27Oct/110

## RES1 Data Link Layer (9/12)

The exercises shown that Send and Wait protocol does not offer good performances when propagation delay is long compared to transmission time. In that case it is better to send several unacknowledged frames instead of waiting for acknowledgments before sending another frame. We study HDLC protocol which can be view as the ancestor of all other anticipation protocols. There is an error in the slides. Instead of  an attentive reader will have noticed that the exact formulae is $\sum_{i=0}^{\infty} i.x^i=\frac{x}{(1-x)^2}$. But the result is correct.