Table of Contents
Advertisement
AT&K00 - (disable V.42bis on ISDN call)
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 reply to an XID frame with a V.42bis request. If it is
able to execute V.42bis; it will ignore the XID or reply to the XID
frame with a 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 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 is 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 ect.
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
connections. 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. A special I/O card on the
computer side is required in this situation.
X.75 ISDN Communications 69
Advertisement
Table of Contents
