AT+i Programmer's Manual - SE Spezial-Electronic AG

27.3 RAS Theory of Operation
iChip RAS Server
When a remote client dials into iChip, the modem RING strings are transferred by iChip
(which defaults to transparent mode) to the host. When the RAS feature is enabled (RAU
contains a value), iChip picks up the line and negotiates a PPP connection by issuing the
ATA (modem) command after RAR RING strings have been received.
If the host chooses to manage a direct (modem-to-modem) data connection, it can pick up
the line before RAR RING strings have arrived by issuing the ATA modem command.
During RAS PPP negotiations, iChip will replies only to (+++) (escape sequence) and
AT+iRPn commands. Specifically, iChip replies ―Connecting as RAS‖ to the AT+iRP2
(iChip status) command. The escape sequence can be used to abort the RAS session at
any time. The AT+iRP2 command is the only means for the host processor to determine
that a PPP session is in progress. iChip manages the RAS protocol internally and does not
transfer any information to the host. Any other commands received from the host are
disregarded by iChip.
Once the PPP connection has been fully negotiated and established, iChip responds to all
AT+i commands as when it is online. Specifically, iChip replies ―RAS Connected‖ to the
AT+iRP2 command.
As part of the PPP negotiation, iChip assigns itself the default IP 192.168.0.1 and
allocates 192.168.0.2 as the client IP. However, if the client requests a specific IP, iChip
always grants the client‘s request and uses the client‘s IP minus 1 as its own IP.
The following restriction to the minus 1 rule applies: If the IP requested by the client
minus 1 is an IP address that ends with 0x00 or 0x255 as the last nibble, iChip assigns
itself with the client‘s IP plus 1 instead of minus 1. This is done to assure that the IP that
iChip assigns itself never violates the rule that defines that a network or host IP segment
may not be all binary 1‘s, nor all binary 0‘s.
After a RAS PPP connection is established, iChip automatically activates the internal web
server, if the AWS parameter is set to a non-zero value. Thus, the remote client can
browse iChip‘s website.
27.3.1 Auto PPP RAS Mode
iChip allows combining RAS and direct modem-to-modem communication sessions. A
special mode, named Auto PPP RAS, supports dialing into the iChip with a PPP dialer or
a regular modem.
Auto PPP RAS mode is enabled by enabling RAS mode and adding a +100 offset to the
RAR parameter, where [-100] determines the number of RINGS after which iChip
automatically picks up the line and negotiates a PPP connection. The host processor can
instruct the modem to pick up the line beforehand by issuing the ATA (modem)
command or by setting the modem to auto-answer after less than [-100] RING
strings. This is normally done in order to manage a direct modem-to-modem (non-PPP)
communication session.
When iChip is in the Auto PPP RAS mode, it monitors the data stream following the
modem CONNECT line. If the first character transmitted by the remote end is (~) (0x7E),