Physical Layer Innovation: Silicon Photonics
So in two previous posts, I discussed the innovations required at the transport, network, and link layer of the communications protocol stack to take advantage of 100Gb/s networks . Let’s now talk about the physical layer. A 100Gb/sec signaling rate implies a 10ps symbol period.
Frankly, this is just not possible on a commercial basis with current technology. It is not practically feasible on copper nor on optical interfaces. At this rate the electrical and optical pulses just can’t travel any useful distance without smearing into each other and getting corrupted.
So there are two possible solutions. The first is to use 4 parallel connections each running @25Gb/sec. The second is to use a single channel with a 25Gb/sec symbol rate but to send four bits per symbol. This can be done either through techniques like Pulse Amplitude Modulation (PAM4) or optically by sending four different colors of light on a single fiber using Wavelength Division Multiplexing (WDM) techniques. Continue reading