collusion among NASDAQ market makers". These results led to an antitrust lawsuit being filed against NASDAQ. As part of NASDAQ's settlement of the antitrust charges, NASDAQ adopted new order handling rules that integrated ECNs into the NASDAQ system. Shortly after this settlement, the SEC adopted Regulation ATS, which permitted ECNs the option of registering as stock exchanges or else being regulated under a separate set of standards for ECNs. At that time major ECNs that became active were Instinet and Island (part of Instinet was spun off, merged with Island into Inet, and acquired by NASDAQ), Archipelago Exchange (which was acquired by the NYSE) and Brut (now acquired by NASDAQ). ECNs enjoyed a resurgence after the adoption of SEC Regulation NMS, which required "trade through" protection of orders in the market, regardless of where those orders are placed. In the past, many ECNs were "closed book"—i.e., allowing participants to interact only with other participants in that network. However, increasingly ECNs have adopted an "open book" format, addressing the potential fragmented liquidity by integrating orders with those of other ECNs or market makers, thus increasing the overall pool of orders. Function ECNs are used as stock exchanges for off-the-floor trading. To trade with an ECN, one must be a subscriber or have an account with a broker that provides direct access trading. ECN subscribers can enter orders into the ECN via a custom computer terminal or network protocols. The ECN will then match contra-side orders (i.e. a sell-order is "contra-side" to a buy-order with the same price and share count) for execution. The ECN will post unmatched orders on the system for other subscribers to view. With ECN, buyers and sellers are able to trade anonymously, with the trade execution reports listing the ECN as the party. ECNs allow its customers to trade outside traditional trading hours. Some ECN brokers may offer additional features to subscribers such as negotiation, reserve size, and pegging, and may have access to the entire ECN book (as opposed to the "top of the book") that real-time market data regarding depth of trading interest. ECNs are also considered to be effective for handling small orders. ECNs are generally facilitated by electronic negotiation, a type of communication between agents that allows cooperative and competitive sharing of information to determine a proper price. Negotiation types The most common paradigm is the electronic auction type. As of 2005, most