## 2 Related Work
### QSFP28 Optical Transceiver Hardware Standard  
The QSFP28 optical transceiver complies with the QSFP multi-source agreement (QSFP-MSA) specification. SFF-8665 is a standard document for QSFP28 optical transceiver solutions [15]. The QSFP28 specification documents to be referenced for the manufacture of optical transceivers consist of documents on the electrical characteristics of optical transceivers, instrument specifications such as optical transceiver connectors and sizes, cage specifications on the host side to operate with inserted optical transceivers, and management interface specifications that can be viewed as software specifications. The SFF-8679 specification describes the hardware and electrical characteristics that the QSFP28 optical transceiver must fulfill and defines the connector specification for the QSFP28 optical transceiver; the function of each pin; and the interface for the data, control, and status signals of the optical transceiver [16]. In addition, the electronic static discharge (ESD) specification and the instrument-related specifications of the optical transceiver as well as the temperature and timing specifications are defined. Figure 2 shows the QSFP28 optical transceiver and host interface structure proposed in the standard document.  
The hardware configuration of the QSFP28 optical transceiver consists of a quad optical transmitter and quad optical receiver for transmission and reception, a clock data recovery (CDR) IC for processing the transmission signals, a micro-controller unit (MCU) for operation of the QSFP28 optical transceiver, and a host connector for sending and receiving control and data signals with the host. There are up to four channels for data  
Figure 1: Comparison of non-return-to-zero (NRZ) and level-four pulse-amplitude modulation (PAM4) signals [3].  
transmission and reception. The functions of control and alarm signals are summarized in Table 2. The power consumption of the QSFP28 optical transceiver is divided into eight classes according to the available host system capacity, with each class defining the maximum allowable power. If the optical transceiver's power consumption exceeds the specified consumption capacity, the QSFP28 optical transceiver is applied with an LPMode signal to enter the low-power state. The QSFP28 optical transceiver in a low-power state should have a power consumption of less than 1.5 W. Another standard document defines temperature specifications for normal operation of the optical transceiver from a minimum of 0 \({}^{\circ}\)C to a maximum of 70 \({}^{\circ}\)C based on the QSFP28 optical transceiver case. Finally, the timing specification for the response speed of the QSFP28 optical transceiver according to the hardware control signal of the host and the timing specification for alarm generation are defined.  
\begin{table}
\begin{tabular}{c c} \hline \hline
**Name** & **Function** \\ \hline ModPrsL & Optical transceiver insertion status to host \\ SCL/SDA & Two-wire serial interface \\ ModSell. & Module selection, when using multiple modules \\ Resell & Optical transceiver reset \\ intL & Optical transceiver alarm interrupt \\ RxLOSL & Optical transceiver loss of signal \\ LPMode & Low power mode enabled \\ TxDis & Disable the optical transmission \\ \hline \hline \end{tabular}
\end{table}
Table 2: Control and alarm signals with host connector for QSFP28 [16].  
Figure 2: QSFP28 interface block diagram between host system and transceiver [16].