ASYNCHRONOUS PROTOCOLS

A number of Asynchronous Data Link protocols have been developed over the last several decades, some of which are shown in Figure(1) .
Now a day, these protocols are employed mainly in modems. Due to its inherent slowness (stemming from the required additions of start and stop bits and extended spaces between frames), Asynchronous transmission at this level is being replaced by higher-speed synchronous mechanisms.

Figure(1) : Asynchronous Protocols

Asynchronous protocols are not complex and are inexpensive to implement. In Asynchronous transmission a data unit is transmitted with no timing coordination between sender and receiver.
A receiver does not need to know exactly when a data unit is sent, It only needs to recognize the beginning and the end of the unit. This is accomplished by using extra bits (start and stop bits) to frame the data unit.

Asynchronous protocols used primarily in modems, feature start and stop bits and variable length gaps between characters.

A variety of Asynchronous data Link layer protocols have been developed, we will discuss only a few of them.

1. XMODEM
In 1979 Ward Christiansen designed a file transfer protocol for telephone-line communication between PCs. This protocol ,now known as XMODEM, is a half-duplex stop-and-wait ARQ protocol. The frame with its fields is shown in Figure(2).

Figure(2) - XMODEM frame

The first field is a one-byte start of header (SOH). The second field is a two-byte header. The first header byte, sequence number, carries the frame number. The second header byte is used to check the validity of the sequence number. The fixed data field holds 128 bytes of data(binary. ASCII. Boolean, text, etc).
The last field, CRC checks for errors in the data field only.
In this protocol, transmission begins with the sending of a NAK frame frame from the receiver to the sender. Each time the sender sends a frame, it must wait for an acknowledgment (ACK) before the next frame can be sent. If instead a NAK is received the received by the sender after a specified amount of time. Besides a NAK or an ACK. the sender can receive a cancel signal (CAN), which aborts the transmission.

2.YMODEM

YMODEM is a protocol similar to XMODEM, with the following major differences :

The data unit is 1024 bytes
Two CANs are sent to abort a transmission.
ITU-T CRC-16 is used for error checking.
Multiple files can be sent simultaneously.

3.ZMODEM

ZMODEM is a newer protocol combining features of both XMODEM and YMODEM.

4.BLAST

Blocked Asynchronous transmission (BLAST) is more powerful than XMODEM. It is full-duplex with sliding window flow control. It allows the transfer of data and binary files.

5.Kermit

Kermit, designed at Columbia University, is currently the most widely used Asynchronous protocol. This file transfer protocol is similar in operation to XMODEM, whith the sender waiting for a NAK before it starts transmission. Kermit allows the transmission of control characters as text using two steps.

First, the control character, which is used as text is transformed to a printable character by adding a fixed number to its ASCII code representation.

Second, the "#" character is added to the front of the transformed character. In this way, a protocol character used as text is sent as two characters. When the receiver encounters a "#" character. It knows that this must be dropped and that the next character is a control character. If the sender wants to send a "#" character, it will send two of them.

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