Technical Reference for ZyXEL omni.net series

Error Correction and Data Compression with V.120

With V.120, the default setting is for LAPD error correction only. No data compression will be negotiated unless V.42bis is enabled by setting [AT&K44].

With the &K44 setting, the omni.net will try to connect using V.42bis data compression. If the remote device does not support V.42, then LAPD error correction will be used.

When a connection is made using V.42bis compression, the following connect message will be displayed:

&CONNECT 115200/V120 56000/LAPD/V42b.

It takes extra time for the calling ISDN TA to negotiate V.42bis. If you know in advance that the called site has no V.42bis capability, it is advisable to issue the [AT&K00] command beforehand in order to get a quick connection.

Since the V.42bis algorithm needs an error-free transmission channel between the compression and decompression processes, it can only work with a protocol with error control competence. X.75 and V.120 are such protocols that can be used together with V.42bis data compression. V.110, on the other hand, is just an R-interface layer 1 adaptation protocol without error-control and is thus inadequate for V.42bis.

Data Compression with X.75

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.

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