Invoking V.42Bis Data Compression - ZyXEL Communications Omni TA128 User Manual

Chapter 8 - X.75 ISDN Communications
Bundle Type
Selection
V.42bis
In-band Bundle
Negotiation
End Point
Discrimination

Invoking V.42bis Data Compression

The following AT commands are used to switch the V.42bis data compression on or off
for ISDN data calls when using X.75 or V.120 protocols.
AT&K44
AT&K00
For X.75, to negotiate compression parameters with the remote ISDN terminal, we
exchange XID frames before the Link Layer is established. The calling site will send an
XID frame with V.42bis request to the called site. If the called site understands this XID's
meaning, it will either reply to an XID frame with V.42bis request. If it is able to execute
V.42bis; it will ignore the XID or reply an XID frame with V.42bis reject or empty
information field.
The calling site will assume that the remote site is unable to execute V.42bis if it gets no
reply for a period of time after sending the request XID. In this situation, normal
connection without data compression will be established.
It takes about 2 seconds for the calling ISDN TA to send XID and wait until time out. If
you know in advance that the called site has no V.42bis capability, it would be better to
issue the AT&K00 command beforehand in order to get a quick connection.
Although not defined in X.75, XID frame is based on the encoding in ISO Standard 8885
and being used in V.42/V.42bis. In addition to the compression parameters, XID can be
used to negotiate the packet parameters as window size, packet size, and so on. Thus we
appeal to other vendors to use this scheme to make X.75 more compatible. Any
suggestion on this issue is highly appreciated.
If you are interested in the V.42bis negotiation procedure, you can use the embedded
protocol analyzer to capture and analyze the exchanged XID frames.
Bundle Connection with V.42bis Data Compression
If both sites have set AT&K44 to enable V.42bis negotiation then XID frames will be
exchanged through the main B channel which corresponds to the call established by the
first SETUP message.
Only one data compression channel will be used in bundle connection. That means the
compression is done before packet disassembly and the decompression is done after
packet assembly. The compression ratio of V.42bis is commonly recognized as up to 4:1
for text files. If the line speed is 128K bps, then the DTE speed may reach 512K bps.
This makes the DTE's normal RS-232 serial port unsuitable for bundle applications.
Special I/O card on the computer side is required in this situation for external models.
S100=0
Yes (AT&K44)
No
N/A
(enable V.42bis on ISDN call)
(disable V.42bis on ISDN call)
S100=1
Yes (AT&K44)
No
N/A
8-4
N/A
No
Yes
Optional