Reference Guide for the Model XM128 ISDN Digital Modem
The devices in the two sites then use X.75 Multiple Link Protocol or cFos channel bundling protocol to coordinate the two B channels. The former would need an overhead of 2 octets for each packet. The following is an example of the connect message reported to the DTE. The success of bundle connection initiation is indicated by this message:
CONNECT 460800 / X.75M 128K / V42b
If any B channel is unavailable in any site, the bundle initiation falls back to single channel connection. The following message may appear in this case:
CONNECT 460800 / X.75 64000 / V42b
Dialing Prestored Phone Numbers
The 40 phone numbers stored in NVRAM can also be used to place a bundle call:
•ATDSn (dials the (n+1)th phone number twice for both the bundle connections; n=0 to 39)
•ATDSn+ (dials the (n+1)th phone number for the first connection and dials the (n+2)th phone number for the second connection;
•ATDSn+Sm (dials the (n+1)th phone number for the first connection and the (m+1)th phone number for the second connection;
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 (enables V.42bis on ISDN call)
•AT&K00 (disables V.42bis on ISDN call)
For X.75 to negotiate compression parameters with the remote ISDN terminal, you exchange XID frames before the Link Layer is established. The calling site sends an XID frame with V.42bis request to the called site. If the called site understands the meaning of this XID, it replies to an XID frame with a V.42bis request. If it is able to execute V.42bis, it ignores the XID or replies using an XID frame with a V.42bis reject or empty information field.
The calling site assumes 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 is established.
It takes about 2 seconds for the calling Model XM128 modem to send XID and wait until timing 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.
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