What can happen at a Token Ring station? (Select
  all correct answers)
• a. examination of the destination address
• b. regeneration of the frame
• c. passing of the frame to the next station
• Answer a,b,c
• 11 (b) (2 points)
• In Token Ring, where is the token when a data
  frame is in circulation? (Select the correct
  answer)
• a. at the receiving station
• b. at the sending station
• c. circulating in the ring
• d. none of the above
• Answer b
• 11 (c) (2 points)
• In token Ring, when a frame reaches its
  destination station and is received by the
  destination station successfully, which of the
  following occurs at the destination station?
  (Select all correct answers)
• a. The message is copies
• b. Four bits in the frame are changed
• c. the message is taken off the ring and
  replaced by the token
• Answer a, b

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• 12 (a) (5 points)
• Imagine the length of a 10Base5 cable is 2,500
  meters. If the speed of the propagation in a
  thick coaxial cable is 60 percent of the speed of
  light (the speed of light is 3.0 108 meters per
  second), how long does it take for a bit to
  travel from the beginning to the end of the
  cable? Ignore any propagation delay in the
  equipment
• Answer
• speed of propagation 3.0 108 meters per
  second 0.6 1.8 108 meters per second
• 2500 meters / 1.8 108 meters per second
  13.89 10-6 seconds
• 12 (b) (5 points)
• The maximum time it takes to sense a collision
  is when data is sent from one end of the cable
  and the collision happens at the other end.
  Remember that the signal needs to make a round
  trip. The date rate of 10Base5 is 10 Mbps.
  Calculate the minimum size of an Ethernet frame
  for collision detection to work properly based on
  the information and answer of 12(a).
• Answer
• 2 13.89 10-6 seconds 27.8 10-6 seconds
• 10 106 bits/seconds 27.8 10-6 seconds 278
  bits

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• 13 (5 points)
• Six stations (S1 S6) are connected to an
  extended LAN through transparent bridges (B1 and
  B2), as shown i the figure next page. Initially,
  the forwarding tables are empty. Suppose the
  following stations transmits frames
• - S2 transmits to S1
• - S5 transmits to S4
• - S3 transmits to S5
• - S1 transmits to S2
• - S6 transmits to S5
• Fill in the forwarding tables of B1 and B2 with
  appropriate entries after frames have been
  completely transmitted.

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S3
S4
S6
S1
S2
S5
LAN1
LAN2
LAN3
port 2
B1
B2
port 1
port 1
port 2
Port
Station
Port
Station
1
S2
1
S2
2
S5
2
S5
S3
2
1
S3
S1
1
S6
2
B1 forwarding table
B2 forwarding table