# What is the average time for a node to transmit on the shared medium for Ethernet? (IEEE 802.3)

Suppose a shared ethernet link L, in which for nodes T1-T4 and a router R are connected:

--------------------------- R
|     |     |     |
T1    T2    T3    T4


L has bit-rate C=100 Mbps, and propagation delay is PROP = 10 msec. Every node transmits packet with probability Pb = 10%. The mean packet length is P(length)=100 KBytes

Note that ethernet uses CSMA/CA

What is the average time for a node to transmit on the shared medium?

What I've tried:

We know that on carrier sense WAIT_BEFORE_RETRANSMITTING = 5*PROP = 50 ms.

We also know transmission_delay = P(length) / C = 8 ms in our case

So, based on those results, I suppose that the average time for a node to transmit on the shared medium is 50 + 8 = 58 ms

• What did you try? Where did you get stuck? We're happy to help you understand the concepts but just solving exercises for you is unlikely to achieve that. You might find this page helpful in improving your question. – dkaeae Aug 23 '19 at 11:08
• @dkaeae I added my attempt, but I'm so unsure about it I initially chose to not include it in the post. Thanks for the tip in any case – Dimitris Aug 23 '19 at 11:30