T4 - Enunciat problemes T4 Ethernet PDF

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Enunciat problemes T4 Ethernet...

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INFRAESTRUCTURES DE XARXA ETHERNET - HOMEWORK 1.

Considering the network of the following sketch, which is the occupation level of point A? switch# To#Internet#

A#

hub# switch# 20# computers#

switch# AP#and## switch# 20# computers#

AP#and## switch# 20# computers#

20# computers#

hub#

20# computers#

We know that: -

Ethernet links work at 100 Mbps. All computers receive 0,1 Mbps of data from Internet. All computers transmit 0,5 Mbps of data with the following destinations: -

2.

50% to Internet. 40% equally distributed among all computers of the network. 10% broadcast.

Two hosts (A and B) are connected to an Ethernet and we assume that both have unlimited data to transmit. The frames belonging to A shall be denoted as A1, A2, ..., An and the frames of B as B1, B2, ..., Bn. Backoff retransmissions are in the interval (0, CW-1). Where CW stands for Contention Window. A and B try to transmit frames A1 and B1 at the same time. The frames collide and both hosts select a random waiting time as specified by the standard (binary-exponential-backoff). Station A's waiting time is WA = 0 time slots and station B's is WB = 1 slots. A “wins” the contention for the medium access and retransmits frame A1 while B waits for A to finish. Station B sends frame B1 again after the transmission of A1 has finished. Unfortunately, frame B1 collides with A2 as A is also trying to transmit. a) Calculate the probability that A wins this contention after the second collision. b) Assume that station A wins the second contention and frame A2 is transmitted while B1 still has to wait. Calculate the probability for A also winning the next (third) contention. c) Calculate the probability of A winning the i-th contention, when A is in its first collision and B is in the i-th collision.

1

d) The term Capture Effect denotes a situation in which A wins all contentions after the first collision. Host B cannot send data until the maximum number of retransmissions has been reached and the current frame is dropped. Give the probability of this event. e) Is the capture effect still relevant in today's Ethernet based networks?

SOLUTIONS: 1. We compute all data flows: -

From Internet = 10 Mbps To Internet = 25 Mbps Broadcast = 5 Mbps To other computers = 0,5 Mbps * 0,4 * (40*60 + 20*80 + 20*80 + 20*99) / 99 = 15,31 Mbps

Total load = 55,31 Mbps Occupation level = 55,31%

2.

a) Prob[A wins] = 5/8. b) Prob[A wins] = 13/16. c) Prob[A wins] =

!"#$ %& . !"#$

d) Prob[Capture effect] = 10,38%. e) Nowadays Ethernet networks are mostly switched, therefore, there are no collisions and no capture effect.

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