4. 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 bandwidth-delay product, R‧tprop.
(b) Consider sending a file of 400,000 bits from Host A to Host B. Suppose the file is
sent continuously as one big message. What is the maximum number of bits that
will be in the link at any given time?
(c) Provide an interpretation of the bandwidth-delay product.
(d) What is the width (in meters) of a bit in the 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.
a) 40,000 bits
b) 40,000 bits
c) The bandwidth-delay product of a link is the maximum number of bits that can
be in the link
d) 1 bit is 250 meters long, which is longer than a football field
e) s/R
5. Refer to problem 4.
(a) How long does it take to send the file, assuming it is sent continuously?
(b) Suppose now 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 acknowledgment 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?
(c) Compare the results from (a) and (b).
a) ttrans + tprop = 400 msec + 40 msec = 440 msec
b) 10 * (ttrans + 2 tprop) = 10*(40 msec + 80 msec) = 1.2 sec
c) It takes a longer time to send the file if it is broken up into packets and each
packet has to be acked before the next one can be sent out.