Issue link: http://www.epageflip.net/i/1043933
November/December 2018 I 25 on-off keying (OOK) previously used in optical transmission. Instead of simply transmitting a 1 or 0, PAM-4 transmission doubles the amount of information that can be sent in a single time period by using four signal levels. PAM-4 signaling will be incorporated into new Ethernet and Fibre Channel standards, as well as MSAs and proprietary solutions as speeds increase. It will allow for 50 Gb/s per lane transmission using today's 25 Gbaud/s lasers, and as laser speeds increase further, will allow even higher lane speeds. Of course, nothing is free. PAM-4 signaling will require better receiver sensitivity than OOK in order to detect the different levels. As seen in Figure 2, four "eyes" will need to be detected, rather than the single eye found in OOK. The sensitivity requirements can be reduced using several compensation methods, including equalization and/or forward error correction. FIGURE 2: Comparison of OOK sensitivity to PAM-4. Latest Ethernet Standards The first Ethernet standards for 40 Gb/s (40GBASE-SR4) and 100 Gb/s (100GBASE-SR10) were adopted in 2010. Both used parallel fibers to support higher bit rates on short reach (SR) links (the "4" or "10" following the "SR" designates the number of fiber pairs in the link). Each fiber pair carried 10 Gb/s bi-directionally, so four pairs were needed to support 40 Gb/s and 10 pairs were used to support 100 Gb/s. Typically, a 12-fiber multi-fiber push on (MPO) connector was used as the interface for 40 Gb/s, while a 24-fiber MPO connector was used for 100 Gb/s (two 12-fiber MPOs could also be used). 40GBASE-SR4 has been widely deployed. Initially very popular for server breakout applications where one 40 Gb/s switch port was used to support four 10 Gb/s servers, native 40 Gb/s switch-to-switch links quickly followed. 40GBASE-eSR4 modules are also available to support longer distance links. While not defined in the standard, transceiver manufacturers are increasingly developing "beyond the standards" products that can support longer link distances found in some data centers. Other than some limited application in service provider central offices, 100GBASE-SR10 has been less widely deployed, in large part because newer standards requiring fewer fiber pairs have been developed. Both PMDs support up to 150 m link lengths on OM4, but as noted previously, 40 Gb/s transceivers are available that can support longer links. Soon after its introduction, it became obvious that 100GBASE-SR4 could be used in breakout situations similar to early 40 Gb/s installations, and a single-pair 25GBASE-SR standard was written in 2016. The application space is similar to that for 40 Gb/s breakout, with four 25 Gb/s servers supported by a single 100GBASE-SR4 switch port. Additional 200 and 400 Gb/s Ethernet solutions were introduced in 2017. Here, only one multimode PMD was included. 400GBASE-SR16 uses 16 fiber pairs, each transmitting at 25 Gb/s. This PMD is not expected to be widely deployed as there is reluctance on the part of end users to deploy 32 fiber cables to support a single link. Newer multimode Ethernet standards being developed will use fewer fiber pairs by incorporating PAM-4 technology and WDM. The second PMD is 400GBASE4.2. This will be the first standards-based application that will exhibit the reach advantage of wideband OM5 fiber.