Download Homework 1

Homework 1
January 19, 2013
1. Suppose users share a 3Mbps link. Also suppose each user requires 150
kbps when when transmitting, but transmits only 10 percent of the time.
(a) When circuit switching is used, how many users can be supported?
(b) Now suppose that packet switching is used for the remainder of this
problem. Find the probability that a given user is transmitting.
2. (in class) NASAs Curiosity rover was successfully landed on Mars in August this year. Assuming the transmission will be over the direct 32 Kb/s
wireless link between the rover and NASAs Deep Space Network (DSN)
on Earth. The distance between Mars and Earth is 200 x 106 km. The
propagation speed of the wireless signal is 2 x 108 m/s.
How long it would take for the rover to transmit a 1600 byte packet from
Mars back to Earth? (answer the question in seconds, with a precision of
0.1s)
3. Continue the previous question. The direct link does not always work,
since Mars is spinning and sometimes the rover can be out-of-the-sight
from Earth. When that happens, the communication between the rover
and the Earth will be relayed by NASAs satellite, odyssey, orbiting around
the Mars. Assuming the satellite does store and forward. The distance
between the rover to the satellite is 400 km and the distance between the
satellite and the Earth is 200x106 km. The wireless link between the rover
to the satellite is 2Mb/s, and the link between the satellite to the Earth
is 64 Kb/s. The propagation speed of the wireless signal on both links is
2x108 m/s.
4. How long does it take a packet of length 1000 bytes to propogate over a
link of distance 2000 km, propogation speed 2. 108 m/s, and transmission
rate 2 Mbps?
5. If the Round-Trip Time (RTT) from London to Houston is 90ms and all
links in the network have a 100 Mbits/second data-rate, how much data
can fit in the pipe from London to Houston and back? Express your answer
in megabytes, rounded to three decimal places. Assume 1MB = 220 Bytes
6. Imagine that now you want to send data directly to a geostationary satellite. If the satellite is 30, 000 km away, and you send data at 1Mb/s, how
much data can fit in the pipe from you to the satellite? Express your
answer in bytes. Assume the speed of light in the void/air is 3x108 m/s.
1