What are the differences between perfect competition and monopoly competition?

Ans) In a monopoly, you are gaining an unfair benefit over any competition because you own so many infrastructures. Monopolies used to be called as trusts, which is why you sometimes hear of Anti-Trust Law violations.

At one time, AT&T owned each phone line, each phone and every piece of phone equipment in the country. They monopolized the industry; how could you compete with them when they owned everything? Likewise, the Post Office has an excellent infrastructure for delivering mail, but they do not have a monopoly due to FedEx and UPS and DHL have all found ways to carve out a healthy piece of the parcel moving business, so though UPS always grumbles about the Post Office, they do OK in competition.