## 4 Network Layer: The Main Building Block of Fixed Networks
### F1G PONs
#### 4.1.4 F2g--2000-2005 (Wideband Era)
A three-way handshaking takes place: A REGISTER_REQ message is generated by the ONU/ONT, the OLT replies with the REGISTER_ACK message, and by the REGISTER_ACK message, the ONU/ONT echoes the registration parameters back to the OLT. * _Report Handling: ONUs_/ONTs generate REPORT messages, whereby they transmit bandwidth requests to the OLT, which processes these requests to proceed with the corresponding bandwidth assignments. * _Gate Handling_: The GATE messages are used by OLT to assign the timeslots in which the users are allowed to transmit. These assignments are based on the bandwidth requirements of the users (REPORT messages). The first implemented DBA algorithm is the IPACT (interleaved polling with adaptive cycle time) [20], where the OLT polls ONUs/ONTs in a round-robin fashion to assign timeslots. After receiving the REPORT messages from all ONUs/ONTs, the OLT calculates the start and end times of each ONU/ONT (timeslot assignment) and sends them with GATE messages (Figure 7). Then, each ONU/ONT schedules the start and end times of packets in its queues. When an ONU/ONT does not have data to transmit, it will send a erPORT message with zero requirements. In this case, the OLT will calculate a timeslot duration equal to the time required to send only the next REPORT message from that ONU/ONT. Various methods (services) have been proposed to determine the timeslot duration of each ONU/ONT:  
* Fixed--The OLT ignores the ONU/ONT requirements and always grants the maximum timeslot duration. It results in a transmission of a constant/maximum cycle time. This method/service would be an easy choice in the traffic environment of multiple service-classes [21]. * Limited--The OLT grants the requested timeslot duration, but no more than a maximum value. This results in a transmission of the shortest cycle time. * Gated--The OLT grants the requested timeslot duration without a limitation. That is, each ONU/ONT transmits all data of its queue in one transmission cycle. This results in a significant increase of the transmission cycle time. For this reason, there is a limit to the queue length that each ONU/ONT possesses. * Constant credit--The OLT grants the requested timeslot duration plus a constant duration. This fixed quantity is added to include in the next ONU/ONT transmission the data that have reached the ONU/ONT from the time it sent the REPORT message until the beginning of its next transmission. * Linear credit--The OLT grants the requested timeslot duration plus a duration which is proportional to the requested one. * Elastic--In this service, there is no maximum duration for each timeslot, but a maximum limit on cycle time. If only one ONU/ONT has data to send, it can receive a timeslot duration equal to the sum of the maximum durations of all ONUs/ONTs. Several variations of the IPACT algorithm have been introduced, which, however, do not support differentiated services [22, 23]. Services should be differentiated according to their QoS requirements, such as bandwidth, latency, delay variation (jitter) and packet loss ratio. An effective way to provide differentiated QoS quality assurance to an EPON is to use priority queues. The network traffic is classified into different categories with corresponding QoS priority levels (802.1Q) and for each traffic category, a separate queue is placed in each ONU. The following eight traffic categories (at the link layer) are defined (priorities from high to low--802.1Q) [24]:  
1. _Network control_: Time- and safety-critical traffic of OAM (Operation Administration and Management) network functions (e.g., routing protocol frames)--no user traffic;
2. _Internetwork control_: No user traffic;
3. _Voice_: Time-critical traffic with a delay <10 ms (e.g., real-time voice);
4. _Video_: Time-critical with a delay <10 ms (e.g., real-time video);
5. _Controlled load_: Loss-sensitive traffic (e.g., streaming multimedia traffic);
6. _Excellent effort_: Loss-sensitive traffic;  
Figure 7: Example of REPORT and GATE messages to define ONU-timeslot durations.