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This elementary problem begins to explore propagation delay and transmission delay, two central concepts in data networking. Consider two hosts, A and B, connected by a single link of rate R bps. Suppose that the two hosts are separated by m meters, and suppose the propagation speed along the link is s meters/sec. Host A is to send a packet of size L bits to Host B.

  1. Express the propagation delay, $d _{prop}$, in terms of m and s.
  1. Determine the transmission time of the packet, $d _{trans}$, in terms of L and R
  2. Ignoring processing and queuing delays, obtain an expression for the end-to-end delay.
  3. Suppose Host A begins to transmit the packet at time t = 0. At time t = $d _{trans}$, where is the last bit of the packet?
  4.  Suppose $d _{prop}$ is greater than $d _{trans}$ . At time t = $d _{trans}$, where is the first bit of the packet?
  5. Suppose $d _{prop}$ is less than $d _{trans}$. At time t = $d _{trans}$, where is the first bit of the packet?
  6. Suppose s = $2.5 \times 10^8$, L = 120 bits, and R = 56 kbps. Find the distance mso that $d _{prop}$ equals $d _{trans}$.

1 Answer

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a) d m s prop m/s seconds.
b) d L R trans L/R seconds.
c) d (m/ s L/ R) end to end (m/s+L/R) seconds.
d) The bit is just leaving Host A.
e) The first bit is in the link and has not reached Host B.
f) The first bit has reached Host B.
g) Want
m=L*s/R

=536km

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