## 3 The Third Industrial Revolution (TIR) and Energy Internet
### Real-Time Polling  
Next-generation long-reach PONs are expected to span optical ranges of up to 100 km. Moreover, long-reach PONs are expected to operate at a line rate of 10 Gb/s and accommodate 2,000 to 4,000 ONUs. The increased propagation delay of long-reach PONs may lead to a significantly increased idle time and delay if ONUs use conventional report-grant mechanisms, as specified in the multipoint control protocol (MPCP) of IEEE 802.3 ah EPON via the so-called REPORT and GATE messages. Figure 5a shows an example of conventional polling in an EPON with two ONUs. Conventional polling imposes a critical constraint on the downstream transmission of GATE messages: they may not be transmitted before the arrival of the REPORT message to the OLT. Since the OLT cannot generate a GATE before receiving a REPORT, the cycle duration in conventional polling cannot fall below the maximum round-trip time \(\text{RTT}_{\max}\) between the OLT and the most distant ONU. This feature is the crucial limitation of conventional polling: it places an upper-bound on the frequency of REPORTs and data transmissions emanating from any ONU. As a result, the average frame delay grows with increasing round-trip propagation delay. To illustrate this, consider the worst-case scenario where a frame is generated an infinitesimal time after the transmission of a REPORT message. In that case the frame experiences a delay equal to the cycle duration before its bandwidth requirement is reported to the OLT. The fact that the cycle duration is lower-bound by \(\text{RTT}_{\max}\) hence compounds the delay and capacity problem in next-generation long-reach PONs. The first candidate to alleviate the delay problem of conventional polling in next-generation long-reach PONs is the well-known multi-thread polling (MTP). MTP creates multiple interleaved polling cycle instances, _i.e._, \"threads\". This is illustrated in Figure 5b, where the REPORTs, GATEs, and data transmissions are identified by a thread number. Each thread is a complete polling cycle process, where all ONUs are polled once. MTP succeeds in reducing frame delays by giving ONUs multiple opportunities to report their queue size and to transmit data within the RTT time-period. Figure 5: Conventional and low-latency polling schemes. The second polling candidate for next-generation long-reach PON is our proposed _real-time polling (RTP)_, illustrated in Figure 5c. RTP uses an additional separate reporting channel that allows ONUs to report increases in their queue size in real time. By using a separate control channel, reporting can be done independently of the upstream data traffic transmission on the legacy upstream data wavelength channel, _i.e._, upstream data and control transmissions are decoupled. Each ONU has the opportunity to report a queue increment of \(\Delta_{Q}\) bytes every reporting period \(T_{s}\) by transmitting a Queue Increment Report (QIR) to the OLT. In addition, REPORTs continue to be issued at the end of each granted data transmission window on the conventional upstream wavelength channel. RTP can be realized by using optical coding (OC). OC-enabled ONUs apply remote encoding, reflection, and ON-OFF modulation of a pulse stream generated at the OLT. The applied codes may be orthogonal and the operation is performed in an interference-free environment. The minimum queue increment size is equivalent to the minimum frame size, leading to \(T_{s}=5\)\(\upmu\)s in 1G-EPON. This means that the pulse train is generated at a frequency of 200 KHz, which is practical using off-the-shelf switches. Smaller increment sizes do not yield higher performance as the Ethernet frames are not fragmented to lower granularity. These OC enhancements allow the OLT to receive out-of-band queue status updates, _i.e._, QIRs, every \(T_{s}\geq 5\)\(\upmu\)s, which is much faster than in conventional PONs, thus enabling more accurate grant sizing. Note that there are two crucial differences between QIRs and REPORT messages.