What is a "dark fibre"in terms of stock trading? Explain the recent controversy pertaining to National Stock Exchange (NSE) and algorithm trading.

Dark fibre networks are some dedicated communication line which provides a significant advantage over the internet service providers (ISPs) by facilitating better scalability and speed.
Recently it was alleged that some brokers got preferential access to the trading system in NSE through the co-location facility wherein the brokers can buy “rack space” for their servers at the stock exchange, and thus with the help of early login and dark fibre a trader can have a split-second faster access to the data feed of the exchange. In simple sense, it was alleged that a selected few were given illegal and prioritised access over others so as to deal in multi-legged option orders. With such access the traders were given an advantage of 10-15 milliseconds in comparison with other normal traders which is quite significant in High-Frequency Tradings (HFTs).  As soon as the matter came to light, SEBI formed an expert committee under its Technical Advisory Committee (TAC) to examine the allegations against NSE.  It was found by the expert committee that preferential access was given to certain stock brokers since they were able to log on to multiple dissemination servers through multiple IPs. Moreover, it was also possible for a single member to have multiple logins to a single dissemination server with the help of multiple IPs assigned to it and it was because of which certain stock brokers had a substantial advantage by logging in first or second or third. While the case is on, NSE has sought settlement process in regard to the allegations of unfair access so as to do out-of-court settlement wherein the case can be settled by paying hefty penalties.