## 2 Beyond 100 Gb/S Era
### Ethernet Standards and Optical Transceiver Applications
#### 2.3.2 Ethernet Standards and Optical Transceiver Applications  
The legacy Common Public Radio Interface (CPRI) and Open Base Station Standard Initiative (OBSAI) lines mainly focus on 3G/4G mobile fronthaul (FH) networks. Moreover, the legacy CPRI/OBSAI line has standard temperature modules (0 to 70 \({}^{\circ}\)C) that can be used in conventional telecom environments. Industrial temperature (\(-\)45 to 85 \({}^{\circ}\)C) modules can also be deployed in harsh environments such as unheated outdoor cabinets and mobile BS towers. In the 3G/4G networks, high networking speeds are not required between the remote radio unit (RRU) and the baseband unit (BBU) compared with 5G mobile network configurations. Consequently, 10G FH SFP+ optical transceivers can be used to satisfy the transmission requirements. Furthermore, based on the base station (BS) manufacturer standard preference, these transceivers can be coded either for the CPRI or OBSAI bit rates. CPRI Options 2 to 8 with line bit rates that range from 1228.8 Mbps to 10.138 Gbps and OBSAI RP3 \(\times\) 2 to RP3 \(\times\) 8 with rates of 1.536 Gbps to 6.144 Gbps can be employed to support 10G-FH. Moreover, with the heterogeneous wireless network evolution, the demarcation point between the FH, midhaul (MH), and backhaul (BH) is becoming indistinct and thin. This imposes stringent requirements on mobile transport networks, demanding the redesign of the interfaces [1]. Therefore, it is imperative to develop an innovative transport network architecture, capable of supporting the multiple _hauls_ simultaneously, as depicted in Figure 1. In this regard, the transport network should be able to support heterogeneous transmission media such as high-capacity copper, optical (e.g., wireless/fiber-based), and wireless (e.g., mmWave/THz) for the network elements' interconnection. Furthermore, they should be able to converge to support different FH link solutions such as the existing protocols (CPRI) and 5G-specific protocols (eCPRI, Next-Generation Fronthaul Interfaces (NGFI)), envisaged 6G-specific protocols, latency-sensitive Ethernet, Ethernet-based backhaul, and generic packet-based solutions [68]. This results in the idea of an integrated transport network known as the \"any-haul or X-haul\" for all-generation (X-G) coexistence (3G/4G/5G/6G) [69]. The need to support X-haul in the 5G and beyond networks imposes new demands on optical transceivers. In this context, there is a growing need for high-rate optical transceivers as a higher cell site density is required. Therefore, as summarized in Table 2, 25 G/100 G optical transceivers are required for the 5G optical FH network to support massive MIMO, while carrier-grade 100 G/200 G/400 G transceivers are required for the MH/BH. In view of this, eCPRI is a better solution compared with CPRI/OBSAI [70]. In Section 3.3, we present various enabling technologies for the optical transceivers to support 5G and beyond applications. \begin{table}
\begin{tabular}{c c c c c c c} \hline
**Speed** & **Nomenclature** & **Standard** & **Medium** & **Form Factor** & **Interface** & **Reach** \\ \hline
400 GBASE-SR8 & 802.3 cm & Multimode Fiber & OSFP-DD & MPO (16) & OM3\(-\)70 m \\  & & (@850 nm) & QSFP-DD & MPO (2 \(\times\) 12) & OM4\(-\)100 m \\ \cline{2-6}  & 400 GBASE-DR4 & 802.3 bs & Single-mode Fiber & CFPS & MPO 12 \({}^{\ast}\) & GS2—500 m \({}^{\ast}\) \\
400 GbE (400 Gb/s) & & (WDM) & QSFP-DD & SN Connector \({}^{\ast}\) & \\ \cline{2-6}  & 400 GBASE-FR4 & 802.3 cu & Single-mode Fiber & CFPS & LC Duplex \({}^{\ast}\) & OS2—2 km \({}^{\ast}\) \\  & & (WDM) & QSFP-DD & & \\ \cline{2-6}  & 400 GBASE-2 FR4 & 802.3 bs & Single-mode Fiber & OSFP & CS Connector & OS2—2 km \\ (2 \(\times\) 200 C-FR4) & & (WDM) & QSFP-DD & & \\ \hline \end{tabular} \({}^{a}\) Widely used transceivers. SR: Short range; LR: Long range; ER: Extended range; BIDI: Bidirectional; SFP+: Enhanced small form-factor pluggable; QSFP: quad small form-factor pluggable; CFP: C form-factor pluggable; WDM: Wavelength division multiplexing.  
\end{table} 
Table 1: Cont.