(c)(4 pt.) In CSMA/CD, after the fifth collision, what is the probability that a node chooses K=4? The result K=4 corresponds to a delay of how many seconds on a 10Mbps Ethernet?
(d)(6 pt.) Suppose nodes A and B are on the same 10 Mbps Ethernet bus, and the propagation delay between the two nodes is 245 bit times (i.e. time to transmit 245 bits). Suppose A and B send frames at the same time, the frames collide, and then A and B choose different values of Kin the CSMA/CD algorithm. Assuming no other nodes are active, can the retransmission from A and B collide? For our purposes, it suffices to work out the following example. Suppose A and B begin transmission at t=0 bit times. They both detect collisions at t=245 bit times. They finish transmitting a jam signal at 245+48=293 bit times. Suppose KA=0 and KB=1. At what time does B schedule its retransmission? At what time does A begin transmission? (Note: The nodes must wait for an idle channel after returning to Step 2 – see protocol. Also, the node must sense idle channel for 96 bit times before it transmits.) At what time does A’s signal reach B? Does B refrain from transmitting at its scheduled time? ririrpqppqpqpipT-+=--+=å¥=-11)1)(1(22

Chuah and Sabidur
ECS152A/EEC173A
4
Time,
Event
0
and
begin transmission
245
and
detect collision
293
and
finish transmitting jam signal
293+245 = 538
538+96=634
's last bit arrives at
;
detects an idle
channel
A starts transmitting
293+512 = 805
(805+96=901)
634+245=879
B returns to Step2
B must sense idle channel for 96 bit times before
it transmits
A’s transmission reaches B
Because
's retransmission reaches
before
's scheduled retransmission time (805+96),
refrains from transmitting while
retransmits. Thus
and
do not collide. Thus the
factor 512 appearing in the exponential backoff algorithm is sufficiently large.
t
A
B
A
B
A
B
B
A
A
A
B
B
B
A
A
B