In a GPON Network, upstream and downstream data packets are transmitted in wavelengths in the 1290-1330nm and 1480-1500nm ranges respectively. The GPON system uses the wavelength division multiplexing (WDM) technology to transmit different wavelengths in the upstream and downstream directions on the same ODN network. That is, a single fiber carries data of different wavelengths in the upstream and downstream directions at the same time.
GPON Downstream Transmission
In the GPON downstream transmission, the backbone optical fiber from the OLT GPON port is divided into N branch fibers through an 1:N optical splitter, and the data is transmitted to branch fibers in broadcast mode. Each branch fiber bears the same data as the backbone fiber. After broadcast data reaches an ONU, the ONU only accepts data intended for it, and discards data not intended for it.
GPON Upstream Transmission
In the GPON upstream transmission, data from different ONUs is transmitted upstream through the respective branch optical fibers.The data is converged after passing through optical splitters. In the upstream direction, time division multiplexing (TDM) is used to transmit data.The upstream link is divided into different timeslots. The timeslot for each ONU to transmit data is centrally scheduled and authorized by the OLT.
An ONU can transmit is data upstream only in the timeslot allocated to it by the OLT。Even if there is no data to be transmitted，the ONU needs to send a blank frame.If an ONU does not comply with the rule and sends data in a timeslot allocated to another ONU, data collision occurs, causing packet loss. A timeslot is a microscopic concept.Generally, the traffic or bandwidth is used to indicate the uplink transmission authorized obtained by an ONU. How to allocate upstream timeslots(namely bandwidth)is an important technology of the OLT. Dynamic bandwidth assignment(DBA) servers this purpose.
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