What are the asynchronous protocols in data link layer? Discuss in detail with examples.

Answer

Asynchronous protocols in data link layer treat each character in a bit stream independently. A variety of Asynchronous protocols have been developed. We will discuss some of the important ones:

• XMODEM
• YMODEM
• ZMODEM
• BLAST
• KERMIT

XMODEM

It is a simple file transfer protocol used to transfer files from one pc to another in asynchronous communications. It is error correcting modem protocol developed in 1978, which transmits data in 128 byte blocks. There are two more versions of the XMODEM file transfer protocol. One, XMODEM 1K transmits data in 1kilobyte blocks and uses more checking. Another XMODEM-CRC incorporates a 2 byte cyclic redundancy check to detect transmission errors.

YMODEM

YMODEM is a variation of the XMODEM file transfer protocol. It divides the data into blocks, where each block consists of the start-of-header character, a block number, 1 kilobyte of data and a checksum. It also incorporates the capabilities to send multiple files in the same session and to abort transfer of files by transmitting two CAN (cancel) characters in row.

ZMODEM

ZMODEM is an error correcting protocol for modems. It is a newer protocol and combines the features of XMODEM and YMODEM. Modems that agree on using the ZMODEM protocol send data in 512-byte blocks. If a block arrives and an error is detected, a "NAK" (negative acknowledgement) is returned and the block is resent.

BLAST

Blast is a Blocked Asynchronous transmission protocol. It is more powerful than XMODEM. BLAST uses full duplex operation with sliding window flow control. It allows transfer of data and binary files.

KERMIT

It is used to transfer files between PC’s and mainframes over standard telephone lines. It detects transmission errors by checking each block of data. Kermit is a widely known protocol used in numerous software packages designed for communications over telephone lines. Information is transmitted in variable-length units (packets), which are normally up to 96 bytes long, and each packet is checked for transmission errors. Control characters are converted into standard printable ASCII characters (decimal codes 32 through 126) and can thus be transferred without the risk of misinterpretation at the receiving end.