Introduction to the GPON System
Mainstream PON technologies include broadband passive optical network (BPON), Ethernet passive optical network (EPON), and gigabit passive optical network (GPON). Adopting the ATM encapsulation mode, BPON is mainly used for carrying ATM services. With the obsolescence of the ATM technology, BPON also drops out. EPON is an Ethernet passive optical network technology. GPON is a gigabit passive optical network technology and is to date the most widely used mainstream optical access technology.
- In the GPON network, such as GPFD, the OLT is connected to the optical splitter through a single optical fiber, and the optical splitter is then connected to ONUs. Different wavelengths are adopted in the upstream and downstream directions for transmitting data. Specifically, wavelengths range from 1260 nm to 1360 nm in the upstream direction and from 1480 nm to 1500 nm in the downstream direction.
- The GPON adopts WDM to transmit data of different upstream/downstream wavelengths over the same ODN. Data is broadcast in the downstream direction and transmitted in the TDMA mode (based on timeslots) in the upstream direction.
GPON Downstream Transmission
All data is broadcast to all ONUs from the OLT. The ONUs then select and receive their respective data and discard the other data.
Main features:
- Supports point-to-multipoint (P2MP) multicast transmission.
- Broadcasts the same data to all ONUs and differentiates ONU data by GEM port ID.
- Allows an ONU to receive the desired data by ONU ID.
GPON Upstream Transmission
In the upstream direction, each ONU can send data to the OLT only in the timeslot permitted and allocated by the OLT. This ensures that each ONU sends data in a given sequence, avoiding upstream data conflicts.
Main features:
- Supports time division multiple access (TDMA).
- Transits data on an exclusive timeslot.
- Couples optical signals on an optical splitter.
- Detects and prevents collisions through ranging.
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