Light in glass is slower than light (or radio waves) in air. Also where they're unable to put point-to-point microwave towers, like the Atlantic ocean, they use

Light in glass is slower than light (or radio waves) in air. Also where they're unable to put point-to-point microwave towers, like the Atlantic ocean, they use HF/shortwave (as in 3-30MHz stuff) radio, because frequencies in that range bounce off the ionosphere. And apparently, the added lag from the increased path length is less than the added lag from glass's optical index, so HF wins out. HF is, however, very noisy and the bandwidth (amount of bulk bits per second you can send) is not too high (even though the end-to-end latency for a single given bit can be very low). It's not pure HF -- there's a fibre link alongside the HF link where stuff like protocol negotiations, encryption keys, channel sounding, feedback on data received and RF conditions, and agreement on what kind of modulation/coding schemes to use for upcoming data (and what transmitted bits are to mean in terms of higher-layer protocol (something about price informations? I don't know much about HFT)). Likewise, the availability of GPS makes time/frequency synchronisation easier (and combined with sounding signals and feedback over fibre), which very likely allows for eliminating latency-inducing complexity in the receiver. It's a far cry from the traditional data-over-HF scenario, which is assumed only to be relevant when other (higher bandwidth) links such as fibre or satellite or point-to-point microwave are unavailable, so everything must be carried out over the RF channel (look at ALE).