COSC6377 Fall 2001

1

• COSC6377 Fall 2001
• Midterm 1
• Student Name
• Student ID
• Student Signature

2

• 1 (a) (5 points)
• You have been asked to review a proposal
  submitted by a consulting firm for the cabling
  scheme for your companys new office building.
  Four computers, A, B, C, and D, are located in
  different offices
• Computer A to the hub 525 feet
• Computer B to the hub 675 feet
• Computer C to the hub 450 feet
• Computer D to the hub 285 feet
• The consulting firm has recommended that you
  implement 10BaseT Category 5 UTP wire for your
  companys network. This recommendation violates
  the UTP and 10BaseT specifications. Which of the
  following are true?
• a. The distance from computer A to the hub
  exceeds the maximum cable length specified by
  10BaseT.
• b. The distance from computer B to the hub
  exceeds the maximum cable length specified by
  10BaseT.
• c. The distance from computer C to the hub
  exceeds the maximum cable length specified by
  10BaseT.
• d. The distance from computer D to the hub
  exceeds the maximum cable length specified by
  10BaseT.
• Answer

3

• 1 (b) (3 points)
• A repeater is a device that regenerates signals
  so that they can travel on additional cable
  segments. Which of the following statements are
  true of repeaters?
• a. Repeaters can be used to solve the problem of
  crosstalk.
• b. Repeaters can be used to solve the problem of
  attenuation.
• c. Segments joined by a repeater must use the
  same media access method.
• Answer
• 1 (c) (2 points)
• Which of the following devices can be used to
  resolve broadcast storms?
• a. routers
• b. repeaters
• c. bridges
• d. hubs
• Answer

4

• 2 (a) (2 points)
• When a host on network A sends a message to a
  host on network B, which address does the router
  look at?
• a. port
• b. IP
• c. physical
• d. none of the above
• Answer
• 2 (b) (2 points)
• Select all statements which are true.
• (a) Physical address is a hierarchical
  addressing structure.
• (b) Physical address is a flat addressing
  structure.
• (c) Physical address for Ethernet is 4 bytes
  long.
• (d) Physical address for Ethernet is 6 bytes
  long.
• Answer
• 2 (c) (1 point)
• For the following three protocols TCP, UDP,
  IP,
• (a) which of the protocols is/are
  connection-oriented?

5

• 3(a) (3 points)
• An add/drop multiplexer (ADM) operates in what
  SONET layers ? (Select all correct answers)
• (a) line
• (b) section
• (c) photonic
• (d) path
• Answer
• 3 (b) (2 points)
• What is the maximum number of VT1.5s (VT stands
  for virtual tributary) that STS-1 can
  accommodate?
• (a) 3
• (b) 9
• (c) 28
• (d) 29
• Answer

6

• (5 points)
• For a channel with bandwidth W 10M Hz, ideal
  pulses are sent at the Nyquist rate. How many
  levels are needed for each pulse so that the
  channel can transmit at a rate of 8.0107
  bits/second?
• Answer

7

• 5 (a). (4 points)
• Complete the following Manchester encoding and
  Differential Manchester encoding based on the
  given bit sequence (Notethe trailing element of
  the preceding symbol of the bit sequence is
  high)

0
1
1
0
0
1
0
1
Manchester encoding
time
Differential Manchester encoding
time
8

• 5 (b). (4 points)
• The bit sequence of token ring starting
  delimiter is JK0JK000. Encode this bit sequence
  using the following diagram (Notethe trailing
  element of the preceding symbol of the J bit is
  high)
• 5.(c) (2 points)
• From synchronizations point of view, which of
  the two encoding schemes is better Unipolar NRZ
  or Manchester Encoding?
• Answer
• From bit rate requirements perspective, which
  of the two encoding schemes is better Polar NRZ
  or Differential Manchester Encoding?
• Answer

0
J
0
K
K
J
0
0
time
9

• 6(a) (4 points)
• Under a light load, which LAN has a smaller
  delay Ethernet or token ring?
• Answer
• Under a high load, which LAN has a smaller
  delay Ethernet or token ring?
• Answer
• 6(b) (6 points)
• An Ethernet frame is about to undergo the third
  retransmission attempt. The mini-slot time is
  51.2 microseconds (51.2 10-6 seconds). Based
  on Binary Exponential Back-off algorithm, what is
  the possible minimum waiting time (not including
  the inter-gap time) and the possible maximum
  waiting time induced by the algorithm?
• Answer

10

• 7 (a) (2 points)
• Single parity check algorithms can detect
• (a) Single-bit errors (1 inverted bit)
• (b) Double-bit errors (2 inverted bits)
• (c) Three bit errors (3 inverted bits)
• Select all correct answers.
• Answer
• 7 (b) (8 points)
• Internet checksum
• Assuming that a header consists of 6 16-bit
  words listed below, calculate the internet
  checksum .
• word 1 10000000 00000001
• word 2 10000000 00000001
• word 3 00000000 00000001
• word 4 00000000 00000001
• word 5 00000000 00000000
• word 6 00000000 00000000
• Answer

11

• 8 (a) (8 points)
• Let g1(x) x 1 and let g2(x) x3 x2 1.
  Given the information bits as (1,1,0,1,1,0),
  find the codeword if g(x) g1(x)g2(x) is used as
  the generating polynomial.
• Answer

12

• 8 (b) (2 points)
• On the error-detecting capabilities of g(x)
• Can g(x) detect single-bit error? Answer
  yes or no.
• Can g(x) detect odd numbers of inverted
  bits? Answer yes or no.
• Answer

13

• 9 (10 points)
• Eight stations (S1 S8) are connected to an
  extended LAN through transparent bridges (B1, B2,
  and B3) as shown in the figure provided in next
  page. Initially, the forwarding tables are
  empty. Suppose the following stations transmit
  frames
• S1 -gt S5
• S6 -gt S1
• S7 -gt S1
• S8 -gt S7
• S2 -gt S1
• Fill in the three forwarding tables in the next
  page with appropriate entries after the frames
  have been completely transmitted.
• Answer

14
Answer
Figure for problem 9
S5
S1
S2
S3
S4
LAN1
LAN2
LAN3
port 2
B1
B2
port 1
port 1
port 2
Address Port
Address Port
port 3
Address Port
B1 forwarding table
B2 forwarding table
S8
S6
S7
B3 forwarding table
LAN4
LAN5
B3
port 1
port 2
15

• 10 (a) (5 points)
• Perform bit de-stuffing for the following bit
  sequence
• 111011111001111101111110
• Answer
• 10(b) (5 points)
• The PPP byte stuffing method uses an escape
  character defined by 0x7D (01111101). When the
  flag, which is 0x7E, is observed inside the
  frame, the escape character is placed in front of
  the flag and the flag is exclusive-ORed with
  0x20. That is, 0x7E is encoded as 0x7D 0x5E. An
  escape character itself (0x7D) is encoded as 0x7D
  0x5D. (a) How many frames are contained in the
  following byte sequence?
• (b) What are the contents of each frame (not
  including the flag of the frame)
• 0x7E 0xEE 0x03 0x7D 0x5E 0xFE 0x24 0x7D 0x5D
  0x7E 0xEE 0x03 0x7D 0x5D 0x62 0x7D 0x5E 0x7E
• Answer
• (a)
• (b)

16

• 11 (a) (3 points)
• . One way to generate orthogonal spreading
  sequence is to use the Walsh function. For
  example
• 1 -gt 1 1 -gt 1 1 1 1
• 1 0 1 0 1 0
• 1 1 0 0
• 1 0 0 1
• A necessary condition of the CDMA spreading
  sequences is that these sequences are orthogonal
  sequences. Are the following four spreading
  sequences orthogonal ? If not, show why?
• (1, 1, 1, 1)
• (1, -1, 1, -1)
• (1, 1, -1, 1)
• (1, -1, -1, 1)
• Answer

17

• 11 (b) (2 points)
• Based on Hamming rule, what is the minimum
  number of check bits required for 8 information
  bits?
• Answer

18

• 12 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 b2
• b5 b2 b3
• b6 b3 b1
• 12 (a) (4 points)
• Calculate the check bits and weights in the
  following table (Note the weight of a codeword
  is defined as the number of 1s in a codeword)
• Answer

19

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