Hayes modem command AT+CRM=0, AT+CRM=1, etc.

[roger88]

I see this command being used this way in a system consisting of a PC hooked to a cellphone:

AT+CRM=0 for circuit data service
AT+CRM=1 for a generic PPP Internet service
AT+CRM=2 for a cellular digital packet data service

I can't find the +CRM command in Hayes command reference materials.

Is +CRM part of the Hayes command set?
Or part of an extended set?
Or does this appear to be a one-off original extension for just the purpose of determining the type of service connection as I describe above?
What is the significance of the plus sign?

 

[robertjo24]

This is what I found in a AT reference manual.

Set Rm Interface Protocol +CRM
Description: The numeric parameter is used for set the Rm interface protocol.
Values: <Val> 0: Asynchronous Data or Fax
1: Packet data service, Relay Layer Rm interface
2: Packet data service, Network Layer Rm interface, PPP
Note: The values 3 and 4 are currently not supported.
3: Packet data service, Network Layer Rm interface, SLIP
4: STU-III Service