Let us consider a statistical time division multiplexing of packets. The…
2007
Let us consider a statistical time division multiplexing of packets. The number of sources is 10. In a time unit, a source transmits a packet of 1000 bits. The number of sources sending data for the first 20 time units is 6, 9, 3, 7, 2, 2, 2, 3, 4, 6, 1, 10, 7, 5, 8, 3, 6, 2, 9, 5 respectively. The output capacity of multiplexer is 5000 bits per time unit. Then the average number of backlogged of packets per time unit during the given period is
- A.
5
- B.
4.45
- C.
3.45
- D.
0
Attempted by 15 students.
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Correct answer: B
Solution overview: compute backlog by tracking how packets accumulate when arrivals exceed service capacity.
Step 1: Convert capacity to packets: 5000 bits per time unit = 5 packets per time unit (1000 bits per packet).
Step 2: Use the recurrence for backlog:
backlog_t = max(0, backlog_{t-1} + arrivals_t - 5), with backlog_0 = 0.
Time 1: arrivals = 6 → backlog = max(0, 0 + 6 - 5) = 1
Time 2: arrivals = 9 → backlog = max(0, 1 + 9 - 5) = 5
Time 3: arrivals = 3 → backlog = max(0, 5 + 3 - 5) = 3
Time 4: arrivals = 7 → backlog = max(0, 3 + 7 - 5) = 5
Time 5: arrivals = 2 → backlog = max(0, 5 + 2 - 5) = 2
Time 6: arrivals = 2 → backlog = max(0, 2 + 2 - 5) = 0
Time 7: arrivals = 2 → backlog = max(0, 0 + 2 - 5) = 0
Time 8: arrivals = 3 → backlog = max(0, 0 + 3 - 5) = 0
Time 9: arrivals = 4 → backlog = max(0, 0 + 4 - 5) = 0
Time 10: arrivals = 6 → backlog = max(0, 0 + 6 - 5) = 1
Time 11: arrivals = 1 → backlog = max(0, 1 + 1 - 5) = 0
Time 12: arrivals = 10 → backlog = max(0, 0 + 10 - 5) = 5
Time 13: arrivals = 7 → backlog = max(0, 5 + 7 - 5) = 7
Time 14: arrivals = 5 → backlog = max(0, 7 + 5 - 5) = 7
Time 15: arrivals = 8 → backlog = max(0, 7 + 8 - 5) = 10
Time 16: arrivals = 3 → backlog = max(0, 10 + 3 - 5) = 8
Time 17: arrivals = 6 → backlog = max(0, 8 + 6 - 5) = 9
Time 18: arrivals = 2 → backlog = max(0, 9 + 2 - 5) = 6
Time 19: arrivals = 9 → backlog = max(0, 6 + 9 - 5) = 10
Time 20: arrivals = 5 → backlog = max(0, 10 + 5 - 5) = 10
Sum of backlog values over 20 time units = 89 packets.
Average backlog = 89 / 20 = 4.45 packets per time unit.
Therefore the average number of backlogged packets per time unit during the given period is 4.45.