-
第一章
(1.2
1.3
节
)
ate the total time required to transfer
a 1,000-KB
?
le in the
following cases, assuming an
RTT of 100
ms, a packet size of 1-KB data, and an initial 2
×
RTT of
< br>“
handshaking
”
before
data is sent.
(a)
The bandwidth is 1.5 Mbps, and data
packets can be sent continuously.
(b)
The bandwidth is 1.5 Mbps,
but after we
?
nish sending
each data packet
we must wait
one RTT before sending the next.
(c)
The
bandwidth is
“
in
?
nite,
”
meaning
that we take transmit time to be
zero, and up to 20 packets can be sent
per RTT.
(d)
The
bandwidth is
in
?
nite, and during the
?
rst
RTT we can send
one
packet
(21
?
1), during the second
RTT we can send two packets
(22
?
1),
during the third we can send four
(23
?
1), and so on.
(A
justi
?
cation
for
such an exponential
increase will be given in Chapter 6.)
7. Consider a point-to-
point link 2 km in length. At what bandwidth would
propagation delay (at a
speed
of
2
×
108m/sec)
equal
transmit
delay
for
100-byte
packets?
What
about
512-byte
packets?
“
wide
”
is a bit on a 1-Gbps link? How long is
a bit in copper wire, where the speed of
propagation is 2.3
×
108
m/s?
e a 100-Mbps point-to-point link is
being set up between Earth and a new lunar colony.
The distance from the moon to Earth is
approximately 385,000 km, and data travels over
the link
at the speed of
light
—
3
×
108 m/s.
(a) Calculate the minimum RTT for the
link.
(b)
Using the RTT as the delay, calculate
the delay
×
bandwidth product for
the link.
(c)
What is the
signi
?
cance of
he
delay
×
bandwidth
product
computed
in (b)?
(d)
A
camera on the
lunar base takes pictures of Earth and saves them
in digital
format to disk. Suppose Mission Control
on Earth wishes to download the
most current
image, which is 25 MB. What is the minimum amount
of
time that will elapse between when the
request for the data goes out and
the transfer is
?
nished?
18. Calculate
the latency (from
?
rst bit
sent to last bit received) for the following:
(a)
A
10-Mbps Ethernet with a single store-and-forward
switch in the path,
and a packet size of 5,000
bits. Assume that each link introduces a propaga-
tion delay of 10 ?
s, and
that the switch begins retransmitting immediately
after it has
?
nished receiving the
packet.
(b)
Same as (a) but with three switches.
(c)
Same as (a)
but assume the switch implements
“
cut-
through
”
switching: it
is able to begin
retransmitting the packet after the
?
rst 200 bits have been
received.
第二章
（除
2.7
2.9
节）
the
NRZ,
Manchester,
and
NRZI
encodings
for
the
bit
pattern
shown
in
Figure
2.46.
Assume that the NRZI signal starts out
low.
er an ARQ
algorithm running over a 20-km point-to-point
?
ber link.
(a)
Compute the
propagation delay for this link, assuming that the
speed of
light is 2
×
108
m/s in the
?
ber.
(b)
Suggest a suitable timeout value for
the ARQ algorithm to use.
(c)
Why
might
it
still
be
possible
for
the
ARQ
algorithm
to
time
out
and
retransmit a frame, given this timeout
value?
text
suggests that the sliding window protocol can be
used to implement
?
ow
control. We
can
imagine
doing
this
by
having
the
receiver
delay
ACKs,
that
is,
not
send
the
ACK
until
there
is
free
buffer
space
to
hold
the
next
frame.
In
doing
so,
each
ACK
would
simultaneously acknowledge the receipt
of the last frame and tell the source that there
is now free
buffer space available to
hold the next frame. Explain why implementing
?
ow control in this way
is not a good idea.
A
and
B be two stations attempting to
transmit on an Ethernet. Each has steady queue of
frames ready to send;
A
’
s frames will be numbered
A
1, A2 , and so on, and
B
’
s similarly. Let T
= 51.2 ?
s be the exponential
backoff base unit. Suppose A
and B
simultaneously attempt to send
frame
1,
collide,
and
happen
to
choose
backoff
times
of
0
×
T
and
1
×
T,
respectively,
meaning
A
wins the race and
transmits A
1 while
B waits.
At the end of this transmission, B will
attempt to retransmit B1 while
A
will attempt to transmit A2 . These
?
rst attempts will collide,
but
now A
backs off for
either 0
×
T or 1
×
T, while
B backs off for time equal to one of 0
×
T, . . . , 3
×
T.
(a) Give the
probability that A
wins this second
backoff race immediately after this
?
rst
collision ,
that is, A
’
s
?
rst choice of backoff time
k
×
51.2 is less
than B
’
s.
(b)
Suppose A
wins this second
backoff race. A
transmits A
3 , and when it is
?
nished, A
and B
collide again as A
tries to transmit A4
and B tries once
more to
transmit B1. Give the probability that
A
wins this third backoff
race immediately after the
?
rst collision.
(c)
Give a reasonable lower bound for the
probability that A
wins all the re-
maining backoff
races.
(d)
What then
happens to the frame B1?
This scenario is known as
the Ethernet capture effect.
48. Repeat the
previous exercise, now with the assumption that
Ethernet is p -persistent with p
=
0.33
(that is, a waiting station transmits
immediately with probability p
when the line goes idle,
and
otherwise
defers
one
51.2-?
s slot
time
and
repeats
the
process).
Y
our
timeline
should
meet
criterion
(1)
of
the
previous
problem,
but
in
lieu
of
criterion
(2),
you should
show
at
least
one
collision and at least
one run of four deferrals on an idle line. Again,
note that many solutions are
possible.
第三章
（
3.1
3.2
节）
the
example
network
given
in
Figure
3.30,
give
the
virtual
circuit
tables
for
all
the
switches
after
each
of
the
following
connections
is
established.
Assume
that
the
sequence
of
connections is cumulative, that
is, the
?
rst
connection is still up when the second connection
is
established, and so on. Also assume
that the VCI assignment always picks the lowest
unused VCI
on each link, starting with
0.
(a)
Host
A
connects to host B.
(b)
Host C connects to host
G
.
(c)
Host E connects to host I.
(d)
Host D connects to host B.
(e)
Host F
connects to host J.
(f)
Host H connects to host A.
the
network
given
in
Figure
3.31,
give
the
datagram
forwarding
table
for
each
node.
The
links
are
labeled
with relative
costs;
your
tables should
forward
each
packet
via
the
lowest-cost
path to its destination.