1) Propagation Delay and Transmission Delay. Consider two hosts, A and B, connec
ID: 3663397 • Letter: 1
Question
1)
Propagation Delay and Transmission Delay. Consider two hosts, A and B, connected by a 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 per second. Host A sends a packet of L bits to Host B.
Express the propagation delay, dprop, in terms of m and s.
Determine the transmission time of the packet, dtrans, in terms of L and R.
Ignoring processing and queuing delags, obtain an expression for the end-to-end delay.
If host A begins to transmit at time t = 0. At time dtrans, where is the last bit of the packet?
Suppose dprop is greater than dtrans. At time = dtrans, where is the first bit of the packet?
Suppose dprop is less than dtrans. At time = dtrans, where is the first bit of the packet?
Suppose s = 200,000,000, L = 220 bits, and R = 56 kbps. Find a distance m so that dprop equals dtrans.
2)
We are sending real-time voice from Host A to Host B over a packet switched network (VoIP) Host A converts analog voice into a digital 64 kbps bit stream on the fly. Host A then groups the bits into 56-byte packets. There is one link between Host A and B. Its transmission rate is 1 Mbps and its propagation delay is 10 msec. As soon as Host A gathers a packet, it sends it to host B. As soon as Host B receives an entire packet, it converts the packets bits into an analog signal. How much time elapses from the time a bit is created (from the original signal at Host A) until the bit is decoded (as part of the analog signal at Host B)?
Explanation / Answer
1)
a. Express the propagation delay, dprop, in terms of m and s.
Answer:
dprop = m/s seconds.
b. Determine the transmission time of the packet, dtrans, in terms of L and R.
Answer:
dtrans = L/R seconds.
c. Ignoring processing and queuing delays, obtain an expression for the
end-to-end delay.
Answer:
d end to end = (m/ s + L / R) seconds.
d. 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?
Answer:
The bit is just leaving Host A.
e. Suppose dprop is greater than dtrans. At time t = dtrans, where is the first bit of the
packet?
Answer:
The first bit is in the link and has not reached Host B.
f. Suppose dprop is less than dtrans. At time t = dtrans, where is the first bit of the
packet?
Answer:
The first bit has reached Host B.
g. Suppose s = 2.5108, L = 120 bits, and R = 56 kbps. Find the distance m so
that dprop equals dtrans.
Answer:
m= L/R(s) =( 220 x 2x108)/56x 103= 785km
2)
Before transmit this bit, it has to wait other bits in this group.
t1 = 56x8/(641000) = 7ms
dtrans = 568bits/1Mbps = 0.448ms
dprop = 10ms
T = t1 + dtrans + dprop = 17.448msBefore transmit this bit, it has to wait other bits in this group.
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