## 5 Physical Layer: Five Decades of Evolution (from 45 Mb/s to 100+ Tb/s)
### Physical Layer Evolution in Access
#### 5.3.2 State-of-the-Art Technology in Optical Transceivers  
Representative commercially available transceivers for data center interconnection and access applications, along with their physical layer characteristics, are shown in Table 12. Various options of pluggable transceivers are illustrated in this table, each tailored to a different bit rate and transmission reach. Such pluggable modules with rates of 100G and beyond are attractive, as they can provide decreased power consumption while they require a smaller space compared with their lower rate counterparts. In addition, individual ports of a network element can be plugged with any needed type of transceiver.  
From Table 12, we can observe that the vast majority of transceivers focus mainly on O and C-band and, to a lesser extent, to parts of S and L-band. This is expected, as the more mature technology is focusing on the second and third transmission windows, where fiber dispersion and attenuation are minimized, respectively, resulting in cheaper components due to their mass production. However, the vast majority of the low-loss fiber spectrum, that is more than half of the O, S and L bands, and the whole spectrum of E and U-band, remain untapped. This designates a strong need to accelerate toward manufacturing cost-effective transceivers for these wavelengths in order to increase the number of supported users within a single fiber. This is especially important for cases with limited fiber availability when network operators have to pay a rent to a government body/agency in order to exploit fiber resources.  
Next, power consumption is an important issue, as its minimization directly leads to lower OpEx costs. Further, in PON links where optical amplification is not considered, a candidate cost-effective solution is the direct digital to optical domain (D/O) conversion instead of using the eDAC intermediaries [57, 58]. The main purpose of eDACs is to drive multilevel analog signals into the optical modulators; however, in the context of ultra-high-speed photonic interconnects, eDAC technology is increasingly expensive and is becoming increasingly difficult to keep scaling up the data rate. In this way, direct D/O conversion can provide significant advantages in terms of performance, energy efficiency, complexity and cost reduction.  
In overall, we can conclude that the \"wish-list\" for next generation transceivers includes lower power consumption, higher power budget, flexibility in terms of transmission wavelength and reach, via a low-cost platform.  
\begin{table}
\begin{tabular}{c c c c c c} \hline \hline
**Type** & **Band** & **Line Rate** & **Distance** & **Output** & **Sensitivity** \\ \hline \hline CFP8 Transceiver & O & 400 Gb/s & \textless{}2 km & N/A & N/A \\ CFP4/CFP2 & O & 100 Gb/s & \textless{}10 km & N/A & N/A \\ Transceiver & & & & & \\
200G and 400G & & & & & \\ Coherent & C & 400 Gb/s & N/A & \textgreater{}\textgreater{}\textgreater{}\textgreater{}\textgreater{}\textgreater{}\textgreater{}\textgreater{}\textgreater{}\textgreater{}\textgreater{} \textgreater{}\textgreater{}\textgreater{}\textgreater{}\