The ASYNC/SYNC conversion module is inserted on top of OSI layer 3 and is used for automatic framing type detection and conversion. Together with the B-channel autodetection, this module enables automatic discrimination of the incoming call type and thus allows to set up a RAS server that is able to accept digital, analog, wireless, and PIAFS calls on the same number by a standard ASYNC PPP application. See Incoming RAS Call Type Autodetection for more information.
The ASYNC/SYNC conversion module is controlled by the AT+iF and AT+iG commands. If you select modem profile 14 (AT&F14 or ATZ14), correct values will be set for most applications and B-channel protocol detection will be enabled. See AT-Command Set for more information.
At the beginning, the ASYNC/SYNC conversion module escapes every control character when it explodes a sync frame because this is the initial state for PPP over asyncronous lines (expected by RNA). Thus 0xffffffff is set as the initial receive ACCM. If the peer does not send an ACCM with its configuration request, a null ACCM added to such a request by default, to prevent the overhead of escaping every control character.
The ASYNC/SYNC conversion module assumes that such frames will never contain unescaped control characters for asynchronous RNA frames. Thus, 0x00000000 is set like the initial transmit ACCM. Because some routers (for example Cisco and NetGW) reject an ACCM on synchronous links, the ASYNC/SYNC PPP conversion module removes (but remembers) the ACCM from outgoing configuration requests by default.