hellcatjr, Did some research on ECN trading about 2 years ago. Remember a few good points.
The volumn on ECN is not reported or not counted on the reports we see and in some cases is much more than what is reported.
There is a SEC paper which describes the entire system and is available on the SEC site.
At that time there were 11 or 12 different firms that you could trade ECN's.
The system matches buys and sells and has a seperate clearing system.
As I remember the SEC paper did a very good job of explaning the entire system but did little to control it. That is from memory and can be corrected by anyone with more knowledge.
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