CS 356 (Lam), Fall 2013

Homework 2 (due at beginning of class on Sept. 25, 2013, Wed.)


PROBLEM 1  (20 points total)

Consider 6 jobs that have gone through a system during the
time interval [0, 15], where time is in seconds, as follows:

	Job	Arrival Time	Departure Time

	 1	     0.5	     4.5

	 2	     1.5	     3.0

	 3	     6.0	    11.0

	 4	     7.0	    14.0

	 5	     8.5	    10.0

	 6	    12.0	    13.0	

For the time duration [0, 15]:

(a)  calculate throughput rate;

(b)  plot number of jobs in system as a function of time from 0 to 15 and
     calculate the average number over the duration [0, 15];

(c)  calculate the average delay of the 6 jobs.

Verify that Little's Law is satisfied by the results in (a), (b) and (c).

Note: For Problems 2 and 3 below, assume that packet lengths have an 
Exponential distribution so that average delay formulas for the M/M/1 
special case are applicable.

PROBLEM 2  (20 points total)

Consider two single-server systems. System A has an arrival rate of
60 packets/second and system B has an arrival rate of 80 packets/second.
Each server has a transmission rate of 100 kilobits/second, and an 
average packet length of 1000 bits.

(a)  Calculate the average delay (waiting plus service) for packets
     of system A.  

(b)  Repeat the calculation in part (a) for system B.

(c)  What is the average number of packets (queueing and being served)
     in both systems A and B.

(d)  Apply Little's Law to calculate the average delay over all
     packets served by both systems A and B.  (You are required to 
     apply Little's Law, instead of using some other approach.
     This requires some thinking.)
     HINT: Think of a boundary that surrounds both system A and 
           system B.  You make observations about all arrivals and 
           all departures in and out of this boundary.
     


PROBLEM 3  (10 points)

Suppose the two packet sources of systems A and B are combined
and served by a single queue and a single server.  What is the 
transmission rate of this server that will provide an average delay
equal to that in part (d) of Problem 2 ?