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Unformatted text preview: EE 450 Homework 1
Summer ’09 ● Nazarian
Name: _________________________________________
Assigned: Thursday June 4, 2009
Due: Thursday June 11, 2009, 8:00 am Lecture 8:00
Score: ________ 1. (5 points) What is the difference between a host and an end system? List
the types of end systems. Is a web server an end system?
2. (each part 5 points)
(a) What is network topology?
(b) Write at least 5 different topologies.
(c) Which topology is the most expensive?
3. (each part 5 points)
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.
(a) Express the propagation delay, dprop in terms of mand s.
(b) Determine the transmission time of the packet, dtrans in terms of Land R.
(c) Ignoring processing and queuing delays, obtain an expression for the
endtoend delay.
(d) Suppose Host A begins to transmit the packet at time t=0. At time
t=dtrans, where is the last bit of the packet?
(e) Suppose dpropis greater than dtrans . At time t=dtrans, where is the first bit of
the packet?
(f) Suppose dpropis less than dtrans . At time t=dtrans, where is the first bit of
the packet?
(g) Suppose s=2.5*108, L=100 bits and R=28 kbps. Find the distance m so
that dprop equals dtrans. 4. (each part 10 points)
Consider an application which transmits data at a steady rate (e.g., the sender
generates a N bit unit of data every k time units, where k is small and fixed).
Also, when such an application starts, it will stay on for relatively long
period of time. Answer the following questions, briefly justifying your
answer:
(a) Would a packetswitched network or a circuitswitched network be more
appropriate for this application? Why?
(b) Suppose that a packetswitching network is used and the only traffic in
this network comes from such applications as described above. Furthermore,
assume that the sum of the application data rates is less that the capacities of
each and every link. Is some form of congestion control needed? Why? 5. (20 points)
In this problem we consider sending voice from Host A to Host B over a
packetswitched network (VoIP). Host A converts analog voice to a digital
64 kbps bit stream on the fly. Host A then groups the bits into 48byte
packets. There is one link between host A and B; its transmission rate is
1 Mbps and its propagation delay is 2 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
coverts the packet's bits to an analog signal. How much time elapses from
when a bit is created (from the original analog signal at A) until a bit is
decoded (as part of the analog signal at B)?
6. (20 Points)
A packet switch receives a packet and determines the outbound link to
which the packet should be forwarded. When the packet arrives, one other
packet is halfway done being transmitted on this outbound link and three
other packets are waiting to be transmitted. Packets are transmitted in order
of arrival.
Suppose all packets are 1,000 bytes and link rate is 1 Mbps. What is the
queuing delay for the packet? More generally, what is the queuing delay
when all packets have length L, the transmission rate is R, x bits of the
currentlybeingtransmitted packet have transmitted, and n packets are
already in the queue? 7. (10 points)
Suppose that between a sending host and a receiving host there is exactly
one packet switch. The transmission rates between the sending host and the
switch and between the switch and the receiving host are R1 and R2,
respectively. Assuming that the router uses storeandforward packet
switching, what is the total endtoend delay to send a packet of length L?
(Ignore queuing and propagation delay.)
8. (each part 5 points)
Suppose two hosts, A and B, are separated by 10,000 kilometers and are
connected by a direct link of R=1 Mbps. Suppose the propagation speed over
the link is 2.5×108 meters/sec.
(a) Calculate the bandwidthdelay product, R×dprop
(b) Consider sending a file of 400,000 bits from Host A to Host B. Suppose
the file is sent continuously as one large message. What is the maximum
number of bits that will be in the link at any given time?
(c) Provide an interpretation of the bandwidthdelay product.
(d) What is the width (in meters) of a bit in link? Is it longer than a football
field?
(e) Derive a general expression for the width of a bit in terms of the
propagation speed s, the transmission rate R, and the length of the link m.
9. (each part 5 points)
Consider problem 8 but now with a link of R=1 Gbps.
(a) Calculate the bandwidthdelay product, R×dprop
(b) Consider sending a file of 400,000 bits from Host A to Host B. Suppose
the file is sent continuously as one large message. What is the maximum
number of bits that will be in the link at any given time?
(c) What is the width (in meters) of a bit in link?
10. (each part 10 points)
Refer again to problem 8.
(a) How long does it take to send the file, assuming it is sent continuously?
(b) Suppose the file is broken up into 10 packets with each packet containing
40,000 bits. Suppose that each packet is acknowledged by the receiver and
the transmission time of an acknowledgement packet is negligible. Finally,
assume that the sender cannot send a packet until the preceding one is
acknowledged. How long does it take to send the file? 11.
(a) (each part 1 point) What are the different layers of OSI?
(b) (10 point) Why does OSI have 7 layers (Not 1 or 50)?
12. (10 points)
What is the first piece of information that the source needs to know about
the destination? And what is the first task the source needs to perform before
sending the message? 13. (10 points)
Why does the diagram illustrate the peertopeer protocols for all 7 layers
with dashed lines? ...
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 Spring '06
 Zahid

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