Power Leveling on PONs

Power leveling attempts to equalize the signal levels an OLT receives from its associated ONUs. An OLT receives separate bursts of transmission from each of its associated ONUs during an upstream frame transmission time, and each ONU signal will experience a different level of attenuation on the PON depending on the distance from the OLT.

Besides distance from the OLT, another factor that affects the OLT’s received signal levels is variations is ONU transmit level. Each ONU may transmit at a different nominal level owing to use of different optical transmitters and variations in manufacturing for the same type of transmitters.

ONUs can be anywhere from 0 to 20 km from the OLT, and the attenuation of the intervening fiber makes the received light arrive at very different levels at the OLT. Typical fiber used in access networks (such as SMF-28)  introduces about 0.35 dB of loss for each kilometer of distance at 1310nm, which is the transmit wavelength for ONUs on a GPON. With a differential range of 20km on a PON, an ONU at 20 km of distance from the OLT may experience 7 dB of additional loss compared to an ONU at 0 km. An ONU situated far from an OLT (say greater than 15 km) can use a higher optical transmit power, and an ONU close to the OLT (say less than 5 km) can use a lower optical transmit power to achieve approximately the same receive signal level at the OLT. The graphic below shows how power leveling can reduce the range of optical levels received at the OLT.

There are two methods for initiating a change of power level in an ONU: the ONU can initiate the level change or the OLT can initiate the level change. The ONU will initiate a level change when that ONU responds to a certain number of serial number requests without a message from the OLT assigning an ONU ID. And it will repeat this process until it receives a proper message assigning it an ONU ID. The OLT will request that an ONU change its transmitted power level when the OLT measures an unacceptably high BER for transmissions received from this ONU.

Power leveling  reduces the range of signal levels an OLT must handle, and it can allow for lower cost optics at the OLT. Without power leveling, the OLT must be able to handle a greater range of receive signals while maintaining a very low bit error rate (BER).

© 2009, The Product Group LLC. All rights reserved.

Related posts:

1. Extending GPON’s Reach Carriers have two fundamental options for extending the reach of a Gigabit PON (GPON) network. One option is to deploy GPON Optical Line Terminals (OLTs)...