When data is communicated using the physical devices on a networks, most often synchronous communication is used. HDLC stands for High Level Data link Control it is an International standard for a data link network. Besides HDLC, a communication system may use other protocols for synchronous or asynchronous transmission of data from a device. The other protocols are X.25, frame relay, ATM, DSL and ADSL. Ethernets and token rings are other protocols used in LAN networks. Sometimes an application protocol is also used. Some of the application protocols are HTTP, SMTP, HTTPS, POP3, ESMTP, TELNET, FTP, DNS, IMAP4 and Bootp.