## 4 Network Layer: The Main Building Block of Fixed Networks
### F1G PONs
#### 4.1.7 F5g--2018--Today (Gigaband Era)  
**50G-EPON--IEEE802.3 ca:** New reasons for a new PON standard emerge as technology and communication needs evolve. On the one hand, the number of users per fiber must be increased (for economic reasons); on the other hand, more and more users need larger and dedicated bandwidth for several reasons, including wireless fronthaul. Another motivation is to reach these targets at a reasonable cost, lower than the cost of NG-PON2. Given that Internet traffic is ubiquitous, a new EPON standard emerged (2020) at 50 Gb/s (50G-EPON) [11,34]. _Splitting ratio and Maximum reach:_ 50G-EPON offers backward compatibility with EPON and 10G-EPON, having the same minimum splitting ratio of 1:32 and a typical reach of 20 km. _DS/US Rates and Wavelengths:_ To minimize the system cost in 50G-EPON, ONUs/ONTs need to support only one or two fixed wavelengths (avoiding tunable transceivers). The transmission rate is 50 Gb/s in both downlink and uplink, or 50 Gb/s in downlink and 25 Gb/s or 10 Gb/s in uplink. Moreover, 50G-EPON supports coexistence with GPON (1290-1330 nm upstream) and XG(S)PON: Two downlink wavelength ranges (1356-1360 nm or 1340-1344 nm) and three uplink wavelength ranges (1260-1280 nm, 1290-1310 nm, or 1318-1322 nm) are supported. _QoS:_ The multi-point MAC control (MPMC) sublayer of the Data Link Layer contains the protocols required to reconcile the 50G-EPON into the Ethernet framework, i.e., allocation of transmission resources, ONU/ONT discovery and registration as well as DBA. To achieve high data rates, the innovative technique of dynamic wavelength channel bonding is applied. The so-called MCRS (Multi-Channel Reconciliation Sublayer) enables data in one MAC instance to be transmitted over multiple signals (envelopes) on multiple MCRS channels. The basic transmission unit, called Envelope Quantum (EQ), is 72 bits (64 bits of data, plus 8 bits). Multiple EQs formulate a frame. Frames are distinguished in full frames and in fragmented frames (such as the IPv4 fragmentation). A transmitted signal is an envelope that contains frames, i.e., EQs. EQs in one envelope are interleaved over multiple MCRS channels to achieve a higher transmission rate. As far as the transceiver optics are concerned, 25 Gb/s avalanche photodiodes (APD) with non-return-to-zero (NRZ) modulation are adopted for receivers, while the optical modulation amplitude (OMA) minus penalty methodology is adopted for transmitters. A major challenge is the introduction of burst-mode transmission from ONUs/ONTs. To this end, ONUs/ONTs require burst-mode laser diodes with a fast on-off switch and control loop for laser bias current. In addition, burst-mode receivers of high sensitivity are required in both OLT and ONUs/ONTs. The following specifications are given for the 50G-EPON standard: 128 ns maximum values for laser on/off time, 800 ns receiver settling time, and 400 ns clock data recovery time. **50G TDM-PON--ITU-T Rec. G.9804.3:** As was anticipated, the ITU-T followed IEEE in extending the transmission rate of PONs to 50 Gb/s [35, 36, 37]. The ITU-T has announced the 50G TDM PON as an evolution of GPON and XG(S)PON [36]. _Splitting ratio and Maximum reach:_ The passive device is connected to the OLT with a single-mode optical fiber over a distance of 20 km at least (10 km in the case of wireless, latency sensitive applications--for instance, 5G), reaching 60 km with reach extenders. The maximum difference in fiber lengths is set to 40 km. The minimum splitting ratio is 1:256 (using Reed-Solomon-based Forward Error Control). _DS/US Rates and Wavelengths_: The 50G TDM PON offers a transmission rate of 49.7664 Gb/s \(\cong\) 50 Gb/s both in the downlink and uplink, and asymmetric transmission at 50 Gb/s in the downlink (wavelength in the range 1340-1344 nm) and 24.8832 \(\cong\) 25 Gb/s in the uplink. Alternatively, the asymmetric transmission in the uplink is 12.4416 \(\cong\) 12.5 Gb/s or 9.95328 \(\cong\) 10 Gb/s; additional asymmetric transmission rates combined under the TDMA scheme are possible.