  The circle travelling delay between OLT (Optical Line Terminal) and ONU
(Optical Network Unit) is one of most important items in dynamic bandwidth
allocation (DBA) algorithms in PON, called RTT (Round Trip Time). The RTT is
taken into account when OLT assigns the start times for upstream bandwidth
grants. In most case, RTT is estimated before making bandwidth allocation
decisions in dynamic bandwidth allocation algorithms. If the estimated RTT is
incorrect, the bandwidth allocation decisions are not matched with bandwidth
requests of channels. Thus, performance of PON can get worse by deviation of
RTT. There are several reasons that cause the RTT to be varying, such as
processing delay, distance of OLT and ONU, changing in fiber refractive index
resulting from temperature drift, and degree of accuracy of RTT estimation
methods. In this paper, we evaluate the impacts of RTT inaccuracy on
performance of DBA and identify levels of collision and waste of bandwidth. By
this way, we propose a method to remedy the performance degradation encountered
by the situation

Networking and Internet Architecture