Information Transfer Protocols
Communication relies on protocols. In order to ensure clear and clean communication without any errors, the device on each end must follow a very strict set of rules. If either device violates any of the rules, the communication will fail. These rules are called File Transfer Protocols (FTPs).
All the necessary protocols should be included with the software that came with the modem. After communication is established with the host (usually the computer that receives the call), it can be asked what type of protocol to use. The call initiator can then select the matching protocol before starting a file transfer. Both computers must use the same protocol. There are five basic protocols used by modems.
ASCII
This protocol uses the standard ASCII character set, just like typing directly from a keyboard. ASCII protocol has no error-checking or compression features. It is simple, uncomplicated, and is used just to get simple character-based data. It is not a good protocol for transferring program files.
Xmodem
Xmodem is the next level of protocol. The advantage of Xmodem is that it includes error detection, which makes it more suitable for transferring program files. It transfers 128-byte blocks of data and one checksum (error-checking) character. The receiving computer calculates a new checksum and compares it to the one transmitted. If they are the same, the receiving computer transmits an ACK. If they are different, it sends back a NAK, and the transmitting computer then retransmits the data block. The protocol uses parity error checking, which is not perfect. If two errors were to occur-that is, if the first error were to change the parity bit, and the second error were to change it back to its original state-the second would cancel the first, and no error would be reported. The result can be a corrupted file or random characters on the display.
Ymodem
Ymodem is faster than Xmodem. Ymodem transfers data in 1024-byte blocks; therefore, less time is required to verify data with ACKs and NAKs.
Zmodem
Zmodem shares all the features found in Xmodem and Ymodem protocols. It also adds a few new features, including crash recovery, automatic downloading, and a streaming file transfer method. This is the protocol of choice for most situations.
Kermit
This protocol is rarely used today. It was the first of the synchronous protocols for uploading and downloading data to and from a mainframe computer.