## 4 Network Layer: The Main Building Block of Fixed Networks
### F1G PONs
#### 4.1.4 F2g--2000-2005 (Wideband Era)
_Best effort_: No time-critical or loss-sensitive traffic (typical Internet traffic);
* 8. _Background traffic._ Data packets from users are routed to the ONU and placed in the corresponding queues depending on the traffic category. If the corresponding queue is full when a high priority packet arrives, then this packet replaces a low priority packet. Moreover, if on arrival of a low priority packet, the corresponding queue is full, this packet is lost. Therefore, DBA algorithms that can provide differentiated QoS support for different types of data traffic are needed. To this end, the DBA algorithms were extended to provide QoS support. **GPON--ITU-T Rec. G.984.x**: The ever-increasing bandwidth requirements of the users and the need to support not only the ATM traffic but, mainly, the Internet traffic were the motivation for the development of Gigabit-PON (GPON) (2004) as an expansion of the BPON. GPON concurrently supports and leverages SONET, SDH and Generic Framing Protocol (GFP). _Splitting ratio and Maximum reach:_ The maximum splitting ratio is 1:128, and the maximum ONU-OLT distance is 20 km. _DS/US Rates and wavelengths_: The downlink rates are 1244.16 Mb/s or 2488.32 Mb/s using the spectral region of 1480-1500 nm, while the uplink rates are 155.52, 622.08 and 1244.16 Mb/s in the spectral region of 1260-1360 nm [6, 8, 10, 16]. In addition, the symmetric uplink-downlink rate of 2488.32 Mb/s is used. Moreover, GPON supports both ATM (fixed-length packets) and Ethernet (variable-length packets) traffic by GEM (GPON encapsulation method, ITU-T Rec. G.7041). Using GEM frame fragmentation at the sender, GPON reduces the latency and thus efficiently supports real-time traffic [6, 10]. _QoS_: The GPON frame has a constant duration of 125 \(\upmu\)s [16]. The mode of transmission is broadcast (TDM) in the downlink and TDMA in the uplink [10]. Since the distances between the OLT and ONUs/ONTs are dissimilar, coordination is needed in the uplink so that the transmitting frame matches the time of the assigned time slot in the TDMA process. To this end, the OLT communicates with each ONU/ONT and, based on the round-trip time (RTT), determines a delay for each ONU/ONT in reverse order of the RTT so that the propagation delay between the OLT and ONUs/ONTs is equalized. In uplink, a QoS guarantee is provided in a similar way to BPON, based on SLA. An uplink frame includes a burst of time slots, which is called T-CONT (transmission container). A T-CONT contains ATM cells or GEM frames exclusively (not both). Each ONU/ONT assigns traffic flows with the same characteristics to one or more T-CONT types out of five possible T-CONT types. The four types of T-CONTs correspond to four traffic priority classes, while the fifth type corresponds to any other traffic class (based on the available bandwidth and combination of T-CONTs). Each T-CONT must be uniquely identified in the GPON by the so-called Alloc_ID [10]. Based on Alloc_IDs, the uplink bandwidth is dynamically assigned to the users according to the SLA and the applied DBA [25]. _Security:_ Security is also provided in a similar way with BPON; the data transmitted down to ONUs/ONTs are encrypted based on the AES mechanism [6].