Epon system uplink using 1310nm wavelength, downlink using 1490nm wavelength, the use of optical fiber ITU-TG. 652 Fiber, we know that G. 652 fiber 0 dispersion wavelength is 1300~1324nm interval, the uplink wavelength right in this interval, Therefore, the spectral characteristics of ONU are not high, and the FP lasers can be used. For a long distance epon system, the Olt must use a narrow-spectrum DFB laser to reduce the dispersion cost for the downlink 1490nm not 0 dispersion wavelength range.
Three of the time in the Epon protocol is the most important one is the system's maximum RTT, one is the registration window time and one is the DBA polling cycle. When the extended transmission distance, the farthest from the Olt ONU the largest RTT, assuming that the furthest ONU is 70km, then the RTT is 2x (70000/2*10^8) =700US (the optical signal in the vacuum in the speed of 3x10^8m/s, in the optical fiber in accordance with the 2x10^8 meter), Therefore, in this case Epon system Registration window should be at least 700us. From the above analysis can be seen with the length of the transmission distance, the registration time will inevitably be lengthened. Another consideration is the DBA's polling cycle, which shows from Figure 6 that the DBA polling cycle should be at least greater than the system's largest RTT (that is, the farthest ONU round trip time), and that, in the long term, DBA efficiency is low, and in most cases, It is recommended that the SBA (static bandwidth allocation) algorithm be used instead of DBA.