Eicon Networks ISDN NT1 Spécifications télécharger pdf (Page 56)

Synchronous and Asynchronous PPP

Synchronous and asynchronous PPP are basically the same, but they are not compatible with each other.

Both can be used to send binary data.

One of the chief difference is that asynchronous PPP can be sent across an asynchronous interface. Such

interfaces have a flow control mechanism used to regulate the flow of data so that no component (eg. buffer,

modem) is overwhelmed. There is often a choice of flow control: hardware or software. This is demonstrated

in the panel below provided by Microsoft on Windows 98.

The software flow control looks for two characters in the data stream. These are the control (non-printable)

characters XON and XOFF. XOFF stops the interface from transmitting until it sees an XON character. The

data that asynchronous PPP is transporting might contain these characters (PPP is bit-transparent). This

could have disastrous consequences on the transport of the data.

Asynchronous PPP uses a technique called byte stuffing. If a control character is found in the data stream

that might have some significance to the data link, an extra character (call an escape character) is inserted

before the character in question. The control character is converted into something else using a table which

maps control characters into something innocuous to the data link.

When the data is received, the escape character is removed and the reverse mapping is performed on the

following character. This recovers the original data. There is an overhead to this process which, depending

on the data being transmitted, could be as high as 50% additional data.

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Eicon Networks ISDN NT1 Spécifications Page 56