ZyXEL Communications ACCESSING INTERNET & INTRANET omni series - Manual (Page 75)

V.120 ISDN Communications 63
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 TA will be able to identify the V.120 protocol
automatically with the Multi Auto-answer routine. Otherwise, the
handshake will fail.
Selecting V.120 for Germany National
ISDN (1TR6)
1TR6 data connections are achieved by setting the Service
Indicator to 7 (Daten_bertragung 64Kbps) and the Additional
Information octet is used to select B channel protocols. Since there
is no pre-defined code for asynchronous V.120, TA uses the