Before traveling to the U.S., citizens of many foreign countries need to obtain a visa. Detailed information is available in the U.S. Department of State, Visa Section:
It is the sole responsibility of the individuals, who would like to attend the conference, to apply and receive proper visa to enter the United States. If you need an entry visa, you must apply in advance and allow adequate time for your visa application to be processed. We strongly recommend you to apply for visa 3-4 months before the conference.
If you need an invitation letter from the ITEC’18 Organizing Committee, you must send a request to the General Chair, Bulent Sarlioglu at email@example.com. An invitation letter will be sent to you only after the registration fee is paid and your registration is complete. Your request must include (1) your full name, complete mailing (postal) address, and email address, (2) conference registration number, (3) paper number and/or presentation title in case you are an author and/or presenter, and (4) indication that you need the invitation letter in electronic format or paper copy. If you need the invitation letter in electronic format, it will be sent to you via email. Paper copies will be sent via regular mail.