This preview shows pages 1–2. Sign up to view the full content.
Armin Tavakoli Naeini
(StudenID:260414299)
Exercise 1:Time
1.
(a)
Since the time received by process P from the server is t=10 : 54 : 23 : 674
and t
roundtrip
=24 ms, we can find the time that P should be set to (t
p
)
from this
formula: t
p
= t + t
roundtrip
/2
=> t
p
=
10 : 54 : 23 : 686 (hr : min : sec : ms)
(b)
The accuracy is determined from the following formula: +/ (t
roundtrip
/2
min).
So in this case in which the min time is 3 ms would be +/ (12 – 3) = +/ 9 ms.
(c)
Since the Process’s time is more than the time to which we want to set , we can
not set back the clock because some applications (like
±²³´
in Linux)
could
timestamp events under the assumption that clocks always advance. (actually
the process P’s clock is 319 ms fast) (10 : 54 : 24 : 005)  (10 : 54 : 23 : 686)
= 319 ms
In our case we have the errant clock, let’s call it
E
and the hardware clock
H
(which is supposed to advance at a perfect rate). Now we can construct a
software clock (
S
) for the process P such that for example after
x
milliseconds
we can replace the errant clock with the software clock in good conditions:
S= c (ET) + T
, where T =
10 : 54 : 24 : 005
and
c
is
to be found
.
We know that when E = T + x then S should be: S= T + 319 ms (because we
want the S clock to be corrected after the determined time of
x
ms) so we have:
T+319 = c(T+ x  T) +T => c = 319/x . (note that we have the value of
x
).
So we reach the following formula for
S:
S= 319/x (ET) + T
(when T <= E <= T+x)
2. With Lamport clocks, nothing can be said about the relationship between
two events
a
and
b
This preview has intentionally blurred sections. Sign up to view the full version.
View Full Document
This is the end of the preview. Sign up
to
access the rest of the document.
This note was uploaded on 01/15/2011 for the course ECE 6161 taught by Professor Khkjk during the Winter '10 term at Concordia Canada.
 Winter '10
 khkjk

Click to edit the document details