Selling tickets:
Visitors buy tickets as usual at the box office. 
The tickets, however, are more than paper- they bear electronic information. Tickets may take the form of "smart cards", "magnetic strips", Barcode, RFID contactless, etc. Tickets may be printed / pre-encoded prior to sale, or printed / encoded at the Point-of-Sale (POS) - at actual selling time.

Entry Readers ("Validators"):
Once the visitor has a ticket, he/she goes to the entrance, and inserts or presents it to the Validator. The Validator normally located on the turnstile.
The Turnstile may be low (tripod) or full height.

Ticket's Validation:
The Validator reads the ticket's encoded data, checks and verifies that data according to pre-defined parameters. 
In standalone Access system, the validator itself checks and verifies all the parameters. 
In Online Access System, the ticket's ID is send to the host computer which hold a main data base of all tickets. The host checks and verifies the ticket ID validity in it's data base.

Ticket's canceling:
Once the ticket have been used, it must be canceled to avoid second use. 
In standalone Access system, if all parameters are found to be in order, the Validator updates the memory in the card (ticket) about this entry, to avoid misuse with this particular card (ticket).
In Online Access system, the host update the main data base, that this ticket's number have been used. 
Then, the Validator opens the turnstile, and let the visitor to enter.