Eicon Networks ISDN NT1 Especificações Página 56
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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.
- ISDN: Theory & Practice 1
- Basic Concepts 2
- ISDN Services 3
- B & D channels 5
- Fractional PRI 7
- B and D channel protocols 7
- B channel charateristics 8
- Summary 10
- ISDN Networks 12
- Bearer Capabilities 15
- Voice, Fax, and Modems 18
- Soft-fax and Soft-modem 22
- Terminology 25
- The NT1 Plus or Super NT1 26
- The S-bus 27
- Device Addressing 30
- Broadcast address 31
- Call Setup 33
- Finishing a call 36
- Receiving calls 36
- Stop other devices ringing 37
- Connection collision 38
- Managing inbound calls 40
- Rate Adaption 44
- Bit stuffing 45
- Connecting GSM data to ISDN 45
- Conversion 45
- Cabling 47
- Power provided by some NT1s 49
- Termination 50
- More Termins 52
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