Multitech CDMA User's Guide download pdf (Page 62)

Appendix A – Working With the IWF

CDMA 1xEV-D0 AT Commands Reference Guide (Document S000482B) 62

Remote-only

Some commands require the IWF to provide the response. The local modem treats these as unrecognized

commands. If configured (using +CXT), the local modem will open the airlink and pass the command to the

IWF, then relay the response to the local host; otherwise the local modem returns ERROR for unrecognized

commands. These commands work on the transport layer in the airlink between the two modems. A

command such as +CGCAP is in this class.

Still other commands are only meaningful if there is an established call because the data involved is transient

and only exists in the presence of a call. +CMIP (page 61) is used to query for the current IP address of the

modem. The modem is assigned an IP address by the network only when there is an active IS 95B call.

This is an example of another type of remote-only command.

Remote-only commands are noted in the reference with the tag “(Remote)”. If the tag is not shown, the

command may be shared. Shared commands are “stored” at the local modem and are sent to configure the

IWF when the airlink is established.

Airlink Control

Most of the time, the modem is operating independently (idle)—without an airlink established to the IWF or

beyond. When AT commands are issued to query, configure, and set up a call, the modem may

automatically establish the airlink as needed. When the call is terminated, or the modem is not actively

configuring the IWF, the airlink is dropped to free the radio band for other users.

Knowing when the transport layer is active and when it is not is critical to proper interpretation of the result

codes. If the link is not in place, remote-only commands give the ERROR result code. The command is

unknown to the local modem or is related to a data object only available from the IWF. The same command

responds differently when the airlink is established.

Establishing the Airlink

For the local modem to communicate with the IWF, an airlink is opened between them. The local modem can

open this link without initiating a call through the PSTN or Quick Net Connect to a remote system. There are

two primary methods to establish the airlink:

• Initiate or answer a call. Using ATD to initiate a call or ATA to answer a call causes the local modem

to open the link.

• Issue a remote-only command (or any command unrecognized by the local modem) with the modem

set using +CXT=1 (Cellular Extension enabled).

If +CXT=0 (disabled), then the local modem gives the ERROR result code for unrecognized

commands. When the cellular extension is enabled, the modem will open the airlink and pass the

command to the IWF for processing.

If the modem can find a channel but is not permitted to register, it attempts to establish the airlink result in

the NO CARRIER result.

Initializing the Airlink

Note: To control the configuration process, the local modem and the IWF have a

standard default setting. Unlike standard wire line modems that allow a user

configuration (profile) to be saved and restored on reset, CDMA modems

support only the factory defaults on reset. A nonstandard user configuration

must be sent to the local modem after any reset, ATZ, or power-cycle.

The local modem automatically initializes the airlink each time it is opened. The initialization process involves

first sending the IWF all needed AT commands to configure it to match the (non-default) settings of the local

modem itself. This ensures that both modems are synchronized. The second step to initializing the link is to

send the IWF the contents of the +CFG string (page 59). These are commands selected by the user to

configure the IWF for a particular operation.

Once the airlink is established, the local modem will pass the command that initiated the link (ATD, ATA, or

the unrecognized command line) to the IWF.

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