Bundle Connection With V.42Bis Data Compression; Selecting V.120 For European Isdn (Dss1) - ZyXEL Communications omni TA128 User Manual

interface layer 1 adaptation protocol without error-control and is
thus inadequate for V.42bis.

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.

Selecting V.120 for European ISDN (DSS1)

With European ISDN, V.120 is an option in the Bearer Capability
(BC) information element, which is a mandatory information
element in the SETUP message. Although we can specify V.120 in
the Low-Layer-Compatibility (LLC) information element, some
switches just don't deliver the LLC. Other switches do deliver the
LLC, but the V.120 selection will be discarded midway.
If the called TA doesn't get any B channel protocol information
from the incoming SETUP message and the remote device is a
ZyXEL ISDN device, the TA128 will be able to identify the V.120
protocol automatically with the Multi Auto-answer routine.
Otherwise, the handshake will fail.
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