In 2017, Russell Dearnley, senior manager of Data Center Connectivity at Panduit Corp, argued that data centres would only cope with the zettabyte era if the cabling between machines kept pace with traffic. His case was that fibre optics was the most efficient way to move large volumes of data, and that four technical developments were pushing speeds up.
His starting point was the Cisco Visual Networking Index forecast of that period. It projected that global IP traffic would nearly triple between 2016 and 2021. Annual traffic had passed 1 zettabyte (one billion terabytes) in 2016 and was forecast to reach 3.3 ZB a year by 2021, with video making up 82% of it. Cisco also expected cloud traffic to rise 3.7-fold, from 3.9 ZB a year in 2015 to 14.1 ZB by 2020, and for 92% of workloads to be handled by cloud data centres by then, against 8% in traditional ones.
The problem: moving data between machines
Dearnley said every data centre faced the same basic task of moving and handling all that data. In his words, operators needed pipes big enough to carry it from one machine to another. Applications and workloads were shifting to cloud architectures, and more devices were connecting to cloud and IP networks. Together these produced a volume, velocity and variety of data that he said was unprecedented.
Four ways to raise fibre speeds
The first was faster lasers. Dearnley said 10 Gbps had been about the fastest on-off switching rate until then. Lasers switching at 25 Gbps were starting to appear, and he expected 50 Gbps lasers within two to three years, a fivefold jump in a short period.
The second was parallel optics. A 40 Gb signal could be split into four 10 Gb streams sent over four fibres and recombined at the far end. A 100 Gb signal could likewise be divided into 25 Gb lanes and multiplexed back together.
The third was advanced modulation. Plain on-off keying is the simplest pulse amplitude modulation scheme. PAM-4 raises the number of amplitude levels from two to four, each carrying a two-bit value (00, 01, 10 or 11). That doubles the bit rate in the channel without needing more bandwidth.
The fourth was wavelength division multiplexing, which sends more than one wavelength down the same fibre. Dearnley pointed to Cisco’s BiDi optics as the first commercial example. They used one wavelength in each direction, and because the two do not interfere, a single fibre carried twice the data.
SWDM and the OM5 fibre standard
The SWDM Alliance was promoting shortwave wavelength division multiplexing as a cost-effective way to carry several channels over duplex multimode fibre at 40 Gbps and above. Its focus at the time was 40 Gbps, built from four 10 Gbps wavelengths multiplexed onto one fibre pair. Dearnley described SWDM as a combination of the four developments above.
To support SWDM, and at least legacy OM4 applications, the Telecommunications Industry Association published a specification for Wideband Multimode Fiber (WBMMF) in June 2017. It was to be called OM5 in the third edition of ISO/IEC 11801, the generic cabling standard for customer premises.
Single-mode versus multimode
Dearnley also noted that hyperscale and web-scale operators, who deploy millions of circuits, were using more single-mode links. As volumes grew, the price gap between multimode optics, then the usual choice inside data centres, and single-mode optics, which offer longer reach and more bandwidth, was narrowing.
He did not expect multimode to disappear. His view was that the two would serve different applications, and that single-mode would become more significant, particularly in large facilities. The main obstacle was bringing single-mode optics down to a cost that data centres could justify.
Where the OM5 debate went
Later technical material shows how the OM5 definition was settled. OM5 is specified at both 850 nm and 953 nm. It requires 4,700 MHz·km of modal bandwidth at 850 nm, the same as OM4, plus 2,470 MHz·km at 953 nm. It was created to support SWDM, whose four-wavelength variant (SWDM4) uses 850, 880, 910 and 940 nm over one duplex fibre pair.
Whether the extra cost pays off remains contested. Cabling vendor Siemon has pointed to a Cisco white paper concluding that OM5 gives no reach advantage with most Cisco multimode transceivers.
The speed ladder Dearnley described has also kept moving. One engineering guide describes the 800GBASE-SR8 specification being worked on under IEEE 802.3df as using eight multimode lanes at 100G PAM-4 each, so PAM-4 and parallel optics both remain in use.